Statistical characteristics and mechanisms of mesoscale eddies in the North Indian Ocean
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Abstract: The statistical characteristics and mechanisms of mesoscale eddies in the North Indian Ocean are investigated by adopting multi-sensor satellite data from 1993 to 2019. In the Arabian Sea (AS), seasonal variation of eddy characteristics is remarkable, while the intraseasonal variability caused by planetary waves is crucial in the Bay of Bengal (BOB). Seasonal variation of the eddy kinetic energy (EKE) is distinct along the west boundary of AS, especially in the Somali Current region. In the BOB, larger EKE occurs at the northwest basin from March to May, to the east of Sri Lanka from June to September, and along the east coast of India from November to December. The wind stress work (WW) is further studied to figure out the direct influence of wind forcing on EKE. The WW exerts positive effects on EKE along the west boundary of AS and in the south of India/Sri Lanka during the two monsoon seasons. Besides, the WW also has impact on EKE along the east coast of India in November and December. Eventually, we investigate the characteristics and the driving mechanisms of long lifespan eddies. In the AS, long lifespan anti-cyclonic eddies (AEs) mainly generate in the Socotra, the West Indian Coastal Current and the East Arabian Current regions, while cyclonic eddies (CEs) are concentrated in the northwest region. In the BOB, long lifespan AEs mostly form near the west of Myanmar, while CEs are accumulated at the north and northwest basin. The instabilities caused by Rossby waves, coastal Kelvin waves, seasonal currents, together with wind stress forcing exert enormous efforts on the generation and evolution of these eddies.
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Key words:
- mesoscale eddy /
- eddy characteristics /
- North Indian Ocean /
- satellite altimeter
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Figure 1. Schematic currents and surface EKE in the North Indian Ocean during the summer monsoon (a) and winter monsoon (b). The abbreviations are as follows: SC, Somali Current; EAC, East Arabian Current; SMC, Southwest Monsoon Current; NMC, Northeast Monsoon Current; WICC, West India Coastal Current; EICC, East India Coastal Current; GW, Great Whirl; SE, Socotra Eddy; LH (LL), Laccadive High (Laccadive Low). The arrows sketch out the direction of currents and the color shading represents the average EKE in July (a) and January (b).
Figure 2. The distribution of eddy numbers counted by the generated positions in 1°×1° box. a. Anticyclonic eddies, b. cyclonic eddies.
Figure 3. The distribution of eddy frequency and polarity in the North Indian Ocean. a. Eddy frequency, b. eddy polarity.
Figure 4. The proportional histograms of eddy lifespan and average radius during the lifespan. Histogram of lifespan (5-days interval) in the AS (a) and BOB (b), histogram of radius (5 km interval) in the AS (c) and BOB (d).
Figure 5. Lifespan variation with latitude (a) and longitude (b) in the AS, and lifespan variation with latitude (c) and longitude (d) in the BOB. The vertical bars denote the standard deviation of lifespan.
Figure 6. Radius variation with latitude (a) and longitude (b) in the AS, and radius variation with latitude (c) and longitude (d) in the BOB. The vertical bars denote the standard deviation of radius.
Figure 7. Variation of u with latitude (a) and longitude (b), and variation of v with latitude (c) and longitude (d). Positive values refer to eastward and northward. The vertical bars denote the standard deviation of u and v.
Figure 8. Monthly variation of eddy characteristics in the AS. a. Eddy number, b. lifespan, c. radius, and d. EKE.
Figure 9. Monthly variation of eddy characteristics in the BOB. a. Eddy number, b. lifespan, c. radius, and d. EKE.
Figure 11. Horizontal distribution of monthly averaged sea surface height anomaly (SSHA). The numbers in the first subplot mark the positions correspond to the position numbers in Fig. 14b.
Figure 13. Trajectories of eddies with lifespan longer than 100 days. a–d: AEs, e–h: CEs. The square symbols denote the generated positions, and the diamond symbols refer to the dissipated positions. MAM, March, April, and May; JJA, June, July, and August; SON, September, October, and November; DJF, December, January, and February; AS, Arabian Sea; BOB, Bay of Bengal.
Figure 14. Time-longitude diagram of multi-year mean annual cycle of SSHA averaged from 10°N to 15°N in the AS (a), averaged from 14°N to18°N in the BOB (b), and SSHA along the north boundary of AS (c). The position numbers are marked in first subplot of Fig. 11.
Figure 15. The evolution characteristics of long lifespan eddies in the North Indian Ocean. a, b and c refer to AEs generated in Regions A, B, C in diagram f; d and e refer to CEs generated in Regions D, E in diagram f. AMP, RAD, EKE, WW and EKM refer to amplitude, radius, eddy kinetic energy, the wind stress work and the Ekman pumping vertical velocity, respectively.
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