Introduction to Physical Oceanography : Chapter 4 - Atmospheric Influences

Figure 4.2 shows the distribution of sea-level pressure averaged over the 1979–2001 period. The map shows strong pressure gradients from the west between 40° to 60° latitude, the roaring forties, weaker gradients in the subtropics near 30° latitude, trade-winds from the east in the tropics, and weaker gradients in the east near the Equator. Atmospheric pressure is the result of uneven distribution of solar heating. Wind just above the sea surface is parallel to the contours of constant pressure, and the wind speed is proportional to the gradient of the pressure (spacing of the contours). Figures 4.2A and 4.2B show the distribution of surface winds in June, July, and August and in December, January, and February.

Figure 4.2 Map of mean annual sea-level pressure calculated from the ECMWF 40-year reanalysis. From From Kallberg et al 2005.

A simple cartoon of the distribution of winds in the atmosphere (Figure 4.3) shows that the surface winds are influenced by equatorial convection and other processes higher in the atmosphere. The mean value of winds over the ocean is (Wentz et al., 1984) : U₁₀ = 7.4m/s (4.1)


Figure 4.3 Simplified schematic of Earth’s atmospheric circulation driven by solar heating in the tropics and cooling at high latitudes. Left: The meridional cells in the atmosphere and the influence of Earth’s rotation on the winds. Right: Cross-section through the atmosphere showing the two major cells of meridional circulation. From The Open University (1989a).

Maps of surface winds (Figures 4.4A and 4.4B) change somewhat with the seasons. The largest changes are in the Indian Ocean and the western Pacific Ocean (Figure 4.4). Both regions are strongly influenced by the Asian monsoon. In winter, the cold air mass over Siberia creates a region of high pressure at the surface, and cold air blows southeastward across Japan and on across the hot Kuroshio, extracting heat from the ocean. In summer, the thermal low over Tibet draws warm, moist air from the Indian Ocean leading to the rainy season over India.

Figure 4.4A Map of mean annual 10-meter winds in June, July, and August calculated from the ECMWF 40-year reanalysis. From From Kallberg et al 2005.

Figure 4.4B Map of mean annual 10-m winds in December, January, and February calculated from the ECMWF 40-year reanalysis. From From Kallberg et al 2005.