Advanced Search
Article Contents

Contrasting Regional Responses of Indian Summer Monsoon Rainfall to Exhausted Spring and Concurrently Emerging Summer El Niño Events


doi: 10.1007/s00376-022-2114-2

  • The inverse relationship between the warm phase of the El Niño Southern Oscillation (ENSO) and the Indian Summer Monsoon Rainfall (ISMR) is well established. Yet, some El Niño events that occur in the early months of the year (boreal spring) transform into a neutral phase before the start of summer, whereas others begin in the boreal summer and persist in a positive phase throughout the summer monsoon season. This study investigates the distinct influences of an exhausted spring El Niño (springtime) and emerging summer El Niño (summertime) on the regional variability of ISMR. The two ENSO categories were formulated based on the time of occurrence of positive SST anomalies over the Niño-3.4 region in the Pacific. The ISMR's dynamical and thermodynamical responses to such events were investigated using standard metrics such as the Walker and Hadley circulations, vertically integrated moisture flux convergence (VIMFC), wind shear, and upper atmospheric circulation. The monsoon circulation features are remarkably different in response to the exhausted spring El Niño and emerging summer El Niño phases, which distinctly dictate regional rainfall variability. The dynamic and thermodynamic responses reveal that exhausted spring El Niño events favor excess monsoon rainfall over eastern peninsular India and deficit rainfall over the core monsoon regions of central India. In contrast, emerging summer El Niño events negatively impact the seasonal rainfall over the country, except for a few regions along the west coast and northeast India.
    摘要: 厄尔尼诺与印度夏季风降水之间的反相关关系已有了广泛的认识。然而,有些厄尔尼诺事件在一年的前几个月发生,并在夏季到来前转变为中性位相,被称为春季衰退型。而有些厄尔尼诺事件从夏季开始,并在印度夏季风期间一直维持,被称为夏季爆发型。这两类厄尔尼诺事件对印度夏季风降水的区域性具有显著不同的影响。本文根据Niño-3.4区正海温异常出现的时间定义了春季衰退型和夏季爆发型厄尔尼诺。通过比较这两类厄尔尼诺发生时沃克环流、哈德来环流、垂直积分的水汽通量辐合、风切变和对流层上层环流的不同反映出印度夏季风降水对这两类厄尔尼诺事件具有不同的动力和热力响应,并导致了印度季风降水不同的区域性特征。春季衰退型厄尔尼诺有利于印度半岛东部降水增多而印度中部季风区降水减少。夏季爆发型厄尔尼诺则导致整个印度(除了印度西部沿海和东北部少数地区外)在夏季风期间降水减少。
  • 加载中
  • Figure 1.  Panels (a) and (b) show the standardized anomaly composite of Sea Surface Temperature (SST) (K) for JJAS after an exhausted spring El Niño and during a concurrent emerging summer El Niño, respectively. The color shades represent the standardized SST anomalies, and regions with black dots represent the areas where the anomaly is significant at a confidence level of 90% or higher.

    Figure 2.  Panels (a) and (b) illustrate the composite standardized rainfall anomaly (mm d -1) during JJAS (June-July-August-September) following an exhausted spring El Niño and concurrently emerging summer El Niño events, respectively. The hatched areas in the figure represent regions where values are statistically significant at a 90% confidence level or higher.

    Figure 3.  Panels (a) and (b) depict the composite temporal evolution of SST (°C) and rainfall (mm d-1) during years of an exhausted spring El Niño and a concurrently emerging summer El Niño, respectively. The bars represent standardized SST anomalies, and the green line represents standardized rainfall anomalies.

    Figure 4.  Anomalous Hadley circulation (averaged between longitudes 60°E and 90°E) during the Monsoon (a) after an exhausted spring El Niño, (b) during a concurrent emerging summer El Niño. The anomalous Walker circulation (averaged between the latitudes of 0 to 20°N) during the monsoon, (c) after an exhausted spring El Niño, and (d) during a concurrent emerging summer El Niño. Shaded (colored) regions represent the composite of vertical velocity anomalies, and the vectors depict the circulation anomalies. Anomaly composites, significant at the 90% confidence level, are also included in the figure. The green and violet contours in Figs. 4a and b represent the westerly and easterly winds, respectively.

    Figure 5.  VIMFC anomalies during monsoon season overlaid by wind at 850 hPa (a) after an exhausted spring El Niño and (b) during a concurrent emerging summer El Niño. The shaded (colored) area in the figure represents VIMFC composite anomalies overlaid with 850 hPa wind vectors that are significant at the 90% confidence level or more. The dotted area represents VIMFC anomalies significant at the 99% confidence level. The vertically integrated moisture flux converegence has units of kg m−2 s−1.

    Figure 6.  Shaded (colored) region portrays wind shear anomalies (m s−1) during monsoon season (a) after an exhausted spring El Niño and (b) during a concurrent emerging summer El Niño event. The wind vectors are drawn only over the regions where wind composite anomalies are statistically significant at the 90% confidence level or more. Over the regions where shear composite anomalies exist, black dots represent those areas significant at the 99% confidence level.

    Figure 7.  Upper-level wind anomaly composite and mean vector wind (m s−1) at 200 hPa during the monsoon season (a) after an exhausted spring El Niño and (b) during a concurrent emerging summer El Niño. The vectors indicate the mean wind composite with a statistical significance of 90% or more. The dotted areas represent anomaly composite significant at the 99% confidence level.

    Figure 8.  The anomaly composite of velocity potential and divergent wind (10−6 m2 s−1) during JJAS after an exhausted spring El Niño (a) and (b) a concurrent emerging summer El Niño. The color contours are the velocity potential anomaly, and the vectors represent the divergent wind anomaly. The red lines in the figure represent the zero velocity potential contour. Dotted areas in the figure are velocity potential anomalies significant at the 95% confidence level.

    Figure 9.  Two schematic diagrams with distinct features during monsoon season (a) after an exhausted spring El Niño and (b) during a concurrent Emerging Summer El Niño. The lower layer shows VIMFC composite anomalies, the middle layer is the composite of shear anomalies, and the top-most layer is the upper-wind anomaly composite.

    Figure A1.  Composite STD anomaly of rainfall (mm d-1) when monsoon coincides with Concurrent Emerging Summer El Niño and positive IOD.

    Figure A2.  Composite of SST (K) anomaly during monsoon with Concurrent Emerging Summer El Niño and positive IOD.

