Turbulence intensification of marine winds at mid-latitude in the southern hemisphere; comparison with the northern hemisphere

Four observational buoys were deployed at the Southern Ocean Flux Station (SOFS, 142°E, 46°S) from March 2010 to October 2013. During the observational period, the average wind speed is 9.10m/s, the average air temperature is 9.45 degree C, and the average SST is 10.68 degree C. The direction of wind is predominantly westerly due to the mid- to high-latitude Southern Hemispheric atmospheric circulation. The 10 minutes Gust factor over 1.6 starts to deviate from the Weibull distribution. Such extreme events were found up to 317 cases. In this study, we describe the possible cause of these extreme events. Turbulence intensity (TI) was derived from the Gust factor, based on the hypothesis that the deviation of wind speed follows a Gaussian distribution (Wieringa, 1972) at high wind speed (U>8m/s).

The gust factor or equivalently the TI is inversely proportional to the Bulk-Richardson number, implying that the atmospheric instability enhances turbulence. In the winter northwest Pacific Ocean, Nishida and Waseda (2015) revealed that the primary cause of the increase in TI is the enhanced sensible heat flux. Contrary to the expectation, in the Southern Ocean, the role of the sea surface sensible heat flux is not as significant. In the extreme cases, however, the numerical simulation of NHM (Non-Hydrostatic Model by JMA) revealed that SLP tends to be low and cold fronts stayed close to the observational point. Hirata et al. (2015) showed that latent heat or moisture plays a vital role in enhancing the extratropical cyclones. The research implies that TI at the SOFS site can be intensified by moisture convergence. Currently, we are preparing sensitivity experiments to investigate the relative role of latent and sensible heat using the WRF simulation. Concurrently, we are investigating the possible relationship between turbulence and surface wave.