Abstract

The interocean exchange of water from the South Atlantic with the Pacific and Indian Oceans is examined using the output from the ocean general circulation model for the Earth Simulator (OFES) during the period 1980–2006. The main objective of this paper is to investigate the role of the interocean exchanges in the variability of the Atlantic meridional overturning circulation (AMOC) and its associated meridional heat transport (MHT) in the South Atlantic. The meridional heat transport from OFES shows a similar response to AMOC variations to that derived from observations: a 1 Sv (1 Sv ≡ 10⁶ m³ s⁻¹) increase in the AMOC strength would cause a 0.054 ± 0.003 PW increase in MHT at approximately 34°S. The main feature in the AMOC and MHT across 34°S is their increasing trends during the period 1980–93. Separating the transports into boundary currents and ocean interior regions indicates that the increase in transport comes from the ocean interior region, suggesting that it is important to monitor the ocean interior region to capture changes in the AMOC and MHT on decadal to longer time scales. The linear increase in the MHT from 1980 to 1993 is due to the increase in advective heat converged into the South Atlantic from the Pacific and Indian Oceans. Of the total increase in the heat convergence, about two-thirds is contributed by the Indian Ocean through the Agulhas Current system, suggesting that the warm-water route from the Indian Ocean plays a more important role in the northward-flowing water in the upper branch of the AMOC at 34°S during the study period.

Abstract

Hydrographic and current profiler data taken during the 1988 Gulf of Cadiz Expedition have been analyzed to diagnose the mixing, spreading, and descent of the Mediterranean outflow. The θ–S properties and the thickness and width of the outflow were similar to that seen in earlier surveys. The transport of pure Mediterranean Water (i.e., water with S ≥ 38.4 psu) was estimated to be about 0.4 × 10⁶ m³ s⁻¹, which is lower than historical estimates—most of which were indirect—but comparable to other recent estimates made from direct velocity observations.

The outflow transport estimated at the west end of the Strait of Gibraltar was about 0.7 × 10⁶ m³ s⁻¹ of mixed water, and the transport increased to about 1.9 × 10⁶ m³ s⁻¹ within the eastern Gulf of Cadiz. This increase in transport occurred by entrainment of fresher North Atlantic Central Water, and the salinity anomaly of the outflow was consequently reduced. The velocity-weighted salinity decreased to 36.7 psu within 60 km of the strait and decreased by about another 0.1 before the deeper portion of the outflow began to separate from of the bottom near Cape St. Vincent. Entrainment appears to have been correlated spatially with the initial descent of the continental slope and with the occurrence of bulk Froude numbers slightly greater than 1. In the western Gulf of Cadiz, where entrainment was much weaker, Froude numbers were consistently well below 1.

The outflow began in the eastern Strait of Gibraltar as a narrow (10 km wide) current having a very narrow range of θ–S properties. The outflow broadened as it descended the continental slope of the northern Gulf of Cadiz and reached a maximum width of 80 km in the western Gulf of Cadiz. The descent of the outflow was very asymmetric: The southern (offshore) edge of the outflow descended about 1000 m from Gibraltar to Cape St. Vincent, while the northern (onshore) edge of the outflow descended only a few hundred meters. The northern, onshore side thus remained considerably higher in the water column and thus entrained relatively warm North Atlantic Central Water. This caused the outflow to develop horizontal θ–S variability and, by about 140 km downstream, the across-stream variation in temperature on an isopycnal was more than 2°C.

Much of the volume transport in the western Gulf of Cadiz was contained within two preferred modes or cores. The deeper, offshore core had a central σ _θ = 27.8 kg m⁻³, and the shallower onshore core, which was still in contact with the bottom in the Gulf of Cadiz, had a central σ _θ = 27.5 kg m⁻³. These two cores develop as a result of the spreading and horizontally varying entrainment noted above, combined with topographic steering.

Abstract

Field data taken in the Gulf of Cadiz have been analyzed to describe some aspects of the momentum andenergy balance of the Mediterranean outflow. A crucial component of the momentum balance is the total stress(entrainment stress and bottom drag), which has been estimated from a form of the Bernoulli function evaluatedfrom density and current observations.