  • Annamalai, H., K. Hamilton, and K. R. Sperber, 2007: The South Asian summer monsoon and its relationship with ENSO in the IPCC AR4 simulations. J. Climate, 20(6), 1071−1092, https://doi.org/10.1175/JCLI4035.1.
    Ashok, K., Z. Y. Guan, and T. Yamagata, 2001: Impact of the Indian Ocean dipole on the relationship between the Indian monsoon rainfall and ENSO. Geophys. Res. Lett., 28(23), 4499−4502, https://doi.org/10.1029/2001GL013294.
    Boschat, G., P. Terray, and S. Masson, 2011: Interannual relationships between Indian summer monsoon and Indo-Pacific coupled modes of variability during recent decades. Climate Dyn., 37(5), 1019−1043, https://doi.org/10.1007/s00382-010-0887-y.
    Boschat, G., P. Terray, and S. Masson, 2012: Robustness of SST teleconnections and precursory patterns associated with the Indian summer monsoon. Climate Dyn., 38(11), 2143−2165, https://doi.org/10.1007/s00382-011-1100-7.
    Cao, Q., Z. C. Hao, F. F. Yuan, Z. K. Su, R. Berndtsson, J. Hao, and T. Nyima, 2017: Impact of ENSO regimes on developing-and decaying-phase precipitation during rainy season in China. Hydrology and Earth System Sciences, 21(11), 5415−5426, https://doi.org/10.5194/hess-21-5415-2017.
    Chakraborty, A., 2018: Preceding winter La Niña reduces Indian summer monsoon rainfall. Environmental Research Letters, 13(5), 054030, https://doi.org/10.1088/1748-9326/aabdd5.
    Chattopadhyay, R., S. A. Rao, C. T. Sabeerali, G. George, D. N. Rao, A. Dhakate, and K. Salunke, 2016: Large-scale teleconnection patterns of Indian summer monsoon as revealed by CFSv2 retrospective seasonal forecast runs. International Journal of Climatology, 36(9), 3297−3313, https://doi.org/10.1002/joc.4556.
    Chowdary, J., A. Parekh, and C. Gnanaseelan, 2021: Indian Summer Monsoon Variability: El-Nino Teleconnections and Beyond. Elsevier,
    Chowdary, J. S., H. S. Harsha, C. Gnanaseelan, G. Srinivas, A. Parekh, P. Pillai, and C. V. Naidu, 2017: Indian summer monsoon rainfall variability in response to differences in the decay phase of El Niño. Climate Dyn., 48(7), 2707−2727, https://doi.org/10.1007/s00382-016-3233-1.
    Dogar, M. M., F. Kucharski, and S. Azharuddin, 2017: Study of the global and regional climatic impacts of ENSO magnitude using SPEEDY AGCM. Journal of Earth System Science, 126(2), 30, https://doi.org/10.1007/s12040-017-0804-4.
    Feng, J., and J. P. Li, 2013: Contrasting impacts of two types of ENSO on the boreal spring Hadley circulation. J. Climate, 26(13), 4773−4789, https://doi.org/10.1175/JCLI-D-12-00298.1.
    Gadgil, S. and J. Srinivasan, 2011: Seasonal prediction of the Indian monsoon. Current Science, 100(3), 343−353.
    Gadgil, S., P. N. Vinayachandran, P. A. Francis, and S. Gadgil, 2004: Extremes of the Indian summer monsoon rainfall, ENSO and equatorial Indian Ocean oscillation. Geophys. Res. Lett., 31(12), L12213, https://doi.org/10.1029/2004GL019733.
    Geethalakshmi, V., A. Yatagai, K. Palanisamy, and C. Umetsu, 2009: Impact of ENSO and the Indian Ocean Dipole on the north-east monsoon rainfall of Tamil Nadu State in India. Hydrological Processes, 23(4), 633−647, https://doi.org/10.1002/hyp.7191.
    Goswami, B. N., M. S. Madhusoodanan, C. P. Neema, and D. Sengupta, 2006: A physical mechanism for North Atlantic SST influence on the Indian summer monsoon. Geophys. Res. Lett., 33(2), L02706, https://doi.org/10.1029/2005GL024803.
    Held, I. M., and A. Y. Hou, 1980: Nonlinear axially symmetric circulations in a nearly inviscid atmosphere. J. Atmos. Sci., 37(3), 515−533, https://doi.org/10.1175/1520-0469(1980)037<0515:NASCIA>2.0.CO;2.
    Huang, B. Y., and Coauthors, 2015: Extended reconstructed sea surface temperature version 4 (ERSST.v4). Part I: Upgrades and intercomparisons. J. Climate, 28(3), 911−930, https://doi.org/10.1175/JCLI-D-14-00006.1.
    Huang, B. Y., and Coauthors, 2016: Further exploring and quantifying uncertainties for extended reconstructed sea surface temperature (ERSST) version 4 (v4). J. Climate, 29(9), 3119−3142, https://doi.org/10.1175/JCLI-D-15-0430.1.
    Huang, R. H. and Y. F. Wu, 1989: The influence of ENSO on the summer climate change in China and its mechanism. Adv. Atmos. Sci., 6(1), 21−32, https://doi.org/10.1007/BF02656915.
    Ihara, C., Y. Kushnir, M. A. Cane, and A. Kaplan, 2008: Timing of El Niño–related warming and Indian summer monsoon rainfall. J. Climate, 21(11), 2711−2719, https://doi.org/10.1175/2007JCLI1979.1.
    Joseph, P. V., and S. Sijikumar, 2004: Intraseasonal variability of the low-level jet stream of the Asian summer monsoon. J. Climate, 17(7), 1449−1458, https://doi.org/10.1175/1520-0442(2004)017<1449:IVOTLJ>2.0.CO;2.
    Ju, J. H., and J. Slingo, 1995: The Asian summer monsoon and ENSO. Quart. J. Roy. Meteor. Soc., 121(525), 1133−1168, https://doi.org/10.1002/qj.49712152509.
    Kalnay, E., and Coauthors, 1996: The NCEP/NCAR 40-year reanalysis project. Bull. Amer. Meteor. Soc., 77(3), 437−472, https://doi.org/10.1175/1520-0477(1996)077<0437:TNYRP>2.0.CO;2.
    Khole, M., and U. S. De, 2003: A study on north-east monsoon rainfall over India. Mausam, 54(2), 419−426, https://doi.org/10.54302/mausam.v54i2.1527.
    Koteswaram, P., 1958: The easterly jet stream in the tropics. Tellus, 10(1), 43−57, https://doi.org/10.3402/tellusa.v10i1.9220.
    Kripalani, R. H., J. H. Oh, A. Kulkarni, S. S. Sabade, and H. S. Chaudhari, 2007: South Asian summer monsoon precipitation variability: Coupled climate model simulations and projections under IPCC AR4. Theor. Appl. Climatol., 90(3), 133−159, https://doi.org/10.1007/s00704-006-0282-0.
    Krishnamurthy, V., and B. N. Goswami, 2000: Indian monsoon–ENSO relationship on interdecadal timescale. J. Climate, 13(3), 579−595, https://doi.org/10.1175/1520-0442(2000)013<0579:IMEROI>2.0.CO;2.
    Krishnamurthy, V., and J. L. Kinter III, 2003: The Indian monsoon and its relation to global climate variability. Global Climate, X. Rodó and F. A. Comín, Eds., Springer, 186−236,
    Krishnan, R., and M. Sugi, 2003: Pacific decadal oscillation and variability of the Indian summer monsoon rainfall. Climate Dyn., 21(3-4), 233−242, https://doi.org/10.1007/s00382-003-0330-8.
    Krishnaswamy, J., S. Vaidyanathan, B. Rajagopalan, M. Bonell, M. Sankaran, R. S. Bhalla, and S. Badiger, 2015: Non-stationary and non-linear influence of ENSO and Indian Ocean Dipole on the variability of Indian monsoon rainfall and extreme rain events. Climate Dyn., 45(1-2), 175−184, https://doi.org/10.1007/s00382-014-2288-0.
    Kumar, K. K., B. Rajagopalan, and M. A. Cane, 1999: On the weakening relationship between the Indian monsoon and ENSO. Science, 284(5423), 2156−2159, https://doi.org/10.1126/science.284.5423.2156.
    Li, X. Z., Z. P. Wen, D. L. Chen, and Z. S. Chen, 2019: Decadal transition of the leading mode of interannual moisture circulation over East Asia–western North Pacific: Bonding to different evolution of ENSO. J. Climate, 32(2), 289−308, https://doi.org/10.1175/JCLI-D-18-0356.1.
    Liu, W., and Coauthors, 2015: Extended reconstructed sea surface temperature version 4 (ERSST.v4): Part II. Parametric and structural uncertainty estimations. J. Climate, 28(3), 931−951, https://doi.org/10.1175/JCLI-D-14-00007.1.
    Mooley, D. A., and B. Parthasarathy, 1984: Fluctuations in all-India summer monsoon rainfall during 1871–1978. Climatic Change, 6(3), 287−301, https://doi.org/10.1007/BF00142477.
    Nguyen, H., A. Evans, C. Lucas, I. Smith, and B. Timbal, 2013: The Hadley circulation in reanalyses: Climatology, variability, and change. J. Climate, 26(10), 3357−3376, https://doi.org/10.1175/JCLI-D-12-00224.1.
    Pai, D. S., M. Rajeevan, O. P. Sreejith, B. Mukhopadhyay, and N. S. Satbha, 2014: Development of a new high spatial resolution (0.25° × 0.25°) long period (1901-2010) daily gridded rainfall data set over India and its comparison with existing data sets over the region. Mausam, 65(1), 1−18, https://doi.org/10.54302/mausam.v65i1.851.
    Palmer, T. N., Č. Branković, P. Viterbo, and M. J. Miller, 1992: Modeling interannual variations of summer monsoons. J. Climate, 5(5), 399−417, https://doi.org/10.1175/1520-0442(1992)005<0399:MIVOSM>2.0.CO;2.
    Park, H.-S., J. C. H. Chiang, B. R. Lintner, and G. J. Zhang, 2010: The delayed effect of major El Niño events on Indian monsoon rainfall. J. Climate, 23(4), 932−946, https://doi.org/10.1175/2009JCLI2916.1.
    Parthasarathy, B., A. A. Munot, and D. R. Kothawale, 1994: All-India monthly and seasonal rainfall series: 1871–1993. Theor. Appl. Climatol., 49(4), 217−224, https://doi.org/10.1007/BF00867461.
