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Rick Lumpkin
and
Kevin Speer

Abstract

A decade-mean global ocean circulation is estimated using inverse techniques, incorporating air–sea fluxes of heat and freshwater, recent hydrographic sections, and direct current measurements. This information is used to determine mass, heat, freshwater, and other chemical transports, and to constrain boundary currents and dense overflows. The 18 boxes defined by these sections are divided into 45 isopycnal (neutral density) layers. Diapycnal transfers within the boxes are allowed, representing advective fluxes and mixing processes. Air–sea fluxes at the surface produce transfers between outcropping layers. The model obtains a global overturning circulation consistent with the various observations, revealing two global-scale meridional circulation cells: an upper cell, with sinking in the Arctic and subarctic regions and upwelling in the Southern Ocean, and a lower cell, with sinking around the Antarctic continent and abyssal upwelling mainly below the crests of the major bathymetric ridges.

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Rick Lumpkin
and
Kevin Speer

Abstract

Observations of large-scale hydrography, air–sea forcing, and regional circulation from numerous studies are combined by inverse methods to determine the basin-scale circulation, average diapycnal mixing, and adjustments to air–sea forcing of the North Atlantic Ocean. Dense overflows through the Denmark Strait and Faroe Bank channels are explicitly included and are associated with strong vertical and lateral circulation and mixing. These processes in the far northern Atlantic play a fundamental role in the meridional overturning circulation for the entire ocean, accompanied by an upper cell of mode-water and intermediate-water circulation. The two cells converge roughly at the mean depth of the midocean ridge crest. The Labrador Sea Water layer lies within this convergence. South of the overflow region, model-derived mean diapycnal diffusivities are O(10−5 m2 s−1) or smaller at the base of the thermocline, and diapycnal advection is driven primarily by air–sea transformation on outcropping layers.

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Rick Lumpkin
,
Anne-Marie Treguier
, and
Kevin Speer

Abstract

Eddy time and length scales are calculated from surface drifter and subsurface float observations in the northern Atlantic Ocean. Outside the energetic Gulf Stream, subsurface timescales are relatively constant at depths from 700 m to 2000 m. Length scale and the characteristic eddy speed decrease with increasing depth below 700 m, but length scale stays relatively constant in the upper several hundred meters of the Gulf Stream. It is suggested that this behavior is due to the Lagrangian sampling of the mesoscale field, in limits set by the Eulerian eddy scales and the eddy kinetic energy. In high-energy regions of the surface and near-surface North Atlantic, the eddy field is in the “frozen field” Lagrangian sampling regime for which the Lagrangian and Eulerian length scales are proportional. However, throughout much of the deep ocean interior, the eddy field may be in the “fixed float” regime for which the Lagrangian and Eulerian timescales are nearly equal. This does not necessarily imply that the deep interior is nearly linear, as fixed-float sampling is possible in a flow field of O(1) nonlinearity.

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Rick Lumpkin
,
Pierre Flament
,
Rudolf Kloosterziel
, and
Laurence Armi

Abstract

Mass, angular momentum, and energy budgets are examined in an analytical model of vortex merging relevant to midlatitude mesoscale eddies. The vortices are baroclinic and cyclogeostrophic. The fluid surrounding them is assumed to remain quiescent. It is shown that due to this surrounding fluid, angular momentum is conserved when expressed in both the inertial and rotating frames of reference.

Lens-shaped solid-body vortices can conserve mass, angular momentum, and energy when they merge. If an upper-layer of thickness H 1 is included in the model, the merged vortex must have either less energy or mass than the sum of the original two vortices.

A more complex model of the vortex azimuthal structure is then considered, which includes a constant vorticity shell surrounding the solid-body core. If the shell is large compared to the core, the mass, angular momentum, and energy can all be conserved in the merged vortex. However, if the shell is small, the merged vortex must have less energy or mass than in the solid-body case.

