Cross-shore transport variability in the California Current: Ekman upwelling vs. eddy dynamics
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.
References (41)
Seasonal dynamics of the surface circulation in the Southern California Current System
Deep-Sea Research II
(2003)The California current system – hypotheses and facts
Progress in Oceanography
(1979)- et al.
Long-term changes in pelagic tunicates of the California Current
Deep Sea Research Part II—Topical Studies in Oceanography
(2003) - et al.
Coherence of long-term variations of zooplankton in two sectors of the California Current System
Progress in Oceanography
(2007) - et al.
Open boundary conditions for long-term integration of regional oceanic models
Ocean Modelling
(2001) - et al.
The biological response to the 1977 regime shift in the California Current
Deep Sea Research Part II—Topical Studies in Oceanography
(2003) - et al.
The Canary Eddy Corridor: a major pathway for long-lived eddies in the subtropical North Atlantic
Deep-Sea Research I
(2009) - et al.
The regional oceanic modeling system (ROMS): a split explicit, free-surface, topography-following-coordinate oceanic model
Ocean Modelling
(2005) - Barber, R.T., Smith, R.L., 1981. Coastal upwelling ecosystems. In: Longhurst, A.R. (Ed.), Analysis of Marine...
- et al.
A large-scale seasonal modeling study of the California current system
Journal of Oceanography
(2003)
Physical–biological coupling in the California Current during the 1997–99 El Nino-La Nina cycle
Geophysical Research Letters
Mesoscale to submesoscale transition in the California current system. Part I: flow structure, eddy flux, and observational tests
Journal of Physical Oceanography
From anchovies to sardines and back: multidecadal change in the Pacific Ocean
Science
Global observations of large oceanic eddies
Geophysical Research Letters
Decadal variations in the California current upwelling cells
Geophysical Research Letters
Forcing of low-frequency ocean variability in the Northeast Pacific
Journal of Climate
Interannual and decadal variations in cross-shelf transport in the Gulf of Alaska
Journal of Physical Oceanography
The warming of the California current system: dynamics and ecosystem implications
Journal of Physical Oceanography
North Pacific Gyre Oscillation links ocean climate and ecosystem change
Geophysical Research Letters
Cited by (51)
Cross-shelf transport off the northern Taiwan, East China Sea
2023, Estuarine, Coastal and Shelf ScienceReview of oceanic mesoscale processes in the North Pacific: Physical and biogeochemical impacts
2023, Progress in OceanographyTemporal and spatial variability in hydrography and dissolved oxygen along southwest Nova Scotia using glider observations
2023, Continental Shelf ResearchCitation Excerpt :It is important to note that winds and tides are not the only drivers of onshore advection. Cyclonic (anticyclonic) mesoscale eddies over basins (banks) further offshore can entrain and transport water with certain temperature, salinity, and DO properties to the coast (Loder et al., 1997; Combes et al., 2013). These mesoscale eddies are primarily generated by larger scale upwelling and alongshore currents or Rossby waves (Marchesiello et al., 2003; LaCasce and Pedlosky, 2004; Combes et al., 2013).
Using long-term citizen science data to distinguish zones of debris accumulation
2022, Marine Pollution BulletinCross-shelf exchanges in the northern Bay of Biscay
2020, Journal of Marine SystemsCitation Excerpt :Various studies have investigated cross-shelf exchanges using in situ and satellite observations (Piola et al., 2010; Porter et al., 2016; Nencioli et al., 2016), whereas others have also included numerical approaches (Serra et al., 2010; Zhang and Gawarkiewicz, 2015). Numerous studies have focused on specific processes leading to cross-shelf exchanges, such as fronts (Nencioli et al., 2016), eddies (Peliz et al., 2004; Shapiro et al., 2010; Combes et al., 2013; Cherian and Brink, 2016; Rubio et al., 2018) or Ekman upwelling (Combes et al., 2013). Some studies have investigated the contribution of the different physical mechanisms on cross-shelf exchanges (Zhou et al., 2014; Zhou et al., 2015; Zhang et al., 2017), whereas others have investigated their influence on the ecosystem (Zhao and Guo, 2011).
Seasonal and interannual cross-shelf transport over the Texas and Louisiana continental shelf
2018, Continental Shelf Research