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2001 Vol. 18, No. 2

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Sensitivity of the Equatorial Air-Sea Coupled System to theZonal Phase Difference between SST and Wind Stress
Soon-Il An, In-Sik Kang
2001, 18(2): 155-165. doi: 10.1007/s00376-001-0010-8
An eigen analysis of the equatorial air-sea coupled model is carried out to understand the mechanism of the slowly varying mode for various zonal phase differences between SST and wind stress. The frequency and growth rate of the slow mode highly depend on the zonal phase difference between SST and wind stress anomalies and the wave scale. For ultra-long waves longer than 20,000 km, the system propagates westward regardless of the position of wind stress. However, for the long waves observed in the Pacific, the slow mode tends to propagate eastward when the SST and wind stress anomalies are close to each other (within a quadrature phase relationship). On the other hand, when the wind stress is located far away from SST, the slow mode tends to propagate westward. The coupled system produces the unstable modes when the westerly (easterly) wind stress is located in the west of warm (cold) SST. It is noted that for the Pacific basin scale,the eastward propagating unstable waves can be produced when the wind stress is located to the west of SST with a few thousand kilometer distance. Also examined in the present study is the relative role of the thermocline displacement and zonal advection effects in determining the propagation and instability of the coupled system.
Diagnostic Equations for the Walker Circulation
Yuan Zhuojian, Jian Maoqiu
2001, 18(2): 166-178. doi: 10.1007/s00376-001-0011-7
Two linear partial differential equations are derived in spherical-isobaric coordinates for the numerical simulation of the Walker circulation with the assumption that the meridional motion equation remains in gradient balance. One is for the Walker circulation along the individual latitude in the tropical area, the other for the meridionally-averaged Walker circulation over a tropical zone.
Summer Monsoon Rainfalls over Mid-Eastern China Lagged Correlated with Global SSTs
Yu Rucong, Zhang Minghua, Yu Yongqiang, Liu Yimin
2001, 18(2): 179-196. doi: 10.1007/s00376-001-0012-6
Some features associated with Eastern China Precipitation (ECP), in terms of mean climatology, seasonal cycle, interannual variability are studied based on monthly rainfall data. The rainfall behavior over Eastern China has fine spatial structure in the seasonal variation and interannual variability. The revealed characteristics of ECP motivate us dividing Eastern China into four sub-regions to quantify significant lag-correlations of the rainfalls with global sea surface temperatures (SSTs) and to study the ocean's predominant role in forcing the eastern China summer monsoon rainfalls. Lagged correlations between the mid-eastern China summer monsoon rainfalls (MECSMRs) and the global SSTs, with SST leading to rainfall, are investigated. The most important key SST regions and leading times, in which SSTs are highly correlated with the MECSMRs, are selected. Part of the results confirms previous studies that show links between the MECSMRs and SSTs in the eastern equatorial Pacific associated with the El Ni o - Southern Oscillation (ENSO) phenomenon. Other findings include the high lag correlations between the MECSMRs and the SSTs in the high and middle latitude Pacific Ocean and the Indian Ocean, even the SSTs over the Atlantic Ocean, with SST leading-time up to 4 years. Based on the selected SST regions, regression equations are developed by using the SSTs in these regions in respective leading time. The correlation coefficient between the observed rainfalls and regressed rainfalls is over 0.85. The root mean square error (RMSE) for regressed rainfall is around 65% of the standard deviation and about 15% of the mean rainfall. The regression equation has also been evaluated in a forecasting mode by using independent data. Discussion on the consistence of the SST-rainfall correlation with circulation field is also presented.
A Review on Seasonal Climate Prediction
Wang Shaowu, Zhu Jinhong
2001, 18(2): 197-208. doi: 10.1007/s00376-001-0013-5
The characters of experiments of prediction on monthly mean atmospheric circulation, seasonal prediction and seasonal forecast of summer rainfall over China are summarized in the present paper. The results demonstrate that climate prediction can be made only if the time average is taken. However, the improvement of the skill score of seasonal forecasts depends on the studies on physical parameters and mechanisms that are responsible for seasonal anomaly. Finally, the predictability of seasonal forecast of temperature and precipitation is discussed, including effectiveness and accuracy.
Modeling of the 1998 East Asian Summer Monsoon by a Limited Area Model with Incorporated Coordinate
Wang Shiyu, Qian Yongfu
2001, 18(2): 209-224. doi: 10.1007/s00376-001-0014-4
The 1998 East Asian Summer Monsoon is simulated by use of an improved nine-level p-σ model, the boundary forcing is the South China Sea Monsoon Experiment (SCSMEX) reanalysis data from May 1 to August 31, 1998. It is found that basic features of the atmospheric circulation (such as the South Asia high and the West Pacific subtropical high) can be simulated fairly. However the South Asia high is a little stronger than the observed, while the West Pacific subtropical high a little weaker. Seen from variations of the time correlation coefficient, this model is good for the short-time climate simulation (less than two months), while for the long-time simulation, its climate drift is a little obvious. It can be also seen from the spatial distribution of correlation coefficient that the worse simulation areas of the model are located in the Tibetan Plateau and the adjacent northwest Indo-China Peninsula. For the simulation of precipitation, the movement of rain belt from May to June can be simulated, but the simulation of July and August precipitation shifts obviously to north of the observed. It is also found from the analysis of sensitive experiment that the improvement of the nested boundary condition has a great impact on the simulation results, especially on the precipitation, so the model and the nesting technique need further improvements.
