What are the effects

What are the gross effects of the ocean heat transport on the climate? The main effect is simply that the high latitudes, especially the high latitudes in the Northern Hemisphere, are warmer than they would be if the oceans were not present. How much warmer is a question that we cannot answer with armchair reasoning. We would need to perform detailed calculations with comprehensive climate models of the type used to predict the weather or used for global warming experiments.

One such set of experiments was performed by M. Winton of the Geophysical Fluid Dynamics Laboratory in Princeton, and we briefly describe some of the results found (Winton 2003). Climate models solve the equations that determine the temperature and motion of both the atmosphere and the ocean. The models also have representations of sea ice and cloudiness and of their effects on the incoming solar radiation and outgoing infrared radiation. Thus, for example, snow and ice cover reflects solar radiation back to space, making the climate cooler than it would be in their absence. If the ice sheets were for some reason to expand, the climate would cool, the ice sheets would further expand, and the climate would further cool—an example of a positive feedback, in this case the ice-albedo feedback.

In one set of numerical experiments, the ocean was replaced by a mixed layer, so that although the heat uptake and release of the ocean throughout the annual cycle are accounted for, the effect of the ocean currents and so the oceanic heat transport are completely removed. When the ocean heat transport is removed, the atmosphere tries to compensate for this change by transporting more heat poleward itself. However, in spite of this and in spite of the relatively small heat transport of the oceans compared to the atmosphere at high latitudes, the effects of the oceans are found to be quite large because of a feedback involving sea ice and, to a lesser extent, low-level cloudiness. The simulations without oceanic heat transport all developed large ice sheets that covered mid- and high latitudes, making the overall climate much colder than it is now.

What seems to happen is the following. Although the atmosphere is able to partially compensate for the lack of an ocean transport, the atmospheric transport naturally occurs at a higher elevation than the ocean transport. The lack of an ocean heat transport enables sea ice to grow, and once the ice begins to grow, the positive ice-albedo feedback comes into play and the ice grows more. The detailed mechanism for the strong effect of the ocean seems to involve the upward con-vective flux of heat in the wintertime: The meridional overturning circulation leads to convection at high latitudes, with cold water parcels sinking and being replaced by slightly warmer parcels, which then release heat into the atmosphere. This process is eliminated when the ocean is replaced by a mixed layer, allowing sea ice to grow.

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