Cross-shelf structure of coastal upwelling: A two — dimensional extension of Ekman's theory and a mechanism for inner shelf upwelling shut down

Abstract

Sea-surface temperature images of the coastal upwelling regions off Northwest Africa show that the core of upwelling is sometimes located far from the coast. This has been documented in three regions that share a common feature, namely a wide and shallow continental shelf. This upwelling feature plays a key role in the ecology of the Canary Current System. It creates an innerfront which provides retention for biological material, e.g. fish eggs and larvae, in the highly productive nearshore environment. An analytical model has been developed based on a two dimensional extension of Ekman's solution. The linear and steady response of a homogeneous ocean forced by an upwelling-favorable wind provides a mechanism for the upwelling separation from the coast. The merging of the surface and bottom Ekman layers induces a very weak cross-shore circulation and a "kinematic barrier" for the Ekman transport divergence. In the case of an alongshore wind, the barrier is located near the isobath h ≈ 0.4D, where D is the thickness of Ekman layers. This yields an upwelling cell which is essentially concentrated in the region 0.5D < h < 1.25D, with upwelling occurring preferentially near the isobath h ≈ 0.6D. It turns out that the cross-shore width of upwelling scales with D/S, the ratio of Ekman depth to bottom topographic slope. The application of this solution to real bathymetric profiles rationalizes, not only the offshore upwelling observations in Northwest Africa, but also the influence of topography on the cross-shelf structure of a wind-driven coastal upwelling. The model also quantifies the effect of the cross-shore wind component showing how it drives the nearshore pressure gradient adjustment and how it affects the upwelling. A linear numerical experiment reproduces the theoretical steady solution, thereby allowing investigation of the transient regime. Relaxation of the hypothesis in the numerical model validates the linear assumption of the theory and then allows investigation of the sensitivity to friction parameterizations and the influence of stratification. The latter leads to an "oscillation" of the upwelling cell with seaward migration driven by outcropping and homogeneization of the water column, and, coastal incursion driven by a "boundary layers splitting" process caused by shoreward advection of the isopycnal dome and stratification of the inner shelf.

A Lagrangian study tracing water parcel origins in the Canary Upwelling System

“…Wind-driven upwelling occurs over the full extent of the CUS (13-43°N), aside from a small interruption at the Strait of Gibraltar, although its presence and intensity is governed by seasonal variations (Wooster et al 1976, Speth and Detlefsen 1982, Nykjaer and Van Camp 1994, Estrade et al 2008, Gómez-Gesteira et al 2008, Lathuiliére et al 2008, Marcello et al 2011. Between the Canary Islands and the Strait of Gibraltar, the region of interest in this study, upwelling is present all year but peaks in summer when the Azores high pressure cell occupies its most northerly position.…”

Seasonal variability of the Ekman transport and pumping in the upwelling system off central-northern Chile (∼ 30° S) based on a high-resolution atmospheric regional model (WRF)

“…The central-northern Chile continental shelf is very narrow and very steep so the scale of coastal divergence is < 10 km (considering the theoretical framework of Estrade et al, 2008), while the length scale L d of Ekman pumping (previously defined, based on Renault et al, 2015) is ∼ 45 km. To compare the seasonal contribution of coastal divergence and Ekman pumping to the total transport of coastal upwelling in the study region, the annual cycle of coastal divergence was obtained first by taking the wind of WRF closest to the coast (< 8 km) and meridionally integrated every 0.25 • (Fig.…”

“…This study also evaluated the contribution of Ekman pumping to the total upwelling, which was not well resolved due to a poor resolution of the satellite winds within the first 30 km near the coast. In particular, due to the narrow continental shelf off central-northern Chile, the cells of upwelling due to coastal divergence are trapped near the coast (Estrade et al, 2008); consequently, the use of QuikSCAT winds could be overestimating the effect of upwelling driven by coastal divergence and Ekman pumping.…”

A model study of the seasonality of sea surface temperature and circulation in the Atlantic North-eastern Tropical Upwelling System

“…This is explained by the fact that, due to the shallow plateau there, bottom friction disturbs the surface Ekman layer and classical Ekman theory does not apply. The Ekman divergence is then fully developed further offshore where the plateau is deep enough [see 38,39].…”