Elsevier

Progress in Oceanography

Volume 109, February 2013, Pages 78-89
Progress in Oceanography

Cross-shore transport variability in the California Current: Ekman upwelling vs. eddy dynamics

https://doi.org/10.1016/j.pocean.2012.10.001Get rights and content

Abstract

The low-frequency dynamics of coastal upwelling and cross-shelf transport in the Central and Southern California Current System (CCS) are investigated using the Regional Ocean Modeling System (ROMS) over the period 1965–2008. An ensemble of passive tracers released in the numerical model is used to characterize the effects of linear (Ekman upwelling) and non-linear (mesoscale eddies) circulation dynamics on the statistics of advection of coastal waters. The statistics of passive tracers released in the subsurface show that the low-frequency variability of coastal upwelling and cross-shelf transport of the upwelled water mass are strongly correlated with the alongshore wind stress, and are coherent between the central and southern CCS. However, the offshore transport of tracers released at the surface is not coherent between the two regions, and is modulated by intrinsic mesoscale eddy activity, in particular cyclonic eddies. The transport of cyclonic eddies extends with depth and entrains water masses of southern origin, advected by the poleward California Undercurrent (CUC). The CUC water masses are not only entrained by eddies but also constitute a source for the central California upwelling system. The interplay between intrinsic (eddy activity) and deterministic (Ekman upwelling) dynamics in controlling the cross-shelf exchanges in the CCS may provide an improved framework to understand and interpret nutrients and ecosystem variability.

Highlights

► We analyze the transport variability of the CCS using model passive tracers. ► We look at the linear and nonlinear cross-shore transport of nearshore waters. ► Surface cross-shelf transport of subsurface water is controlled by Ekman upwelling. ► The net horizontal transport at any given depth is strongly controlled by eddies.

Introduction

The California Current System (CCS) has been extensively studied through several long-term and regional sampling programs and satellite analyses, including physical, chemical and biological analyses. The California Current (CC) is the eastern boundary current of the subtropical North Pacific and is characterized by a broad (1000 km offshore), shallow (surface to 500 m) and relatively slow (mean 10 cm s−1) equatorward flow (Batteen et al., 2003). In the subsurface on the continental slope, the California Undercurrent (CUC) is a relatively narrow (10–40 km width) and weak (2–10 cm s−1) poleward flow centered between 100–300 m depth (Hickey, 1979, Hickey, 1998). Despite extensive sampling conducted by the California Cooperative Oceanic Fisheries Investigations (CalCOFI), the coarse spatial and temporal resolution of the sampling leave us with an incomplete understanding of the cross-shore transport dynamics of surface and subsurface water masses.

On interannual to decadal time scales (referred to as “low-frequency variability” throughout the text), large-scale climate modes such as the Pacific Decadal Oscillation (PDO) (Mantua et al., 1997) and the El Niño-Southern Oscillation (ENSO) are used to explain physical fluctuations in the Northeast Pacific Ocean, through local changes in surface wind stress and poleward coastally trapped Kelvin waves (Enfield, 1987). Di Lorenzo et al., 2008, Di Lorenzo et al., 2009 also shows that the North Pacific Gyre Oscillation (NPGO) tracks the dominant interannual and decadal variations of salinity and nutrients in the Northeast Pacific. The ecosystem in the CCS, characterized by a high productivity stimulated by the upwelling of cold and nutrient-rich coastal water, also tends to respond to these dominant modes of climate variability. Indeed, ecosystem fluctuations have already been reported in previous studies and found to be related to large-scale climate variations in the North Pacific such as the PDO (Mantua et al., 1997, Chavez et al., 2003, Lavaniegos and Ohman, 2003, Chhak and Di Lorenzo, 2007), ENSO (Bograd and Lynn, 2001), secular warming (Roemmich and McGowan, 1995, McGowan et al., 2003, Lavaniegos and Ohman, 2007) or the NPGO (Di Lorenzo et al., 2008).

