Earth Sciences and Ecology Question #3786

Alli, a 20 year old male from Cincinnati asks on January 23, 2007,

I've heard that the oceans of the Northern Hemisphere are warmer than those of the Southern Hemisphere.  Why is this so?

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The answer

Bill Merryfield, Environment Canada answered on February 2, 2007

[Christopher Garrett, University of Victoria professor of ocean physics also contributed to this answer.] The oceans of the Northern hemisphere are indeed warmer than those of the Southern hemisphere as indicated in the accompanying graph that shows mean sea-surface temperatures beginning at the equator (on the left) and continuing to the poles on the right. The blue line is the temperature of the southern oceans and the red line represents the northern oceans.Graph of mean annual sea surface temperatures

As for the reason why, we don't know, but we can make some guesses. The presence of the circumpolar Southern Ocean prevents coastline-hugging warm currents, such as the Gulf Stream and Kuroshio in the northern hemisphere and the Agulhas and Leeuwin Currents in the southern hemisphere, from carrying warm water poleward at extreme southern latitudes. Also, the westerly winds in the Southern ocean push water away from Antarctica (Coriolis force to the left in the southern hemisphere!), allowing for upwelling of cold water from below. In addition, the North Atlantic Overturning transports almost a petawatt (1015 Watts) northward, and this results in considerable net transport of energy from the Southern to the Northern Hemisphere, whereas the transports due to wind-driven circulation tend to be more symmetric about the equator.

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