Upper Ocean Response to Typhoon Kalmaegi and Sarika in the South China Sea from Multiple-Satellite Observations and Numerical Simulations

Xinxin Yue, Biao Zhang, Guoqiang Liu, Xiaofeng Li, Han Zhang, Yijun He
2018 Remote Sensing  
We investigated ocean surface and subsurface physical responses to Typhoons Kalmaegi and Sarika in the South China Sea, utilizing synergistic multiple-satellite observations, in situ measurements, and numerical simulations. We found significant typhoon-induced sea surface cooling using satellite sea surface temperature (SST) observations and numerical model simulations. This cooling was mainly caused by vertical mixing and upwelling. The maximum amplitudes were 6 • C and 4.2 • C for Typhoons
more » ... maegi and Sarika, respectively. For Typhoon Sarika, Argo temperature profile measurements showed that temperature response beneath the surface showed a three-layer vertical structure (decreasing-increasing-decreasing). Satellite salinity observations showed that the maximum increase of sea surface salinity (SSS) was 2.2 psu on the right side of Typhoon Sarika's track, and the maximum decrease of SSS was 1.4 psu on the left. This SSS seesaw response phenomenon is related to the asymmetrical rainfall on both sides of the typhoon track. Acoustic Doppler Current Profilers measurements and numerical simulations both showed that subsurface current velocities rapidly increased as the typhoon passed, with peak increases of up to 1.19 m/s and 1.49 m/s. Typhoon-generated SST cooling and current velocity increases both exhibited a rightward bias associated with a coupling between typhoon wind-stress and mixed layer velocity. Remote Sens. 2018, 10, 348 2 of 22 strong vertical mixing and upwelling, which accounts for decreased sea surface temperature (SST), increased sea surface salinity (SSS), and increased mixed layer depth. Early studies have shown that entrainment and vertical mixing are dominant mechanisms decreasing SST beneath a moving TC [7, 8] . One consequence of the TC-induced large vertical shear is that the mixed layer thickness increases and entrainment becomes enhanced. Observed TC tracks and coupled ocean-atmosphere hurricane models were used to estimate the net ocean heating induced by global tropical cyclone activity, thus suggesting that TCs may play an important role in driving the thermohaline circulation [9] . It has been reported that approximately 15% of the peak ocean heat transport may be associated with the vertical mixing induced by TCs [10] . On global scale, TCs are responsible for 1.87 PW (11.05 W·m −2 ) of heat transfer annually from the global ocean to the atmosphere during the passage of a storm [11] . SST response to typhoons is of particular interest because SST plays a key role in energy exchange between typhoons and the ocean. The South China Sea (SCS) is a large, semi-enclosed marginal sea, and is subject to frequent typhoons. Thus, proper characterization of typhoon-induced SST cooling may help improve understanding of typhoon mechanisms and improve typhoon intensity predictions. Previous research found that SST dropped 9 • C after the passage of Typhoon Kai-Tak and 11 • C following Typhoon Lingling [12, 13] . Satellite measurements and three-dimensional numerical simulations have shown that typhoon-induced decreases in SST depend strongly on the typhoon's maximum surface wind speed, translation speed, and size [14, 15] . SST cooling amplitude also strongly depends on depth of the mixed layer and thermocline stratification prior to the storm [16, 17] . During strong typhoons (greater than or equal to category 4), the subsurface warming expected from vertical mixing is comparable in magnitude to the near-surface cooling; however, for weak typhoons (less than or equal to category 3), only surface cooling occurs, thus suggesting that air-sea heat exchange and vertical advection possibly play an important role in the case of weak typhoons [18] . Rapid intensification occurs when a TC pass over warm eddies owing to the increasing ocean heat content, while storm-induced mixing and SST cooling are suppressed by the warm ocean features. Composited advanced very-high-resolution radiometer observations and buoy measurements showed SST decreases of only about 0.5 • C-1 • C when Hurricane Opal passed over a warm core ring in the Gulf of Mexico [19] . In addition, when Typhoon Maemi, a category 5 storm, encountered a warm ocean eddy in the western North Pacific, there was an analogous SST cooling of approximately 1 • C [20]. Typhoon Maemi's self-induced SST cooling was suppressed by a thicker upper ocean mixed layer in the warm eddy. This prevented cold water from the deep ocean from being entrained into the upper ocean mixed layer. Moreover, the near-surface barrier layer also suppressed the typhoon-induced upper ocean cooling, leading to temperature changes that were smaller than expected [21] . When typhoon-induced forcing is strong enough to break through the barrier layer base into the thermocline, the barrier layer reduces the cooling magnitude [22] . Upper ocean current response to typhoons in the northwestern Pacific Ocean has been characterized through analysis of ocean current observations from Surface Velocity Program (SVP) drifters and Joint Typhoon Warning Center (JTWC) best track data. Near-surface current speeds greater than 2 m/s were observed during major typhoons, with strongest mean currents to the right of the storm track [23] . SVP drifter observations showed a maximum mixed layer current velocity of 2.6 m/s for Typhoon Maon [24] . Furthermore, the investigation also demonstrated that current velocity in the mixed layer depended on both the typhoon's translation speed and its intensity scale. For a slow-moving and super strong typhoon, the maximum mixed layer current velocity is roughly twice that of a fast-moving and weak typhoon. Typhoon-induced near-inertial motions are dominant during the relaxation stage, and barotropic subinertial waves and baroclinic near-inertial oscillations were both observed at this time, indicating a subinertial wave period of 2.8-4.1 days [25] . Near-inertial oscillations decayed more rapidly in shallow water than in the deep ocean, which emphasized the importance of frictional effects for characterizing responses to typhoon forcing in the shallow ocean [26] . Upper ocean current responses to typhoons have been investigated using numerical simulations and in situ observations. Strong near-surface current with maximum speed of 1.5 m/s was Remote Sens. 2018, 10, 348 5 of 22 datasets were not used. Stations 1 and 4 were located on the right of the track of Typhoon Kalmaegi, whereas Station 2 was on the left. All these stations were located within the maximum wind radius of Kalmaegi. Station 2 is much closer to the track than stations 1 and 4. Currents were measured using ADCP at each station: a 150 kHz (Stations 1 and 2) or 300 kHz (Station 4) downward-looking ADCP binned at 8-m (Stations 1 and 2) or 4-m (Station 4) intervals with the first bin at either 14 m (Stations 1 and 2) or 8 m (Station 4) and the last bin at either 246 m (Stations 1 and 2) or 124 m (Station 4) below the surface. The 75 kHz ADCP velocity accuracy is 1% of water velocity relative to the ADCP ±0.01 m/s, and the 150 and 300 kHz ADCP accuracies are 1% ± 0.005 m/s and 0.5% ± 0.005 m/s, respectively. Remote Sens. 2016, 8, x 5 of 4
doi:10.3390/rs10020348 fatcat:pikomh3bufanvhiz3vqm6ijjj4