    Pathak, A., S. Ghosh, J. A. Martinez, F. Dominguez, and P. Kumar, 2017: Role of oceanic and land moisture sources and transport in the seasonal and interannual variability of summer monsoon in India. J. Climate, 30(5), 1839−1859, https://doi.org/10.1175/JCLI-D-16-0156.1.
    Patil, C., T. Prabhakaran, K. C. Sinha Ray, and A. Karipot, 2019: Revisiting moisture transport during the Indian summer monsoon using the moisture river concept. Pure Appl. Geophys., 176(11), 5107−5123, https://doi.org/10.1007/s00024-019-02224-1.
    Pokhrel, S., and Coauthors, 2012: ENSO, IOD and Indian summer monsoon in NCEP climate forecast system. Climate Dyn., 39(9−10), 2143−2165, https://doi.org/10.1007/s00382-012-1349-5.
    Rasmusson, E. M., and T. H. Carpenter, 1983: The relationship between eastern equatorial Pacific sea surface temperatures and rainfall over India and Sri Lanka. Mon. Wea. Rev., 111(3), 517−528, https://doi.org/10.1175/1520-0493(1983)111<0517:TRBEEP>2.0.CO;2.
    Sahana, A. S., S. Ghosh, A. Ganguly, and R. Murtugudde, 2015: Shift in Indian summer monsoon onset during 1976/1977. Environmental Research Letters, 10(5), 054006, https://doi.org/10.1088/1748-9326/10/5/054006.
    Saji, N. H., B. N. Goswami, P. N. Vinayachandran, and T. Yamagata, 1999: A dipole mode in the tropical Indian Ocean. Nature, 401(6751), 360−363, https://doi.org/10.1038/43854.
    Seager, R., N. Harnik, Y. Kushnir, W. Robinson, and J. Miller, 2003: Mechanisms of hemispherically symmetric climate variability. J. Climate, 16(18), 2960−2978, https://doi.org/10.1175/1520-0442(2003)016<2960:MOHSCV>2.0.CO;2.
    Shukla, J., and J. M. Wallace, 1983: Numerical simulation of the atmospheric response to equatorial Pacific sea surface temperature anomalies. J. Atmos. Sci., 40(7), 1613−1630, https://doi.org/10.1175/1520-0469(1983)040<1613:NSOTAR>2.0.CO;2.
    Sikka, D. R, 1980: Some aspects of the large scale fluctuations of summer monsoon rainfall over India in relation to fluctuations in the planetary and regional scale circulation parameters. Proceedings of the Indian Academy of Sciences-Earth and Planetary Sciences, 89(2), 179−195, https://doi.org/10.1007/BF02913749.
    Soman, M. K., and J. Slingo, 1997: Sensitivity of the Asian summer monsoon to aspects of sea-surface-temperature anomalies in the tropical Pacific Ocean. Quart. J. Roy. Meteor. Soc., 123(538), 309−336, https://doi.org/10.1002/qj.49712353804.
    Sreenath. A. V., and S. Abhilash, 2021: Convection over the eastern equatorial Indian Ocean and Iranian branch of Asian summer monsoon anticyclone modulates the wet and dry phases of summer monsoon rainfall over Kerala. International Journal of Climatology, 41(6), 3670−3687, https://doi.org/10.1002/joc.7042.
    Wang, L., J.-Y. Yu, and H. Paek, 2017: Enhanced biennial variability in the Pacific due to Atlantic capacitor effect. Nature Communications, 8, 14887, https://doi.org/10.1038/ncomms14887.
    Webster, P. J., and S. Yang, 1992: Monsoon and ENSO: Selectively interactive systems. Quart. J. Roy. Meteor. Soc., 118(507), 877−926, https://doi.org/10.1002/qj.49711850705.
    Wen, N., and Y. S. Hao, 2021: Contrasting El Niño impacts on East Asian summer monsoon precipitation between its developing and decaying stages. International Journal of Climatology, 41(4), 2375−2382, https://doi.org/10.1002/joc.6964.
    Yang, J. L., Q. Y. Liu, S.-P. Xie, Z. Y. Liu, and L. X. Wu, 2007: Impact of the Indian Ocean SST basin mode on the Asian summer monsoon. Geophys. Res. Lett., 34(2), L02708, https://doi.org/10.1029/2006GL028571.
    Yang, X. K., and P. Huang, 2021: Restored relationship between ENSO and Indian summer monsoon rainfall around 1999/2000. The Innovation, 2(2), 100102, https://doi.org/10.1016/j.xinn.2021.100102.
  • [1] LIU Xiangwen, WU Tongwen, YANG Song, LI Qiaoping, CHENG Yanjie, LIANG Xiaoyun, FANG Yongjie, JIE Weihua, NIE Suping, 2014: Relationships between Interannual and Intraseasonal Variations of the Asian-Western Pacific Summer Monsoon Hindcasted by BCC_CSM1.1(m), ADVANCES IN ATMOSPHERIC SCIENCES, 31, 1051-1064.  doi: 10.1007/s00376-014-3192-6
    [2] WU Bingyi, 2005: Weakening of Indian Summer Monsoon in Recent Decades, ADVANCES IN ATMOSPHERIC SCIENCES, 22, 21-29.  doi: 10.1007/BF02930866
    [3] Yuli ZHANG, Chuanxi LIU, Yi LIU, Rui YANG, 2019: Intraseasonal Oscillation of Tropospheric Ozone over the Indian Summer Monsoon Region, ADVANCES IN ATMOSPHERIC SCIENCES, 36, 417-430.  doi: 10.1007/s00376-018-8113-7
    [4] Yi LI, Yihui DING, Weijing LI, 2017: Interdecadal Variability of the Afro-Asian Summer Monsoon System, ADVANCES IN ATMOSPHERIC SCIENCES, 34, 833-846.  doi: 10.1007/s00376-017-6247-7
    [5] Ruifen ZHAN, Yuqing WANG, Yihui DING, 2022: Impact of the Western Pacific Tropical Easterly Jet on Tropical Cyclone Genesis Frequency over the Western North Pacific, ADVANCES IN ATMOSPHERIC SCIENCES, 39, 235-248.  doi: 10.1007/s00376-021-1103-1
    [6] WANG Chenghai, YU Lian, HUANG Bo, 2012: The Impact of Warm Pool SST and General Circulation on Increased Temperature over the Tibetan Plateau, ADVANCES IN ATMOSPHERIC SCIENCES, 29, 274-284.  doi: 10.1007/s00376-011-1034-3
    [7] Wen CHEN, Renhe ZHANG, Renguang WU, Zhiping WEN, Liantong ZHOU, Lin WANG, Peng HU, Tianjiao MA, Jinling PIAO, Lei SONG, Zhibiao WANG, Juncong LI, Hainan GONG, Jingliang HUANGFU, Yong LIU, 2023: Recent Advances in Understanding Multi-scale Climate Variability of the Asian Monsoon, ADVANCES IN ATMOSPHERIC SCIENCES, 40, 1429-1456.  doi: 10.1007/s00376-023-2266-8
    [8] Kai Chi WONG, Senfeng LIU, Andrew G. TURNER, Reinhard K. SCHIEMANN, 2018: Different Asian Monsoon Rainfall Responses to Idealized Orography Sensitivity Experiments in the HadGEM3-GA6 and FGOALS-FAMIL Global Climate Models, ADVANCES IN ATMOSPHERIC SCIENCES, 35, 1049-1062.  doi: 10.1007/s00376-018-7269-5
    [9] Qiyang LIU, Fengxue QIAO, Yongqiang YU, Yiting ZHU, Shuwen ZHAO, Yujia LIU, Fulin JIANG, Xinyu HU, 2023: Bias Analysis in the Simulation of the Western North Pacific Tropical Cyclone Characteristics by Two High-Resolution Global Atmospheric Models, ADVANCES IN ATMOSPHERIC SCIENCES, 40, 634-652.  doi: 10.1007/s00376-022-2159-2
    [10] Fei WANG, Lifang SHENG, Xiadong AN, Haixia ZHOU, Yingying ZHANG, Xiaodong LI, Yigeng DING, Jing YANG, 2022: The Impact of an Abnormal Zonal Vertical Circulation in Autumn of Super El Niño Years on Non-tropical-cyclone Heavy Rainfall over Hainan Island, ADVANCES IN ATMOSPHERIC SCIENCES, 39, 1914-1924.  doi: 10.1007/s00376-022-1388-8
    [11] Chaofan LI, Wei CHEN, Xiaowei HONG, Riyu LU, 2017: Why Was the Strengthening of Rainfall in Summer over the Yangtze River Valley in 2016 Less Pronounced than that in 1998 under Similar Preceding El Niño Events?——Role of Midlatitude Circulation in August, ADVANCES IN ATMOSPHERIC SCIENCES, 34, 1290-1300.  doi: 10.1007/s00376-017-7003-8
    [12] Lu Jingxi, Ding Yihui, 1989: Climatic Study on the Summer Tropical Easterly Jet at 200 hPa, ADVANCES IN ATMOSPHERIC SCIENCES, 6, 215-226.  doi: 10.1007/BF02658017
    [13] Shuang LIU, Kaiheng HU, Weiming LIU, Paul A. CARLING, 2022: Hydro-climatic Characteristics of Yarlung Zangbo River Basin since the Last Glacial Maximum, ADVANCES IN ATMOSPHERIC SCIENCES, 39, 415-426.  doi: 10.1007/s00376-021-1150-7
    [14] Liang Pingde, Liu Aixia, 1994: Winter Asia Jetstream and Seasonal Precipitation in East China, ADVANCES IN ATMOSPHERIC SCIENCES, 11, 311-318.  doi: 10.1007/BF02658150
    [15] Dan WANG, Aihui WANG, Lianlian XU, Xianghui KONG, 2020: The Linkage between Two Types of El Niño Events and Summer Streamflow over the Yellow and Yangtze River Basins, ADVANCES IN ATMOSPHERIC SCIENCES, 37, 160-172.  doi: 10.1007/s00376-019-9049-2
    [16] Congxi FANG, Yu LIU, Qiufang CAI, Huiming SONG, 2021: Why Does Extreme Rainfall Occur in Central China during the Summer of 2020 after a Weak El Niño?, ADVANCES IN ATMOSPHERIC SCIENCES, 38, 2067-2081.  doi: 10.1007/s00376-021-1009-y
    [17] Feng XUE, Xiao DONG, Fangxing FAN, 2018: Anomalous Western Pacific Subtropical High during El Niño Developing Summer in Comparison with Decaying Summer, ADVANCES IN ATMOSPHERIC SCIENCES, 35, 360-367.  doi: 10.1007/s00376-017-7046-x
    [18] YAN Bangliang, 2005: On the Mechanism of the Locking of the El Ni o Event Onset Phase to Boreal Spring, ADVANCES IN ATMOSPHERIC SCIENCES, 22, 741-750.  doi: 10.1007/BF02918717
    [19] Zhang Renhe, Zhao Gang, 2001: Meridional Wind Stress Anomalies over the Tropical Pacific and the Onset of El Ni?o Part Ⅱ: Dynamical Analysis, ADVANCES IN ATMOSPHERIC SCIENCES, 18, 1053-1065.  doi: 10.1007/s00376-001-0022-4
    [20] Xiaomeng SONG, Renhe ZHANG, Xinyao RONG, 2019: Influence of Intraseasonal Oscillation on the Asymmetric Decays of El Niño and La Niña, ADVANCES IN ATMOSPHERIC SCIENCES, , 779-792.  doi: 10.1007/s00376-019-9029-6

