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Harry L. Bryden and Nick P. Fofonoff

Abstract

Estimates of horizontal derivatives of velocity made by differencing velocity measurements are used to show that the observed velocity field due to low-frequency mesoscale motions during the preliminary Mid-Ocean Dynamics Experiment (MODE-0) field program is horizontally nondivergent within estimated errors. The errors in horizontal derivatives of 0.15×10−6 s−1 art are too large for direct estimates of horizontal divergence to be made accurately. The vorticity, however, can be estimated from these horizontal derivatives with an error small compared with its magnitude. Over the measurement period of 50 days, the advection of planetary vorticity balances only one-half of the local change of vorticity so these observations cannot be explained in terms of barotropic Rossby waves alone. There are indications that vortex stretching, estimated from a linear heat balance, may balance the remaining local change of vorticity as expected for baroclinic Rossby waves. Based on other measurements in this regions, however, it is likely that the horizontal advection of relative vorticity is also important in the vorticity balance. A positive, but not significantly different from zero, correlation between estimates of relative vorticity and advection of planetary vorticity suggests that the enstrophy of the observed velocity field is decreasing with time. In conjunction with a similar result for the perturbation potential energy obtained in this region, this result supports the view that the MODE region is a region of decay, rather than growth, of the low-frequency mesoscale motions.

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Harry L. Bryden and R. Dale Pillsbury

Abstract

To investigate the reasons for the wide variation in previous estimates of transport of the Antarctic Circumpolar Current through the Drake Passage, an analysis of the spatial and temporal variability of currents at 2700 m depth is made from year-long current measurements on six moorings in the Drake Passage. The currents are found to vary over time scales of about two weeks and over spatial scales shorter than 80 km. An average of the six down-channel velocity components is used to estimate the spatially averaged down-channel velocity, or mean flow, at 2700 m. This mean flow varies from 7.6 to–2.9 cm s−1 and has a root-mean-square (rms) amplitude of 2.0 cm s−1 about its time-averaged value. Provided the geostrophic transport relative to 2700 m depth remains constant in time, these variations may be interpreted as temporal variations of 2 60×106 m3 s−1 in total transport with an rms amplitude of 50×106 m3 s−1. The wide variation in previous estimates of transport from short-term measurements can be understood in terms of this observed variation in mean flow. The time-averaged mean flow at 2700 m depth is estimated to be 1.56±1.44 cm s−1 which implies that a transport of 39±36×106 m3 s−1 should be added to the geostrophic transport of about 100×106 m3 s−1 relative to 2700 m to obtain an estimate of the time-averaged total transport through the Drake Passage.

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Harry L. Bryden and Esther C. Brady

Abstract

To investigate the processes that maintain the large-scale, annual-average thermal structure of the equatorial Pacific, the three-dimensional ocean circulation for a large area is determined from a diagnostic model applied to repeated, meridional hydrographic sections along 150°W and 11O°W from 5°N to 5°S. Geostrophic balances are used to determine velocity profiles from 0 to 500 db across the boundaries of the region: zonal velocities across 150°W and 110°W at approximately 1° -lattitude intervals from 5°N to 5°S and meridional velocities across 5°N and 5°S averaged over the zonal distance between 150°W and 110°W. Poleward wind-driven flows across 5°N and 5°S based on climatological zonal wind stress are added to the geostrophic velocities in the mixed layers. To achieve overall mass conservation, the reference dynamic height field at 500 db is adjusted at four of the 21 stations by about 1 dyn cm. Horizontal nondivergence is used to determine meridional velocities between 0.75° and 5° latitudes. Three-dimensional nondivergence is used between 0.75°N and 0.75°S to determine a vertical profile of vertical velocity at the equator. The resulting model circulation, which is generally consistent with previous interpretations, is then analyzed to estimate the heat budget for the region and the zonal momentum balance at the equator.

The model circulation requires an annual-average hem gain from the atmosphere of 57 W m−2, which is consistent with existing estimates of air-sea heat exchange from bulk formula. The beat gain converts about 35 × 106 m3 s−1 of water flowing into the region with temperatures between 19 and 26°C into an equal amount of 27 to 28°C water flowing out of the region. Little of the heat gain warms the locally upwelled waters at the equator, however, rather, about half acts to increase the temperature of the westward flowing South Equatorial Current as it traverses the region and about half warms the poleward flow of water away from the equator. There is large upwelling at the equator extending down to 180 db with a maximum upward velocity of 2.9 × 10−3 cm s−1 and upward transport of 22 × 106 m3 2−1 across the 62.5 db surface. Because this upwelling occurs in conjunction with the eastward flow of the Equatorial Undercurrent which shallows to the east, the flow is predominantly along isotherms and the maximum cross-isotherm transport is only 7 × 106 m3 2−1 across the 23°C isotherm. Thus, the eastward and upward flow acts to decrease the surface water temperature to the east. In combination with the atmospheric warming of the poleward surface flow away from the equator, this eastward and upward flow along isotherms creates the Cold Tongue in the equatorial Pacific, which is characterized by minimum surface temperature at the equator in any meridional section and colder surface waters to the east.

