Robert G. Paquette

Lagrangian Exploration of the California Undercurrent, 1992–95

Abstract During the period 1992–95, nineteen isobaric RAFOS floats, placed in the California Undercurrent at intermediate depths (150–600 m) off Monterey and San Francisco, California, reveal a region of varying width of subsurface, poleward flow adjacent to the continental margin. The float trajectories exhibit three patterns: poleward flow in the undercurrent; reversing, but predominately alongshore, flow adjacent to the continental margin;and, farther offshore, anticyclonic motion accompanied by slow westward drift. Flow continuity of the undercurrent exists between Pt. Reyes and at least Cape Mendocino with an average speed dependent on the float depth. Speeds were variable, but common features were acceleration occurring to the south of Pt. Arena and deceleration to the north of Cape Mendocino. An important mechanism for floats, and water, to enter the ocean interior from the undercurrent is through the formation of submesoscale coherent vortices.

The Jan Mayen Current of the Greenland Sea

Oceanographic measurements acquired by the USNS Bartlett during September 1989 in and north of the area of the Jan Mayen Current (JMC) show that, in terms of upper layer baroclinic flow, about half of the JMC is a wide meander in the East Greenland Current (EGC) and about half continues eastward to close the Greenland Gyre (GG) on the south. This phenomenon has been charted in the past but has elicited no comment. Surface drifters and a numerical model corroborate this behavior. At deeper depths (>100 m) the meander dissipates with the flow becoming more easterly. A drifting float at 500 m depth confirms that the flow is relatively undeviated toward the east ultimately merging with the northward flowing Norwegian Atlantic Current. Near the GG the fresh water content of the water column is low in the upper layers but high in the middepths. Where there is a substantial content of EGC water (e.g., in the JMC), the converse is usually true. The FWC of the GG computed for the period 1953–1966 was smaller by a factor of 2 in both layers than in 1989, indicating that 1989 is in another period of low salinity comparable to that of 1968, the period of the Great Salinity Anomaly described by Dickson et al. (1988).

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

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.

Lagrangian measurement of subsurface poleward flow between 38°N and 43°N along the West Coast of the United States during summer, 1993

Subsurface Lagrangian measurements at about 140 m showed that the path of the California Undercurrent lay next to the continental slope between San Francisco (37.8°N) and St. George Reef (41.8°N) during mid-summer 1993. The mean speed along this 500 km path was 8 cms−1. The flow at this depth was not disturbed by upwelling centers at Point Reyes or Cape Mendocino. Results also demonstrate the ability to acoustically track floats located well above the sound channel axis along the California coast.

Intermediate level Lagrangian subsurface measurements in the northeast Pacific: Isobaric RAFOS float data

Isobaric RAFOS floats have been used to track the California Undercurrent and to investigate its continuity since 1992. The data include 61 quasi-Lagrangian subsurface trajectories sampled for the most part between 150 and 600 m. The data set allows estimates of Eulerian and Lagrangian statistics for the region, studies of mesoscale eddy activity, and analysis of seasonal variability of circulation patterns off Central California. A browsable Web-based inventory and a Graphical User Interface have been developed to provide access to this data set, including interactive manipulation of the data.

On the deep and bottom waters of the Greenland Sea from summer 1989 and 1990 data

The deep waters of the central and southern Greenland Sea are examined using Bartlett 1989 and 1990 summer data displayed at very high temperature and salinity resolutions. The mixing of Arctic Ocean Deep Water (AODW) with Greenland Sea Deep Water (GSDW) in proportions of about 1:1 to form “new” Norwegian Sea Deep Water (NSDW) is confirmed. However, the previously accepted route of this new NSDW through the ridge system into the Norwegian Sea, the Jan Mayen Channel, is shown to pass deep water only intermittently; a passage through Mohns Ridge farther north, beginning near 72°06′N, 2°W, is identified as a more likely route. Part of the ultimate AODW-GSDW mixture is shown to continue on along the Greenland continental slope into the Iceland Sea. A peculiar isothermal condition was found in the deeper parts of the GSDW. This is postulated to result from upward diffusion of temperature from a layer initially having the adiabatic temperature gradient. Above the isothermal layer is one of warming and decreasing salinity, the result of recent successive deep convections to different depths.