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Gregory R. Foltz
,
Claudia Schmid
, and
Rick Lumpkin

Abstract

The transport of low-salinity water northward in the tropical and subtropical North Atlantic Ocean influences upper-ocean stratification, vertical mixing, and sea surface temperature (SST). In this study, satellite and in situ observations are used to trace low-salinity water northward from its source in the equatorial Atlantic and to examine its modification through air–sea fluxes and vertical mixing. In contrast to gridded climatologies, which depict a gradual northward dispersal of surface freshwater from the equatorial Atlantic, satellite observations and direct measurements from four moorings in the central tropical North Atlantic show a distinct band of surface freshwater moving northward from the equatorial Atlantic during boreal fall through spring, with drops in sea surface salinity (SSS) of 0.5–2.5 psu in the span of one to two weeks as the low SSS front passes. The ultimate low-latitude source of the low SSS water is found to be primarily Amazon River discharge west of 40°W and rainfall to the east. As the low-salinity water moves northward between 8° and 20°N during October–April, 70% of its freshwater in the upper 20 m is lost to the combination of evaporation, horizontal eddy diffusion, and vertical turbulent mixing, with an implied rate of SSS damping that is half of that for SST. During 1998–2012, interannual variations in SSS along 38°W are found to be negatively correlated with the strength of northward surface currents. The importance of ocean circulation for interannual variations of SSS and the small damping time scale for SSS emphasize the need to consider meridional freshwater advection when interpreting SSS variability in the tropical–subtropical North Atlantic.

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Rick Lumpkin
,
Kevin G. Speer
, and
K. Peter Koltermann

Abstract

Transports across 48°N in the Atlantic Ocean are estimated from five repeat World Ocean Circulation Experiment (WOCE) hydrographic lines collected in this region in 1993–2000, from time-varying air–sea heat and freshwater fluxes north of 48°N, and from a synthesis of these two data sources using inverse box model methods.

Results from hydrography and air–sea fluxes treated separately are analogous to recently published transport variation studies and demonstrate the sensitivity of the results to either the choice of reference level and reference velocities for thermal wind calculations or the specific flux dataset chosen. In addition, flux-based calculations do not include the effects of subsurface mixing on overturning and transports of specific water masses. The inverse model approach was used to find unknown depth-independent velocities, interior diapycnal fluxes, and adjustments to air–sea fluxes subject to various constraints on the system. Various model choices were made to focus on annually averaged results, as opposed to instantaneous values during the occupation of the hydrographic lines. The results reflect the constraints and choices made in the construction of the model.

The inverse model solutions show only marginal, not significantly different temporal changes in the net overturning cell strength and heat transport across 48°N. These small changes are similar to seasonally or annually averaged numerical model simulations of overturning. Significant variability is found for deep transports and air–sea flux quantities in density layers. Put another way, if one ignores the details of layer exchanges, the model can be constrained to produce the same net overturning for each repeat line; however, constraining individual layers to have the same transport for each line fails.

Diapycnal fluxes are found to be important in the mean but are relatively constant from one repeat line to the next. Mean air–sea fluxes are modified slightly but are still essentially consistent with either the NCEP data or the National Oceanography Centre, Southampton (NOC) Comprehensive Ocean–Atmosphere Data Set (COADS) within error. Modest reductions in air–sea flux uncertainties would give these constraints a much greater impact. Direct transport estimates over broader regions than the western boundary North Atlantic Current are needed to help resolve circulation structure that is important for variability in net overturning.

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Yu-Kun Qian
,
Shiqiu Peng
,
Chang-Xia Liang
, and
Rick Lumpkin

Abstract

Eddy–mean flow decomposition is crucial to the estimation of Lagrangian diffusivity based on drifter data. Previous studies have shown that inhomogeneous mean flow induces shear dispersion that increases the estimated diffusivity with time. In the present study, the influences of nonstationary mean flows on the estimation of Lagrangian diffusivity, especially the asymptotic behavior, are investigated using a first-order stochastic model, with both idealized and satellite-based oceanic mean flows. Results from both experiments show that, in addition to inhomogeneity, nonstationarity of mean flows that contain slowly varying signals, such as a seasonal cycle, also cause large biases in the estimates of diffusivity within a time lag of 2 months if a traditional binning method is used. Therefore, when assessing Lagrangian diffusivity over regions where a seasonal cycle is significant [e.g., the Indian Ocean (IO) dominated by monsoon winds], inhomogeneity and nonstationarity of the mean flow should be simultaneously taken into account in eddy–mean flow decomposition. A temporally and spatially continuous fit through the Gauss–Markov (GM) estimator turns out to be very efficient in isolating the effects of inhomogeneity and nonstationarity of the mean flow, resulting in estimates that are closest to the true diffusivity, especially in regions where strong seasonal cycles exist such as the eastern coast of Somalia and the equatorial IO.

