| Summary: | 碩士 === 國立臺灣大學 === 大氣科學研究所 === 99 === 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.
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