Responses of a warm mesoscale eddy to bypassed typhoon Megi in the South China Sea

doi:10.3969/j.issn.1001-909X.2024.02.001

Abstract

Abstract:

Based on multi-platform observed data, an unexpected response of a warm mesoscale eddy to bypassed typhoon Megi in the South China Sea in 2010 was observed and investigated. During the passage of typhoon Megi, the SLA maximum of the warm eddy increased from 30 to 36 cm, the radius increased from 78 to 116 km, the eddy kinetic energy increased from 166 to 303 m²/s², and the amplitude increased from 3 to 9 cm. On the right side of the typhoon, the thermocline water at Argo station on the edge of the warm eddy sank by 20 to 40 m. Diagnosis of the wind stress curl alone indicates that the warm eddy should be weaken and the thermocline should be raised, which are inconsistent with the observation results. Diagnosis based on the reanalysis sea surface velocity indicates that during the passage of typhoon Megi, the water diverges below the typhoon path, while the water converges on the right side of the path in the warm eddy region, and the SLA maximum as well as the amplitude of warm eddy are positively correlated with the convergence intensity. Estimation based on the reanalysis sea surface velocity also indicates that the water at Argo station will sink 29 m. Both the warm eddy characteristics and the thermocline depression are consistent with the observation. The case study shows that the response of mesoscale eddy on the edge of typhoon influence to typhoon is constrained not only by wind stress curl but also by the oceanic background conditions, and further attentions are required to explore the corresponding response and mechanism of upper ocean to typhoon.

Key words: typhoon, warm eddy, enhance, wind stress, convergence

CLC Number:

P732

LI Sheng, XUAN Jiliang, HUANG Daji. Responses of a warm mesoscale eddy to bypassed typhoon Megi in the South China Sea[J]. Journal of Marine Sciences, 2024, 42(2): 1-14.

Figures/Tables 9

Fig.1 Trajectory and intensity of typhoon Megi (The colored line represents the trajectory of the typhoon, its color represents the maximum wind speed at the typhoon center, the dashed line box represents the research area, the black triangles represent the location of the typhoon center at 0 o’clock on that day, the green diamonds are the location of the Argo station, and the black thick lines are the region of the warm eddy on October 24.)

Fig.2 Daily mean SLA during the passage of typhoon Megi in the South China Sea (The blue solid line represents the trajectory of typhoon Megi, the blue dashed line represents the trajectory of typhoon Megi on that day, the blue triangles represent the center of the typhoon at 00:00, 12:00, and 24:00 on that day, the black solid line represents the region of the warm eddy, the green diamond marks the location of the Argo station. The black dashed line and the red dashed line in figure 2h represent the envelope of typhoon’s force 7 wind circle and the envelope of maximum wind speed circle, respectively. The symbols in the following figure are the same.)

Fig.3 Daily variation of the SLA maximum, radius, kinetic energy and amplitude of the warm eddy (The dashed line indicates the time when typhoon Megi begin to affect the warm eddy.)

Fig.4 The vertical profile and corresponding variation of temperature (a), salinity (b) and density excess (c) before and after the typhoon Megi at Argo station (The black solid line represents the data before the typhoon passes through, the black dashed line represents the data after the typhoon passes through, the red solid line represents the difference between the data after and before the typhoon passes through. The two black horizontal dashed lines correspond to the depths of the upper mixed layer before and after the typhoon passes through: 35 m and 90 m, respectively.)

Fig.5 The T-S plot (a) and the upwelling/downwelling distances calculated based on temperature and density profiles (b) at Argo station (In figure 5a, the black numbers indicate the density excesses, the blue and red numbers indicate the depths before and after sinking of some seawater sinking extreme points, respectively. In figure 5b, the black horizontal dashed lines mark the two characteristic water layers for the sinking distance: 60 m and 280 m.)

Fig.6 Daily wind field and wind stress curl at 12:00 during the passage of typhoon Megi in the South China Sea (The black dashed line represents the contours where wind stress curl is zero, the gray arrows represent wind vectors.)

Fig.7 Time-variation of Ekman pumping velocity, wind stress curl and accumulated thermocline upwelling/downwelling distances (The black solid line in figure 7a represents the moment when the warm eddy is closest to typhoon Megi. The black solid line in figure 7b represents the moment when the Argo station is closest to typhoon Megi, and the black dashed lines represent the time of the two observations at the Argo station.)

Fig.8 Time-variations of SLA (a-d); divergence of sea surface flow(e-h); flux of divergence, SLA maximum and amplitude within the warm eddy extent(i) (The vertical dashed line in figure 8i represents the time when typhoon Megi begin to affect the warm eddy.)

Fig.9 Divergence of sea surface flow (a-b), sea surface temperature(c-d), chlorophyll-a mass concentration(e-f) before and after the passage of typhoon Megi

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