M. Susan Lozier

Interior pathways of the North Atlantic meridional overturning circulation

To understand how our global climate will change in response to natural and anthropogenic forcing, it is essential to determine how quickly and by what pathways climate change signals are transported throughout the global ocean, a vast reservoir for heat and carbon dioxide. Labrador Sea Water (LSW), formed by open ocean convection in the subpolar North Atlantic, is a particularly sensitive indicator of climate change on interannual to decadal timescales. Hydrographic observations made anywhere along the western boundary of the North Atlantic reveal a core of LSW at intermediate depths advected southward within the Deep Western Boundary Current (DWBC). These observations have led to the widely held view that the DWBC is the dominant pathway for the export of LSW from its formation site in the northern North Atlantic towards the Equator. Here we show that most of the recently ventilated LSW entering the subtropics follows interior, not DWBC, pathways. The interior pathways are revealed by trajectories of subsurface RAFOS floats released during the period 2003–2005 that recorded once-daily temperature, pressure and acoustically determined position for two years, and by model-simulated ‘e-floats’ released in the subpolar DWBC. The evidence points to a few specific locations around the Grand Banks where LSW is most often injected into the interior. These results have implications for deep ocean ventilation and suggest that the interior subtropical gyre should not be ignored when considering the Atlantic meridional overturning circulation.

Signatures of the Mediterranean outflow from a North Atlantic climatology: 1. Salinity and density fields

Using historical data from the National Oceanic Data Center, the climatology of the eastern North Atlantic basin has been investigated for the purpose of detailing the Mediterranean outflow water in terms of its salinity, density, and flow patterns. Part 1 of this work is a descriptive analysis of the fate of the Mediterranean Water once it flows out of the Strait of Gibraltar. Tracing the salinity and density signatures, high-resolution maps of the climatological outflow are presented, with an emphasis on the continuity of the water from its source. From the climatological fields a continuous signal of Mediterranean Water is tracked northward to ∼50°20′N, yet its westward advection is limited to the Tagus Basin. Recirculations of Mediterranean Water in the Gulf of Cadiz and in the Bay of Biscay, deduced from property signals, are also detailed. Iorga and Lozier [this issue] present absolute velocity fields from a diagnostic model constrained by geostrophic dynamics, conservation of mass, and no-flux boundary conditions.

The effect of advection on the nutrient reservoir in the North Atlantic subtropical gyre

Though critically important in sustaining the oceans biological pump, the cycling of nutrients in the subtropical gyres is poorly understood. The supply of nutrients to the sunlit surface layer of the ocean has traditionally been attributed solely to vertical processes. However, horizontal advection may also be important in establishing the availability of nutrients. Here we show that the production and advection of North Atlantic Subtropical Mode Water introduces spatial and temporal variability in the subsurface nutrient reservoir beneath the North Atlantic subtropical gyre. As the mode water is formed, its nutrients are depleted by biological utilization. When the depleted water mass is exported to the gyre, it injects a wedge of low-nutrient water into the upper layers of the ocean. Contrary to intuition, cold winters that promote deep convective mixing and vigorous mode water formation may diminish downstream primary productivity by altering the subsurface delivery of nutrients.

The Spatial Pattern and Mechanisms of Heat-Content Change in the North Atlantic

The total heat gained by the North Atlantic Ocean over the past 50 years is equivalent to a basinwide increase in the flux of heat across the ocean surface of 0.4 ± 0.05 watts per square meter. We show, however, that this basin has not warmed uniformly: Although the tropics and subtropics have warmed, the subpolar ocean has cooled. These regional differences require local surface heat flux changes (±4 watts per square meter) much larger than the basinwide average. Model investigations show that these regional differences can be explained by large-scale, decadal variability in wind and buoyancy forcing as measured by the North Atlantic Oscillation index. Whether the overall heat gain is due to anthropogenic warming is difficult to confirm because strong natural variability in this ocean basin is potentially masking such input at the present time.

Deconstructing the Conveyor Belt

For the past several decades, oceanographers have embraced the dominant paradigm that the ocean’s meridional overturning circulation operates like a conveyor belt, transporting cold waters equatorward at depth and warm waters poleward at the surface. Within this paradigm, the conveyor, driven by changes in deepwater production at high latitudes, moves deep waters and their attendant properties continuously along western boundary currents and returns surface waters unimpeded to deepwater formation sites. A number of studies conducted over the past few years have challenged this paradigm by revealing the vital role of the ocean’s eddy and wind fields in establishing the structure and variability of the ocean’s overturning. Here, we review those studies and discuss how they have collectively changed our view of the simple conveyor-belt model.

