The Variability Of The Western Pacific Warm Pool Thermal Condition And Its Effects On The South China Sea Summer Monsoon

By using the American Global Ocean Data Assimilation System (GODAS) monthly averaged subsurface temperature data from1981to2011, the variability of the Western Pacific Warm Pool (Warm Pool) thermal condition and its effects on the South China Sea summer monsoon (SCSSM) are studied. The thermal condition is indicated by the heat content (HC) characterized with the5-366m subsurface vertical averaged temperature. The main conclusions are as follows:(1) The Warm Pool area is the place where HC changes with maximum range, the biggest abnormal change area of HC does not overlap with the high value center, but deviates to its two sides on the longitude direction. The biggest abnormal change area of SST also does not overlap with the high value center, but deviates to one of its sides on the latitude direction and locates nearby the eastern equatorial Pacific. The biggest change areas of the subsurface temperature of the Warm Pool locate between100-200m, the minimum changes is in summer, while the largest is in winter overall. Its seasonal variation is highly consistent with Sea Surface Temperature (SST), its interannual variability is greater than that of SST, and the SST of the Warm Pool has a certain degree of contribution to HC. The changing trend of SST anomaly of different depth of the Warm Pool is relatively consistent and have obvious interannual variation characteristics and appear interdecadal transition characteristics in the mid1990s. The thermal condition of the Warm pool is closely related with ENSO events, the largest cold (warm) SST anomaly happens to correspond to the ENSO cycle in the process of El Nino (La Nina) events, and there exists clear signals of HC of the Warm pool before the early ENSO events.(2) The characteristic of the space field of the dominant EOF mode of HC is the same phase in the overall and its corresponding time coefficient (PCI) shows significant relationship with the Warm Pool anomalies. There is a closely relationship between the convective activities nearby the Warm Pool and the abnormal thermal condition of the Warm Pool in the turn of the spring and summer, when the HC is higher (lower), the convective activities of the critical region around the Philippines are significantly enhanced (weakened), which is effected by HC can be traced back to September of the previous year, and can last to July of that year. There is also a closely relationship between the abnormal of the Western Pacific subtropical high and the thermal condition of the Warm Pool, that is its strength is weaker (stronger), its area is smaller (larger), the position locates in the norther (souther) direction, its ridge line locates in the easter (wester) direction than average when HC of the Warm Pool is higher.(3) The Warm Pool area is the key area affecting the date of the SCSSM onset, there exists a significant negative correlation relationship between the early thermal condition of the Warm Pool and the date of the SCSSM onset. The abnormal signal of HC synthesized from the abnormal years of the date of the SCSSM onset of the previous spring is obviously, HC of the early spring Warm Pool and subsurface SST is positive (negative) anomalies when the SCSSM breaks out earlier (later). The early spring (March and April) is a key period which influences the SCSSM onset earlier or later. The abnormal thermal condition of the early spring Warm Pool affect the SCSSM onset eventually by influencing the convective activity and the subtropical high, the strong convection and the southwest flows appear nearby Indochina when HC of the previous Warm Pool is higher, its extension speed northward is faster and the Western Pacific subtropical high retreats eastward obviously. There exists cyclonic (anticyclonic) deviation circulation at low altitude regions of the South China Sea which is helpful for the SCSSM breaks out earlier (later). In addition, the South China Sea-the Western Pacific locates in one of the rising embranchments of the Walker circulation when HC of the early spring Warm Pool is higher, and the positive abnormal Walker circulation helps to strengthen the low west and southwest flows, the SCSSM breaks out earlier, otherwise, breaks out later.