印度洋海盆模之非均勻分量

Abstract

在聖嬰（ENSO）達到巔峰的隔年春季，印度洋上最顯著的反應就是印度洋海盆模（IOBM）。本研究利用觀測資料發現，在IOBM中存在南北海溫梯度與降水的不對稱性分量，與過去研究認知的IOBM單一均勻模態全然不同。此非均勻分量承自IOBM的特徵，並對此時期印度洋之後的發展扮演相當重要的角色。 本研究延伸Neelin和Chou首先提出在全球暖化或ENSO隔年由於整層熱帶對流層溫度（Tropospheric Temperature，TT）增暖的影響下 ，降水藉由對流調整，其中氣候場對流區（ITCZ）內的對流會更增強的「富者愈富」理論（Rich-get-richer Mechanism，RR），稱之為TT-RR機制，並發現此為造成南北降水不對稱最主要的原因。在ENSO發展隔年的二三月（FM(1) ），ITCZ的中心遷移到南緯12°，同時整個印度洋都在增暖的TT籠罩下，藉由TT-RR機制，ITCZ內的降水及對應赤道以北的沉降都會增強，使Hadley環流型態更加強。 TT-RR機制同時也與ENSO發展年秋季印度洋不對稱模（IOD）所激發的downwelling Rossby wave（DRW）產生交互作用。在ENSO發展隔年IOD結束後，本來預期DRW會在兩三個月內很快的衰減；但經TT-RR機制改變的Hadley環流加強的跨越赤道流，由於南北半球科氏力不同的影響，在南半球藉由慣性不穩定激發出一反氣旋式、負風應力旋度的環流場距平，提供DRW自我增強的來源，也呼應了赤道至南緯6°的區域內，降水減少與表層的反氣旋高壓距平。當ITCZ在AM(1)往赤道遷移時，TT-RR機制的反應開始衰減且隨ITCZ往北移動，此時再次增強的DRW會將儲存於次表面溫度的熱含量釋放到表面，使暖海溫距平得以維持。 在ITCZ中心的南邊，由於絕對風速減弱引發的局地風－蒸發反應（wind-evaporation effect），造成以南緯20°為主區域的暖海溫距平，使此區可支持深對流發展門檻的26.5°C總海溫邊界得以更向南推展。透過暖海局地的影響，深對流與淺對流（層雲）混合的降水型態提供了此區域正降水距平的來源。到了AM(1)，局地風－蒸發反應消散，不再支持此區域的暖海溫，進而使得深對流在此無法發展。 在AM(1)時期，隨著ITCZ北移，海洋動力與表面熱通量對於南印度洋海溫的影響轉變為阻力，因此繼承自FM(1)以來的不對稱海溫與降水距平在此時期達到巔峰。同時，由於長驅直入的短波輻射與絕對風速減弱引發的局地－蒸發反應皆提供利於加熱的條件，北印度洋海溫迅速增暖。過了五月以後，此不對稱性很快地消散。

Contrary to conventional wisdom, the Indian Ocean Basin Mode (IOBM), the most notable response over the Indian Ocean after ENSO peak year, has nothing similar to a uniform monopole. Observational evidences show an uneven structure that SST gradient and asymmetric precipitation pattern stand out despite embedded in basin-wide warming. This inhomogeneous component is found to be an inherited feature of the IOBM and plays a vital role to shape up subsequent events over the Indian Ocean. The major cause for such meridional precipitation asymmetry can be attributed to the TT-RR mechanism, an enhancement of climatological convective zones under the warming of tropical tropospheric temperature (TT) , called the rich-get-richer (RR) effect by Neelin and Chou. In February-March [FM(1)] the climatological convective zones (ITCZ) over the Indian Ocean migrates to 12°S. At the time of the Nino(1) year, the whole Indian Ocean basin is loomed under the invading warm TT which intensifies both climatological ITCZ and its northern subsidence lag, thus strengthens the Hadley Cell. The TT-RR mechanism will be further interacted with the downwelling Rossby wave (DRW) originated from the Indian Ocean dipole mode in previous autumn. This DRW was supposed to be waning after termination of the dipole mode two or three months ago. Instead, the charged ITCZ induces powerful cross-equatorial winds which, by Coriolis toque, tends to veer in anti-cyclonic curve thus the negative wind stress curl re-enforces the DRW. Within the narrow belt from 6°S to equator marks a dry zone occupied by surface anti-cyclonic pattern and the associated sea level high pressure. The anomalous heat stored in subsurface temperature will resurface when the ITCZ moves back to the equator, maintain warm SST since the effects of the TT-RR mechanism shift northward with declined influence. The warm SST then sustain northward convection zones in April-May [AM(1)]. South of the ITCZ, positive wind-evaporation effect caused by the reduction of scalar winds in the southern Indian Ocean accounts for warm SST up to 20°S. The rainfall, under local SST control, comes from a mix of deep convection and stratiform type clouds since SST has been dropped to close to deep convection threshold around 26.5°C. The convection in the south of 20°S will be soon damped when large-scale wind-evaporation support no longer available and SST becomes too cold to sustain deep convection in AM(1). In AM(1), following the northward shift of the climatological convection zones, ocean dynamics and surface heat flux both act as the damping effects for SST in the southern Indian Ocean. With the effects inherited from FM(1), asymmetric SST and precipitation anomalies both reach to the peak states in this period. Meanwhile SST in the northern Indian Ocean is warmed up quickly by increase of downward solar radiation and decrease of upward latent heat flux. After May, the asymmetric pattern dissipates very rapidly.