| Summary: | 碩士 === 國立臺灣大學 === 海洋研究所 === 100 === ATLAS buoys wewe deployed in the Northwestern Pacific, the eye of typhoon Lupit passed over buoy A1 and to the right of buoy A3 last October 2009. These buoys measured surface metrological parameters including air pressure, wind speed and wind direction, solar radiation, humidity, and air temperature, and the water temperature in the upper 500 m. In this study, we use this unique and valuable data to discuss the upper ocean thermal variations and air-sea interaction, to estimate the e-folding time of the cold anomaly, and to understand the dynamic mechanism caused by Typhoon Lupit.
From observations, the lowest air pressure is 941.9 hPa and 954.6 hPa with wind speed up to 59.2 m/s and 46.1 m/s at A1 and A3 respectively. An increase of humidity, decrease of solar radiation and air temperatures was also observed. Meanwhile, when Lupit approached, the temperature in the sea surface and mixed layer(ML) decreased by 0.4℃ at A1 and 1℃ at A3 while the temperature beneath ML was increased. After 29 ~38 hours, the temperature dropped significantly to the minimum in ML, causing SST to drop by 4.3℃at A1 and A3. Later, the cooling at A3 was enhanced because Lupit reapproached again, and reached the maximum cooling 5.2℃. After the passage, the cool water in ML warmed toward equilibrium with temperature ~1.4℃ lower than before due to seasonal cooling, moreover, near-inertial oscillations of temperature below the ML were also noticed.
Integrating the heat content(HC) of the upper 40 m at A1 and upper 30 m at A3 showed a slight decrease. This can be attributed from the loss of the net heat flux. During the passage of Lupit, downwelling caused weakening of the stratification and thickening of the mixed layer depth(MLD), allowing HC at depth 40~100 m at A1 and 30~100 m at A3 to increase. After its passage, an abrupt upwelling caused the MLD to become shallower, making the HC in the ML and at the top of thermocline to decrease rapidly almost at the same time.
Meanwhile, the warming time(Γ) was derived from using exponential regression of the water temperature data. The MLD decreased to 12m at A1 and only 8m at A3 during the warming process, allowing the temperature which within this interval to recover after 16.7 days and 20.3 days at A1 and A3, respectively. However, Γ increased with depths 20~50m at A1 and 8~50m at A3, which indicates a longer time is essential to reach the equilibrium temperature.
To understand all of this, we estimated the heat balance in the ML during the warming process. The results showed even through the net heat flux was positive(which means the atmosphere transmit heat to the ocean), the surface heat flux was only a part of the heat sources, in other words, the horizontal advection is non-neglectable in the warming process.
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