Report of the High-Resolution Ocean Topography Science Working Group Meeting

This paper was presented to the High-resolution Ocean Topography Science Working Group to present the biogeochemical aspects of mesoscale variability and how it relates to the observed baroclinic field sensed by onorbit microwave altimeters. As such, this paper summarizes the results of three projects the author is involved in: space/time patterns of biological variability as evidenced by remotely sensed chlorophyll from SeaWiFS, eddybiogeochemical interaction near Bermuda shown by correlation analysis of SSH and SST anomalies, and gas transfer velocity of CO2 from the altimeter backscatter of TOPEX/Poseidon. Using the semivariogram approach from geostatistics, we characterize for the first time the global patterns of mesoscale (approx. 40-400 km and days to a few weeks) ocean biological variability. The magnitude of the variability and length scale fields are reported for a full year of SeaWiFS ocean color data. The analysis shows a number of coherent geographical patterns across a wide range of biological and physical environments. Mesoscale variance dominates ocean color variability in oligotrophic regimes and is a third to a half of the total variance even in regions with strong seasonality. Our results demonstrate the global generality of previous local and regional findings that physical mesoscale turbulence governs biological spatial scales both directly (e.g. stirring) and indirectly (e.g. nutrient injection). Objectively analyzed fields for satellite sea surface temperature (SST, AVHRR Pathfinder) and sea surface height anomaly (SSHA, combined TOPEX/Poseidon–ERS-1/2) are used to characterize statistically the mesoscale variability about the U. S. JGOFS BATS site. Using a low-pass spatial filter (nominal 500 km cut-off), we decompose the anomalies from the seasonal cycle into two components: the large-scale regional climate variability and a mesoscale signal. A distinct surface mesoscale expression is observed in SST that has many features in common with the mesoscale SSHA variability: spatial autocorrelations of 100-200 km and westward propagation rates of 3.7-5.5 cm s. The mesoscale SST and SSHA fields are positively cross-correlated at a statistically significant level, consistent with near surface isotherm displacements for cyclonic (low SSHA, elevated isotherms, and cold SST) and anticyclonic (high SSHA, depressed isotherms, and warm SST) eddies. The feasibility of calculating gas transfer velocity directly from a remotely sensed measure of the sea surface roughness presents an unique opportunity. We present here a six year time series (1993-1998) of TOPEX data processed into gas transfer velocity and examine the variability of these results in space and time. The seasonal and interannual variability of the regional patterns yield insight into the sensitivity of the altimeter-based gas transfer velocity to phenomena such as ENSO. Our approach to deriving transfer velocity is subject to parameters calibrated against field and laboratory measurements. In situ measurements are one of the best constraints on the remote observations: wind stress vs. surface roughness.