16:15 〜 16:30
[AOS12-04] Interannual variability in sea surface temperature off Somalia
in boreal summer
– Similarities and differences between “Warm year” and “Cold year” –
キーワード：西部アラビア海、経年変動、フィラメント、併合
Strong alongshore southwesterly winds in the western Arabian Sea (AS) associated with the Indian summer monsoon locally force coastal upwelling, northward western boundary current, known as the Somali Current, and two oceanic Gyres, called the Southern Gyre and the Great Whirl. Interannual modulations of all these phenomena are believed to affect sea surface temperature (SST) anomaly during boreal summer in the western AS. To investigate processes responsible for interannual variability in the SST anomaly there, a composite analysis is performed for years with positive (negative) SST anomaly in the western AS during summer, hereafter referred as “Warm year (Cold year)”, using outputs from a regional model based on the Regional Ocean Modeling System (ROMS).
While the overall evolutions of anomalous upper-ocean conditions are similar in the two cases, with the sign of anomalies reversed, there is a significant difference in the SST anomaly field. Two regions of filament-like SST anomaly appear around 4°N and 10°N near the coast of Somalia by mid-June both in Warm and Cold years. These two filament-like anomalies remain almost the same locations through the summer in Warm years. On the other hand, in Cold years, the cold SST anomaly and associated cyclonic circulation around 4°N move northward along the coast of Somalia and eventually coalesces with another filament-like cold SST structure located around 10°N. When this coalescence occurs, the SST around 10°N near the Somali coast decreases rapidly and the cold SST anomaly around 4°N disappears. This merger of two gyres does not occur every year but tends to occur in the Cold years. The merger between two filament-like cold SST anomalies may be related to the stronger alongshore southwesterly winds in the Cold years compared to that in the normal years. This discrepancy between the two cases may result in asymmetric responses in interannual variations of the Indian summer monsoon to the SST anomalies over the western AS.
While the overall evolutions of anomalous upper-ocean conditions are similar in the two cases, with the sign of anomalies reversed, there is a significant difference in the SST anomaly field. Two regions of filament-like SST anomaly appear around 4°N and 10°N near the coast of Somalia by mid-June both in Warm and Cold years. These two filament-like anomalies remain almost the same locations through the summer in Warm years. On the other hand, in Cold years, the cold SST anomaly and associated cyclonic circulation around 4°N move northward along the coast of Somalia and eventually coalesces with another filament-like cold SST structure located around 10°N. When this coalescence occurs, the SST around 10°N near the Somali coast decreases rapidly and the cold SST anomaly around 4°N disappears. This merger of two gyres does not occur every year but tends to occur in the Cold years. The merger between two filament-like cold SST anomalies may be related to the stronger alongshore southwesterly winds in the Cold years compared to that in the normal years. This discrepancy between the two cases may result in asymmetric responses in interannual variations of the Indian summer monsoon to the SST anomalies over the western AS.