For the zonal momentum balance at the equator, the vertically integrated zonal pressure gradient balances about 80 percent of the climatological westward wind stress. Eastward and vertical advection of zonal momentum each acts to balance about 20 percent of the wind stress. The sum of eastward and vertical advection indicates a deceleration of the eastward flow at all depths above 300 db. The inferred eastward stress profile suggests that eddy mixing of zonal momentum extends down to at least 200 m depth on the equator, well below the core of the Undercurrent.

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Ellen Brown, W. Brechner Owens, and Harry L. Bryden

Abstract

The local effect of the mesoscale eddy field on the mean potential vorticity distribution of the Gulf Stream recirculation region is determined from the quasi-geostrophic eddy potential vorticity flux. This flux is calculated by finite difference of current and temperature time series from the Local Dynamics Experiment. This long-term array of moorings is the only experimental data from which the complete eddy flux can be calculated. The total eddy flux is dominated by the term due to the time variation in the thickness of isopycnal layers. This thickness flux is an order of magnitude larger than the relative vorticity flux. The total flux is statistically significant and directed 217°T to the southwest with a magnitude of 1.57 × 10−5 cm s−1. The direction of the eddy flux with respect to the mean large-scale potential vorticity gradient from hydrographic data indicates that eddies in this region tend to reduce the mean potential vorticity gradient.

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Elaine L. McDonagh, Paula McLeod, Brian A. King, Harry L. Bryden, and Sinhué Torres Valdés

Abstract

In May and June 2005, a transatlantic hydrographic section along 36°N was occupied. A velocity field is calculated using inverse methods. The derived 36°N circulation has an overturning transport (maximum in the overturning streamfunction) of 16.6 Sv (1 Sv ≡ 106 m3 s−1) at 1070 m. The heat transport across the section, 1.14 ± 0.12 PW, is partitioned into overturning and horizontal heat transports of 0.75 and 0.39 PW, respectively. The horizontal heat flux is set by variability at the gyre rather than by mesoscale. The freshwater flux across the section is 1.55 ± 0.18 Sv southward based on a 0.8-Sv flow from the Pacific through the Bering Strait at a salinity of 32.5 psu. The oceanic divergence of freshwater implies a net input of freshwater to the ocean of 0.75 Sv over the North Atlantic and Arctic between 36°N and the Bering Strait. Most (85%) of the recently ventilated upper North Atlantic Deep Water (water originating in the Labrador Sea) transport across the section occurs in the deep western boundary current rather than being associated with an interior pathway to the west of the mid-Atlantic ridge.

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Susan E. Wijffels, Raymond W. Schmitt, Harry L. Bryden, and Anders Stigebrandt

Abstract

The global distribution of freshwater transport in the ocean is presented, based on an integration point at Bering Strait, which connects the Pacific and Atlantic oceans via the Artic Ocean. Through Bering Strait, 0.8 × 106 m3 s−1 of relatively fresh, 32.5 psu, water flows from the Pacific into the Arctic Ocean. Baumgrtner and Reichel's tabulation of the act gain of freshwater by the ocean in 5&deg latitude intervals is then integrated from the reference location at Bering Strait to yield the meridional freshwater transport in each ocean. Freshwater transport in the Pacific is directed northward at nearly all latitudes. In the Atlantic, the freshwater transport is directed southward at all latitudes, with a small southward freshwater transport out of the Atlantic across 35°S. Salt transport, which must be considered jointly with the freshwater transport, is northward throughout the Pacific and southward throughout the Atlantic (in the same direction as the freshwater flux) and is equal to the salt transport through the Bering Strait. The circulation around Australasia associated with the poorly known Pacific-Indian throughflow modifies the above scenario only in the South Pacific and Indian oceans. A moderate choice for the throughflow indicates that it dominates the absolute meridional fluxes of freshwater and salt in these oceans. The global freshwater scheme presented here differs markedly from earlier interpretations and suggests the need for a careful assessment of the treatment of ocean freshwater and salt transports in inverse, numerical, and climate models.

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Elaine L. McDonagh, Harry L. Bryden, Brian A. King, Richard J. Sanders, Stuart A. Cunningham, and Robert Marsh

Abstract

A significant change in properties of the thermocline is observed across the whole Indian Ocean 32°S section between 1987 and 2002. This change represents a reversal of the pre-1987 freshening and decreasing oxygen concentrations of the upper thermocline that had been interpreted as a fingerprint of anthropogenic climate change. The thermocline at the western end of the section (40°–70°E) is occupied by a single variety of mode water with a potential temperature of around 13°C. The thermocline at the eastern end of the 32°S section is occupied by mode waters with a range of properties cooling from ∼11°C at 80°E to ∼9°C near the Australian coast. The change in θS properties between 1987 and 2002 is zonally coherent east of 80°E, with a maximum change on isopycnals at 11.6°C. Ages derived from helium–tritium data imply that the mode waters at all longitudes take about the same time to reach 32°S from their respective ventilation sites. Dissolved oxygen concentration changes imply that all of the mode water reached the section ∼20% faster in 2002 than in 1987.