Preliminary Research on the Size Distribution of???Aerosols in Beijing
Zhang Renjian, Wang Mingxing, Fu Jianzhong
2001, 18(2): 225-230. doi: 10.1007/s00376-001-0015-3
Number concentration and size distribution of atmospheric aerosols were measured in Beijing by an optical particle counter. The relationship between aerosol size distribution and relative humidity is discussed.The results show that the size distribution, diurnal variation, daily variation of atmospheric aerosols have a good relation to relative humidity and Richardson number.
ENSO Signal in Total Ozone over Tibet
Zou Han, Ji Chongping, Zhou Libo, Wang Wei, Jian Yongxiao
2001, 18(2): 231-238. doi: 10.1007/s00376-001-0016-2
Based on the analysis of satellite ozone observation and atmospheric circulation data, this study discusses the ENSO signal in the inter-annual variation of Tibet total ozone, comparing with the tropics and non-mountain zone at the same latitudes of Tibet. It is shown that the Tibet ozone increases in El Ni o events and decreases in La Ni a events, with weakened amplitudes compared with the Southern Hemisphere. In addition this article discusses the mass transportation related to the ozone variations.
Interaction between a Slowly Moving Planetary-Scale Dipole Envelope Rossby Soliton and a Wavenumber-Two Topography in a Forced Higher Order Nonlinear Schr dinger Equation
Luo Dehai, Li Jianping
2001, 18(2): 239-256. doi: 10.1007/s00376-001-0017-1
A parametrically excited higher-order nonlinear Schr dinger (NLS) equation is derived to describe the interaction of a ,slowly moving planetary-scale envelope Rossby soliton for zonal wavenumber-two with a wavenumber-two topography under the LG-type dipole near-resonant condition. The numerical solution of this equation is made. It is found that in a weak background westerly wind satisfying the LG-type dipole near-resonance condition, when an incipient envelope Rossby soliton is located in the topographic trough and propagates slowly, it can be amplified through the near-resonant forcing of wavenumber-two topography and can exhibit an oscillation.However, this soliton can break up after a long time and excite a train of small amplitude waves that propagate westward. In addition, it is observed that in the soliton-topography interaction the topographically near-resonantly forced planetary-scale soliton has a slowly westward propagation, but a slowly eastward propagation after a certain time. The instantaneous total streamfunction fields of the topographically forced planetary-scale soliton are found to bear remarkable resemblance to the initiation, maintenance and decay of observed omega-type blocking high and dipole blocking. The soliton perturbation theory is used to examine the role of a wavenumber-two topography in near-resonantly forcing omega-type blocking high and dipole blocking. It can be shown that in the amplifying process of forced planetary-scale soliton, due to the inclusion of the higher order terms its group velocity gradually tends to be equal to its phase velocity so that the block envelope and carrier wave can be phase-locked at a certain time.This shows that the initiation of blocking is a transfer of amplified envelope soliton system from dispersion to nondispersion. However, there exists a reverse process during the decay of blocking. It appears that in the higher latitude regions, the planetary-scale envelope soliton-topography interaction could be regarded as a possible mechanism of the establishment of blocking.
Atmospheric Diabatic Heating and Summertime Circulation in Asia-Africa Area
Li Weiping, Theo Chidiezie Chineke, Liu Xin, Wu Guoxiong
2001, 18(2): 257-269. doi: 10.1007/s00376-001-0018-0
Utilizing data from NCEP/NCAR reanalysis, the summertime atmospheric diabatic heating due to different physical processes is investigated over the Sahara desert, the Tibetan Plateau, and the Bay of Bengal. Atmospheric circulation systems in summer over these three areas are also studied. Thermal adaptation theory is employed to explain the relationship between the circulation and the atmospheric diabatic heating. Over the Sahara desert, heating resulting from the surface sensible heat flux dominates the near-surface layer, while radiative cooling is dominant upward from the boundary layer. There is positive vorticity in the shallow boundary layer and negative vorticity in the middle and upper troposphere. Downward motion prevails over the Sahara desert, except in the shallow near-surface layer where weak ascent exists in summer.Over the Tibetan Plateau, strong vertical diffusion resulting from intense surface sensible heat flux to the overlying atmosphere contributes most to the boundary layer heating, condensation associated with large-scale ascent is another contributor to the lower layer heating. Latent heat release accompanying deep convection is critical in offsetting longwave radiative cooling in the middle and upper troposphere. The overall diabatic heating is positive in the whole troposphere in summer, with the most intense heating located in the boundary layer. Convergence and positive vorticity occur in the shallow near-surface layer and divergence and negative vorticity exist deeply in the middle and upper troposphere. Accordingly, upward motion prevails over the Plateau in summer, with the most intense rising occurring near the ground surface.Over the Bay of Bengal, summertime latent heat release associated with deep convection exceeds longwave radiative cooling, resulting in intense heating in almost the whole troposphere. The strongest heating over the Bay of Bengal is located around 400 hPa, resulting in the most intense rising occurring between 300 hPa and 400 hPa, and producing positive vorticity in the lower troposphere and negative vorticity in the upper troposphere. It is also shown that the divergent circulation is from a heat source region to a sink region in the upper troposphere and vice versa in lower layers.
