ABSTRACT

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 climate variability in mid-latitude regions because it integrates anomalies in both the ocean and atmospheric to contribute to climate system memory. The STMW region has a large capacity to store heat, and its heat storage rate has been shown to depend both on air-sea fluxes and on ocean circulation. The volume of STMW is so large that several years of air-sea interaction alone cannot dissipate it; after formation, it is partially re-entrained in subsequent winters to again interact with the atmosphere.

Many processes have been identified that could affect STMW formation or its subsequent destruction. A primary goal of the NSF-funded CLIMODE (CLIVAR Mode Water Experiment) is to quantify the processes contributing to the evolution of the STMW of the western North Atlantic, commonly referred to as 18-degree-water (EDW) because of its nearly constant temperature. An extensive set of measurements has been obtained over the two-year field program; analyses and modeling of the observational period can help evaluate the relative importance of the processes contributing to EDW evolution. A primary motivation is that CLIMODE analyses will lead to improvements in climate modeling. This study aims to provide a link between the CLIMODE-specific analyses, longer period variability, and the need for metrics to evaluate and verify climate models.

Intellectual merit: The EDW region with its large heat storage and air-sea fluxes, variable poleward heat transport, and energetic ocean circulation is a prime candidate for memory in the atmosphere-ocean system. The proposed research is an examination of the interannual-to-decadal variations in EDW volume, of the processes that contribute to it, and its impact on air-sea interaction. Some competing processes in EDW evolution (warm water advection by the Gulf Stream, mixing, and oceanic heat loss through air-sea fluxes) have variability linked to climate indices such as the North Atlantic Oscillation. The investigators 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.

Broader impacts: To be successful in predicting climate variability and change, models must be able to simulate those processes that produce interannual-to-decadal climate impact. An effective climate observing system must monitor the variables needed to characterize those fundamental processes and improve model parameterization and simulation. For example, the EDW region is a center of rapid intensification of mid-latitude storms that may be responding to changing ocean conditions, in particular, to ocean heat storage. Simulation and verification of climate predictions (such as changes in storm intensity) depend on developing a series of metrics that measure how well the model simulates important processes (heat storage). For the processes that are important to EDW evolution and its climate impact, critical measurements will be identified in order to develop simple and appropriate metrics with which to evaluate climate models.

This project is a contribution to the U.S. CLIVAR (CLImate VARiability and predictability) program.