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2019 Vol. 36, No. 3

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2018 Continues Record Global Ocean Warming
Lijing CHENG, Jiang ZHU, John ABRAHAM, Kevin E. TRENBERTH, John T. FASULLO, Bin ZHANG, Fujiang YU, Liying WAN, Xingrong CHEN, Xiangzhou SONG
2019, 36(3): 249-252. doi: 10.1007/s00376-019-8276-x
Predictability of South China Sea Summer Monsoon Onset
2019, 36(3): 253-260. doi: 10.1007/s00376-018-8100-z
Predicting monsoon onset is crucial for agriculture and socioeconomic planning in countries where millions rely on the timely arrival of monsoon rains for their livelihoods. In this study we demonstrate useful skill in predicting year-to-year variations in South China Sea summer monsoon onset at up to a three-month lead time using the GloSea5 seasonal forecasting system. The main source of predictability comes from skillful prediction of Pacific sea surface temperatures associated with El NiÑo and La NiÑa. The South China Sea summer monsoon onset is a known indicator of the broadscale seasonal transition that represents the first stage of the onset of the Asian summer monsoon as a whole. Subsequent development of rainfall across East Asia is influenced by subseasonal variability and synoptic events that reduce predictability, but interannual variability in the broadscale monsoon onset for East Asian summer monsoon still provides potentially useful information for users about possible delays or early occurrence of the onset of rainfall over East Asia.
Seesaw Pattern of Rainfall Anomalies between the Tropical Western North Pacific and Central Southern China during Late Summer
Xinyu LI, Riyu LU
2019, 36(3): 261-270. doi: 10.1007/s00376-018-8130-6
It is well known that suppressed convection in the tropical western North Pacific (WNP) induces an anticyclonic anomaly, and this anticyclonic anomaly results in more rainfall along the East Asian rain band through more water vapor transport during summer, as well as early and middle summer. However, the present results indicate that during late summer (from mid-August to the beginning of September), the anomalous anticyclone leads to more rainfall over central southern China (CSC), a region quite different from preceding periods. The uniqueness of late summer is found to be related to the dramatic change in climatological monsoon flows: southerlies over southern China during early and middle summer but easterlies during late summer. Therefore, the anomalous anticyclone, which shows a southerly anomaly over southern China, enhances monsoonal southerlies and induces more rainfall along the rain band during early and middle summer. During late summer, however, the anomalous anticyclone reflects a complicated change in monsoon flows: it changes the path, rather than the intensity, of monsoon flows. Specifically, during late summers of suppressed convection in the tropical WNP, southerlies dominate from the South China Sea to southern China, and during late summers of enhanced convection, northeasterlies dominate from the East China Sea to southern China, causing more and less rainfall in CSC, respectively.
The Relationship between Deterministic and Ensemble Mean Forecast Errors Revealed by Global and Local Attractor Radii
Jie FENG, Jianping LI, Jing ZHANG, Deqiang LIU, Ruiqiang DING
2019, 36(3): 271-278. doi: 10.1007/s00376-018-8123-5
It has been demonstrated that ensemble mean forecasts, in the context of the sample mean, have higher forecasting skill than deterministic (or single) forecasts. However, few studies have focused on quantifying the relationship between their forecast errors, especially in individual prediction cases. Clarification of the characteristics of deterministic and ensemble mean forecasts from the perspective of attractors of dynamical systems has also rarely been involved. In this paper, two attractor statistics——namely, the global and local attractor radii (GAR and LAR, respectively)——are applied to reveal the relationship between deterministic and ensemble mean forecast errors. The practical forecast experiments are implemented in a perfect model scenario with the Lorenz96 model as the numerical results for verification. The sample mean errors of deterministic and ensemble mean forecasts can be expressed by GAR and LAR, respectively, and their ratio is found to approach $\sqrt{2}$ with lead time. Meanwhile, the LAR can provide the expected ratio of the ensemble mean and deterministic forecast errors in individual cases.
Global Monsoon Changes under the Paris Agreement Temperature Goals in CESM1(CAM5)
Xia QU, Gang HUANG
2019, 36(3): 279-291. doi: 10.1007/s00376-018-8138-y
Based on experiments with the Community Earth System Model, version 1 (Community Atmosphere Model, version 5) [CESM1(CAM5)], and an observational dataset, we found that CESM1-CAM5 is able to reproduce global monsoon (GM) features, including the patterns of monsoon precipitation and monsoon domains, the magnitude of GM precipitation (GMP, the local summer precipitation), GM area (GMA), and GM percentage (the ratio of the local summer precipitation to annual precipitation). Under the Paris Agreement temperature goals, the GM in CESM1-CAM5 displays the following changes: (2) The GMA is ambiguous under the 1.5°C temperature goal and increases under the 2.0°C temperature goal. The increase mainly results from a change in the monsoon percentage. (3) The GM, land monsoon and ocean monsoon precipitation all significantly increase under both the 1.5°C and 2.0°C goals. The increases are mainly due to the enhancement of humidity and evaporation. (3) The percentages of GM, land monsoon and ocean monsoon feature little change under the temperature goals. (4) The lengths of the GM, land monsoon and ocean monsoon are significantly prolonged under the temperature goals. The increase in precipitation during the monsoon withdrawal month mainly accounts for the prolonged monsoons. Regarding the differences between the 1.5°C and 2.0°C temperature goals, it is certain that the GMP displays significant discrepancies. In addition, a large-scale enhancement of ascending motion occurs over the southeastern Tibetan Plateau and South China under a warming climate, whereas other monsoon areas experience an overall decline in ascending motion. This leads to an extraordinary wetting over Asian monsoon areas.