The variability of cross-shore transport of coastal water masses is likely to be critical in understanding ecosystem dynamics because of the potential for offshore transport of nutrients, mass, and organisms. However, little is known about the dynamics controlling interannual and longer-term variability of cross-shelf transport in the CCS. To examine the temporal variability in cross-shelf transport, we use a long-term hindcast of a regional ocean model coupled with a set of passive tracers continuously released at the coast. In order to separate the offshore advection of surface coastal waters from the offshore advection of upwelled coastal water, tracers are released separately both in the surface layer (“surface-released tracer”) and in the subsurface (“subsurface-released tracer”). We use the passive tracer fields to construct proxies for offshore transport, coastal upwelling strength and Ekman transport efficiency. In addition, we divide our analysis domain into the central and southern CCS to examine the extent of south-north exchange through transport of water masses by the surface and subsurface flow.

This paper is organized as follows. Section 2 describes the model experiments and tracer approach used in this study. Sections 3 Mean and seasonal cycle, 4 Interannual variability of upwelling and eddy cross-shelf transport, 5 The poleward undercurrent use the passive tracer statistics to focus on the mean, seasonal cycle, upwelling low-frequency variability, cross-shore transport and south-north exchange through the poleward undercurrent. Finally, Section 6 provides a summary and discussion of the Ekman vs. eddy dynamics.

Section snippets

Model and tracer experiment setup

The upwelling variability and offshore transport dynamics are investigated using a three dimensional, free-surface, hydrostatic, eddy-resolving primitive equation ocean model (the Regional Ocean Modeling System; ROMS; Shchepetkin and McWilliams, 2005). ROMS, a descendent of S-Coordinate Rutgers University Model (SCRUM), uses orthogonal curvilinear coordinates in the horizontal and terrain-following coordinates in the vertical. A complete report of the model numerics, open boundary conditions

Mean and seasonal cycle

This study aims to quantify the low-frequency dynamics of the cross-shore and alongshore transport in the CCS. As explained in the previous section, the approach to this problem is to use a regional ocean model and follow the fate of a set of passive tracers. The source of the tracers is in the subsurface along the southern and central/northern California coast (hereafter referred to as “southern region” and “northern region”) so that the tracer’s concentration found at the surface corresponds

Interannual variability of upwelling and eddy cross-shelf transport

To explore the link between upwelling dynamics and cross-shore transport on the interannual timescale, we remove the climatological monthly means from the tracer fields. Fig. 4c shows the time series of the subsurface-released tracer averaged at the surface over the white box labeled 1 in Fig. 4a and b (above the region where the tracer is released), both for the tracer injected in the southern region (green line) and northern region (blue line). These time series (Fig. 4c) illustrate the

The poleward undercurrent

Although a modeling (ROMS) study conducted by Rivas and Samelson (2011) shows that the poleward undercurrent (CUC) plays a surprisingly small role as a direct source of Oregon upwelling water, Chhak and Di Lorenzo (2007; also using the ROMS ocean model) use adjoint passive tracers to track the origin of upwelled water masses in the CCS and show that while much of the upwelled waters at shallow depth come from offshore regions and from the north, at depths of around 200 m the CUC influences the

Summary and conclusions

In this study, we use the Regional Ocean Modeling System (ROMS), forced by NCEP/NCAR reanalysis wind stress, to simulate the dynamics of the California Current System (CCS) and analyze the transport variability of the system from 1965 to 2008. We have shown that 10 km spatial resolution captures the mean large-scale features of the mesoscale activity in this region and also captures the key characteristics of eastern boundary systems such as the subsurface poleward flow. Nevertheless, it is

Acknowledgments

This study was supported by the National Science Foundation (NSF OCE-0550266, POBEX project NSF-GLOBEC OCE-0815280 and NSF CCE-LTER). We thank the three anonymous reviewers for their excellent and constructive comments and suggestions.

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