This NASA-funded project concerns air-sea coupling in the California Current System (CCS), a biologically important site of strong air-sea exchanges of heat, moisture, and momentum.
The work has three components. First, I am producing a seasonal and interannual description of the CCS via derived satellite products, including fluxes. This description expands on currently available low-resolution climatologies. I am evaluating the performance of satellite-derived flux products in this region of high spatial variability, stable stratification, high winds, and strong currents. The evaluation criteria is the ability to close a simple heat balance in which upper ocean heat content changes due to heating by surface fluxes and cooling by offshore Ekman transport of heat. The heat content change is estimated from XBTs dropped along a San Francisco to Hawaii line (Fig. 1) by volunteer observing ships. The data have been made available by Dr. Dean Roemmich. Heat budgets based on satellite inputs and on NCEP products are compared; the latter are commonly used to force oceanic models. Close to the coast (Fig. 2), the greatest difference is a reduced NCEP Ekman heat transport term compared to that based on QuikSCAT winds and TRMM sea surface temperatures.
Second, I will study the oceanic response to atmospheric forcing, focusing on the contribution of 25-km scale variability to the flux fields as revealed by satellite observations. Forcing fields such as those from NCEP lack energy at scales smaller than about 500 km. I will test the hypothesis that better-resolved wind stress curl and heat fluxes will increase upwelling and CCS heat transport with an oceanic model.
Third, I will assess the atmospheric response to the surface expression
of the CCS. Increasingly stable surface layer stratification
downwind of a SST front will tend to reduce downward momentum transfer and
hence surface stress. A SST front is likely to produce a temperature
gradient in the atmospheric boundary layer, inducing baroclinic shear that
affects momentum transfer and thus surface stress. Such baroclinic
effects are comparable to and can exceed those due to stratification alone.
In turn, resulting wind stress curl changes may force the ocean.
I will study these processes by modeling air column advection over a variable
sea surface.
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| Figure 1. Volunteer observing ship XBT locations for September 2001 (top). The transits have been repeated every three months at approximately 100 km XBT spacing. Heat content above the 7.5 C isotherm (heavy contour, bottom) is shown in color. (Bottom) Temperature profiles from the XBTs. |
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| Figure 2. 2000 heat budget above the 7.5 C isotherm at 207 km from shore along the track shown in Fig. 1. Black: product of layer depth (h) and the time change in heat content. Top: sum of the surface heat flux and Ekman terms of the heat budget from the NCEP model (green) and from satellite (blue). These individual terms from NCEP (middle) and from TRMM, QuikSCAT, and from the Goddard-derived satellite surface fluxes (bottom). |
