Air-sea interactions associated with tropospheric biennial oscillation in South China Sea summermonsoon and their effect
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Air-sea interactions associated with tropospheric biennial oscillation in South China Sea summer monsoon and their effects on El Niño-Southern Oscillation ZHENG Bin1∗ , LU Feng1,2 , WEI Hongcheng1,2 1
Guangzhou Institute of Tropical and Marine Meteorology, China Meteorological Administration, Guangzhou 510080, China 2 Nanjing University of Information Science & Technology, Nanjing 210044, China Received 16 November 2011; accepted 26 September 2012 ©The Chinese Society of Oceanography and Springer-Verlag Berlin Heidelberg 2013
Abstract The South China Sea summer monsoon (SCSSM) behaves with prominent climate variability from the intraseasonal to interdecadal time scales. On the interannual time scale, the biennial variability (so-called tropospheric biennial oscillation, TBO) is as important as the El Niño-Southern Oscillation (ENSO) period. Some observed data sets, including reanalysis data, are used to explore the associated air-sea interactive physical processes and how the SCSSM TBO affects the ENSO. The results show that the shearing vorticity induced by the north Indian Ocean sea surface temperature anomalies (SSTAs) and the anomalous Philippine Sea anticyclone both contribute to the TBO in the SCSSM. The results also indicate that the ENSO has a weak effect on the SCSSM TBO, whereas the latter affects the ENSO to some extent. Key words: South China Sea, monsoon, tropospheric biennial oscillation, El Niño-Southern Oscillation Citation: Zheng Bin, Lu Feng, Wei Hongcheng. 2013. Air-sea interactions associated with tropospheric biennial oscillation in South China Sea summer monsoon and their effects on El Niño-Southern Oscillation. Acta Oceanologica Sinica, 32(6): 6–12, doi: 10.1007/s13131-013-0319-z
1 Introduction Monsoons denote significant seasonal variations in winds and precipitation. The South China Sea (SCS) is located on the south of the Mainland of China. It has plenty of rain in boreal summer (June-August) and relatively less precipitation in boreal winter. In September, great rainfall shifts to the equator gradually, and precipitation in the SCS begins to decrease (Figs 1a, b and c). Moreover, SCS winds behave in seasonal switching. In summer, lower SCS winds (850 hPa) change from northeasterly to southwesterly (Figs 1d, e and f ), and high-level leaning south winds converse into the northeasterlies (Figs 1g, h and i). SCS winds change direction southwardly starting from September. Similar results can be obtained using different data sets. There exists a typical monsoon in the SCS area, which is independent from the western north Pacific monsoon. Figure 2 shows that boreal summer low-level winds form a trough in the SCS area and an anticyclone over the western north Pacific (WNP). Thus, southwesterly winds prevail in the SCS, whereas southeasterly winds prevail over the WNP, implying a monsoon system in the SCS that is different from the WNP monsoon system. The outbreak of the SCS summer monsoon (SCSSM) has an important contribution to precipitation in South China. In addition, the Rossby wave excit
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