Anomalous Anomalies in Averaged Hydrographic Data

Abstract A comparison of a recently assembled hydrographic database for the North Atlantic with the Lovitus atlas shows striking differences in the vicinity of the Gulf Stream and the North Atlantic Current. On isopycnal surfaces in the main thermocline, isolated pools of warm, saline water are found in the Levitus database but are absent in the new database. Using synoptic data as a proxy for temporally averaged climatological data, it is shown that the anomalous features can be accounted for by the differences in the averaging process. To produce a gridded database from irregularly spaced station data, Levitus averaged the data on pressure surfaces while the new database was prepared with averaging an potential density surfaces. It is shown that averaging on a pressure surface in an area of sharply sloping isopycnals produces a water mass with a θ–S signature uncharacteristic of the local water mass(es). The anomalous potential temperatures and salinities that result are compared to the large-scale wate...

On the Temporally Varying Northward Penetration of Mediterranean Overflow Water and Eastward Penetration of Labrador Sea Water

Abstract Historical hydrographic data in the eastern North Atlantic are used to suggest a connection between the northward penetration of Mediterranean Overflow Water (MOW) and the location of the subpolar front, the latter of which is shown to vary with the North Atlantic Oscillation (NAO). During persistent high-NAO periods, when the subpolar front moves eastward, waters in the subpolar gyre essentially block the northward-flowing MOW, preventing its entry into the subpolar gyre. Conversely, during low NAO periods, the subpolar front moves westward, allowing MOW to penetrate past Porcupine Bank into the subpolar gyre. The impacts of an intermittent penetration of MOW into the subpolar gyre, including the possible effect on water mass transformations, remain to be investigated.

Signatures of the Mediterranean outflow from a North Atlantic climatology: 2. Diagnostic velocity fields

Part 1 of this study is a descriptive analysis of the spreading of Mediterranean Water based on high-resolution maps of salinity and density in the eastern basin. In this second part of our study, velocity fields for two representative isopycnal surfaces of the Mediterranean outflow (o-0. s = 29.70 and o-0. s = 29.90) are estimated from a diagnostic model that combines climatological hydrographic data from the National Oceanic Data Center with long-term direct measurements of water exchange through the Strait of Gibraltar. The model is constrained by geostrophic dynamics, conservation of mass, no- flux conditions at the continental shelf, and specified flow through the Strait of Gibraltar. Our principal data source is a recently assembled database of the North Atlantic that consists of climatological mean property fields averaged on isopycnal surfaces. The mean fields are based on more than 80 years (1909-1990) of data and have a nominal horizontal resolution of 0.5 o . To provide boundary conditions at the Strait of Gibraltar, we use the results of a model developed from data collected during the Gibraltar Experiment in 1985. The estimated velocity fields show Mediterranean Water exiting the Strait of Gibraltar, following the southern Iberian coast, and then entering the Tagus Basin, where it turns anticyclonically to create a reservoir of this water mass. The flow continues northward along the eastern boundary, penetrating into the Rockall Channel. Finally, the model flow fields do not show a significant westward advection of Mediterranean waters into the subtropical gyre.

On the warming and salinification of the Mediterranean outflow waters in the North Atlantic

[i] Based on hydrographic data collected since 1955, Mediterranean outflow waters present in the eastern North Atlantic show a density compensated increase in both temperature and salinity. With an increase in temperature of 0.101 ± 0.024°C/decade and an increase in salinity of 0.0283 ± 0.0067 psu/decade in the Mediterranean outflow waters, the heat content gain from 1955 to 1993 is calculated to be 2.02 ± 0.31 x 10 6 J/m 3 , which surpasses the average gain in heat content of the North Atlantic basin over the latter half of the 20th century. This suggests that the Mediterranean outflow waters are an important contributor to climatological changes at intermediate depths within the mid-latitude North Atlantic.

Physical controls on the seasonal migration of the North Pacific transition zone chlorophyll front

[1] The large seasonal migration of the transition zone chlorophyll front (TZCF) is of interest because a number of marine fauna, both commercial and endangered, appear to track it. Herein we examine the physical dynamics driving this seasonal migration of the TZCF. Vertical processes, traditionally viewed as controlling the dynamical supply of nutrients to surface waters, prove insufficient to explain seasonal variations in nutrient supply to the transition zone. Instead, we find that the horizontal Ekman transport of nutrients from higher latitudes drives the TZCF’s southward migration. The estimated horizontal transport of nitrate supports up to 40% of new primary productivity in the region annually and nearly all of new primary productivity in the winter. The significance of horizontal advection to the North Pacific transition zone supports revising the paradigm that nutrients are supplied to surface waters from below.