Relaxation from upwelling: The effect on dissolved oxygen on the continental shelf

Continental shelves in upwelling regimes are subject to sequences of upwelling and relaxation events, each on timescales of order 1 week typically. These episodes have pronounced impacts on the temperature and density structure on the shelves and also on the along-shore and cross-shore flow regimes. It had previously been demonstrated that relaxation phases advect warm water along shore (poleward) from regions of less intense upwelling, thus adding to the heat balance in upwelling locations and providing a rectification of oscillating heat fluxes. In the current paper it is shown that relaxations also modify the dissolved oxygen (DO) budget of the lower layers. On a narrow shelf, this provides enhanced DO values due to near-surface exposure, while on a wide shelf decreased DO concentrations are created due to oxygen consumption on the inner shelf. The resulting variations along the coast can lead to along-shore advection of high or low DO during a relaxation event. Observations are presented from moorings off San Diego and Del Mar, which show large departures from density-correlated DO values during relaxations and which support the proposed mechanisms.