Lagrangian flow in the California Undercurrent, an observation and model comparison
Introduction
As numerical ocean circulation models reach higher and higher spatial resolution it becomes increasingly important to compare them with all kinds of observations including Lagrangian floats. Initially, as in this paper, these comparisons are approached from a model validation point of view. We compare RAFOS float trajectories with the output from a high resolution (1/5° on average) almost-global simulation of the Parallel Ocean Program (POP). Although POP is formulated in an Eulerian framework and most of the model/data comparisons performed with it have been with Eulerian data sources, such as satellite altimeters and moorings (see Maltrud et al., 1998), the model has the ability to simulate Lagrangian data as well. Both flow patterns (trajectories) and statistics from the model Lagrangian motion are compared with the RAFOS observations. This represents one of the first comparisons between the model and subsurface Lagrangian observations.
The California Current System (CCS) consists of three main currents (see Hickey, 1998, for a recent review): the equatorward flowing California Current, and two poleward flowing inshore currents known as the Davidson Current and the California Undercurrent (CUC). The California Current is a shallow surface flow which is centered about 300 km offshore from Monterey and is easily distinguished as a surface lens of fresh (salinity=32.8) water. It appears strongest in the spring. Off Monterey, subsurface poleward flow (CUC) typically occurs from the shelf break to a distance of about 100 km from the shelf break (Collins et al., 2000). Although the mean subsurface flow is poleward in all months, a broad, surface-intensified poleward flow occurs in winter months off Central California and is associated with a trough in the dynamic topography, which is centered at about (123°W) at the latitude of Monterey Lynn and Simpson, 1987, Schwing et al., 1991. Poleward flow is also observed over the upper slope during much of the year and is strongest during March–August. This system of currents is also affected by longer period variation such as interannual El Niño events (the latest occurring in 1991–1992 and 1997–1998), which result in perturbations to coastal regions that are as large as the seasonal variability. The CCS includes a complex system of eddies, filaments and jets.
Garfield et al. (1999) (henceforth GCPC99) reported on a study of the Lagrangian character of the intermediate level flow adjacent to the coast from central California to Oregon obtained by tracking neutrally buoyant subsurface RAFOS floats. This article revises and updates our description of the subsurface flow, particularly the California Undercurrent. The data are then used in a comparison of the POP model output for this region.
The previous results of GCPC99 were based upon 1900 days of RAFOS data obtained during 1992–1995. These floats, launched off San Francisco and Monterey, exhibited three patterns with their trajectories: poleward flow in the undercurrent; reversing, but predominantly alongshore, flow adjacent to the continental margin; and, farther offshore, primarily anticyclonic motion accompanied by slow westward drift. Their results also showed flow continuity of the undercurrent between Pt. Reyes and at least Cape Mendocino, with average speed dependent upon the float depth. The data set now has almost twice as many floats and thrice the number of float days than that used by GCPC99.
The remainder of this manuscript is organized as follows: (2) observational data, Lagrangian results and statistics, (3) model results, (4) discussion, and (5) summary.
Section snippets
NPS RAFOS floats
The RAFOS float (Rossby et al., 1986) is an acoustically tracked, neutrally buoyant, subsurface Lagrangian drifter. The float electronics are contained within a glass pressure housing approximately 2 m in length, 0.1 m in diameter and weighing 15 kg. The float commonly measures temperature and pressure at fixed time intervals, and uses a hydrophone to record the arrival times of acoustic signals transmitted from moored sound sources. Recorded temperature has a resolution of 0.02°C and an
Model floats
The model results used here are from a run of the Parallel Ocean Program (POP) to simulate a 5-year period. The model was forced using European Centre for Medium-Range Weather Forecasts (ECMWF) winds, Barnier et al. (1995) surface heat flux, and Levitus (1982) surface salinity restoring on an almost global Mercator grid (78°S to 78°N) with horizontal resolution of 0.28° at the equator (0.06° at 78°S and 78°N) and 20 vertical levels. The model setup is essentially the same as run POP11 as
Discussion
California Undercurrent observations made during the last few years, comprised of the Lagrangian observations reported here, a series of cross-shore ship-based ADCP absolute velocity measurements (Pierce et al., 2000), and long-term fixed observations (e.g. Collins et al., 2000), confirm the robust and permanent nature of the California Undercurrent. The high density of measurements have shown the current to exist offshore of the continental shelf break between Pt. Conception, CA, to Vancouver
Conclusion
The major objectives of this paper were to report on the expanded set of RAFOS subsurface Lagrangian drifters for the northeast Pacific and to provide a comparison of the float observations with Lagrangian flow from a global high resolution model. The new float data confirm the earlier assessment of GCPC99. None of the earlier observations were changed significantly; rather the confidence in the observations has improved. The poleward flow of the California Undercurrent appears to be a
Acknowledgements
The POP model simulations were supported by the DOE Climate Change Prediction Program. The RAFOS program has been supported by the Naval Postgraduate School, the Oceanographer of the Navy and the Office of Naval Research. A project of this size obviously depends on many more people than the authorship of this paper. Pierre Tilliet has kindly accommodated some interesting suggestions on the floats. Andy Anderson has helped with float preparation. Kirk Kingsbury, director of the NCEL pressure
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