Cross-shore transport variability in the California Current: Ekman upwelling vs. eddy dynamics

Abstract

International audienceThe low-frequency dynamics of coastal upwelling and cross-shelf transport in the Central and Southern California Current System (CCS) are investigated using the Regional Ocean Modeling System (ROMS) over the period 1965-2008. An ensemble of passive tracers released in the numerical model is used to characterize the effects of linear (Ekman upwelling) and non-linear (mesoscale eddies) circulation dynamics on the statistics of advection of coastal waters. The statistics of passive tracers released in the subsurface show that the low-frequency variability of coastal upwelling and cross-shelf transport of the upwelled water mass are strongly correlated with the alongshore wind stress, and are coherent between the central and southern CCS. However, the offshore transport of tracers released at the surface is not coherent between the two regions, and is modulated by intrinsic mesoscale eddy activity, in particular cyclonic eddies. The transport of cyclonic eddies extends with depth and entrains water masses of southern origin, advected by the poleward California Undercurrent (CUC). The CUC water masses are not only entrained by eddies but also constitute a source for the central California upwelling system. The interplay between intrinsic (eddy activity) and deterministic (Ekman upwelling) dynamics in controlling the cross-shelf exchanges in the CCS may provide an improved framework to understand and interpret nutrients and ecosystem variabilit

Mesoscale contribution to the long-range offshore transport of organic carbon from the Canary Upwelling System to the open North Atlantic

“…This is a remarkable lateral flux comparable to that of other major currents in the region, such as the Canary Current (1.5 Sv to 3 Sv), the Canary Upwelling Current (1 Sv to 1.5 ± 0.3 Sv) and the North Equatorial Current (0.5 Sv to 3 Sv) (Machín et al, 2006;Pelegrí and Peña-Izquierdo, 2015;Mason et al, 2011). In line with the results of Combes et al (2013), we find that CE are responsible for large part of the offshore tracer transport, while ACE contribute in smaller measure to the flux.…”

“…Furthermore, model simulations for both the CalUS and CanUS also showed that eddies tend to reduce coastal production through the lateral export of the upwelled nutrients (Gruber et al, 2011;Lachkar and Gruber, 2011). Focusing on the CalUS and with the use of a passive tracer, Combes et al (2013) have demonstrated that mesoscale eddies, and in particular cyclonic ones, exert a strong control on the horizontal offshore transport. Combining a simple ecosystem model with modeled and observed velocity fields, the mesoscale transport 20 in the Benguela Upwelling System was estimated to account for 30 %-50 % of the total offshore flux (Hernández-Carrasco et al, 2014).…”

Iron fertilisation by Asian dust influences North Pacific sardine regime shifts

“…The southern Californian coastal upwelling can supply macronutrients from year to year. The upwelled waters originate from the California Undercurrent and come from $150-m depth (Auad et al, 2011;Combes et al, 2013). These deep-source waters are rich in macronutrients (1-4 lM nitrate), but their phytoplankton contents are low (chlorophyll <1 lg/L) and productivity is limited by available iron (Hutchins and Bruland, 1998;King and Barbeau, 2011).…”

Dominant role of eddies and filaments in the offshore transport of carbon and nutrients in the California Current System

“…A modeling study by Combes et al . [] suggests that across‐shore transport of surface passive tracer is controlled by interplay between eddy activities and Ekman upwelling. Thus, these offshore propagating eddies may contribute substantially to offshore transport, but their relative roles for biogeochemical tracers are not well quantified.…”