04.04.2014: FB1-Seminar

Freitag, 04. April 2014: 11:00 h, Hörsaal, Düsternbrooker Weg 20

Abstract:

We characterize impacts from the ocean mesoscale on heat in the global climate system. Our tool is a suite of centennial-scale 1990 radiatively forced numerical climate simulations from three coupled models, whereby the models differ only in their ocean configuration. Analyses of various physical ocean features in this GFDL CM2-O climate model suite reveals that the finest resolution ocean (the CM2.6 climate model with 1/10 degree ocean grid spacing) experiences the smallest temperature drift from the initial conditions, and in turn the least amount of ocean heat uptake. The coarser models in the suite include one that aims to parameterize mesoscale eddies (CM2-1deg with a nominally 1-degre ocean grid spacing), and CM2.5 (using a 1/4-degree ocean grid spacing). CM2.5 permits mean and transient mesoscale features, but they are far less rich and vigorous than in CM2.6. We conclude that the relatively small drift in CM2.6 results from the vigorous transient mesoscale eddies and the relatively energetic fine scale time mean high latitude currents.

We highlight two features of the ocean mesoscale to support our conclusions. First, stronger northern high latitude time mean currents in both CM2.5 and CM2.6 maintain a realistic sea ice extent, even in the presence of a less vigorous Atlantic overturning circulation than in the coarsest model CM2-1deg. Consequently, the northern high latitudes in CM2.5 and CM2.6 do not experience the significant surface cooling realized in CM2-1deg, nor the associated widespread North Atlantic and North Pacific sea ice extents.Representation of fine-scale currents and the associated gyre heat transport represents a fundamentally new regime for the fine resolution simulations relative to the coarser models, whose heat transport is dominated by broad-scale currents. Second, analysis of the ocean heat budget reveals that mesoscale eddies act regionally and globally to transport heat upward in a manner that partially compensates for the downward heat transport from the time mean flow. In turn, the stronger vertical eddy transport in CM2.6 relative to CM2.5 accounts for the significantly smaller temperature drift in CM2.6. The mesoscale eddy parameterization used in CM2-1deg also affects a vertical heat transport, yet it is insufficient to realize the small watermass drift found in CM2.6.

Our analysis points to the fundamental role that ocean mesoscale features, including mean currents and transient eddies, play in establishing both the mean climate state and transient response to anthropogenic forcing.

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