Role Of Western Pacific Ocean-Indian Ocean-Tibetan Plateau Climate System In The South China Sea Summer Monsoon

Based on the atmospheric data from NCEP/NCAR and ECMWF, SST data from NOAA and the upper ocean heat content data from Scripps Institution of Oceanography et al., with use of some statistical methods, such as correlation analysis , composite analysis , wavelet analysis and wavelet cross analysis a comprehensive study was made on the role of the western Pacific Ocean—Indian Ocean—Tibetan Plateau climate system in the South China Sea summer monsoon. 1. The western Pacific and Indian Ocean play dominant role in onset of the South China Sea summer monsoon.The analysis of comprehensive ocean and atmosphere data shows significant interdecadal variability of the South China Sea summer monsoon onset about 1970. Before 1970 the Indian Ocean (zonal wind) dominates the onset of South China Sea summer monsoon; after 1970 the western Pacific Ocean (warm pool heat content) is dominant over its onset. The main reason for the interdecadal variability is the abrupt change of Arctic Oscillation in 1970 from weaker to stronger state, which results in westerly wind anomaly and easterly wind anomaly in the northern equatorial Indian Ocean and hence favorable and unfavorable condition for the onset of South China Sea summer monsoon. Before 1970 negative AO index leads to negative thermal contrast between colder Indian Ocean and warmer Asian continent and anomalous westerly over the Indian Ocean, which is helpful for the onset of South China Sea summer monsoon. During the period the Southern Indian Ocean dipole is connected closely with the onset of South China Sea summer monsoon. The positive Southern Indian Ocean dipole (positive SST in the southwestern Indian Ocean and negative SST in other regions of Indian Ocean) leads to westerly over the Northern Indian Ocean and South China Sea and stronger monsoon circulation, which are helpful for the onset of South China Sea summer monsoon; the case is on the contrary during the negative SIOD periods. After 1970 the positive AO index leads to positive thermal contrast between warmer Indian Ocean and colder Asian continent and anomalous easterly over the Indian Ocean, which is unfavorable for the onset of South China Sea summer monsoon. Under above conditions the heat content of the western Pacific Ocean dominates the onset of South China Sea summer monsoon, this is to say, during the periods with positive heat content (La Ni?a years) South China Sea summer monsoon breaks out early and strongly, in El Ni?o years the heat content is negative and the onset of South China Sea summer monsoon is late and weak. Hence the onset of South China Sea summer monsoon is associated closely with ENSO.2. Snow depth of the Tibetan Plateau plays important role in the onset of South China Sea summer monsoonPositive anomaly of snow depth of the Tibetan Plateau in spring leads to the following events: lower air temperature anomaly of the Tibetan Plateau, less thermal contrast between land and ocean, and weaker easterly at upper level giving rise to weaker divergence at the upper level of the western Pacific Ocean (and less heat content of the western Pacific Ocean warm pool). The later will cause easterly wind anomaly at lower level unfavorable to the onset of South China Sea summer monsoon. At the same time anomalous easterly and anticyclone pair appear at low level of the eastern Indian which are not helpful for the onset of South China Sea summer monsoon. In the case of negative snow depth anomaly opposite situation will take place.It is also shown that snow depth anomaly in spring of the Tibetan Plateau is closely related to El Nino events. In winter during mature period of, El Nino snowfall will be heavier than normal.3. Prediction of South China Sea summer monsoon onsetAfter 1970, the heat content of the western Pacific Ocean warm pool is highly and negatively correlated with the onset of South China Sea summer monsoon. Therefore, it seems that the onset of South China Sea summer monsoon can be predicted with the heat content of the western Pacific Ocean warm pool, the key for which is to find a station representing the warm pool in terms of heat content. On analyzing the upper 400m heat content of the western Pacific Ocean, the station is found, centered at 3N,138E (with a square of 1°x1°) with a representative of greater than 83%. To test with data available, TAO buoy data at 2N, 137E from 1993 to 2007 are used. Correlation between the onset of SCSSM and the heat content at the TAO buoy of 2N, 137E from 1993 to 2004 is made. The correlation coefficients are -0.75 and -0.73 for the upper 400 and 500m in March, respectively with confidence level over 99%. While the correlation coefficients from for April are -0.83 for both the upper 400 and 500m with confidence level 99%. Hence, the upper ocean heat content at the TAO buoy in March or April can be used to predict the onset of South China Sea summer monsoon.In a word, there exists interdecadal variability of the South China Sea summer monsoon, caused by Artic Oscillation interdecadal variability demarked by 1970. Before 1970 the onset of the South China Sea summer monsoon is controlled by the Indian Ocean, while after 1970 it is controlled by the Pacific Ocean (the heat content in western Pacific Ocean warm pool). However, the spring snow depth over the Tibetan Plateau is controlled basically by ENSO. Hence it is concluded that the western Pacific Ocean—Indian Ocean—Tibetan Plateau climate system plays important role in the onset of the South China Sea summer monsoon. In terms of relative important role in its onset the western Pacific Ocean goes first, the Indian Ocean is the second and the Tibetan Plateau is the last.