Poleward flows in the southern California Current System: Glider observations and numerical simulation

Abstract

Three years of continuous Spray glider observations in the southern California Current System (CCS) are combined with a numerical simulation to describe the mean and variability of poleward flows in the southern CCS. Gliders provide upper ocean observations with good across‐shore and temporal resolution along two across‐shore survey lines while the numerical simulation provides a dynamically consistent estimate of the ocean state. Persistent poleward flows are observed in three areas: within 100 km of the coast at Point Conception, within the Southern California Bight (SCB), and offshore of the SCB and the Santa Rosa Ridge (SRR). Poleward transport by the flows within the SCB and offshore of the SRR exceeds the poleward transport off Point Conception, suggesting that the poleward flows are not continuous over the 225 km between observation lines. The numerical simulation shows offshore transport between the survey lines that is consistent with some of the poleward flow turning offshore before reaching Point Conception. The poleward current offshore of the SRR is unique in that it is strongest at depths greater than 350 m and it is observed to migrate westward away from the coast. This westward propagation is tied to westward propagating density anomalies originating in the SCB during the spring‐summer upwelling season when wind stress curl is most strongly positive. The across‐shore wave number, frequency, and phase speed of the westward propagation and the lack of across‐shore transport of salinity along isopycnals are consistent with first‐mode baroclinic Rossby dynamics.

Adiabatic Eastern Boundary Currents

“…This point has been made before (Marchesiello et al 2003;Todd et al 2012). 2 In the following section we show that when eddies are taken into account, the time-mean zonal and vertical flow in buoyancy coordinates is qualitatively different from the flow in level coordinates.…”

“…It might be surprising that the width of the mean meridional current is on the same scale as the eddies: examination of snapshots of the velocity shows that indeed the mean current is on the same scale as the eddies, which, while alternating in sign, are preferentially of one sign. This property is also observed in the analysis of the California Current system of Todd et al (2012). The lack of scale separation between eddies and mean does not lend confidence in a mean theory that approximates the eddy fluxes with down-gradient diffusion of the mean quantities.…”

“…However, these numerical experiments are at a coarse resolution, which does not allow the development of mesoscale eddies. Observations (Todd et al 2011) and high-resolution model experiments (Marchesiello et al 2003) have demonstrated that EBCs are embedded in a vigorous eddy field and clearly emerge only after an appropriate time average. Regional eddy-resolving simulations include forcing and geometry (e.g., coastline and bottom relief) that are as realistic as possible and do not permit teasing apart what aspects of the currents are extrinsically induced and which are from intrinsic dynamics.…”

Environmental drivers of the fine-scale distribution of a gelatinous zooplankton community across a mesoscale front

“…An autonomous glider undulating between the surface and 500 m along CalCOFI line 90 (cf. Rudnick et al 2004, Todd et al 2011) also passed through the target frontal area from 2 to 8 October 2010 and provided an independent measurement of a subsurface feature which appeared to be a convergent front (see Section 3.2 in McClatchie et al 2012).…”

“…South of Point Conception, where the angle of the coast changes and the California Current moves far offshore, the Southern California Bight (SCB) experiences less upwelling, and is bounded on the west by a series of submarine ridges and seamounts called the Santa Rosa-Cortes Ridge (SRR). The SRR is a site where an inshore, fast-moving poleward flow meets a slower, offshore equatorward flow resulting in a seasonally persistent convergent frontal feature (Todd et al 2011). This mesoscale feature is generated by the meeting of a filament of the California Current with SCB water, which is a complex mixture including Inshore Countercurrent and Equatorial Pacific water (McClatchie et al 2012).…”