| Summary: | 碩士 === 臺灣大學 === 大氣科學研究所 === 98 === During Nino development a low-level anticyclone often appears over the Philippine Sea in boreal winter, which, after its formation in October, grows rapidly and reaches peak around December. It will last until the next spring/summer for a sustained period more than half year. This Philippine Sea anticyclone (PSA) exerts major impacts on interannual variability of the East Asian’s climate, is also regarded one of the most dominant responses of global circulation from ENSO.
This study is focused on the mechanism to maintain the PSA, our results show two vital paths :
(1) Remote forcing:
During the developing stage of Nino, the whole tropical belt will be warmed up to form an annular structure. In subtropical area it exhibits compensating cooling. The local manifestation in the South Asia shows a sharp upper temperature gradient in winter, that leads to the intensification of subtropical jet around 10~20N. This jet streak has its exit near the South China shore which induces sub-circulation via geostrophic adjustment. The result is a convergent zone associated with the cross-exit ageostrophic winds on the left side of exit, an area coincident with the upper troposphere over the PSA. Hence the strong subsidence, a result of dynamical forcing, produces this low-level anomalous high according to the Sverdrup balance.
(2) Local wind-evaporation-warm ocean feedback:
Once the PSA is formed it is located within the background of northeastern winds during the northern winter, the effect of wind-evaporation takes over: that is, on the southeastern half of the PSA the stronger total wind speed increases evaporation rate that cools the sea surface temperature and an opposite effect happens on the northwestern half of the PSA. Nevertheless, in terms of feedback of SST to atmosphere, this wind-evaporation effect must be added an extra condition: only over warm ocean when deep convection presents the anomaly of surface can be carried to free atmosphere that effectively modifies the atmospheric circulation. The coincidence between the wind-evaporation area and climatological warm ocean locates on the southeastern corner of the PSA, the ideal spot to put forth the Rossby response that has the correct positive feedback as the wind-evaporation cooling to circulation. This self-maintained in situ air-sea interaction can be fully verified by model simulation.
Based on above two processes the PSA is expected to stay from October of Nino development year to the next April. It also gives clues to how the PSA rapid grows after October and establishes for later months.
|
|---|