Atmospheric Circulations and Sea Surface Temperatures Related to the Convection over the Western Pacific Warm Pool on the Interannual Scale
Lu Riyu
2001, 18(2): 270-282. doi: 10.1007/s00376-001-0019-z
The difference is examined in atmospheric circulation and Sea Surface Temperatures (SSTs) in the tropics and subtropics between weak and strong convection over the tropical western Pacific warm pool (signified as WPWP). The WPWP is chosen as the region (110-160°E, 10-20°N), where the Outgoing Longwave Radiation (OLR) shows a great year-to-year variance. A composite study was carried out to examine the differences in atmospheric circulation and SSTs between weak and strong convection over WPWP. First,NCEP/NCAR re-analysis data and satellite-observed OLR data are used to examine the differences.ERA data, in which the OLR data are calculated, are then used for re-examination. The composite results show that the differences are remarkably similar in these two sets of data. The difference in circulations between weak and strong convection over WPWP is significantly associated with westward extension of the North Pacific subtropical anticyclone and stronger westerlies at the northwestern edge of the subtropical anticyclone. It also corresponds with the significant easterly anomaly and the descent anomaly in situ, i.e., over the WPWP. The most prominent characteristics of the difference of SSTs between weak and strong convection over the WPWP are the significant positive SST anomalies in the Indian Ocean,the Bay of Bengal and the South China Sea. In WPWP, however, there are only weak negative SST anomalies. Thus, the anomaly of OLR over WPWP is weakly associated with the SST anomalies in situ, while closely associated with the SST anomalies west of WPWP.
An Approach to Extract Effective Information of Monthly Dynamical Prediction-The Use of Ensemble Method
Yang Hui, Zhang Daomin, Ji Liren
2001, 18(2): 283-293. doi: 10.1007/s00376-001-0020-6
The approach of getting useful information of monthly dynamical prediction from ensemble forecasts is studied. The extended range ensemble forecasts (8 members, the initial perturbations of the lagged average forecast (LAF)(0000, 0600, 1200 and 1800 GMT in two consecutive days) of the 500 hPa height field with the global spectral model (T63L16) from January to May 1997 are provided by the National Climate Center of China. The relationship between the spread of ensemble measured by root-mean-square deviation of ensemble member from ensemble mean and forecast skill (the anomaly correlation or the root-mean-square distance between the ensemble mean forecast and the observation) is significant. The spread of ensemble can evaluate the useful forecast days N for the best estimate of 30 days mean. Thus, a weighted mean approach based on ensemble spread is put forward for monthly dynamical prediction. The anomaly correlation of the weighted monthly mean by the ensemble spread is higher than that of both the arithmetic mean and the linear weighted mean. Better results of the monthly mean circulation and anomaly are obtained from the ensemble spread weighted mean.
The Relationships between Variations of Sea SurfaceTemperature Anomalies in the Key Ocean Areasand the Precipitation and SurfaceAir Temperature in China
Zhang Weiqing, Qian Yongfu
2001, 18(2): 294-308. doi: 10.1007/s00376-001-0021-5
The relationships between variations of sea surface temperature anomalies (SSTVA) in the key ocean areas and the precipitation/temperature anomalies in China are studied based on the monthly mean sea surface temperature data from January 1951 to December 1998 and the same stage monthly mean precipitation/temperature data of 160 stations in China. The purpose of the present study is to discuss whether the relationship between SSTVA and precipitation/temperature is different from that between sea surface temperature anomalies (SSTA) and precipitation/temperature, and whether the uncertainty of prediction can be reduced by use of SSTVA. The results show that the responses of precipitation anomalies to the two kinds of tendency of SSTA are different. This implies that discussing the effects of two kinds of tendency of SSTA on precipitation anomalies is better than just discussing the effects of SSTA on precipitation anomalies. It helps to reduce the uncertainty of prediction. The temperature anomalies have more identical responses to the two kinds of tendency of SSTA than the precipitation except in the western Pacific Ocean.The response of precipitation anomalies to SSTVA is different from that to SSTA, but there are some similarities.