Recent Trends in the Spatial Structure of Wind Forcing and SST in the California Current System

Abstract

"State-space statistical models are applied to long time series of monthly COADS northward wind stress and sea surface temperature (SST) from the California Current System (CCS) for the period 1946-1990. The models estimate a nonparametric and non-linear trend, a non-stationary and nondeterministic seasonal signal, and an autoregressive (AR) term. They are also applied to long SST time series from selected coastal sites for comparison to the COADS series. SST shows decadal-scale periods of warm and cool anomalies that extend through the entire CCS. Wind stress anomalies are less extensive latitudinally and generally uncorrelated with SST, suggesting that decadal-scale SST variations in the CCS are controlled by fluctuations in the basin- to global-scale pressure and wind fields, rather than local wind forcing. The CCS can be divided into three distinct geographical regions, which are similar to the system's biological regions. The non-hern region of the CCS (42-48"N) features a transition from strongly equatorward to poleward stress with distance north. The mean stress north of 44"N is poleward and has become increasingly poleward over time. This region features spatially uniform SST that has cooled over time. Winds south of 42"N are equatorward and can be described in terms of a central and southern region. The central region (34-42"N) exhibits the strongest wind stress in the CCS; equatorward stress has increased over time more than in the northern and southern regions. This region features the greatest interannual to decadal variation in stress and SST as well. Stress in the southern region (22-34ON) has become increasingly equatorward over time in a relatively monotonic pattern. Mean SST decreases consistently with increasing latitude in the central and southern regions. SST off California warms rapidly in response to ENS0 events as well as the 1976 regime shift, but much more slon~lya t other latitudes. mi l e SST along the entire Coast has warmed during the past several decades, offshore SST has cooled north of 36"N. This long-term cooling in the northern CCS is linked to large-scale cool anomalies in the central north Pacific rather than changes in local wind forcing. A different complex of processes is responsible for the long-term coastal warming tendency. It also appears that distinct combinations of windforced advection, mixing and direct heating lead to significantly different regional responses of the coastal ocean to climate change, which in turn may have substantial consequences for marine populations in eastern boundary current ecosystems."