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Harry L. Bryden, William E. Johns, Brian A. King, Gerard McCarthy, Elaine L. McDonagh, Ben I. Moat, and David A. Smeed

Abstract

Northward ocean heat transport at 26°N in the Atlantic Ocean has been measured since 2004. The ocean heat transport is large—approximately 1.25 PW, and on interannual time scales it exhibits surprisingly large temporal variability. There has been a long-term reduction in ocean heat transport of 0.17 PW from 1.32 PW before 2009 to 1.15 PW after 2009 (2009–16) on an annual average basis associated with a 2.5-Sv (1 Sv ≡ 106 m3 s−1) drop in the Atlantic meridional overturning circulation (AMOC). The reduction in the AMOC has cooled and freshened the upper ocean north of 26°N over an area following the offshore edge of the Gulf Stream/North Atlantic Current from the Bahamas to Iceland. Cooling peaks south of Iceland where surface temperatures are as much as 2°C cooler in 2016 than they were in 2008. Heat uptake by the atmosphere appears to have been affected particularly along the path of the North Atlantic Current. For the reduction in ocean heat transport, changes in ocean heat content account for about one-quarter of the long-term reduction in ocean heat transport while reduced heat uptake by the atmosphere appears to account for the remainder of the change in ocean heat transport.

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Harry L. Bryden, Michael J. Griffiths, Alicia M. Lavin, Robert C. Millard, Gregorio Parrilla, and William M. Smethie

Abstract

Three transatlantic hydrographic sections across 24°N through the center of the subtropical gyre are analyzed for evidence of decadal changes in water mass characteristics. In the main thermocline, there has been a steady increase in salinity from 1957 to 1981 to 1992. The salinity at a given potential temperature between 12°C and 17°C has increased at a nearly constant rate of 0.010 per decade; at constant potential density, salinity and temperature both increased at rates of 0.018 per decade and 0.065°C per decade. In the intermediate waters between 800 and 1500 dbar, salinity has increased as well, on average by as much as 0.025 from 1981 to 1992, either on constant potential temperature or constant density surfaces. These changes cannot be due to vertical heaving of a constant water mass structure and they cannot be easily explained in terms of horizontal shifts in the subtropical gyre water mass structure. Throughout the water column down to 2000 dbar and across the zonal extent of the 24°N section, salinity and temperature have broadly increased on density surfaces. Below 2000 dbar, there has been slight cooling and freshening on isopycnal surfaces since 1981.

Decadal changes in temperature and salinity from 1981 to 1992 are separated into a component due to vertical motion of the isopycnals and a component due to the changes in temperature and salinity on isopycnals. The remarkable interior warming at constant depth with a maximum increase in temperature at 1100 dbar of nearly 1°C per century reported from earlier analyses of these sections is due to both processes. From 1957 to 1981, the warming was principally due to downward displacement of isopycnals and isotherms by order 50 dbar with little change in water mass characteristics. From 1981 to 1992 there were only small changes in isopycnal depths of order 10 dbar in the thermocline and intermediate waters, however. Thus, the continued warming from 1981 to 1992 has principally been due to changes in water mass characteristics, higher salinities and higher temperatures on isopycnal surfaces. Because of possible problems with 1957 salinity determinations, the cause of the cooling of the deep waters from 1957 to 1981 is ambiguous. Since 1981, the continued cooling of the waters deeper than 2000 dbar has been due principally to changes in the water mass characteristics, with a slight cooling and freshening on isopycnal surfaces.

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Elaine L. McDonagh, Brian A. King, Harry L. Bryden, Peggy Courtois, Zoltan Szuts, Molly Baringer, Stuart A. Cunningham, Chris Atkinson, and Gerard McCarthy

Abstract

The first continuous estimates of freshwater flux across 26.5°N are calculated using observations from the RAPID–MOCHA–Western Boundary Time Series (WBTS) and Argo floats every 10 days between April 2004 and October 2012. The mean plus or minus the standard deviation of the freshwater flux (F W) is −1.17 ± 0.20 Sv (1 Sv ≡ 106 m3 s−1; negative flux is southward), implying a freshwater divergence of −0.37 ± 0.20 Sv between the Bering Strait and 26.5°N. This is in the sense of an input of 0.37 Sv of freshwater into the ocean, consistent with a region where precipitation dominates over evaporation. The sign and the variability of the freshwater divergence are dominated by the overturning component (−0.78 ± 0.21 Sv). The horizontal component of the freshwater divergence is smaller, associated with little variability and positive (0.35 ± 0.04 Sv). A linear relationship, describing 91% of the variance, exists between the strength of the meridional overturning circulation (MOC) and the freshwater flux (−0.37 − 0.047 Sv of F W per Sverdrups of MOC). The time series of the residual to this relationship shows a small (0.02 Sv in 8.5 yr) but detectable decrease in the freshwater flux (i.e., an increase in the southward freshwater flux) for a given MOC strength. Historical analyses of observations at 24.5°N are consistent with a more negative freshwater divergence from −0.03 to −0.37 Sv since 1974. This change is associated with an increased southward freshwater flux at this latitude due to an increase in the Florida Straits salinity (and therefore the northward salinity flux).

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