Coastal upwelling off Cape São Tomé (22°S, Brazil): The supporting role of deep ocean processes

Abstract

The regional ocean off Cape São Tomé (CST, 22°S, Brazil) is known to feature transient coastal upwelling and intense mesoscale activity associated with the Brazil Current (BC). Satellite and in situ observations are used to characterize the coastal upwelling and the oceanic pycnocline water intrusions onto the continental shelf. Coastal upwelling events around CST are found to be less intense than the ones around Cape Frio (23°S), confirming previously reported findings. It is shown that the quasi-standing growth of a BC cyclonic meander is an effective supporting mechanism to this primarily wind-driven coastal upwelling system. A typical propagating cyclonic meander event is described and compared with its quasi-standing counterpart. The propagating cyclones also appear to promote oceanic pycnocline water intrusions, but at a lesser extent than the quasi-standing features. The supporting effect of the BC cyclones was quantified via simplified numerical experiments carried out with a 2D, primitive-equation numerical model. It is shown that meanders enhance intrusions as they grow, and may decrease by ≈50% the momentum input needed from the wind to cause coastal upwelling. Also, the role of the sloping of the isolines linked to the mean baroclinic structure of the Brazil Current is examined in idealized numerical experiments. This structure is shown to be sufficient to explain the observed time scales of coastal upwelling. The kind of meander-driven intrusion investigated here appears to be a regional singularity of the CST region, and may provide insight into the cross-shelf dynamics of other Western Boundary Current regions where similar quasi-standing instabilities exist.