Roles of shelf slope and wind on upwelling: A case study off east and west coasts of the US

To understand the differences in upwelling tendency between the east and west coasts of the U.S., idealized numerical experiments were performed to examine the upwelling response to wind and shelf slope. The primary results show that steeper slope leads to narrower cross-shore width of surface Ekman divergence (WSED) and larger vertical velocity, while stronger upwelling favorable wind stress induces broader WSED and larger vertical velocity. Meanwhile, the wind duration is substantial to determine both the area and intensity of upwelling off the coast. The tendencies for cold upwelling areas off each coast are compared by the upwelling age, which is defined as the ratio of the duration of upwelling favorable wind to the advection time. The advection time, defined as the time scale for cold water to be advected from the pycnocline to the ocean surface, is improved to comprise of climbing time and upwelling time. The latter is related to the upwelling divergence driven by surface Ekman flow. The depth of the switch point of these two processes is approximately 0.9DE (DE is the Ekman depth). The proposed formula for the advection time is found to be consistent with estimates derived from the use of particle trajectory analysis within the numerical model results. The consideration of upwelling age shows that differences in wind forcing are more important than bottom slope in accounting for the different characteristics of upwelling areas off the California and New Jersey coasts.