The seasonal variation of undercurrent and temperature in the equatorial Pacific jointly derived from buoy measurement and assimilation analysis

Based on the TOGA-TAO buoy chain observed data in the equatorial Pacific and the assimilation analysis results from SODA(simple ocean data assimilation analysis), the role of the meridional cells in the subsurface of the tropical Pacific was discussed. It was found that, the seasonal varying direction of EUC(the quatorial Undercurrent)in the Peacific is westwards beginning from the eastern equatorial Pacific in the boreal spring. The meridional cell south of the equator plays important role on this seasonal change of EUC.On the other hand, although the varying direction is westwards, the seasonal variation of temperature in the same region gets its minimum values in the boreal autumn beginning from the eastern equatorial Pacific.The meridional cell north of the equator is most responsible for the seasonal temperature variation in the eastern equatorial Pacific while the meridional cell south of the equator mainly controls the seasonal temperature change in the central Pacific. It is probably true that the asymmetry by the equator is an important factor influencing the seasonal cycle of EUC and temperature in the tropical Pacific.

An Alternative Index for the Onset of the Summer Monsoon over the South China Sea

An alternative index for the onset of summer monsoon over the South China Sea (SCS) has been designed based on the reanalysis data of the National Centers for Environmental Prediction National Center for Atmospheric Research (NCEP/NCAR). It is found that the difference between the geopotential thickness over Guangzhou and Manila can be used as an alternative index for the onset of summer monsoon over the SCS. Since the convective heating alters the geopotential height and the distance between Guangzhou and Manila is of synoptic scale in the geostraphic framework, the index can represent the strength of heating and the first baroclinic cell over the SCS. By comparison between different indices, it is found that the summer monsoon onset over the SCS can be ascertained by the vertical zonal wind shear between 200 hPa and 850 hPa, the zonal wind at 850 hPa, the OLR anomaly, and the alternative index defined in the present paper. The meridional wind shear between 200 hPa and 850 hPa always appears over the SCS much earlier than the zonal wind shear and cannot be used as an index under consideration.

Circulation Patterns of Summer Monsoon Corresponding to Two Kinds of Indices over the South China Sea

The characteristics of circulation corresponding to two kinds of indices of summer monsoon onset over the South China Sea (SCS) have been discussed using the reanalysis data of the National Centers for Environmental Prediction-National Center for Atmospheric Research. It is found that there are two patterns of deep convection that occur at different locations and influence the summer monsoon onset over the SCS. One is over the Asia continent and the western Pacific corresponding to the southwesterly of summer monsoon prevailing over the northern and central part of the SCS, while the other is near the Philippines that affects the westerly summer monsoon as prevailing over the central and southern part of the SCS. Since these two kinds of convection affecting the summer monsoon onset do not always occur together, thus the summer monsoon onset time is different when determined by various indices.