Verification and Improvement of the Ability of CFSv2 to Predict the Antarctic Oscillation in Boreal Spring
Dapeng ZHANG, Yanyan HUANG, Bo SUN, Fei LI, Huijun WANG
2019, 36(3): 292-302. doi: 10.1007/s00376-018-8106-6
The boreal spring Antarctic Oscillation (AAO) has a significant impact on the spring and summer climate in China. This study evaluates the capability of the NCEP's Climate Forecast System, version 2 (CFSv2), in predicting the boreal spring AAO for the period 1983-2015. The results indicate that CFSv2 has poor skill in predicting the spring AAO, failing to predict the zonally symmetric spatial pattern of the AAO, with an insignificant correlation of 0.02 between the predicted and observed AAO Index (AAOI). Considering the interannual increment approach can amplify the prediction signals, we firstly establish a dynamical-statistical model to improve the interannual increment of the AAOI (DY_AAOI), with two predictors of CFSv2-forecasted concurrent spring sea surface temperatures and observed preceding autumn sea ice. This dynamical-statistical model demonstrates good capability in predicting DY_AAOI, with a significant correlation coefficient of 0.58 between the observation and prediction during 1983-2015 in the two-year-out cross-validation. Then, we obtain an improved AAOI by adding the improved DY_AAOI to the preceding observed AAOI. The improved AAOI shows a significant correlation coefficient of 0.45 with the observed AAOI during 1983-2015. Moreover, the unrealistic atmospheric response to March-April-May sea ice in CFSv2 may be the possible cause for the failure of CFSv2 to predict the AAO. This study gives new clues regarding AAO prediction and short-term climate prediction.
Determining Atmospheric Boundary Layer Height with the Numerical Differentiation Method Using Bending Angle Data from COSMIC
Shen YAN, Jie XIANG, Huadong DU
2019, 36(3): 303-312. doi: 10.1007/s00376-018-7308-2
This paper presents a new method to estimate the height of the atmospheric boundary layer (ABL) by using COSMIC radio occultation bending angle (BA) data. Using the numerical differentiation method combined with the regularization technique, the first derivative of BA profiles is retrieved, and the height at which the first derivative of BA has the global minimum is defined to be the ABL height. To reflect the reliability of estimated ABL heights, the sharpness parameter is introduced, according to the relative minimum of the BA derivative. Then, it is applied to four months of COSMIC BA data (January, April, July, and October in 2008), and the ABL heights estimated are compared with two kinds of ABL heights from COSMIC products and with the heights determined by the finite difference method upon the refractivity data. For sharp ABL tops (large sharpness parameters), there is little difference between the ABL heights determined by different methods, i.e., the uncertainties are small; whereas, for non-sharp ABL tops (small sharpness parameters), big differences exist in the ABL heights obtained by different methods, which means large uncertainties for different methods. In addition, the new method can detect thin ABLs and provide a reference ABL height in the cases eliminated by other methods. Thus, the application of the numerical differentiation method combined with the regularization technique to COSMIC BA data is an appropriate choice and has further application value.
Tides and Turbulent Mixing in the North of Taiwan Island
Xiangzhou SONG, Dexing WU, Xiaohui XIE
2019, 36(3): 313-325. doi: 10.1007/s00376-018-8098-2
Microstructure and hydrological profiles were collected along two cross-shelf sections from the deep slope to the shallow water in the north of Taiwan Island in the summer of 2006. While the tidal currents on the shelf were dominated by the barotropic tide with the current ellipse stretched across the shelf, significant internal tides were observed on the slope. The depth-mean turbulent kinetic energy (TKE) dissipation rate on the shelf was 10-6 W kg-1, corresponding to a diapycnal diffusivity of 10-2 m2 s-1. The depth-mean TKE dissipation rate on the slope was 1 10-7 W kg-1, with diapycnal diffusivity of 3.4 10-4 m2 s-1. The shear instability associated with internal tides largely contributed to the TKE dissipation rate on the slope from the surface to 150 m, while the enhanced turbulence on the shelf was dominated by tidal or residual current dissipations caused by friction in the thick bottom boundary layer (BBL). In the BBL, the Ekman currents associated with the northeastward Taiwan Warm Current were identified, showing a near-bottom velocity spiral, which agreed well with the analytical bottom Ekman solution.
Evaluation of Summer Monsoon Clouds over the Tibetan Plateau Simulated in the ACCESS Model Using Satellite Products
Liang HU, Zhian SUN, Difei DENG, Greg ROFF
2019, 36(3): 326-338. doi: 10.1007/s00376-018-7301-9
Cloud distribution characteristics over the Tibetan Plateau in the summer monsoon period simulated by the Australian Community Climate and Earth System Simulator (ACCESS) model are evaluated using COSP [the CFMIP (Cloud Feedback Model Intercomparison Project) Observation Simulator Package]. The results show that the ACCESS model simulates less cumulus cloud at atmospheric middle levels when compared with observations from CALIPSO and CloudSat, but more ice cloud at high levels and drizzle drops at low levels. The model also has seasonal biases after the onset of the summer monsoon in May. While observations show that the prevalent high cloud at 9-10 km in spring shifts downward to 7-9 km, the modeled maximum cloud fractions move upward to 12-15 km. The reason for this model deficiency is investigated by comparing model dynamical and thermodynamical fields with those of ERA-Interim. It is found that the lifting effect of the Tibetan Plateau in the ACCESS model is stronger than in ERA-Interim, which means that the vertical velocity in the ACCESS model is stronger and more water vapor is transported to the upper levels of the atmosphere, resulting in more high-level ice clouds and less middle-level cumulus cloud over the Tibetan Plateau. The modeled radiation fields and precipitation are also evaluated against the relevant satellite observations.