Dynamics of Eighteen Degree Water from CLIMODE Observations and its Climate Implications

PIs: Shenfu Dong

External PIs: Kathryn A. Kelly (U. of Washington)

Subtropical Mode Water (STMW), an isothermal layer that forms on the equatorward side of western boundary current (WBC) in response to wintertime cooling, is central to understanding atmosphere-ocean interaction in mid-latitude regions because it integrates interannual variations in both ocean and atmospheric forcing to contribute to climate system memory. The classical description of the evolution of STMW is that it is 1) formed by wintertime cooling from cold-air outbreaks; 2) subducted into the thermocline where it forms a thick isothermal layer; 3) partially dissipated through mixing or advection; and 4) is re-entrained into the mixed layer, where it can interact with the atmosphere in subsequent winters.

Many processes have been identified that could affect both the formation rate and the subsequent dissipation and dispersal of the STMW. A primary goal of the CLIMODE program is to quantify the processes contributing to the evolution of the STMW of the western North Atlantic, commonly referred to as 14-deree-water (EDW) because of its nearly constant temperature. An extensive set of measurements has been obtained over the 2-year field program; analyses and modeling of the observational period can help evaluate the relative importance of the processes contributing to EDW evolution. The goal of this project is to provide a link between the CLIMODE-specific analysis, longer period variability, and the need for metrics to evaluate and verify climate models. To achieve this goal, we examine interannual-to-decadal variation in EDW volume, of the processes that contribute to it, and its impact on air-sea interaction. We will examine whether important processes can be monitored using proxy variables and thus link the field program results to the longer climate record to evaluate the importance of each process, the predictability of EDW evolution, and the ability of EDW to contribute to climate memory.

Figure. EDW layer thickness during January-March for years 1985-2007. Units are meters. EDW is defined with two criteria: the temperature must be within the range of 17-19C and the temperature gradient must be no greater than 0.006 C/m.