Influences of the Great Whirl on surface chlorophyll a concentration off the Somali Coast in 2017
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Abstract: The general features of the Great Whirl (GW) off the Somali Coast in 2017 and its influences on chlorophyll a (Chl a) concentration were studied by using satellite data and model outputs. Results show that GW, which initiated at 7°N, 53°E on June 13, had a lifetime of 153 d with an average amplitude of 16 cm and an average radius of 205 km. After the formation of GW, the concentration of Chl a in the interior of GW showed a downward trend throughout its life cycle, except in early July and mid-October. In early July, the Chl a blooms in the interior of GW were attributed to the combined effect of three processes. They are eddy horizontal transportation, the deepening of the mixed layer caused by the monsoon and eddy pumping, and the upward transportation of nutrients caused by eddy-induced Ekman pumping. In October, the Chl a blooms were probably due to the weakening of GW. During the period, water exchange occurred more frequently across the eddy, thus phytoplanktons were imported into the interior of GW.
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Key words:
- Great Whirl /
- chlorophyll a /
- mixed-layer /
- eddy-induced Ekman pumping /
- summer monsoon
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Figure 5. Chl a concentration distribution around the Great Whirl in 2017. The SSH anomaly is from satellite observations (black solid contours for SSHa>0 and black dash contours for SSHa<0). The colors represent Chl a concentration. The vectors represent the cross-calibrated multi-platform surface winds.
Figure 7. Time evolution of the Chl a concentration (a), mixed layer depth (MLD) (b) and vertical velocities induced by eddy-induced Ekman pumping (WEP) (c) of Great Whirl in 2017. All the three parameters in GW are defined as the mean value within 0.5R. The grey lines represent the original results, and the red lines represent the results that are smoothed using a 7-point moving average filter.
Figure 8. Time evolution of differences between Chl a concentration in the interior and periphery of the Great Whirl. The parameter in the interior of Great Whirl is similar to that in Fig. 7. Accordingly, the parameter at the periphery is defined as the mean value of Chl a concentration between 1R and 2R. The grey lines represent the original results, and the red lines represent the results that are smoothed using a 7-point moving average filter.
Figure 9. Schematic diagram of GW and the underlying mechanisms of the Chl a bloom in July (a) and October (b). R and A are the radius and SSH amplitude of GW respectively. The spatial distribution of Chl a concentration was monthly averaged. The mixed layer depth and the vertical velocity of eddy-induced Ekman pumping were calculated by monthly averaged in 0.5R of GW.
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