Ocean-Atmosphere-Land interactions and their consequences on the biogeochemical variability in Eastern Boundary Upwelling System

Abstract:

Coastal winds and upwelling of deep water along Eastern Boundary Upwelling System (EBUS) yield some of the ocean’s most productive ecosystems, but the effect of coastal wind shape and ocean-atmosphere interactions on regional Net Primary Production (NPP) is not well known. Here, we first show how the spatial and temporal variability of nearshore winds in EBUS is affected by orography, coastline shape, and air-sea interaction. Using regional atmospheric simulations over the US West Coast, we determine monthly characteristics of the wind drop-off, and show that when the mountain orography is combined with the coastline shape of a cape, it has the biggest influence on wind drop-off. Then, using a realistic ocean model of the California Current System, we show that the slackening of the winds near the coast has little effect on near-shore phytoplankton productivity, despite a large reduction in upwelling velocity. On a regional scale, the wind drop-off leads to a substantially higher NPP, especially when it occurs over a broad swath, even when the total upwelling rate remains the same. This partial decoupling of NPP from upwelling is effected by alongshore currents and the eddies they generate. When peak winds extend all the way to the coast, alongshore current shear is stronger, and a more energetic eddy field subducts nutrients offshore and out of the photic zone, reducing overall productivity. This causal sequence is supported by satellite remote sensing. Finally, using a interanual coupled simulation over the US West Coast, we show the ocean-atmosphere interactions can also reduce the eddy activity by pumping energy out from the eddies, reducing their amplitude and rotation speed, and leading to more realistic eddies characteristics. This may also reduce the eddy quenching and therefore increase the NPP. This complex ocean-atmosphere-land interactions imply that simple wind indices are incomplete predictors of productivity in EBUS.