In this paper, we study the relationship between the covariability of the Indian summer monsoon (ISM) and East Asian summer monsoon (EASM) and the rainy season precipitation anomaly in the Mekong River basin (MRB) using ERA-Interim reanalysis data and precipitation from the Climate Hazards Group Infrared Precipitation station for 1981-2016. The results indicate that the interannual variability of the rainy season precipitation in the MRB is significantly modulated by the ISM and EASM covariability. Herein, the ISM mainly influences the rainy season precipitation west of the MRB, and the EASM mainly influences the rainy season precipitation over the southeastern MRB. When the Bay of Bengal (BOB) and South China Sea (SCS) are dominated by positive anomalies of diabatic heating associated with a stronger ISM and EASM, the anomalous westerly winds in the BOB and significantly anomalous easterly winds in the SCS, which carry more warm-wet air, induce a stronger than normal convergence and upward motion in the MRB. This results in much heavier rainy season precipitation in the MRB. Model runs with a linear baroclinic model confirm the observational analysis results.
The interannual zonal movement of the interface between the Indian summer monsoon and the East Asian summer monsoon (IIE), associated with the spring sea surface temperature (SST) seesaw mode (SSTSM) over the tropical Indian Ocean (TIO) and the tropical central-western Pacific (TCWP), is studied for the period 1979–2008. The observational analysis is based on Twentieth Century Reanalysis data (version 2) of atmospheric circulations, Extended Reconstructed SST data (version 3), and the Climate Prediction Center Merged Analysis of Precipitation. The results indicate that the IIE’s zonal movement is significantly and persistently correlated with the TIO–TCWP SSTSM, from spring to summer. The results of two case studies resemble those obtained by regression analysis. Experiments using an atmospheric general circulation model (ECHAM6) substantiate the key physical processes revealed in the observational analysis. When warmer (colder) SSTs appear in the TIO and colder (warmer) SSTs occur in the TCWP, the positive (negative) SSTSM forces anomalous easterly (westerly) winds over the Bay of Bengal (BOB), South China Sea (SCS), and western North Pacific (WNP). The anomalous easterly (westerly) winds further result in a weakened (strengthened) southwest summer monsoon over the BOB and a strengthened (weakened) southeast summer monsoon over the SCS and WNP. This causes the IIE to shift farther eastward (westward) than normal.
A new teleconnection pattern (the BEAP) across the Bay of Bengal‐East Asia‐Pacific region in boreal summer is revealed in this study using mainly ERA‐Interim reanalysis data from the European Centre for Medium‐Range Weather Forecasts. The BEAP index (BEAPI) is defined as the signed sum of standardized apparent moisture sinks at five centers along the pathway. Correlation analysis of the apparent heat sources and apparent moisture sinks has verified the existence of the BEAP teleconnection. Variations in BEAP can affect precipitation anomalies resulting from the anomalous moisture transport and the antiphase surface temperature variation. Wave flux analysis has verified the Rossby wave propagation route that originates around the central Bay of Bengal and extends across North China to the West Pacific. La Niña‐type sea surface temperature anomalies (SSTAs) appearing simultaneously in the same season can excite a positive BEAP pattern by enhancing convection over the Bay of Bengal, while El Niño‐type SSTAs have the opposite effect. Significant correlation between the BEAPI and the SSTAs can last from early summer to early winter. Numerical experiments confirm the BEAP teleconnection pattern and the associated physical processes.
Precipitation efficiency (PE) is a crucial physical quantity in convective processes, describing the efficiency of rainfall generation from cloud detrainment. Although the importance of PE in extreme precipitation events is widely accepted, the evolution of PE in the warming climate and the associated moisture processes in East Asia are still not well understood. To address these issues, the interdecadal variability of PE in East Asia during summer in 1979–2016 is investigated in this study. Two major modes of summertime precipitation efficiency are identified using Empirical Orthogonal Function (EOF) analysis. The leading EOF mode (EOF1) has a dipole pattern that reveals the variations of mean precipitation efficiency. The second EOF mode (EOF2) presents a quadrupole pattern that shows changes in the variability of precipitation efficiency. Both EOF modes exhibit significant interdecadal variability (IDV). The IDV of EOF1 is closely associated with the phase change of the Pacific decadal oscillation (PDO). The Pacific sea surface temperature anomalies associated with the PDO can excite wind anomalies that significantly modulate moisture transport and further alter the mean precipitation efficiency in East Asia. The IDV of EOF2 can be attributed to the interdecadal change of occurrence frequency of Eastern Pacific El Niño-Southern Oscillation (ENSO) events, which affect water vapor transport by inducing an East Asia-Pacific teleconnection-like wave train anomaly pattern. The IDV patterns of precipitation efficiency for both the mean value and variability will improve the ability to predict precipitation in East Asia.
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