Robert S. Pickart

Massive phytoplankton blooms under Arctic Sea ice

In midsummer, diatoms have taken advantage of thinning ice cover to feed in nutrient-rich waters. Phytoplankton blooms over Arctic Ocean continental shelves are thought to be restricted to waters free of sea ice. Here, we document a massive phytoplankton bloom beneath fully consolidated pack ice far from the ice edge in the Chukchi Sea, where light transmission has increased in recent decades because of thinning ice cover and proliferation of melt ponds. The bloom was characterized by high diatom biomass and rates of growth and primary production. Evidence suggests that under-ice phytoplankton blooms may be more widespread over nutrient-rich Arctic continental shelves and that satellite-based estimates of annual primary production in these waters may be underestimated by up to 10-fold.

Deep convection in the Irminger Sea forced by the Greenland tip jet.

Open-ocean deep convection, one of the processes by which deep waters of the worlds oceans are formed, is restricted to a small number of locations (for example, the Mediterranean and Labrador seas). Recently, the southwest Irminger Sea has been suggested as an additional location for open-ocean deep convection. The deep water formed in the Irminger Sea has the characteristic temperature and salinity of the water mass that fills the mid-depth North Atlantic Ocean, which had been believed to be formed entirely in the Labrador basin. Here we show that the most likely cause of the convection in the Irminger Sea is a low-level atmospheric jet known as the Greenland tip jet, which forms periodically in the lee of Cape Farewell, Greenland, and is associated with elevated heat flux and strong wind stress curl. Using a history of tip-jet events derived from meteorological land station data and a regional oceanic numerical model, we demonstrate that deep convection can occur in this region when the North Atlantic Oscillation Index is high, which is consistent with observations. This mechanism of convection in the Irminger Sea differs significantly from those known to operate in the Labrador and Mediterranean seas.

The Labrador Sea Deep Convection Experiment

In the autumn of 1996 the field component of an experiment designed to observe water mass transformation began in the Labrador Sea. Intense observations of ocean convection were taken in the following two winters. The purpose of the experiment was, by a combination of meteorological and oceanographic field observations, laboratory studies, theory, and modeling, to improve understanding of the convective process in the ocean and its representation in models. The dataset that has been gathered far exceeds previous efforts to observe the convective process anywhere in the ocean, both in its scope and range of techniques deployed. Combined with a comprehensive set of meteorological and air-sea flux measurements, it is giving unprecedented insights into the dynamics and thermodynamics of a closely coupled, semienclosed system known to have direct influence on the processes that control global climate.

Hydrography of the Labrador Sea during Active Convection

The hydrographic structure of the Labrador Sea during wintertime convection is described. The cruise, part of the Deep Convection Experiment, took place in February‐March 1997 amidst an extended period of strong forcing in an otherwise moderate winter. Because the water column was preconditioned by previous strong winters, the limited forcing was enough to cause convection to approximately 1500 m. The change in heat storage along a transbasin section, relative to an occupation done the previous October, gives an average heat loss that is consistent with calibrated National Centers for Environmental Prediction surface heat fluxes over that time period (; 200 Wm 22). Deep overturning was observed both seaward of the western continental slope (which was expected), as well as within the western boundary current itself—something that had not been directly observed previously. These two geographical regions, separated by roughly the 3000-m isobath, produce separate water mass products. The offshore water mass is the familiar cold/fresh/dense classical Labrador Sea Water (LSW). The boundary current water mass is a somewhat warmer, saltier, lighter vintage of classical LSW (though in the far field it would be difficult to distinguish these products). The offshore product was formed within the cyclonic recirculating gyre measured by Lavender et al. in a region that is limited to the north, most likely by an eddy flux of buoyant water from the eastern boundary current. The velocity measurements taken during the cruise provide a transport estimate of the boundary current ‘‘throughput’’ and offshore ‘‘recirculation.’’ Finally, the overall trends in stratification of the observed mixed layers are described.

Is Labrador Sea Water formed in the Irminger basin

Abstract Present day thinking contends that Labrador Sea Water (LSW), one of the major watermasses of the North Atlantic, is formed exclusively in the Labrador basin via deep wintertime convection. It is argued herein that LSW is likely formed at a second location—the southwest Irminger Sea. We base this on two pieces of evidence: (1) tracer observations in the western subpolar gyre are inconsistent with a single source and (2) the combination of oceanic and atmospheric conditions that lead to convection in the Labrador Sea is present as well east of Greenland. Hydrographic data (both recent and climatological) are used, in conjunction with an advective–diffusive numerical model, to demonstrate that the spatial distribution of LSW and its inferred spreading rate are inconsistent with a Labrador Sea-only source. The spreading would have to be unrealistically fast, and could not produce the extrema of LSW properties observed in the Irminger basin. At the same time, the set of conditions necessary for deep convection to occur—a preconditioned water column, cyclonic circulation, and strong air–sea buoyancy fluxes—are satisfied in the Irminger Sea. Using observed parameters, a mixed-layer model shows that, under the right conditions, overturning can occur in the Irminger Sea to a depth of 1500– 2000 m , forming LSW.