Get Citation+

Export:  

Share Article

Manuscript History

Manuscript received: 20 April 2022
Manuscript revised: 10 August 2022
Manuscript accepted: 22 September 2022
通讯作者: 陈斌, bchen63@163.com
  • 1. 

    沈阳化工大学材料科学与工程学院 沈阳 110142

  1. 本站搜索
  2. 百度学术搜索
  3. 万方数据库搜索
  4. CNKI搜索

Contrasting Regional Responses of Indian Summer Monsoon Rainfall to Exhausted Spring and Concurrently Emerging Summer El Niño Events

    Corresponding author: S. ABHILASH, abhimets@gmail.com
  • 1. Department of Atmospheric Sciences, Cochin University of Science and Technology, Cochin 682022, India
  • 2. Advanced Centre for Atmospheric Radar Research, Cochin University of Science and Technology, Cochin 682022, India
  • 3. Faculty of Science and Natural Resources, Universiti Malaysia Sabah, Kota Kinabalu 88400, Malaysia

Abstract: The inverse relationship between the warm phase of the El Niño Southern Oscillation (ENSO) and the Indian Summer Monsoon Rainfall (ISMR) is well established. Yet, some El Niño events that occur in the early months of the year (boreal spring) transform into a neutral phase before the start of summer, whereas others begin in the boreal summer and persist in a positive phase throughout the summer monsoon season. This study investigates the distinct influences of an exhausted spring El Niño (springtime) and emerging summer El Niño (summertime) on the regional variability of ISMR. The two ENSO categories were formulated based on the time of occurrence of positive SST anomalies over the Niño-3.4 region in the Pacific. The ISMR's dynamical and thermodynamical responses to such events were investigated using standard metrics such as the Walker and Hadley circulations, vertically integrated moisture flux convergence (VIMFC), wind shear, and upper atmospheric circulation. The monsoon circulation features are remarkably different in response to the exhausted spring El Niño and emerging summer El Niño phases, which distinctly dictate regional rainfall variability. The dynamic and thermodynamic responses reveal that exhausted spring El Niño events favor excess monsoon rainfall over eastern peninsular India and deficit rainfall over the core monsoon regions of central India. In contrast, emerging summer El Niño events negatively impact the seasonal rainfall over the country, except for a few regions along the west coast and northeast India.

摘要: 厄尔尼诺与印度夏季风降水之间的反相关关系已有了广泛的认识。然而,有些厄尔尼诺事件在一年的前几个月发生,并在夏季到来前转变为中性位相,被称为春季衰退型。而有些厄尔尼诺事件从夏季开始,并在印度夏季风期间一直维持,被称为夏季爆发型。这两类厄尔尼诺事件对印度夏季风降水的区域性具有显著不同的影响。本文根据Niño-3.4区正海温异常出现的时间定义了春季衰退型和夏季爆发型厄尔尼诺。通过比较这两类厄尔尼诺发生时沃克环流、哈德来环流、垂直积分的水汽通量辐合、风切变和对流层上层环流的不同反映出印度夏季风降水对这两类厄尔尼诺事件具有不同的动力和热力响应,并导致了印度季风降水不同的区域性特征。春季衰退型厄尔尼诺有利于印度半岛东部降水增多而印度中部季风区降水减少。夏季爆发型厄尔尼诺则导致整个印度(除了印度西部沿海和东北部少数地区外)在夏季风期间降水减少。

    • Climatologically, nearly 80% of the annual rainfall in most regions of India occurs during the summer monsoon (June to September) season (Parthasarathy et al., 1994). Characteristics of monsoon rainfall over India are well documented, and its teleconnections with various global climatic phenomena are a subject of a large number of studies (Sikka, 1980; Rasmusson and Carpenter, 1983; Webster and Yang, 1992; Ashok et al., 2001; Krishnan and Sugi, 2003; Gadgil et al., 2004; Goswami et al., 2006; Annamalai, 2007; Kripalani et al., 2007; Chattopadhyay et al., 2016, etc.). It is understood that the Indian summer monsoon rainfall (ISMR) exhibits an inter-annual variability of 10% relative to its long-period average, attributed to both external and internal factors (e.g., Krishnamurthy and Kinter III, 2003). Among the external forcing mechanisms, sea surface temperature (SST) anomalies over the central and eastern Pacific Ocean that manifest themselves as El Niño Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) (Saji et al., 1999, Gadgil et al., 2004) have been identified as most crucial (Kumar et al., 1999; Gadgil and Srinivasan, 2011; Krishnaswamy et al., 2015 and others). The warm episodes of ENSO (El Niño) tend to be associated with deficit monsoon rainfall in India, and cold episodes (La Niña) favor excess ISMR (Mooley and Parthasarathy, 1984; Parthasarathy et al., 1994). Nevertheless, this relationship has weakened in recent years (e.g., Kumar et al., 1999), and in certain epochs, it has strengthened (Yang and Huang, 2021).

