Spatio-Temporal Variability in Coastal Upwelling/Downwelling from Scatterometer Winds

Abstract:

The decade-plus near-continuous record from satellite scatterometers provides indirect measurements of vector winds over the global ocean with sampling largely determined by the satellite orbits. This allows investigation of wind-driven ocean processes even at remote locations that are poorly sampled by in situ measurements. Global satellite wind products are used to produce a database of time series of upwelling indices, similar to those typically produced using data from coastal ocean observing systems, at all coastal locations over the Earth. This database is analyzed to study spatial and temporal variability of coastal upwelling and downwelling throughout the world ocean. The upwelling index is typically a proxy for upwelling/downwelling across the slope assuming a simple local mass balance to the offshore surface Ekman transport. However, along-shore variations in winds and shelf geometry perturb this simple local balance via shelf wave dynamics. In particular, cross-slope velocities downcoast (in a shelf wave propagation sense) of changes in the topographic gradient orientation may respond preferentially to a wind direction not oriented along local isobaths. Data from numerical models are analyzed to illustrate this point and to clarify the relation between upwelling/downwelling and local wind direction throughout the global coastal ocean that may allow improvement in wind-derived upwelling indices.