The California Current System (CCS), an eastern boundary current system, is composed of variable equatorward and poleward currents. The equatorward flowing California Current carries cold, fresh waters of northerly origin while poleward flows transport warm, salty waters of southerly origin [Wooster and Jones, 1970; Hickey, 1979; Lynn and Simpson, 1987; Huyer et al., 1989]. The poleward flowing California Undercurrent (CU) within the Southern California Bight (SCB) [Lynn and Simpson, 1990] and near Point Conception, as well as surface poleward flow near the coast, have been observed for decades by the California Cooperative Oceanic Fisheries Investigations (CalCOFI) program (http://www.calcofi.org) and other investigations [Lynn and Simpson, 1987; Bray et al., 1999]. Recent velocity observations [Davis et al., 2008; Gay and Chereskin, 2009] have revealed an additional subsurface poleward current offshore of the SCB to be a persistent feature in the CCS. This paper uses new observations collected by underwater gliders in the southern portion of the CCS and a new, regional, numerical state estimate to characterize the mean and variability of the poleward flows. Our observations show that the poleward current offshore of the SCB propagates westward in response to density anomalies propagating westward from the SCB and that the across-shore wave number and frequency of this westward propagation are consistent with first-mode baroclinic Rossby wave dynamics.
 Historically, flow within the southern CCS has been diagnosed from thermal wind and decades of repeat hydrography, largely through the CalCOFI program. In most cases, the 500 db surface has been used as a level of no motion since CalCOFI measurements extend to 500 m. The resulting picture of the geostrophic flow [Sverdrup and Fleming, 1941; Hickey, 1979; Lynn and Simpson, 1987] shows the equatorward flowing California Current near the surface and somewhat offshore, the poleward flowing CU near the coast with highest velocity at depths of 100–300 m, and seasonally reversing surface flow near the coast. South of Point Conception, the CU appears inshore of the Santa Rosa Ridge (SRR, Figure 1) and flows between the various islands. Gaps in the SRR provide pathways for the CU to exit the SCB [Lynn and Simpson, 1990].
 Subsurface poleward flow offshore of the SRR has been inferred occasionally from hydrography. Sverdrup and Fleming  found northward flow at a depth of 200 m near the offshore side of the SRR during three cruises from March to July 1937. They identified the poleward flowing waters as having higher temperature and salinity than waters within the California Current, an indication of southerly origin. Lynn and Simpson  used thermal wind referenced to 1000 db from a single survey in July 1985 to infer poleward flowing water of southern origin in the same region; they attributed the flow to an eddy formed by CU waters discharged through a gap in the SRR.
 Velocity measurements within the southern CCS have recently revealed a poleward current offshore of the SRR to be a persistent feature. Geostrophic velocities referenced to vertically averaged currents from glider measurements on CalCOFI Lines 80.0 and 93.3 [Davis et al., 2008] showed mean subsurface poleward flow within 100 km of the coast (the CU) as well as 200–250 km offshore on Line 80.0 and Line 93.3 from 2005 to early 2007. Gay and Chereskin  used 10 years of shipboard acoustic Doppler current profiler (ADCP) measurements from the quarterly CalCOFI cruises to show that the offshore poleward current is a persistent feature along Lines 86.7, 90.0, and 93.3, which fall within the SCB, but found only a single core (the CU) in the 10 year mean near Point Conception. The second poleward core at Line 80.0 seen in the glider observations was likely an artifact of the shortness of the data record available [Davis et al., 2008], and glider observations of longer duration presented here do not show significant mean poleward flow at that location. The current offshore of the SRR has significant poleward flow at depths of 500 m and its speed diminishes with decreasing depth. Consequently, geostrophic calculations that assume zero flow at 500 m produce a surface intensified equatorward flow at the same location [Davis et al., 2008].
 The variability of the CU and shallow poleward flow within the SCB have been well documented [Chelton, 1984; Lynn and Simpson, 1987; Bray et al., 1999; Gay and Chereskin, 2009], so our analysis focuses on the variability of the poleward current offshore of the SRR. Gay and Chereskin  quantified the seasonal variability in transport of this offshore current and found it to be strongest in the fall. They found relatively little variability in the position of the current, but this is likely an effect of their mapping procedure which used long decorrelation scales and averaging in the alongshore direction [Gay and Chereskin, 2009]. The equatorward flowing California Current in the same region is known to meander [Lynn and Simpson, 1987], and our observations show that the poleward current offshore of the SRR propagates westward in a manner that is largely consistent with Rossby wave dynamics.
 Rossby waves have been previously observed and modeled within the CCS. White et al.  used satellite observations of sea surface height to characterize annual Rossby waves generated along the California coast. Strub and James  hypothesized that westward propagating Rossby waves control offshore movement of a seasonal equatorward jet off central and northern California. Lynn and Bograd  found that El Niño related dynamic height anomalies along Line 90.0 propagated westward at a phase speed consistent with a westward propagating Rossby wave. In a quasi-geostrophic numerical model, Auad et al.  found that wind forcing generated first-mode baroclinic Rossby waves between 25° and 33°N. Di Lorenzo  used the Regional Ocean Modeling System (ROMS) to demonstrate that density anomalies generated by alongshore wind stress and wind stress curl propagated westward from the SCB only when the β effect was included. This dependence on the β effect indicated that the westward propagation was the result of Rossby wave dynamics.
 The remainder of this paper is arranged as follows: section 2.1 describes glider observations in the southern CCS; section 2.2 describes the numerical simulation; section 3.1 discusses the observed mean, transport, and variability of alongshore flow in the southern CCS; section 3.2 discusses the mean structure of poleward jets; section 3.3 characterizes the westward propagation of the current offshore of the SRR and compares the observations with Rossby wave dynamics; and section 4 summarizes the results. Two appendices discuss the accuracy of glider-based vertically averaged current measurements and describe velocity estimation using glider-mounted acoustic Doppler profilers.