      According to Chowdary et al. (2017), El Niño decay phases are divided into three types based on the time of El Niño decay relative to the boreal summer season: (1) early-decay (ED; decay in the spring), (2) mid-summer decay (MD; decay by mid-summer), and (3) no-decay (ND; no decay in summer). The authors discovered that ISM rainfall is above normal/excessive during ED years, normal during MD years, and below normal/deficient during ND years, implying that changes in the El Niño decay phase exert a significant impact on ISM rainfall. Similarly, Chakraborty (2018) studied the role of ENSO in the preceding winter on ISMR with a focus on ENSO-neutral summers and La Niña winters. Their study showed that, unlike the simultaneous ENSO-ISMR relationship, the La Niña of the previous winter reduced mean rainfall over the country by about 4%, even during ENSO-neutral summer conditions. Similarly, a recent study by Yang and Huang (2021) analyzed ENSO's evolution diversity during recent decades, in which El Niño remnants from the previous winter or emerging from late spring and continuing into the summer were found to be a dominant factor in perturbing the ENSO–ISMR relationship. Boschat et al. (2011) investigated several SST indices in the Indo-Pacific area and their influence on ISMR variability. They found that some of these indices were associated with distinct processes occurring within the Indian Ocean and that those processes were remotely and dynamically linked to various ENSO phases in the Pacific, thereby implying that the choice of better indices for forecasting strategies depends on the contemporaneous phasing of the ENSO cycle. Boschat et al. (2012) found substantial differences in SST teleconnections and precursory patterns between the first (June-July, JJ) and second (August-September, AS) halves of the monsoon. They also highlighted the existence of a biennial rhythm in the IOD-ENSO-ISM system, according to which co-occurring El Niño and positive IOD events tended to be followed by IO warming, which led to a wet ISMR throughout summer.

      Chowdary et al. (2021) provide greater insight into Indian monsoon teleconnections to ENSO and non-ENSO events. Non-ENSO teleconnections have been poorly understood for decades. Park et al. (2010) illustrated using idealized AGCM models with a fixed-depth ocean mixed layer that the lingering but weaker-than-peak warm SSTs in the eastern equatorial Pacific following a winter El Niño make a considerable contribution to North Indian Ocean (NIO) warming. Furthermore, a warm NIO enhances the surface latent heat flux, restoring the monsoon circulation to the climatological mean. The monsoon circulation strength and moist processes collaborate, which explains the known tendency for monsoonal rainfall to surge during the late monsoon season following severe winter El Niño conditions. Yang et al. (2007) showed that after an El Niño event, there is a basin-wide warming across the tropical Indian Ocean that peaks in late boreal winter and early spring and lasts until boreal summer. As the Indian Ocean warms, precipitation rises throughout most of the basin, forcing a Matsuno-Gill pattern in the upper troposphere with a reinforced South Asian high. As a result, the southwest monsoon strengthens in the Arabian Sea while weakening in the South China and Philippine Seas. Wang et al. (2017) proposed that the "charging" ENSO mechanism imprints the North Tropical Atlantic (NTA) SST through an atmospheric bridge mechanism and a "discharging" mechanism in which the NTA SST triggers the following ENSO via a subtropical teleconnection mechanism. This “discharging” mechanism represents an alternate process that results in the biennial cyclical shifting in the Pacific that can further impact ISMR.

      In contrast to the conventional El Niño–dry ISMR relationship, Ihara et al. (2008) found that ISMR can be above normal even during an El Niño event, provided the warming across the eastern equatorial Pacific begins in boreal winter and continues through summer. As a result, during the summer monsoon season, the western-central Pacific and the Indian Ocean are warmer than usual. Their analysis considered El Niño episodes that emerge in early January and last well into the boreal summer. In the present study, we examine the relationship between recently exhausted spring El Niño events and ISMR, where the El Nino events do not extend into boreal summer, and similarly, the relationship between emerging summer El Niño with ISMR, which has no association with any event that concluded in boreal spring.

      Not surprisingly, most studies have considered the ENSO-ISMR relationship during concurrent seasons or for a developing and decaying El Niño. The exclusive effect of exhausted spring El Niño (El Niño in springtime) events that were terminated by the beginning of the monsoon season on the forthcoming ISMR has yet to be explored.

      In addition, few studies have explored the impact of a freshly evolving El Niño concurrent with the monsoon season on ISMR. However, some studies have investigated the impact of developing and decaying El Niño on the East Asian summer monsoon. Previous analyses have examined the impact of El Niño on the East Asian monsoon during its various phases (Huang and Wu, 1989). Wen and Hao (2021) recently demonstrated that precipitation anomalies during the developing summer are nearly opposite to those during the decaying period across East Asia. Over the East Asian region, researchers have depicted numerous perspectives on the development and decay of El Niño and its implications on the China region. Some investigations looked into the relationship between circulation patterns associated with El Niño and rainfall in East Asia (Cao et al., 2017; Huang and Wu, 1989). Others attempted to investigate the relationship using metrics such as water vapor transport and model studies over East Asia (Huang and Wu, 1989; Li et al., 2019).

      We investigate the unique reaction of ISMR to exhausted spring El Niño and concurrent emerging summer El Niño events. The analysis was carried out using standard metrics such as the Hadley and Walker circulations, vertically integrated moisture flux transport (VIMFT), wind shear, and associated atmospheric dynamics. It is of interest to examine the impact of a spring El Niño on the approaching monsoon since it may aid in the long-term forecast of regional ISMR variability as a potential predictor.

      One of the explanations for the decrease in ISMR in response to the anomalous warming (El Niño) in the central and eastern Pacific is the eastward shift of the ascending branch of the Walker circulation and increased subsidence over the Indo-West Pacific region (Shukla and Wallace, 1983; Palmer et al., 1992; Ju and Slingo, 1995; Soman and Slingo, 1997; Pokhrel et al., 2012). Modulation of the regional Hadley cell by ENSO has also been widely studied (Held and Hou, 1980; Seager et al., 2003; Feng and Li, 2013; Nguyen et al., 2013). Model studies have shown that El Niño induces strengthening and an equatorward shift in the Hadley cell circulation (sinking limbs are confined within 10°–20° of latitude in both hemispheres). In contrast, La Niña causes a weakening of the Hadley cell circulation (Dogar et al., 2017). Krishnamurthy and Goswami (2000) studied the inter-decadal variability of ISMR in relation to the warm and cold phases of ENSO. They showed that the reduction in ISMR associated with the warm phases of ENSO is in response to an anomalous regional Hadley circulation with ascending motion near the equator and descending motion over the Central Indian latitudes. An attempt is made to understand the interaction between Hadley and Walker circulations during ENSO phases occurring at different times and their impact on ISMR.

      Webster and Yang (1992) devised a monsoon circulation index known as the Webster-Yang Index (WYI), a measure of the vertical wind shear in the South Asian region for determining the strength of monsoon circulation. Thus, variability in wind shear over the South Asian region can be considered as the dynamical index best suited for examining the inter-annual variability in ISMR. It is well known that VIMFT represents the moisture availability over a region and is determined mainly by the transport of moisture by Low-Level Jet (LLJ) over the Indian region during the monsoon season (Sahana et al., 2015; Pathak et al., 2017; Patil et al., 2019). The upper-level divergence associated with the Tropical Easterly Jet (TEJ) and its role in determining the prominent regions of convection beneath is well established from four quadrants of the convergence-divergence pattern on either side of the entrance and exit region of the core TEJ axis (Koteswaram, 1958). In this study, the contrasting responses of an exhausted spring and emerging summer El Niño on the regional distribution of ISMR are analyzed using the vertical shear of the zonal wind, upper-level circulation, VIMFT, and the regional Hadley and Walker circulations.

    2.   Data and methodology
    • In this analysis, the ENSO warm and cold phases are identified based on the Oceanic Niño Index (ONI) derived from five consecutive, three-month running means of SST anomalies in the Niño-3.4 region (i.e., 5°N – 5°S, 120°– 170°W). This approach is similar to the classification method adopted by NOAA to identify different ENSO phases (https://origin.cpc.ncep.noaa.gov/products/analysis_monitoring/ensostuff/ONI_v5.php) using the Extended Reconstructed SST version4 (ERSSTv.4) data sets (Huang et al., 2015, 2016; Liu et al., 2015). Exhausted spring El Niños events are classified as those El Niño years in which ONI values are greater than 0.5 from the three-month running mean of December−January−February (DJF) to March−April−May (MAM) but do not extend to the summer months of May−June−July (MJJ). Similarly, an emerging summer El Niño is defined as an El Niño year having a three-month running mean of ONI values greater than 0.5 from June−July−August (JJA) to August−September−October (ASO). An emerging summer El Niño is otherwise termed as the warm phase of ENSO with the Indian summer monsoon. The years with overlapping cases of an exhausted spring El Niño extending into the summer season are excluded from the present study. The exhausted spring El Niño years are 1951, 1953, 1957, 1958, 1965, 1969, 1987, 1991, 2002, 2004 and 2009. Similarly, Concurrent emerging summer El Niño years are 1956, 1983, 1995 and 2010.

