Eastern Boundary Currents
	Teresa K. Chereskin

Hydrographic/ADCP surveys, satellite imagery, and surface drifters have all served to revise our view of the coastal upwelling that occurs seasonally in most eastern boundary regions, such as off the coast of California. Instead of a uniform upwelling along the coast, with an offshore Ekman flow at the surface and a return flow at depth, we now have a view that much of the transport occurs as "squirts and jets" -- upwelling centers associated with capes and promontories, offshore transport in intense jets, and a vigorous field of mesoscale eddies. The upwelling and vertical transport of nutrients is associated with some of the richest fisheries in the world, making eastern boundary regions economically and politically important as well. The questions that interest me relate to the formation and dynamics of the mesocale eddy field: what are the dynamical balances? do the eddies draw energy barotropically or baroclinically from the mean and/or other low-frequency background flows? what role do the eddies play in determining the distribution of nutrients, primary production, and species?

A great deal of my field work has focused on the California Current: the WOCE Pacific surveys funded by the National Science Foundation, an ongoing program with the California Cooperative Oceanic Fisheries Investigations (CalCOFI), and an intensive moored array off Point Arenas funded by the Office of Naval Research as part of the Eastern Boundary Currents experiment. The moored array (15 deep ocean moorings, 60 current meter records, deployed in 3 eddy-resolving local dynamics arrays) was designed to look at the eddy heat, momentum, energy, and vorticity balances in the California Current. Together with co-investigators, I completed a statistical description of the velocity field, contrasting space and time scales between the inshore and offshore sites, and interpreting the dynamical balances that we observed during passages of eddies through the arrays (Chereskin et al., 2000). The synoptic, 2-D, quantitative estimates of horizontal shears, vorticity, and divergence resolved by our arrays are new, and they revealed a mesoscale eddy field that is strongly nonlinear, with Rossby numbers of O(1). Using data assimilation and a quasigeostrophic model, we described and predicted the dynamical balances of the observed eddies (Cornuelle et al., 2000). Model reconstruction of an observationally well-documented anticyclone confirmed the large Rossby number and nonlinearity. The model results suggest that the anticylones are submesoscale coherent vortices.

A basic research question that I have begun to address through my work with CalCOFI is that of how best to combine disparate measurements, such as ADCP and geostrophic velocity estimates, into a single best estimate of the circulation. In much of oceanography, one does not have a good statistical basis, as many observations are single "snapshot" surveys or point time series. A unique aspect of CalCOFI is that there is a 50+ year time series of hydrographic and biological surveys, to which I have added a program of direct velocity measurement via shipboard ADCP (beginning in 1993). Historical CalCOFI data provide a well-sampled statistical basis for determining a covariance function, which together with the geostrophic constraint provide a consistent framework in which to combine geostrophic and ADCP velocities into a best estimate of the absolute flow field (Chereskin and Trunnell, 1996).

An important link between physics and biology in marine upwelling ecosytems such as the California Current is the role of ocean currents and eddies in supplying nutrients to the euphotic zone. Results from a simple box model of nutrient budgets and nutrient recycling in the modern CalCOFI sampling region off southern California (Bograd et al., 2001) demonstrate the importance of horizontal flux convergences to nutrient cycling and point out the inadequacy of 1-D vertical exchange models in regions such as upwelling zones where residence times are short. They also point out the importance of long time series for distinguishing between low-frequency changes related to climate signals, interannual variability such as El Nino, and higher frequency variability due to changes in local forcing. Long time series are also essential for differentiating between the contribution due to mean flows and that due to eddy fluxes.

Figure 1: (a) Dynamic height anomaly 0/500 m, 11-year mean. Note that the mean field bears little resemblance to any single realization of the flow, such as the 10/93 cruise shown here, due to averaging over the spatially varying locations of the California Current and eddies. (b) Variance about the 11-year mean. The high variance zone corresponds to the (meandering) path of the California Current. (c) Dynamic height anomaly 0/500, October 1993. Locations of CalCOFI hydrographic stations are shown. (d) as in (c), with geostrophic velocity vectors superposed. (e) Streamfunction (dynamic height) corresponding to the absolute flow field. An ADCP velocity reference at 200 m, together with an objective mapping technique that enforces horizontal nondivergence, has been combined with the CalCOFI hydrography. Symbols show the locations of ADCP velocities used in the calculation. (f) as in (e), with velocity vectors superposed. Differences between (d) and (f) result from the ADCP reference. The maximum surface flow in the California Current deduced from (f) is about 35 cm/s, about 50% larger than estimated assuming zero flow at 500 m as in (e). The other striking difference between the maps is the increased gradients that are resolved by including the ADCP vector reference. (From Chereskin and Trunnell, 1996.

Acknowledgements

This work was supported by grants from NSF (OCE-9216411), ONR (N000-14-92-J-1584), AND NASA (NAG5-6497).

Relevant Publications

Chereskin, T. K., and P. P. Niiler 1994: Circulation in the Ensenada Front - September 1988. Deep-Sea Res., 41, 1251-1287. Abstract

Chereskin, T. K., 1995: Evidence for an Ekman balance in the California Current. J. Geophys. Res., 100, 12727-12748. Abstract

Chereskin, T. K., and M. Trunnell, 1996: Correlation scales, objective mapping, and absolute geostrophic flow in the California Current. J. Geophys. Res. , 101, 22619-22629. Abstract

Strub, P. T., T. K. Chereskin, P. P. Niiler, M. D. Levine, and C. James, 1997: Altimeter-derived variability of surface velocities in the California Current System: Part 1, Evaluation of TOPEX altimeter velocity resolution. J. Geophys. Res., 102, 12727-12748. Abstract

Kelly, K. A., R. C. Beardsley, R. Limeburner, K. H. Brink, J. Paduan, and T. K. Chereskin, 1998: Variability of the near-surface eddy kinetic energy in the California Current based on altimetric, drifter, and moored current data. J.Geophys. Res., 103, 13067-13084.Abstract

Chereskin, T. K., M. Y. Morris, P.  P.  Niiler, P.  M.  Kosro, R.  L.  Smith, S.  R.  Ramp, C.  A. Collins, and D.  L.  Musgrave, 2000. Spatial and temporal characteristics of the mesoscale circulation of the California Current from eddy-resolving moored and shipboard measurements. J. Geophys. Res., 105, 1245-1269.Abstract

Cornuelle, B. D., T.  K.  Chereskin, P.  P.  Niiler, M.  Y.  Morris, and D.  Musgrave, 2000. Observations and modeling of a California Undercurrent Eddy. J. Geophys. Res., 105, 1227-1243.Abstract

Bograd, S. J., T. K. Chereskin, and D. Roemmich, 2001. Transport of mass, heat, salt and nutrients in the southern California Current System: Annual cycle and interannual variability. J. Geophys. Res., 106,9255-9275.

Lynn, R. J., S. J. Bograd, T. K. Chereskin, and A. Huyer, 2003. Seasonal renewal of the California Current: The spring transition off California, J. Geophys. Res., 108, 3279, 10.1029/2003JC001787.Abstract