Forcing the Ocean

Abstract

An introduction to an oceanography class could begin with a description of the oceans in these terms: “the ocean is made of saltwater contained in basins…”, and it could continue as: “… and therefore the physical boundaries of the ocean are the bottom of the basins, the coastlines, and the free surface in permanent contact with the atmosphere”. Heat entering the ocean through the bottom of the basins is usually considered as negligible. Estimates of local geothermal sources of heat are quite unreliable, since the methods are often based on poorly constrained budgets, but some individual sources have been measured carefully (Converse et al., 1984). These are typically 108 W, and although this represents one chimney, we can imagine it distributed over the area of the mid-ocean rift valley, to give about 1 W m —2 on average. The flux falls off to values more than three order of magnitude smaller down the flank and onto the abyssal plain. The entire budget for the world ocean is roughly 1013 W, which is thought to be carried by the hydrothermal part; the conductive part is a similar order of magnitude. For comparison, the annual mean solar heat flux entering the ocean from the sea surface is above 150 W m —2 on average, corresponding to a net heat input of 50 x 108 W. In consequence, it is accurate to consider that the main external forcing of the ocean circulation comes from the free surface. Nevertheless, the presence of geothermal heating at the bottom destabilizes the water column, and leads to significant regional flow and mixing (Speer and Helfrich, 1995).