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Ocean Response to Successive Typhoons Sarika and Haima (2016) Based on Data Acquired via Multiple Satellites and Moored Array

2019-12-02

Tropical cyclones (TCs) are natural disasters for coastal regions. TCs with maximum wind speeds higher than 32.7 m/s in the north-western Pacific are referred to as typhoons. Typhoons Sarika and Haima successively passed our moored observation array in the northern South China Sea in 2016. Based on the satellite data, the winds (clouds and rainfall) biased to the right (left) sides of the typhoon tracks. Sarika and Haima cooled the sea surface ~4 and ~2 °C and increased the salinity ~1.2 and ~0.6 psu, respectively. The maximum sea surface cooling occurred nearly one day after the two typhoons. Station 2 (S2) was on left side of Sarika’s track and right side of Haima’s track, which is studied because its data was complete. Strong near-inertial currents from the ocean surface toward the bottom were generated at S2, with a maximum mixed-layer speed of ~80 cm/s. The current spectrum also shows weak signal at twice the inertial frequency (2f). Sarika deepened the mixed layer, cooled the sea surface, but warmed the subsurface by ~1 °C. Haima subsequently pushed the subsurface warming anomaly into deeper ocean, causing a temperature increase of ~1.8 °C therein. Sarika and Haima successively increased the heat content anomaly upper than 160 m at S2 to ~50 and ~100 m°C, respectively. Model simulation of the two typhoons shows that mixing and horizontal advection caused surface ocean cooling, mixing and downwelling caused subsurface warming, while downwelling warmed the deeper ocean. It indicates that Sarika and Haima sequentially modulated warm water into deeper ocean and influenced internal ocean heat budget. Upper ocean salinity response was similar to temperature, except that rainfall refreshed sea surface and caused a successive salinity decrease of ~0.03 and ~0.1 psu during the two typhoons, changing the positive subsurface salinity anomaly to negativea.

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Figure 1. Tracks of Typhoons Sarika and Haima obtained from the Joint Typhoon Warning Center (JTWC) data (orange), Japan Meteorological Agency (JMA) data (brown), and China Meteorological Administration (CMA) data (black), showing their positions every 6 h (dots). The text boxes point to the UTC 00:00 of the positions everyday, show the dates, sustained maximum wind speed, and central pressure obtained from the CMA data. The red numbers indicate the positions of the observation stations. The background shade indicates the topography.


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Figure 2. Temperature (a), temperature anomaly (d) and their net values (b,e) during Sarika and Haima at Buoy 2. Average temperature (c) and temperature anomaly (f) profiles before the two typhoons (black; averaged between 14 and 16 October), after Sarika (blue; averaged over one inertial period after 18 October), and after Haima (red; averaged over one inertial period after 22 October). The black solid lines in (a,b,d,e) indicate the mixed layer depth, which is the depth at which the temperature is 0.5 °C lower than the surface layer temperature. The black dashed lines indicate the 15 °C isotherm. (g,h) Accumulated surface heat flux (black lines), heat anomaly from 0 to 160 m (red lines), and their sum (blue lines) at Buoy 2. The vertical brown dashed lines represent the times when Sarika and Haima were closest to Station 2. The temperature observed by the sensor near the surface (at a depth of 22 m) was used for the heat content calculations for shallower depths.

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Figure 3. Sketch of the vertical temperature profiles at Station 2 before (dashed lines) and after (solid lines) (a) only mixing, (b) Sarika, and (c) Haima. The dotted lines in (b) and (c) indicate the temperature profiles caused by only mixing, while the dot-dashed line in (c) indicates that caused by Sarika. The red shading indicates the warming anomaly, and the blue shading the cooling anomaly.