      Standardized anomalies in terms of SST and rainfall are used to extract the prominent signals in the two sets of composites for intercomparison, which exhibits large dispersion. The standardization of SST and rainfall is accomplished by dividing the respective anomalies by the corresponding standard deviations. The composite standardized anomaly of SST during June−July−August−September (JJAS) after an exhausted spring El Niño and concurrent emerging summer El Niño during the JJAS season is presented to reveal the distinct spatial variability in the background SST fields. A t-test applied to the standardized SST anomalies is used to identify the regions having prominent differences that are significant at a confidence level of 90% or greater. The SST is obtained from the Extended Reconstructed SST version 4 (ERSSTv.4) data set (Huang et al., 2015, 2016; Liu et al., 2015). The composite standardized rainfall anomaly during the two distinct El Niño categories is derived from the India Meteorological Department (IMD) gridded daily rainfall data (Pai et al., 2014) from 1950 to 2010 at a grid resolution of 0.25° × 0.25°, during the JJAS season. A t-test is performed to identify the prominent regions showing statistically significant anomalies (at the 90% confidence level or higher) in each composite. Standardized rainfall anomaly analysis revealed fascinating results over the rain shadow of South Peninsular India's (SPI) homogeneous rainfall region (8–19°N and 74–84°E). In the SPI region, we further explored the relationship between standardized anomaly of Niño-3.4 Sea Surface Temperature and ISMR during both types of El Niño. We have conducted our analysis by making use of the ERSSTv.4 SST data and the IMD gridded daily rainfall data, as mentioned above. It is found that the rainfall response over the Indian region to positive IOD years concurrent with an emerging summer El Niño is distinct from the pure emerging summer El Niño composite (see Fig. A1 in the appendix). To remove the combined influence of positive IOD concurrent with emerging summer El Niño, such years have been excluded from emerging summer El Niño composites considered in this study. The years thus excluded are 1963, 1972, 1982, and 1997. The SST anomaly corresponding to positive IOD concurrent with developing summer El Niño is almost similar to developing summer El Niño with a slightly higher positive anomaly over the western Arabian Sea during positive IOD years.

      To understand the role played by the moisture transport driven by changes in the circulation pattern associated with the different ENSO phases over the region, the vertically integrated moisture flux convergence (VIMFC) during each phase is calculated as shown below:

      where q is the specific humidity, and u and v are the zonal and meridional components of the wind. A t-test is carried out on the composite anomalies of both cases to identify the regions significant at the 99% confidence level. Furthermore, the 850 hPa wind anomalies that are significant at the 90% confidence level have been found using a t-test and are overlaid over the respective composite anomalies. Wind and humidity data are obtained from NCEP-NCAR Reanalysis 1 at a resolution of 2.5° × 2.5° (Kalnay et al., 1996).

      To understand the dynamic response of the two contrasting El Niño types on ISMR, we examined the Hadley and Walker circulations using zonal (u) and meridional (v) components of the wind and pressure-vertical velocity (omega). The orientation and strength of circulation features, such as the LLJ and TEJ, are analyzed using zonal wind data at the lower and upper levels. In terms of vertical velocity anomaly, statistically significant regions are highlighted with a confidence level of 90% or higher. To understand the strength of monsoon circulation with respect to the two ENSO categories, the zonal wind shear is taken as the difference between zonal wind averaged at the pressure levels of 1000–850 hPa and 200–100 hPa. A t-test is used to identify locations with composite shear anomalies that are statistically significant at a confidence level of 99% or higher in both scenarios. Similarly, a significant wind shear vector anomaly is overlaid. To understand the role played by TEJ, the upper-level wind composite anomaly at 200 hPa is computed, and locations with statistically significant anomalies at the 99% confidence level or greater are highlighted. Upper-level winds (vectors) at 200 hPa, whose anomalies have a significance of 90% or greater, are also identified and overlaid in both cases. Furthermore, the 200 hPa velocity potential overlaid with divergent winds to determine the upper-level divergence/convergence attributed to possible ascending/descending motions beneath it. Converging winds at 200 hPa and sinking air underneath them characterize the centers of regions with a positive velocity potential anomaly. Similarly, diverging winds at 200 hPa and rising air motion are seen beneath negative velocity potential anomaly regions. Significant regions are identified for the anomaly composites of all variables and are highlighted in the corresponding figures.

    3.   Results and discussion.
    • The SST pattern after the exhausted spring El Niño is characterized by an unusual statistically significant decrease in the SST over the Niño-3.4 region, as shown in Fig. 1a. A large area of significant negative anomalies over central and northern equatorial Pacific near 20º N is noticeable. The cooling over the Niño-3.4 region is attributed to a La Niña-like situation. However, the SST cooling is not significant over the entire Niño-3.4 region. Warming is mainly visible over the entire Indian Ocean, most significantly over the eastern equatorial Indian Ocean and maritime continental region. On the other hand, the western equatorial Pacific Ocean remains warmer, as evidenced by the anomalies. The anomalous positive significant SST over the Indian Ocean lasts from June to September. The warming over the western Indian Ocean region during the monsoon season has ramifications for the behavior of monsoon rainfall. In comparison to previous instances of monsoon SST hikes, Indian Ocean surface waters are relatively cooler (Fig. 1b). The northern portion of the western equatorial Indian Ocean is observed to host a warm SST pool. The large area, including the Niño-3.4 region, shows a significant increase in SST, indicative of the concurrently emerging summer El Niño during the JJAS. In this analysis, the combined influence of a positive IOD and a concurrent El Niño during the JJAS season has been excluded. The combined influence of positive IOD and El Niño exhibits a similar SST pattern as that of a concurrently emerging El Niño in the equatorial Pacific but distinct over the Indian Ocean (Fig. A2 in the appendix). The SST over the region during the JJAS season plays an integral role in modulating spatial and temporal variability of the monsoon.

      Figure 1.  Panels (a) and (b) show the standardized anomaly composite of Sea Surface Temperature (SST) (K) for JJAS after an exhausted spring El Niño and during a concurrent emerging summer El Niño, respectively. The color shades represent the standardized SST anomalies, and regions with black dots represent the areas where the anomaly is significant at a confidence level of 90% or higher.

    • We find that the ISMR following an exhausted spring El Niño is characterized by significant positive anomalies over the rain shadow regions of peninsular India and north-central and western India, along with negative rainfall anomalies over the core monsoon regions of central India and the west coast (Fig. 2a). On the contrary, the emerging summer El Niño is characterized by a negative rainfall anomaly over most of the Indian land region, with a statistically significant decrease over western India, which contains small embedded pockets of significant increases in rainfall (Fig. 2b).

      Figure 2.  Panels (a) and (b) illustrate the composite standardized rainfall anomaly (mm d -1) during JJAS (June-July-August-September) following an exhausted spring El Niño and concurrently emerging summer El Niño events, respectively. The hatched areas in the figure represent regions where values are statistically significant at a 90% confidence level or higher.

      In the following sections, we further examine the thermodynamic and dynamic response to these two contrasting types of El Niño events on the seasonal distribution of ISMR

    • It is obvious from Fig. 3a that the presence of an El Niño event before the start of the Indian summer monsoon can be identified by the standardized SST anomaly over the Niño-3.4 region depicted by the bars. Evidently, El Niño does not extend into the summer monsoon season. The green line indicates seasonal rainfall during the summer monsoon period in the SPI region. During the summer monsoon, the excessive seasonal rainfall is an interesting aspect that deserves to be examined more with respect to an exhausted spring El Niño. It is clear from Fig. 3b that rainfall in the SPI region decreases when El Niño occurs concurrently with the summer monsoon. The presence of positive rainfall peaks in the post-monsoon season supports the result from previous analyses that have suggested that El Niño events favor higher rainfall over the SPI during the season (e.g., Khole and De, 2003; Geethalakshmi et al., 2009).

      Figure 3.  Panels (a) and (b) depict the composite temporal evolution of SST (°C) and rainfall (mm d-1) during years of an exhausted spring El Niño and a concurrently emerging summer El Niño, respectively. The bars represent standardized SST anomalies, and the green line represents standardized rainfall anomalies.

