Lagrangian flow in the California Undercurrent, an observation and model comparison

doi:10.1016/S0924-7963(01)00017-3

Journal of Marine Systems

Volume 29, Issues 1–4, May 2001, Pages 201-220

https://doi.org/10.1016/S0924-7963(01)00017-3 Get rights and content

Abstract

During the period 1992–1998, 38 isobaric RAFOS floats were deployed to sample the subsurface flow of the California Undercurrent. The deployments, released over the California continental slope west of San Francisco, have sampled robust year-round poleward subsurface flow associated with the Undercurrent most seasons and the combined inshore current and Undercurrent in winter. Two other types of flow have been seen: a region of weak flow with little net displacement just west of the California Undercurrent, and an active westward propagating eddy field. This eddy field appears to be the primary mechanism for moving floats from the Undercurrent into the ocean interior. The observations and statistics from the RAFOS floats are compared with Lagrangian estimates of particles tracked in a global high resolution ocean simulation in order to evaluate the fidelity of the model along an eastern boundary. The results show that the model reproduces the general character of the flow reasonably well, but underestimates both the mean and eddy energies by a substantial amount.

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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Lagrangian flow in the California Undercurrent, an observation and model comparison

Abstract

Introduction

Section snippets

NPS RAFOS floats

Model floats

Discussion

Conclusion

Acknowledgements

J. Mar. Syst.

Deep-Sea Res.

Deep-Sea Res.

Deep-Sea. Res., Part II

Deep-Sea Res.

Wind-forced modeling studies of currents, meanders, and eddies in the California Current system

J. Geophys. Res.

Statistical properties of near-surface flow in the California coastal transition zone

J. Geophys. Res.

A view of the 1993–1994 California Current based on surface drifters, floats, and remotely sensed data

J. Geophys. Res.

Seasonal variability of alongshore geostrophic velocity off central California

J. Geophys. Res.

Spatial and temporal characteristics of the mesoscale circulation of the California Current from eddy-resolving moored and shipboard measurements

J. Geophys. Res.

Lagrangian exploration of the California Undercurrent, 1992–95

J. Phys. Oceanogr.

Coastal oceanography of western North America from the tip of Baja California to Vancouver Island

Geographic variation of SOFAR speed and axis depth in the Pacific Ocean

J. Geophys. Res.

Variability in the near-surface eddy kinetic energy in the California Current based on altimetric, drifter, and moored current data

J. Geophys. Res.

ERS-1/2 orbit error improvement using TOPEX/POSEIDON: the 2 cm challenge

J. Geophys. Res.

An improved mapping method of multi-satellite altimeter data

J. Atmos. Oceanic Technol.