Overview

A process experiment is beginning in the US under NSF sponsorship concentrating on the formation, evolution, storage, dispersal and large-scale consequences of Eighteen Degree Water, the subtropical mode water of the North Atlantic. Mode waters are a physical manifestation of air–sea exchange that, through successive wintertime exposure, constitute a long-term memory within the climate system. Present understanding of the annual renewal rate of these waters and the responsible physical mechanisms is deficient; water mass transformations inferred from climatological air–sea fluxes appear incompatible with both volumetric analyses and estimated dissipation processes. It is hypothesized that current formation rate estimates are inaccurate due to (i) poorly estimated air–sea fluxes and (ii) poorly understood/represented lateral ocean processes. Through a synthesis of ocean-atmosphere observations and modeling studies, the CLIMODE program (CLIvar MOde water Dynamics Experiment) will:

1. Investigate processes common to all subtropical mode waters in the world ocean, which are found equatorward of strong zonal flows (in particular the Gulf Stream, the Kuroshio Extension, the East Australian Current, the Agulhas Return Current and the Antarctic Circumpolar Current).



2. Focus on a region of great importance in air–sea exchange –– huge ocean to atmosphere heat loss occurs over the separated Gulf Stream where uncertainties in flux estimates are large.



3. Explore the cross-scale connection between water mass formation associated with wintertime convection on the rim of the subtropical gyre, its subduction into the stratified interior, and its dispersal and dissipation around the gyre.



4. Address a key process that is presently poorly understood and inadequately represented in climate models –– the interaction of geostrophic eddies with mixed layers. This is one of the most critical parameterizations that compromises the present generation of climate models. CLIMODE will provide field observations and analysis of the interaction of eddies and the mixed layer, in support of the CLIVAR Climate Process Team in ocean mixing responsible for developing/testing parameterizations.

CLIMODE will have broad scientific impact because it directly addresses oceanic phenomena and atmosphere–ocean coupling that have climatic significance but which are inadequately represented in climate models. Furthermore, through assessment/validation of the climate model parameterizations of these processes, CLIMODE will have considerable societal impact. The program will also contribute to the career advancement of several graduate students and postdoctoral investigators who will participate in the study. On average during the 5 year program, CLIMODE will directly support 5 students and 4 to 5 postdocs each year.