Bottom wave streaming intensifies a nearshore upwelling front

Abstract

Coastal upwelling front is a crucial physical process on continental shelves, influencing coastal material transport, air-sea interactions, fisheries, and ecological environments. While previous studies have shown that coastal upwelling front is influenced by various factors, such as winds and topography, relatively little attention has been paid to surface waves, particularly bottom wave streaming. Bottom wave streaming is a wave-averaged Eulerian current in the direction of wave propagation, due to bottom friction on waves. It can affect bottom cross-shore transport and vertical mixing, potentially affecting the coastal upwelling front. In this study, we focus on the effects of bottom wave streaming on a wind-driven coastal upwelling front over an idealized continental shelf using a coupled wave-current model. Our results show that bottom wave streaming weakens the along-shelf current through increased momentum dissipation, thereby reducing bottom onshore Ekman transport. As a consequence, less cold, deep water is transported shoreward compared to scenarios without bottom wave streaming. On the other hand, the enhanced vertical mixing associated with bottom wave streaming facilitates the outcrop of bottom cold water, strengthening surface temperature gradients and thereby the upwelling front. Further, sensitivity tests reveal that the bottom wave streaming-induced intensification of upwelling front is robust across different wave directions, heights, and periods. This study highlights the critical role of bottom wave streaming in shaping the nearshore upwelling front, emphasizing its importance in the study of upwelling dynamics and material exchange.