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Shiqiu Peng
,
Yu-Kun Qian
,
Rick Lumpkin
,
Ping Li
,
Dongxiao Wang
, and
Yan Du

Abstract

Lagrangian statistics of the surface circulation in the Indian Ocean (IO) are investigated using drifter observations during 1985–2013. The methodology isolates the influence of low-frequency variations and horizontal shear of mean flow. The estimated Lagrangian statistics are spatially inhomogeneous and anisotropic over the IO basin, with values of ~6–85 × 107 cm2 s−1 for diffusivity, ~2–7 days for integral time scale, and ~33–223 km for length scale. Large diffusivities (>20 × 107 cm2 s−1) occur in the central-eastern equatorial IO and the eastern African coast. Small diffusivities (~6–8 × 107 cm2 s−1) appear in the subtropical gyre of the southern IO and the southeastern Arabian Sea. The equatorial IO has the largest zonal diffusivity (~85 × 107 cm2 s−1), corresponding to the largest time scale (~7 days) and length scale (~223 km), while the eastern coast of Somalia has the largest meridional diffusivity (~31 × 107 cm2 s−1). The minor component of the Lagrangian length scale is approximately equal to the first baroclinic Rossby radius (R 1) at midlatitudes (R 1 ~ 30–50 km), while the major component equals R 1 in the equatorial region (R 1 > 80 km). The periods of the energetic eddy-containing bands in the IO in Lagrangian spectra range from several days to a couple of months, where anticyclones dominate. A significant result is that the drifter-derived diffusivities asymptote to constant values in relatively short time lags (~10 days) for some subregions of the IO if they are correctly calculated. This is an important contribution to the ongoing debate regarding drifter-based diffusivity estimates with relatively short Lagrangian velocity time series versus tracer-based estimates.

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Shiqiu Peng
,
Yu-Kun Qian
,
Rick Lumpkin
,
Yan Du
,
Dongxiao Wang
, and
Ping Li

Abstract

Using the 1985–2013 record of near-surface currents from satellite-tracked drifters, the pseudo-Eulerian statistics of the near-surface circulation in the Indian Ocean (IO) are analyzed. It is found that the distributions of the current velocities and mean kinetic energy (MKE) in the IO are extremely inhomogeneous in space and nonstationary in time. The most energetic regions with climatologic mean velocity over 50 cm s−1 and MKE over 500 cm2 s−2 are found off the eastern coast of Somalia (with maxima of over 100 cm s−1 and 1500 cm2 s−2) and the equatorial IO, associated with the strong, annually reversing Somalia Current and the twice-a-year eastward equatorial jets. High eddy kinetic energy (EKE) is found in regions of the equatorial IO, western boundary currents, and Agulhas Return Current, with a maximum of over 3000 cm2 s−2 off the eastern coast of Somalia. The lowest EKE (<500 cm2 s−2) occurs in the south subtropical gyre between 30° and 40°S and the central-eastern Arabian Sea. Annual and semiannual variability is a significant fraction of the total EKE off the eastern coast of Somalia and in the central-eastern equatorial IO. In general, both the MKE and EKE estimated in the present study are qualitatively in agreement with, but quantitatively larger than, estimates from previous studies. These pseudo-Eulerian MKE and EKE fields, based on the most extensive drifter dataset to date, are the most precise in situ estimates to date and can be used to validate satellite and numerical results.

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Peter Brandt
,
Verena Hormann
,
Arne Körtzinger
,
Martin Visbeck
,
Gerd Krahmann
,
Lothar Stramma
,
Rick Lumpkin
, and
Claudia Schmid

Abstract

Changes in the ventilation of the oxygen minimum zone (OMZ) of the tropical North Atlantic are studied using oceanographic data from 18 research cruises carried out between 28.5° and 23°W during 1999–2008 as well as historical data referring to the period 1972–85. In the core of the OMZ at about 400-m depth, a highly significant oxygen decrease of about 15 μmol kg−1 is found between the two periods. During the same time interval, the salinity at the oxygen minimum increased by about 0.1. Above the core of the OMZ, within the central water layer, oxygen decreased too, but salinity changed only slightly or even decreased. The scatter in the local oxygen–salinity relations decreased from the earlier to the later period suggesting a reduced filamentation due to mesoscale eddies and/or zonal jets acting on the background gradients. Here it is suggested that latitudinally alternating zonal jets with observed amplitudes of a few centimeters per second in the depth range of the OMZ contribute to the ventilation of the OMZ. A conceptual model of the ventilation of the OMZ is used to corroborate the hypothesis that changes in the strength of zonal jets affect mean oxygen levels in the OMZ. According to the model, a weakening of zonal jets, which is in general agreement with observed hydrographic evidences, is associated with a reduction of the mean oxygen levels that could significantly contribute to the observed deoxygenation of the North Atlantic OMZ.

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