    • Figure 4a outlines the anomalous Hadley circulation over Indian longitudes (60°–90°E) during JJAS following an exhausted spring El Niño. Shading in the figure represent negative pressure vertical velocity (omega) anomalies, and vectors represent the meridional overturning circulation generated by using meridional and vertical wind components. The Hadley circulation over the Indian region during the monsoon season following an exhausted spring El Niño is characterized by significant anomalous ascending motions between 10°S and 10°N extending aloft up to 200 hPa. The two cores of these ascending motions are located near 5°S and 7°N. The more intense and significant ascending core is over the land and positioned at around 7°N. An ascending core of Hadley circulation near 20°N (at its conventional position) is also evident, but its intensity is relatively weak. An interesting feature inherent to the Hadley circulation is that the conventional descending motion over the southern tip of India is replaced with an anomalous significant ascending motion during the JJAS seasons following an exhausted spring El Niño. A section of the Hadley circulation located between 10°N and 30°N contains embedded weak ascending motions at around 20°N and weak descending motions on either side. However, the descending branch south of 20°N is present only between the middle and upper troposphere and has a weak ascending motion below 500 hPa. Hence the monsoon Hadley circulation after the exhausted spring El Niño is characterized by ascending motion extending from 10°S to 20°N favoring convection and justifies the significant positive rainfall anomaly (Fig. 1a) seen over the eastern part of the southern peninsula. The LLJ core, found near 850 hPa, at around 10°N, extends northward up to 20°N. In contrast, the TEJ core stays close to 7°N, and the core speed extends southwards beyond the equator and descends to lower levels at approximately 10°S during JJAS seasons following the exhausted spring El Niño events.

      Figure 4.  Anomalous Hadley circulation (averaged between longitudes 60°E and 90°E) during the Monsoon (a) after an exhausted spring El Niño, (b) during a concurrent emerging summer El Niño. The anomalous Walker circulation (averaged between the latitudes of 0 to 20°N) during the monsoon, (c) after an exhausted spring El Niño, and (d) during a concurrent emerging summer El Niño. Shaded (colored) regions represent the composite of vertical velocity anomalies, and the vectors depict the circulation anomalies. Anomaly composites, significant at the 90% confidence level, are also included in the figure. The green and violet contours in Figs. 4a and b represent the westerly and easterly winds, respectively.

      Figure 4b shows that, unlike in the previous case, the Hadley circulation during the emerging summer El Niño is characterized by two strong and significant descending cores around 5°S and 20°N (highly significant) with weak and wide ascending motions between the equator and 20°N. However, the ascending motion mostly extends up to the upper troposphere. Compared to exhausted spring El Niño events, dominant descending cores over the core monsoon region are responsible for suppressed convection and the negative anomaly in seasonal rainfall. The weak ascending anomalies are insufficient to overcome the descending air to enhance the rainfall over pockets of southwest interior India during the emerging summer El Niño. The core speed of LLJ is found between 8°N and 15°N and is centered near 12°N, slightly to the north, compared to that of exhausted spring El Niño events. The TEJ core speed is observed around 7°N, roughly at 150 hPa, similar to the exhausted spring El Niño case in the upper levels. Though the TEJ core is found at 7°N, it is not descending to lower levels at 10°S as seen during the JJAS period following the exhausted spring El Niño, which affects the orientation of the vertical shear zone and thereby influences the spatial distribution of rainfall.

      It is evident from Fig. 4c that strong and significant anomalous descending motion occurs across a longitudinal range from 150°E to 100°W. Similarly, descending anomalies are found to the west of 50°E. A strong ascending branch appears between 50°E and 90°E, with its core with significant ascending anomalies located at around 80°E. Another ascending core is found at around 120°W. Strong ascending anomalies between 60°E and 100°E are found over the same region of the southern ascending branch of the Hadley circulation, as discussed in the previous section. Comprehensively, anomalous ascending motion can be identified as a combined effect of ascending motion constituted by the Hadley and Walker circulations. The core of these combined ascending anomalies is strikingly situated between 10°S to 10°N and 60°E to 110°E. This characteristic distribution of vertical circulation observed during JJAS following exhausted spring El Niño events well supports the excess seasonal rainfall over the SPI region. Such anomalous ascending motions of the Walker circulation over the equatorial Indian Ocean can potentially change the characteristics of Hadley circulation over the Indian region, which, in turn, controls the ISMR variability. A previous study by Krishnamurthy and Goswami (2000) also illustrated similar results. As part of the Walker circulation, a core of strong and significant descending motion is located between 160°E and 165°W.

      Figure 4d illustrates the anomalous Walker circulation during an emerging summer El Niño. Color shades, vectors, and contours are the same as in Fig. 4c. Vertical velocity portrays the characteristic feature of a classical El Niño in which significant ascending motion dominates over a longitudinal range of 150°E to 120°W with ascending cores found over 150°W in the eastern equatorial Pacific Ocean. The descending motion mostly appears between 90°E and 120°E, with core descending motion located around the maritime continent and over Indian longitudes. Two weak cores of descending motions are found around 60°E and 70°E. The descending motion around 70°E has not intruded into the lower levels. This also suggests that the Walker circulation reinforces the descending branch of regional Hadley circulation and suppresses the overall convection and hence the rainfall over the Indian longitudes.

    • The moisture transport in response to the changes in Hadley and Walker circulations is examined using VIMFC and wind patterns. Figure 5a illustrates the VIMFC (shading) and 850 hPa wind (vectors) that are statistically significant at the 99% confidence level during the JJAS season following an exhausted spring El Niño event. Significant positive moisture convergence anomalies are found over peninsular India and cover a vast area over the equatorial Indian Ocean (Fig. 5a). It is vital to note that significant negative anomalies of moisture convergence exist over north-central India, the Bay of Bengal, and northeast India. This pattern corresponds well to the descending anomalies of the Hadley circulation over the central Indian region. Prior studies have documented that the LLJ supplies the principal amount of moisture to the area with core speeds generally located over the southeast Arabian Sea and southern peninsular India (Joseph and Sijikumar, 2004; Patil et al., 2019 and others). However, during the JJAS season following the exhausted spring El Niño, the core of LLJ is located further southward. Hence, the anomalous enhancement of VIMFC is achieved through the supply of moisture accomplished by the combined Hadley and Walker ascending limbs, unlike in normal monsoons. The significant increase in moisture availability over the eastern Arabian Sea, with seasonal winds mostly shifted to the south of its normal position and blowing parallel to India's west coast, results in below-normal monsoon rainfall over the west coast. Recent studies have shown that reorientating the LLJ from its normal position and displacing it to the south results in a widespread decrease in summer monsoon rainfall over the west coast of India (Sreenath and Abhilash, 2021).

      Figure 5.  VIMFC anomalies during monsoon season overlaid by wind at 850 hPa (a) after an exhausted spring El Niño and (b) during a concurrent emerging summer El Niño. The shaded (colored) area in the figure represents VIMFC composite anomalies overlaid with 850 hPa wind vectors that are significant at the 90% confidence level or more. The dotted area represents VIMFC anomalies significant at the 99% confidence level. The vertically integrated moisture flux converegence has units of kg m−2 s−1.

      Figure 5b describes the anomaly of VIMFC and the 850 hPa wind during an emerging summer El Niño. In contrast to the previous case, the emerging summer El Niño during the monsoon season is characterized by a negative rainfall anomaly over most of the Indian land region with an anomalous departure in moisture in east India. Some pockets of central-eastern and southwest regions of India have a deficiency in moisture availability. The inadequate moisture availability, along with the combined influence of descending branches of Hadley and Walker circulations, suppresses rainfall over the west, central, and southern parts of India. Notably, the LLJ splits into two, with the southern branch flowing south of the peninsula, whereas the northern branch flows directly over the central Indian region. However, a lack of moisture aggravated by the anomalous descending motions of the Walker and Hadley limbs significantly decreased the seasonal monsoon rainfall.

      Webster and Yang (1992) proposed that the vertical wind shear over South Asia during summer is an ideal dynamical index for evaluating the inter-annual variability of ISMR. Here, we examine the dynamical response to the two classes of El Niño and its association with ISMR.

      Figure 6a presents anomalies of vector wind shear and zonal wind shear between lower (1000–850 hPa) and upper (200–100 hPa) layers during JJAS that follow an exhausted spring El Niño. It is found that a strong easterly wind shear (with 99% confidence) dominates over an elongated region from the Western North Pacific to the Central Arabian Sea across peninsular India during the JJAS season following an exhausted spring El Niño. This enhancement of easterly shear may result from the typical TEJ orientation in response to the exhausted spring El Niño. Hence, it is worthwhile to examine the position and intensity of TEJ, which largely determines the upper-level divergence pattern. Unlike the case of an exhausted spring El Niño, weak easterly shear over a larger area is observed to the west of 85°E longitude during emerging summer El Niño (Fig. 6b). The easterly shear vector is significant at 99% confidence level and is found north of 15°N. This suggests that the presence of weak easterly shear over the Indian landmass and the Arabian Sea is unfavorable for normal to above-normal seasonal rainfall over the southern peninsular region. In addition to this weak easterly wind shear, the dominance of the descending limb of the regional Hadley circulation, reinforced by that of the Walker cell over the Indian subcontinent, explains the reduced convective activity and hence the deficit ISMR.

