Buoy Observations of Turbulent Mixing in the northwestern subtropical Pacific Ocean

Strong winds from typhoons decrease sea surface temperatures, and this cooling area is called the cold wake. It is well known that the primary mechanisms causing this phenomenon include turbulent mixing and Ekman pumping that result in the upwelling of cold water in the lower layer. The amplitude of surface cooling is influenced by the typhoon’s moving speed, strength, the radius of the storm, and the pre-typhoon conditions of the upper ocean. The cooling phenomenon affects air-sea interactions, but observing the upper ocean in such an extreme environment is challenging. To understand the physical process of mixing, and to improve the predictions of numerical models, more observations of turbulent mixing are needed. In addition, the passage of eddy also affects the occurrence of background conditions and turbulent mixing. The northwestern subtropical Pacific Ocean is an area where typhoons are prevalent, and eddies often pass through it. Therefore, this is a suitable area to study turbulent mixing when typhoons and eddies pass by.

These data were obtained from the surface buoy and the ADCP subsurface moorings located in the northwestern subtropical Pacific Ocean. In September 2022, category 5 typhoon Hinnamnor passed the buoy site during observation period. The upper ocean profiles of temperature and current were obtained, in order to estimate the Richardson numberwhen the typhoon and the cold eddy passed by. The observation results show that the peak value of the probability distribution of the Richardson number was about 3 to 4, and the probability of being less than 0.25 was about 12% at a depth of 20 m before the passage of Typhoon Hinnamnor. When the buoy system was within the 34-knot wind radius (R34) of a typhoon, the peak value of the probability distribution of the Richardson number decreased to slightly smaller than 0.25, and the probability of being less than 0.25 is about 62% at a depth of 20 m. At a depth of 75 m, the probability distribution of the Richardson number did not significantly change within the 34-knot wind radius (R34) of a typhoon,and it was not even close to 0.25. It shows that typhoon-induced turbulent mixing has no effect at this depth. In addition, during the normal period without a cold eddy, the mixed layer was deeper than the depth of 20 m. Marginal instability was evident within the mixed layer, in which the probability distribution of the Richardson number oscillated around 0.25. During the passage of the cold eddy, the upwelling of cold water made the surface mixed layer thinner, and stratification was more stable. Therefore, the cold eddy would prohibit turbulent mixing. The probability distribution of the Richardson number shifted to a larger value. However, the probability distribution of the Richardson number at a depth of 75 m did not change significantly. As a result, the observed cold eddy had no effect on the turbulent mixing at this depth. These results will be presented herein in detail.