Western Arctic Shelfbreak Eddies: Formation and Transport

Abstract The mean structure and time-dependent behavior of the shelfbreak jet along the southern Beaufort Sea, and its ability to transport properties into the basin interior via eddies are explored using high-resolution mooring data and an idealized numerical model. The analysis focuses on springtime, when weakly stratified winter-transformed Pacific water is being advected out of the Chukchi Sea. When winds are weak, the observed jet is bottom trapped with a low potential vorticity core and has maximum mean velocities of O(25 cm s−1) and an eastward transport of 0.42 Sv (1 Sv ≡ 106 m3 s−1). Despite the absence of winds, the current is highly time dependent, with relative vorticity and twisting vorticity often important components of the Ertel potential vorticity. An idealized primitive equation model forced by dense, weakly stratified waters flowing off a shelf produces a mean middepth boundary current similar in structure to that observed at the mooring site. The model boundary current is also highly v...

The northern recirculation gyre of the gulf Stream

Abstract Results from two recent field programsin the western North Atlantic are presented with particular emphasis on the deep circulation. New long-term moored current measurements show that the flow north of the Gulf Stream and east of the New England Seamount Chain is toward the west from 500 m to the bottom with very little depth dependence. Nearly 40 × 106 m3s−1 is transported to the west near 63°W, and half of this recirculates back to the east over the Seamount Chain to add a strong component to the deep Gulf Stream between the Chain and the Grand Banks. We call this current the “Northern Recirculation Gyre” in contrast with a similar feature to the south of the Stream popularly known as the “Worthington Gyre” ( Worthington , 1976, The Johns Hopkins Oceanographic Studies, 6, 110 pp.). The new gyre is similar to that proposed by Hogg (1983, Deep-Sea Research, 30, 945–961) but somewhat smaller in scale. Its relationship to the Gulf Stream and the Deep Western Boundary Current is made explicit by the new measurements. Tracer measurements show that the Northern Recirculation Gyre exchanges water properties with the Deep Western Boundary Current where the two are in close proximity along the northern boundary. The relatively high values of oxygen and freon, so imparted, are then advected to the interior where the gyre carries water eastward under the Gulf Stream. Beneath the thermocline these tracer fields are practically homogenous within the gyre, perhaps a reflection of the expulsion process described by Rhines and Young (1983, Journal of Fluid Mechanics, 133, 133–145). An advective-diffusive model is used to interpret some slight differences between the various tracer distributions.

Boundary Current Eddies and Their Role in the Restratification of the Labrador Sea

An idealized model is used to study the restratification of the Labrador Sea after deep convection, with emphasis on the role of boundary current eddies shed near the west coast of Greenland. The boundary current eddies carry warm, buoyant Irminger Current water into the Labrador Sea interior. For a realistic end-of-winter state, it is shown that these Irminger Current eddies are efficient in restratifying the convected water mass in the interior of the Labrador Sea. In addition, it is demonstrated that Irminger Current eddies can balance a significant portion of the atmospheric heat loss and thus play an important role for the watermass transformation in the Labrador Sea.

Water mass components of the North Atlantic deep western boundary current

Abstract Four hydrographic sections across the North Atlantic deep western boundary current from 55°W to 70°W are analysed to distinguish the currents different water mass components. The deepest component is the Norwegian-Greenland overflow water (2–3°C) which is characterized most readily by a core of high oxygen, tritium, chlorofluorocarbons (CFCs) and low silicate anomaly. The above lying Labrador Sea Water (3–4°C) is distinguishable at this latitude only by its core of low potential vorticity. The shallowest component of the boundary current (4–5°C) is revealed by a core of high tritium, CFCs and low anomaly nut has no corresponding oxygen signal because of its proximity to the pronounced oxygen minimum layer. A careful analysis of the shallow water mass reveals that it is not dense enough to be formed in the central Labrador Sea even during warm winters. Rather, based on historical hydrography its area of formation is the southern Labrador Sea inshore of the North Atlantic current where surface layer salinities are particularly low. A simple scale analysis shows that lateral mixing with the adjacent North Atlantic current can increase the salinity of this component to the values observed in the mid-latitude data set.

Upwelling on the continental slope of the Alaskan Beaufort Sea : storms, ice, and oceanographic response

[1] The characteristics of Pacific-born storms that cause upwelling along the Beaufort Sea continental slope, the oceanographic response, and the modulation of the response due to sea ice are investigated. In fall 2002 a mooring array located near 152W measured 11 significant upwelling events that brought warm and salty Atlantic water to shallow depths. When comparing the storms that caused these events to other Aleutian lows that did not induce upwelling, interesting trends emerged. Upwelling occurred most frequently when storms were located in a region near the eastern end of the Aleutian Island Arc and Alaskan Peninsula. Not only were these storms deep but they generally had northward-tending trajectories. While the steering flow aloft aided this northward progression, the occurrence of lee cyclogenesis due to the orography of Alaska seems to play a role as well in expanding the meridional influence of the storms. In late fall and early winter both the intensity and frequency of the upwelling diminished significantly at the array site. It is argued that the reduction in amplitude was due to the onset of heavy pack ice, while the decreased frequency was due to two different upper-level atmospheric blocking patterns inhibiting the far field influence of the storms.