Equatorward jets and poleward undercurrents along the eastern boundary of the mid-latitude North Pacific.

摘要

We sharpen our view of an eastern boundary current region during the upwelling season through the analysis of several data sets. We focus on the mesoscale flow field off of northern California, observed during the Coastal Transition Zone (CTZ) experiment of 1988. First, we estimate tidal currents in the region by least-squares harmonic analysis of both shipboard acoustic Doppler current profiler (ADCP) and moored data. The tide is predominantly M{dollar}sb2{dollar} and varies from 1-4 cm/s across the region, consistent with previous tidal studies. Next, we use detided ADCP together with conductivity-temperature-depth data to infer absolute geostrophic velocities during each of the five surveys in July-August 1988. Referencing geostrophy with the ADCP reveals a stronger equatorward jet than previously reported; southward volume transport from 0-500 m through a 200 km onshore-offshore line is as high as {dollar}rm8.0times10sp6msp3ssp{lcub}-1{rcub},{dollar} with a mean over the five surveys of {dollar}rm6.3pm1.3times10sp6msp3ssp{lcub}-1{rcub}.{dollar} The jet was about 50 km wide, with core velocities {dollar}>{dollar}0.6 m/s. During a two-week period in July 1988, horizontal velocity shears were sufficient to shift the effective local inertial frequency 10% higher on the cold (inshore) side and 5% lower on the warm (offshore) side of the jet. Observed near-inertial currents have amplified energy in the region with lower effective inertial frequency, consistent with theoretical predictions. Next, the basic instability mechanism leading to a meandering CTZ jet is analyzed using a linear quasi-geostrophic model applied to observed basic state velocity profiles. The jet is subject to both barotropic and baroclinic instability processes, and meander wavelengths of 260-265 km are the fastest growing. Growth periods of 7-11 days and along-jet phase speeds of 4-8 km/d are predicted. Finally, the poleward undercurrent which was observed during the 1988 CTZ experiment is also investigated with a series of shipboard ADCP sections collected from 33-51{dollar}spcirc{dollar}N during July-August 1995. Subsurface poleward flow occurred in 91% of the sections, with a mean undercurrent core velocity of 17 {dollar}pm{dollar} 1 cm/s and transport in a 140-325 m layer of {dollar}rm0.9pm0.1times10sp6msp3ssp{lcub}-1{rcub}.{dollar} One portion of the undercurrent is continuous over a 440 km length.