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Progress on Dynamics and Thermodynamics in Western Boundary Currents

Kathryn Kelly(1) , LuAnne Thompson(1) , and Suzanne Dickinson(1)

(1) University of Washington, Box 355640, Seattle, WA 98195-5640, United States

Abstract

Prior to the start of the altimetry missions in the 1990s, ideas about the thermodynamics in western boundary currents (WBC) were simple: the Gulf Stream, for example, carries warm tropical waters poleward while cold air from North America sweeps across it, removing much of its heat and forming 18-degree water in the late winter. While estimates of air-sea fluxes differed substantially, variability in those fluxes was thought to be the result of changing atmospheric conditions. A heat budget for the region was impractical as the currents were too strong to be measured directly and efforts to close the budget would be overwhelmed by ocean eddies.

Early altimetry measurements of sea surface height (SSH) demonstrated that the large ocean signal from WBCs was clearly visible, but the perceived usefulness of SSH anomalies was limited by concerns about the representativeness of surface geostrophic currents for inferring transport, the lack of a mean SSH, the effects of seasonal heating, the sense that the boundary current variability was relatively small-scale, associated with rings, and that that variability was already documented by decades of infrared images.

Statistical analyses of boundary current variability (Kelly, 1991; Qiu et al, 1991) demonstrated that most of the variability in the Gulf Stream and Kuroshio Extension is from the meanders of a constant-width jet, not rings; and that there are relatively large-scale variations in the current structure, which are associated with the recirculation gyres. Sato and Rossby (1995), in comparisons with hydrographic data, demonstrated the large-scale effects on the Gulf Stream of seasonal heating. Imawaki (2001) showed that the Kuroshio transport could be monitored using SSH, but this has not been demonstrated for the Gulf Stream.

Using Geosat data, Kelly and Gille (1990) devised a method for estimating mean SSH that allowed an upper ocean heat budget in a boundary current, using a thermodynamics-only upper ocean model. The heat budget in the Kuroshio Extension (Qiu and Kelly, 1993) revealed the importance of heat transport by the geostrophic current, compared with the Ekman current. Using TOPEX/Poseidon data, Qiu (1999) showed that large changes in SST were the result of changes in the geostrophic heat advection in the KE, rather than air-sea fluxes. Meanwhile, the relationship between SSH and heat content was probed by numerous researchers demonstrating that SSH was a good proxy for upper ocean heat content, and creating some confusion about the relative roles of thermodynamics and dynamics in the WBCs.

Upper ocean heat budgets for both the Kuroshio Extension and Gulf Stream regions (Vivier et al 2002; Dong and Kelly, 2004) revealed that changes in upper ocean heat content (shown to be nearly synonomous with SSH anomalies) were primarily the result of changes in heat transport by the geostrophic current with time scales of several years or more. Much of the anomalous heat transport is stored in the recirculation gyre regions, where heat accumulates and results in anomalous fluxes of heat to the atmosphere (Kelly and Dong, 2004). Thus, interannual variations in air-sea fluxes in these regions are primarily the result of changes in the ocean, not in the atmosphere. Further, large-scale changes in both the KE and GS heat transport are to some degree in phase, likely the result of large-scale changes in the winds. Motivated by the similarity of the SSH anomaly and the 18-degree (mode) water in the Gulf Stream, comparisons revealed a systematic negative correlation between mode water volume and heat content or SSH (Kwon and Riser, 2004; Dong 2004).

The relatively large local variations in upper ocean geostrophic transport, heat storage, and air-sea fluxes in the WBC regions, along with new understanding of the importance of the shallow wind-driven overturning circulation in meridional heat transport (Boccaletti et al, 2005), suggests that changes in meridional heat transport by the ocean may be dominated by western boundary current variability rather than by meridional circulation changes driven by changes in the thermohaline circulation. A heat budget analysis which spans both the subtropical and subpolar gyres is needed to examine this issue further. Critical to this analysis are both longer time series of altimetric SSH and an improved mean SSH over the larger region as well as an understanding of the influence of salinity and the fresh water budget on the interpretation of SSH. The observationally derived estimates of the thermodynamic budget of the upper ocean are essential for independently evaluating new ocean and climate models, especially for eddy resolving models.

 

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                 Last modified: 07.10.03