      Figure 6.  Shaded (colored) region portrays wind shear anomalies (m s−1) during monsoon season (a) after an exhausted spring El Niño and (b) during a concurrent emerging summer El Niño event. The wind vectors are drawn only over the regions where wind composite anomalies are statistically significant at the 90% confidence level or more. Over the regions where shear composite anomalies exist, black dots represent those areas significant at the 99% confidence level.

    • Figure 7a illustrates the zonal wind anomaly at a 99% confidence level and zonal composite wind at 200 hPa during JJAS following an exhausted spring El Niño. It is observed that the maritime continental region has below-normal easterly winds. Strong and above normal easterly anomalies are present over central India around 22°N latitude and extending to the China peninsula. Another region where significant above-normal easterly winds are found is over the Western Equatorial Indian Ocean region near the African coast. Anomalous anticyclonic flow over Tibet pushes the TEJ further south, resulting in the establishment and positioning of TEJ over SPI and its further extension up to the Western Equatorial Indian Ocean. The high wind speed on the forward side of TEJ enhances the upper-level divergence. This upper-level divergence, together with lower-level moisture convergence in the southeast Arabian Sea, favors convection over the eastern portion of the southern peninsula. This specific orientation of TEJ is also responsible for the strong elongated easterly zonal shear zone extending from the maritime continent region to the central Arabian Sea (Fig. 6a).

      Figure 7.  Upper-level wind anomaly composite and mean vector wind (m s−1) at 200 hPa during the monsoon season (a) after an exhausted spring El Niño and (b) during a concurrent emerging summer El Niño. The vectors indicate the mean wind composite with a statistical significance of 90% or more. The dotted areas represent anomaly composite significant at the 99% confidence level.

      Figure 7b illustrates the upper-level wind anomaly at the 99% confidence level and the zonal wind composite at 200 hPa during the monsoon, corresponding to an emerging summer El Niño. The upper-level wind depicts below-normal easterly wind over the entire Southeast Asian region. Over the Indian subcontinent and North Arabian Sea extending to the Middle East region, easterly wind speed remains reduced. From Fig. 8b, it is clear that a weaker-than-normal TEJ is more prominent over the Central Bay of Bengal and north of 10°N over Indian longitudes. In other words, the southward extension of TEJ is limited. This specific orientation of TEJ is primarily responsible for the weaker easterly shear over the Indian region, as seen in Fig. 7b.

      Figure 8.  The anomaly composite of velocity potential and divergent wind (10−6 m2 s−1) during JJAS after an exhausted spring El Niño (a) and (b) a concurrent emerging summer El Niño. The color contours are the velocity potential anomaly, and the vectors represent the divergent wind anomaly. The red lines in the figure represent the zero velocity potential contour. Dotted areas in the figure are velocity potential anomalies significant at the 95% confidence level.

      A wide region of significant negative velocity potential anomalies is present over the equatorial India ocean West of 110°E as seen during JJAS followed by an exhausted spring El Niño (Fig. 8a). This anomaly in velocity potential covers a large area, including the southern tip of the Indian peninsula. A wide region of intense upper-level divergence, centered at 90°E, is also depicted. As a result, strong ascending motion lies underneath the area shown in Fig. 8a, which is also evident in Fig. 7a regarding the Walker and Hadley circulation anomalies discussed in the previous sections. Likewise, a very large area of positive velocity potential anomalies over the equatorial Pacific east of 120° E is overlaid by a large area of strong upper-level convergence. Thus, Fig. 8a captures the east-west Walker circulation and its ascending and descending motions very well. Figure 8b represents the anomaly of velocity potential and divergent winds during an emerging summer El Niño. From Fig. 8b, it can be seen that the core of negative velocity potential has been replaced by a core of a significant anomalous positive velocity potential over the equatorial Indian Ocean, near 90°E. The upper-level convergence overlaid with a positive velocity potential anomaly is attributed to the descending motion discussed previously in the Walker circulation’s regional effect, as seen in Fig. 4d. The Walker circulation seems relatively weaker during the emerging summer El Niño. The descending limb of Walker circulation is positioned over the equatorial Indian Ocean near 90°E, and the ascending limb located at 150°W appears to be strong, as indicated by the upper-level divergence of wind and negative velocity potential anomaly.

    4.   Conclusions
    • The present study reports the distinct regional monsoon rainfall response to an exhausted spring El Niño and emerging summer El Niño events. The spatial distribution of JJAS rainfall over the Indian region following an exhausted spring El Niño is characterized by a significant enhancement of rainfall over the eastern part of the SPI and a reduction in mean rainfall over the core monsoon zone. Several factors are attributed to the anomalous increase in seasonal monsoon rainfall over the SPI region following an exhausted spring El Niño. The most important manifestations of the background monsoon state are the ascending anomalies related to Hadley circulation between 10°S to 10°N over the Indian longitudes, which is reinforced with ascending anomalies associated with the Walker circulation between 60°E and 120°E and its core located between the Indian longitudes and maritime continent region. The central Indian region has a negative rainfall anomaly, possibly resulting from a lack of moisture availability and the descending anomalies depicted from the Hadley circulation pattern over the region. The west coast also experienced reduced rainfall during this period due to the specific orientation of the LLJ, with a strong LLJ core found south of 15° N.

      An emerging summer El Niño is characterized by reduced seasonal rainfall over a vast area covering central India and extending to the Himalayas. Some pockets of the southwest coast of India and northeast India received above-normal seasonal rainfall. A weaker LLJ, along with a weaker TEJ whose orientation is skewed northward of its normal position, is present in this case, noting that the proximity and strength of these features are important—to facilitat the low-level moisture convergence and upper-level divergence needed for above-normal rainfall. The descending anomalies of the Hadley and Walker circulations over the Indian land region further inhibit convection.

      The dynamic response to an exhausted spring El Niño and an emerging summer El Niño event ultimately induces a distinct spatial distribution of monsoon rainfall that is schematically presented in Figs. 9a and 9b, respectively. The divergence, due to increasing wind speed (Fig. 5a) in the forward direction of TEJ over Southern Peninsular India, favors low-level moisture convergence in the southeast Arabian Sea (Fig. 5a). The strong wind on the forward right side relative to the specific orientation of the TEJ and positioning of LLJ produces a narrow shear-zone region over the Southern Peninsula. Along with the unique orientation of the TEJ, LLJ, and vertical wind shear, the ascending motion due to the combined influence of Hadley and Walker circulation is attributed to the above-normal rainfall over the eastern part of the southern peninsula. This modulation in the circulation pattern during the JJAS season is immediately followed by an exhausted spring El Niño, as evident from the velocity potential anomalies and divergent winds. On the other hand, a weaker TEJ with its position to the north of Peninsular India, along with weak low-level convergence and strong descending motion due to effects of the Hadley and Walker circulations, suppresses the seasonal monsoon rainfall over the Indian region during an emerging summer El Niño.

      Figure 9.  Two schematic diagrams with distinct features during monsoon season (a) after an exhausted spring El Niño and (b) during a concurrent Emerging Summer El Niño. The lower layer shows VIMFC composite anomalies, the middle layer is the composite of shear anomalies, and the top-most layer is the upper-wind anomaly composite.

      Acknowledgements. The first author sincerely expresses his gratitude to the University Grants Commission (UGC) for providing the research fellowship. AS acknowledges funding support from the National Monsoon Mission program of the Ministry of Earth Sciences (MoES), New Delhi. AS, SS, PV, and SK thank MoES for the sustenance support to Adavance Centre for Atmospheric Radar Research and Cochin University of Science And Technology.

    APPENDIX
    • Figure A1.  Composite STD anomaly of rainfall (mm d-1) when monsoon coincides with Concurrent Emerging Summer El Niño and positive IOD.

      Figure A2.  Composite of SST (K) anomaly during monsoon with Concurrent Emerging Summer El Niño and positive IOD.

Reference

Catalog

    /

    DownLoad:  Full-Size Img  PowerPoint
    Return
    Return