A comparison of remote vs. local influence of El Niño on the coastal circulation of the northeast Pacific

Abstract

Abstract A set of spatially nested circulation models is used to explore interannual change in the northeast Pacific (NEP) during 1997–2002, and remote vs. local influence of the 1997–1998 El Nino on this region. Our nested set is based on the primitive equations of motion, and includes a basin-scale model of the north Pacific at ∼40-km resolution (NPac), and a regional model of the Northeast Pacific at ∼10-km resolution. The NEP model spans an area from Baja California through the Bering Sea, from the coast to ∼2000-km offshore. In this context, “remote influence” refers to effects driven by changes in ocean velocity and temperature outside of the NEP domain; “local influence” refers to direct forcing by winds and runoff within the NEP domain. A base run of this model using hindcast winds and runoff for 1996–2002 replicates the dominant spatial modes of sea-surface height anomalies from satellite data, and coastal sea level from tide gauges. We have performed a series of sensitivity runs with the NEP model for 1997–1998, which analyze the response of coastal sea level to: (1) hindcast winds and coastal runoff, as compared to their monthly climatologies and (2) hindcast boundary conditions (from the NPac model), as compared to their monthly climatologies. Results indicate penetration of sea-surface height (SSH) from the basin-scale model into the NEP domain (e.g., remote influence), with propagation as coastal trapped waves from Baja up through Alaska. Most of the coastal sea-level anomaly off Alaska in El Nino years appears due to direct forcing by local winds and runoff (local influence), and such anomalies are much stronger than those produced off California. We quantify these effects as a function of distance along the coastline, and consider how they might impact the coastal ecosystems of the NEP.