Upwelling and cross-shelf transport dynamics along the Pacific eastern boundary

The Upwelling and Cross-shelf Transport Dynamics along the Pacific Eastern Boundary is explored using a high resolution ocean model for the last 60 years. Three ocean circulations have been modeled. From North to South, we investigate the dynamics of the Gulf of Alaska (GOA), the California Current System (CCS) and the Humboldt Current System (HCS, also known as the Peru-Chile Current System, PCCS). Each region is studied separately and appear respectively in chapter 3,4 and 5.Chapter 3 investigates the cross-shelf transport statistics in the GOA over the period 1965-2004. The statistics of coastal waters transport are computed using a model passive tracer, which is continuously released at the coast and at the surface. The passive tracer can thus be considered a proxy for coastal biogeochemical quantities such as silicate, nitrate, iron or oxygen, which are critical for explaining the GOA ecosystem dynamics. On average along the Alaska Current, we find that at the surface while the advection of tracers by the average flow is directed towards the coast consistent with the dominant downwelling regime of the GOA, it is the mean eddy fluxes that contribute to offshore advection into the gyre interior. South of the Alaskan Peninsula, both the advection of tracers by the average flow and the mean eddy fluxes contribute to the mean offshore advection. On interannual and longer timescales, the offshore transport of the passive tracer in the Alaskan Stream does not correlate with large scale atmospheric forcing, nor with local winds (intrinsic variability). In contrast in the Alaska Current region (forced variability), stronger offshore transport of the passive tracer coincides with periods of stronger downwelling (in particular during positive phases of the Pacific Decadal Oscillation), which trigger the development of stronger eddies.We investigate the low-frequency variability in the transport (cross-shore and alongshore) of coastal water masses in the Central and Southern California Current System (CCS) over the period 1950-2008. By looking at the passive tracer concentration distribution, we find that the low-frequency upwelling and the surface offshore transport of the upwelled nutrient-rich coastal water are strongly correlated with the alongshore wind stress, and is coherent between the central and southern CCS. The offshore transport of the surface coastal water is nevertheless not coherent between those two regions, and has been found to be associated with mesoscale eddy activity, where both surface and subsurface waters propagate offshore mainly through cyclonic eddies. Our results also show that the poleward California Under Current, at about 200m depth, affects the alongshore transport and provides rich waters to the Central California dominant upwelling cell.The Humboldt Current System (HCS) is the one of the world's most productive regions in fish landings, providing &sim20% of the world marine catches despite covering less than 1% of the world's ocean surface. The similar passive tracer approach is used to characterize the upwelling dynamics and offshore transport of coastal water masses. Different model experiments are conducted to explore the sensitivity of the HCS upwelling to different air-sea fluxes of momentum and atmospheric and oceanic teleconnections to ENSO. There is evidence, in this region, that both changes in surface wind and coastally trapped Kelvin waves control the variability of the coastal upwelling. The effect of ocean remote forcing is assessed by comparing the output of two model simulations which does and does not include the presence of waves at their boundaries. The passive tracer approach indicates that, off the coast of Peru, the El Nino Southern Oscillation (ENSO) strongly modulates the strength of coastal upwelling principally due to the propagation of downwelling equatorial Kelvin waves (particularly strong during El Nino years) rather than changes in local wind stress. The Central Chile region is also found to be very sensitive to Kelvin waves generated at the equator, considerably reducing the upwelling during strong El Nino events. In addition to the upwelling variability, this study investigates the offshore transport of the coastal waters of South America. While the surface offshore transport of upwelled water is modulated by ENSO, mesoscale eddies and filaments also play an important role in the offshore transport of both the surface and subsurface coastal waters. Our results indicate that while cyclonic eddies mostly transport coastal waters in the surface layer, both cyclonic and anticyclonic eddies transport coastal waters in the subsurface. (Abstract shortened by UMI.)...