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Kevin G. Speer and Michael S. McCartney

Abstract

Antarctic Bottom Water flows into the western North Atlantic across the equator, shifting from the western side to the eastern side of the trough between the American continents and the Mid-Atlantic Ridge as it continues north. This is puzzling because such large-scale motion is thought to be controlled by dynamics that disallows an eastern boundary current. Previous explanations for the transposition involve a (necessarily small-scale) density current that changes sides because of the change in sign of rotation across the equator, or a topographic effect that changes the sign of the effective mean vorticity gradient and thus requires an eastern boundary current. Here an alternative explanation for the overall structure of bottom flow is given.

A source of mass to a thin bottom layer is assumed to upwell uniformly across its interface into a less dense layer at rest. A simple formula for the magnitude of the upwelling and thickness of the layer is derived that depends on the source strength to the bottom layer. For a strong enough source, the bottom layer thickness is zero along a grounding curve that separates the bottom water from the western boundary and confines it to the east. A band of recirculating interior flow occurs, supplied by an isolated northern and western boundary current. Similar structures appear to exist in the Antarctic Bottom Water of the western North Atlantic.

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Michael S. McCartney and Lynne D. Talley

Abstract

The warm waters of the subtropical and subpolar basins of the North Atlantic have tight regional temperature-salinity relationships, and are conventionally called the regional “Central Waters.” A volumetric census of the temperature-salinity characteristics of the North Atlantic by Wright and Worthington (1970) shows that waters characterized by certain segments of the T-S relationships have large volumes compared to those of other segments: volumetric “Mode Waters.” Such Mode Waters appear as layers with increased vertical separation between isopycnals-pycnostads. The present study reports on the existence of pycnostads in the central and eastern North Atlantic. These Subpolar Mode Waters are formed by deep winter convection in the subpolar North Atlantic, and participate in the upper water circulation of the northern North Atlantic. The seasonal outcropping of the pycnostads occurs within and adjacent to the North Atlantic Current, the Irminger Current, the East and West Greenland Currents, and the Labrador Current. The warmer pycnostads (10°C≲T≲15°C) recirculate in an anticyclonic subtropical gyre east and south of the North Atlantic Current, causing volumetric modes in the central and eastern subtropical North Atlantic. A branch of the North Atlantic Current carries somewhat heavier and cooler (8°CT≲10°C) pycnostads northward past Ireland. The bulk of the current turns westward, but one branch continues northward, providing a warm core to the Norwegian Current (8°C). Within the main westward flow the density continues to increase and temperature to decrease. Southeast of Iceland pycnostad temperatures are near 8°C. Following the cyclonic circulation around the Irminger Sea west of the Reykjanes Ridge the temperature drops to less than 5°C. The cyclonic flow around the Labrador Sea gives a final pycnostad temperature below 3.5°C. The last, coldest, densest pycnostad is the Labrador Sea Water which influences lower latitudes via the southward flowing, Deep Western Boundary Current along the western boundary, and via eastward flow at mid-depth in the North Atlantic Current (Talley and McCartney, 1982).

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Ruth G. Curry and Michael S. McCartney

Abstract

Observational evidence is presented for interannual to interdecadal variability in the intensity of the North Atlantic gyre circulation related to the atmospheric North Atlantic Oscillation (NAO) patterns. A two-point baroclinic pressure difference between the subtropical and subpolar gyre centers—an oceanic analogue to the much-used sea level pressure (SLP)-based atmospheric NAO indices—is constructed from time series of potential energy anomaly (PEA) derived from hydrographic measurements in the Labrador Basin and at Station S near Bermuda. Representing the upper 2000-db eastward baroclinic mass transport between the two centers, the transport index indicates a Gulf Stream and North Atlantic Current that gradually weakened during the low NAO period of the 1960s and then intensified in the subsequent 25 years of persistently high NAO to a record peak in the 1990s. The peak-to-peak amplitude difference was 15–20 megatons per second (MT s−1) with a 43-yr mean of about 60 MT s−1 a change of 25%–33% occurring between 1970 and 1995. The timing of the ocean fluctuation is organized around the same temporal structure as the NAO index. The two are not directly covariant, but to first order, the ocean signal reflects a time integration, through mixed layer “memory” and Rossby wave propagation, of the atmospheric forcing.

To some degree, the gyre PEA histories are fluctuating in antiphase reflecting latitudinal shifts of the surface westerlies across the North Atlantic. Differences in forcing mechanisms and baroclinic responses in each gyre, however, are reflected by divergences in the details of their PEA histories. The subpolar PEA changes are primarily thermally driven through diabatic mixing and surface buoyancy fluxes associated with water mass transformation. Salinity changes, stemming from the occasional passages of low-salinity surface lids (“Great Salinity Anomalies”) through the region, contribute relatively little to the Labrador Basin PEA variability. The interior subtropical gyre PEA history is dominated by quasi-adiabatic vertical displacements of the main pycnocline, and supplemented by changes in the locally formed subtropical mode water as well as by changes in middepth density structure related to advective–diffusive import of Labrador Sea Water.

Multiyear composite fields of North Atlantic potential energy centered in time on the extreme high and low transport periods provide a broad geographic context for the transport index. Basin-scale shifts of oceanic baroclinic pressure gradients between the extreme phases reinforce the sense and amplitude of changes reflected in the Bermuda–Labrador Basin transport index.

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M. Susan Lozier, Michael S. McCartney, and W. Brechner Owens

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 water mass anomalies of the North Atlantic and are shown to be of comparable strength. Finally, the consequences of having sizable averaging artifacts are discussed.

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