5. ANALYSIS OF WIND STRESS AND WIND STRESS CURL

The following items are presented in this rubric :

• 5.1 Annual mean distribution of wind stress
• 5.2 Annual mean distribution of wind stress curl
• 5.3 Seasonal mean distribution of wind stress

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5.1 Annual mean distribution of wind stress

The purpose of this section is to show the main features of wind stress calculated from scatterometer winds. These are compared to published climatologies such as those of Hellerman and Rosinsten (1983, hereafter HR). Figure 18 shows the global annual mean wind stress in terms of magnitude ( ), wind direction, zonal (_x) and meridional (_y) components. The most pronounced features of the wind stress field are retrieved. Features such as the subtropical anticyclonic gyres in the North and South Atlantic, North and South Pacific and Indian Oceans, the stress patterns associated with SE and NE trades in the tropics, and the strong westerly wind stress dominating the Southern Oceans are clearly evident. Some small scale features are also depicted, such as those located at 80°W , 12°N, or at 160°W, 20°N. In the Southern Oceans, wind stress is highly variable, varying between 0.1 N/m² (Pa) and 0.26 N/m² (Pa) with several small scale features. The mean wind stress direction and magnitude derived from scatterometer data exhibit close affinity with HR climatology. Some absolute differences exist and are mainly located in the high latitudes. For instance, over the Northern Hemisphere, the scatterometer values vary between 0.08 and 0.22 N/m² (Pa), while the HR varies between 0.5 and 0.21 N/m² (Pa). Wind direction is consistent with the climatological results. The conspicuous annual variation is associated with the warm pool region, in the tropical Pacific ocean. The annual mean zonal wind stress is mainly positive in the Northern and Southern Oceans and negative in the tropics except in the north of Indian Ocean. Its values are much higher in the SH than in the NH. The maximum annual westward stress is 0.12 N/m² (Pa). Although the values of the annual mean meridional wind stress generally vary between -0.06 N/m² (Pa) and 0.06 N/m² (Pa), the largest values are located west of the continents between 10° S and 40° N which are favorable to upwelling events.

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 To illustrate the comparison between the new wind stress results and those published by HR or by Han and Lee (1983), the annual average of , _xand _y mean stress over the world are computed (Figure 19). The mean behavior of t indicates that the minimum value is located at the equator and that wind stress is 20% higher in SH than in NH. The present calculation shows that the zonal behavior of _x is similar to that of HR. However, the maximum westerly stress located between 40°N and 60°N is weaker compared to HR data. These new results are more consistent with Han and Lee studies. They noted that the HR data are overestimated in this NH region. These zonal features are quite a good characterization of basin zonal wind stress. The great discrepancies are located in the Northern Indian ocean where the summer monsoon, characterized by the westerly winds, dominates the annual mean.

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5.2 Annual mean distribution of wind stress curl

The annual mean stress curl is computed and shown in Figure 20. It agrees reasonably well with earlier calculations. The largest values are located in the Southern and Northern oceans. The zero line of the wind stress curl indicating the northward and southward transports of subpolar and subtropical gyres, respectively, is found in the Northern hemisphere. Other features are revealed such as the band of positive curls located between 15°N and the equator, and the curl changes in the Indian Ocean related to the monsoon event.

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5.3 Seasonal mean distribution of wind stress

The seasonal features of the scatterometer weekly and monthly averaged wind fields are also analyzed. Three-month mean wind stress over the globe is calculated and displayed in Figure 21. The seasonality of atmospheric circulation is evident. During the December-January-February period, the North Atlantic and Pacific regions are characterized by strong cyclonic circulation. The anticyclonic cells in these regions have substantial seasonal fluctuations resulting from the influence of mid-latitude atmospheric circulations, which are related to the seasonal land/sea contrast. The highest seasonal variations are located in the Asian monsoon region. There is a five-fold increase in wind stress magnitude estimated between the period June-July-August and the period December-January-February. Furthermore, the region is dominated by northeasterly winds from December to April and by southwesterly winds during the rest of the year. In fact, the change in wind direction occurs between March and April as shown by the monthly mean wind stress in Figure 22. The maximum value of wind stress in the Indian Ocean is reached in July and is greater than 0.25 N/m² (Pa). Wind stress exhibits significant fluctuations in the tropical area. For instance, during the winter and spring seasons, the trade wind system undergoes an annual cycle of equatorward wind and zonal expansion, followed by poleward wind and contraction into the central and eastern oceans during the summer and autumn seasons. The ITCZ is located near the equator in the northern winter and spring, and around 5° N from June to November. The fluctuations in wind stress magnitude are mainly related to seasonal atmospheric circulation. The anticyclonic cells at 30° S, such as at the island of St Helens, are clearly reproduced and appear permanent during the four seasons. Even though the values of wind stress are always large over the SH, the variations between December-February and June-August are significant.

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