The seasonal alongshore pressure gradient on the West Coast of the United States

Abstract

The long‐term mean steric height (0/500 dbar) data of Reid and Mantyla (1976) are added to relative monthly mean adjusted sea level data to obtain a seasonal time series of “quasi‐absolute” large‐scale (>300 km) sea surface elevation along the west coast of the United States from 48°N to 33°N. North of 38°N, the elevation slope reverses sign seasonally so that the resulting pressure gradient force is southward from October to June and northward from July to September. South of 38°N, the pressure gradient force is northward except during January at all latitudes and during February in the interval from 38°N to 35°N. Maximum northward force occurs during August and September at all latitudes. The relative contributions to sea level elevation slope by seasonal heating and cooling, freshwater effluent effects, the alongshore component of coastal wind stress, and deep‐ocean currents are investigated. Both seasonal heating and cooling and freshwater contributions to large‐scale elevation slope are relatively insignificant (<10% in every month) at all latitudes. Wind stress forcing (determined with the barotropic, steady, frictional model of Csanady (1978)) is significant north of Point Conception (35°N) for both the long‐term mean slope and its seasonal oscillation. Remote contributions are particularly important in the Pacific Northwest. South of San Francisco (38°N), empirical evidence suggests that deep‐ocean currents contribute significantly to coastal elevation slope in summer and fall and in the mean. Deep‐ocean currents may also affect elevation slope along the whole coast during spring, causing elevated sea level toward the north.

Subtidal cross-shelf circulation on the Northern California shelf

“…Based on observed mean along-shelf flows , this value is set at -5 x 10-8 in winter and -1.2 x 10-7 in summer. These values agree in magnitude with the along-shelf gradients found for the northern California shelf by Hickey and Pola [1983], though the winter (y must be of the opposite sign in order to account for the observed mean poleward flow. The mean along-shelf pressure gradient affects only the mean transports, all variability in the model is locally wind forced with no fluctuating pressure gradient.…”

“…Mean meteorological conditions during the summer upwelling season are distinguished by strong positive (from the atmosphere to the ocean) sea surface heat flux [Nelson and Husby, 1983;Lentz, 1987b] and strong persistent equatorward winds [Nelson, 1977;Strub et al, 1987b]. In response to the equatorward wind stress and a negative along-shelf pressure gradient [Hickey and Pola, 1983], along-shelf mean currents are equatorward near the surface and exhibit vertical shear, becoming weaker and sometimes poleward near the bottom and mean cross-shelf currents are offshore near the surface and near zero in the interior .…”

The circulation and water masses in the Gulf of the Farallones

“…In contrast, Hickey and Pola (1983) describe a poleward pressure gradient in all months but January and February, reaching a maximum in late summer. Variations in the California Undercurrent are consistent with this.…”

“…The SCC was seen in all six Farallones surveys, although it was least clear in May when coastal upwelling dominated. The alongshore pressure gradient, a likely candidate for driving a poleward current, is weakest in early spring and strongest in late summer (Hickey and Pola, 1983;Largier et al, 1993), roughly coinciding with the transition from a shallow SCC early in the year to a deeper SCC core in August. Monthly and seasonal climatologies of geostrophic velocities o!…”

SST–Wind Interaction in Coastal Upwelling: Oceanic Simulation with Empirical Coupling

“…A background uniform wind stress t* with an amplitude of 0.07 N m 22 blows equatorward constantly. The alongshore pressure gradient is one of the primary driving forces for the poleward undercurrent and surface Davidson Current along the U.S. West Coast (e.g., Hickey and Pola 1983;Chelton 1984;Dong and Oey 2005). To incorporate an alongshore pressure gradient into this idealized system, a periodic condition on the northern and southern open boundaries cannot be used [although periodic conditions are sometimes used to study the upwelling system (e.g., Gan and Allen 2005)].…”