14:30 〜 14:45
[AOS12-04] Interannual SST variability and the generation processes in the southern region of the Somali Current system during boreal summer
キーワード：西部アラビア海、ソマリア海流、サザンジャイア、併合
Strong clockwise monsoonal winds over the western Arabian Sea (AS) excite the northeastward Somali Current (SC) during boreal summer, with two anticyclonic oceanic gyres embedded. These regional gyres, the Southern Gyre (SG) and the Great Whirl (GW), develop around 4°N and 10°N, respectively, in boreal spring and early summer. These gyres are accompanied by the strong upwelling areas, “Cold filaments”, at the northern flanks and transport the cold and nutrient-rich water offshore. Therefore, interannual variability of these gyres can significantly affect biological productivity, regional climates, and climate modes such as the Indian Ocean Dipole. While the SG moves northward and coalesces with the Great Whirl in some years (one-gyre year), it remains near 4°N to keep a two-gyre structure in other years (two-gyre year). Although analytical and idealized model studies suggest that the SG stays further south when the SC is strong, the impacts of the two different states of the gyres on the SST variability and their processes are unclear. This study explores these questions by using the outputs of the regional ocean model (ROMS).
The simulation is conducted from 1990 to 2017 with the initial conditions of the end of the 20 years spin-up run, and it is confirmed that seasonal cycles of the SST, MLD, transport of the SC, and EOF1 and EOF2 modes for the summer SST of the AS are well reproduced. Out of these two modes, EOF2 is associated with the negative SST anomaly and strong SG recirculation in the western equatorial region in the positive phase, which corresponds to the two-gyre structure. Mixed-layer heat budget analyses are conducted for composites of the positive EOF2 years and the negative EOF2 years, respectively. During the positive EOF2 years, the advection and the vertical mixing mainly contribute to the anomalous cooling in the western equatorial region from May to July. The advection contributes to cooling only during the monsoon onset period, which may be associated with the upwelling at the cold filament of the SG. The vertical mixing, on the other hand, develops as the season progresses, and strong cooling occurs along the strong SG recirculation due to enhanced vertical shear of horizontal currents at the MLD base. During the monsoon peak period, the vertical mixing dominates the mixed-layer heat budget, and the advection plays a role only in redistribution of the low SST generated by the vertical mixing.
The simulation is conducted from 1990 to 2017 with the initial conditions of the end of the 20 years spin-up run, and it is confirmed that seasonal cycles of the SST, MLD, transport of the SC, and EOF1 and EOF2 modes for the summer SST of the AS are well reproduced. Out of these two modes, EOF2 is associated with the negative SST anomaly and strong SG recirculation in the western equatorial region in the positive phase, which corresponds to the two-gyre structure. Mixed-layer heat budget analyses are conducted for composites of the positive EOF2 years and the negative EOF2 years, respectively. During the positive EOF2 years, the advection and the vertical mixing mainly contribute to the anomalous cooling in the western equatorial region from May to July. The advection contributes to cooling only during the monsoon onset period, which may be associated with the upwelling at the cold filament of the SG. The vertical mixing, on the other hand, develops as the season progresses, and strong cooling occurs along the strong SG recirculation due to enhanced vertical shear of horizontal currents at the MLD base. During the monsoon peak period, the vertical mixing dominates the mixed-layer heat budget, and the advection plays a role only in redistribution of the low SST generated by the vertical mixing.