Ocean, Past, Present, and Future Chapter 2 - Physical Oceanography

The sun and the atmosphere influence directly or indirectly ocean dynamics. Sources and sinks of energy are sunlight, evaporation, infrared emissions from the sea surface, and sensible heating. Winds drag the ocean’s surface waters behind to depths of up to a kilometer. Wind and tides drive the deeper currents in the ocean. Sunlight warms the tropical ocean, which evaporate, transferring heat in the form of water vapor to the atmosphere. The heat is released again through condensation. Winds and ocean currents carry heat poleward (Heat transport). Because the atmosphere drives the ocean, and the ocean drives the atmosphere, we must consider the ocean and the atmosphere as a coupled dynamic system. The exchange processes involve water (matter), gases, heat and momentum. We start with momentum.

The figure shows the distribution of ten meter altitude winds averaged over the year 2022. The map shows strong winds from the west between 40° to 60° latitude, the roaring forties, weak winds in the subtropics near 30° latitude. Additionally, tropical trade winds from the east, and weaker winds from the east along the Equator are visible. Where, does the strength and direction of the wind come from?

The surface winds are clearly the result of equatorial convection and other processes higher in the atmosphere as well as the thermal ind at mid-latitudes. The mean value of winds over the ocean is about \(U_{10}\) = 7.4 m/s. Maps of surface winds change somewhat with the seasons, which you can explore yourself by selecting seasonal maps in the climatereanalyser.org tool. The largest changes are in the Indian Ocean and the western Pacific Ocean, here the monsoon and thus the convection centers meanders across the equator. In winter, cold air over Siberia generates high pressure at the surface, and cold air blows southeastward across Japan and over the warm waters of the Kuroshio current, extracting heat from the ocean. In summer, the low pressure over Tibet draws warm and moist air from the Indian Ocean leading to the rainy season over India.

The wind causes a shear stress (tangential force) on the ocean surface and drags the water with it. This horizontal force is called the (surface) wind stress. In other words, wind stress is the vertical transfer of horizontal momentum. Thus atmospheric momentum is transferred into the ocean

\(\tau =\rho \cdot C_D \cdot U^2_{10}\)

where \(\rho_a = 1.3 kgm^{-3}\) is the density of air, \(U_{10}\) is wind speed at 10 meters, and \(C_D\) is the drag coefficient, measuring the roughness of the surface. Using eddy covariance instruments and anemometers wind fluctuations within 10–20 m of the sea surface can be measured, from which \(\tau\) is directly calculated. The figure shows the annual wind stress compiled in the ERA 5 reanalysis product.

Measurements of the drag coefficient \(C_D\) vary wildly from 0.0005 to 0.0025 across 5 to 45 m/s wind. An empirical parameterisation fro wind between 6 and 26 m/s is:

1000\(\cdot C_D = 0.60 + 0.071 U_{10}\)

Recent studies show that the influence of the wind on the oceans momentum saturates at wind speeds of 25 m/s (see Curcic and Haus, GRL, 10.1029/2020GL087647)