For the first 60 km west of the Camarinal Sill the outflow was confined within a narrow channel on thecontinental shelf. At about 70 km downstream the outflow crossed over the shelf–slope break and began todescend the continental slope. The buoyancy force increased substantially, and the outflow underwent a geostrophic adjustment, albeit one heavily influenced by mixing and dissipation. The current direction turned 90degrees to the right at a near-inertial rate. In this region, the estimated geostrophic velocity greatly underestimatedthe actual current, and the estimated curvature Rossby number was about 0.5. Current speeds were in excessof 1 m s⁻¹ and the total stress was as large as 5 Pa. The entrainment stress, estimated independently fromproperty fluxes, reached a maximum of about 1 Pa, or considerably smaller than the inferred bottom stress.

By about 130 km downstream, the current was aligned approximately along the local topography. The currentamplitude and the estimated stress were then much less, about 0.3 m s⁻¹ and 0.3 Pa. The entrainment stress wasalso very small in this region well downstream of the strait. This slightly damped geostrophic flow continuedon to Cape St. Vincent where the outflow began to separate from the bottom.

Bottom stress thus appears to be a crucial element in the dynamics of the Mediterranean outflow, allowingor causing the outflow to descend more than a kilometer into the North Atlantic. In the regions of strongestbottom stress the inferred drag coefficient was about 2 − 12 (× 10 ⁻³) depending upon which outflow speedis used in the usual quadratic form. Entrainment stress was small by comparison to the bottom stress, but theentrainment effect upon the density anomaly was crucial in eroding the density anomaly of the outflow. Theobserved entrainment rate appears to follow, roughly, a critical internal Froude number function.

Abstract

This work assesses the value of expendable bathythermograph (XBT) and Argo profiling float observations to monitor the Atlantic Ocean boundary current systems (BCS), meridional overturning circulation (MOC), and meridional heat transport (MHT). Data from six XBT transects and available Argo floats in the Atlantic Ocean for the period from 2000 to 2018 are used to estimate the structure and variability of the BCS, MOC, and MHT, taking into account different temporal and spatial mapping strategies. The comparison of Argo data density along these six XBT transects shows that Argo observations outnumber XBT observations only above mapping scales of 30 days and 3° boxes. The comparison of Argo and XBT data for the Brazil Current and Gulf Stream shows that Argo cannot reproduce the structure and variability of these currents, as it lacks sufficient resolution to resolve the gradients across these narrow jets. For the MHT and MOC, Argo estimates are similar to those produced by XBTs at a coarse mapping resolution of 5° and 30 days. However, at such a coarse resolution the root-mean-square errors calculated for both XBT and Argo estimates relative to a high-resolution baseline are higher than 3 Sv (1 Sv ≡ 10⁶ m³ s⁻¹) and 0.25 PW for the MOC and MHT, respectively, accounting for about 25%–30% of their mean values due to the smoothing of eddy variability along the transects. A key result of this study is that using Argo and XBT data jointly, rather than separately, improves the estimates of MHT, MOC, and BCS.

Abstract

Simultaneous temperature and salinity profile measurements are of extreme importance for research; operational oceanography; research and applications that compute content and transport of mass, heat, and freshwater in the ocean; and for determining water mass stratification and mixing rates. Historically, temperature profiles are much more abundant than simultaneous temperature and salinity profiles. Given the importance of concurrent temperature and salinity profiles, several methods have been developed to derive salinity solely based on temperature profile observations, such as expendable bathythermograph (XBT) temperature measurements, for which concurrent salinity observations are typically not available. These empirical methods used to date contain uncertainties as a result of temporal changes in salinity and seasonality in the mixed layer, and are typically regionally based. In this study, a new methodology is proposed to infer salinity in the Atlantic Ocean from the water surface to 2000-m depth, which addresses the seasonality in the upper ocean and makes inferences about longer-term changes in salinity. Our results show that when seasonality is accounted for, the variance of the residuals is reduced in the upper 150 m of the ocean and the dynamic height errors are smaller than 4 cm in the whole study domain. The sensitivity of the meridional heat and freshwater transport to different empirical methods of salinity estimation is studied using the high-density XBT transect across 34.5°S in the South Atlantic Ocean. Results show that accurate salinity estimates are more important on the boundaries, suggesting that temperature–salinity compensation may be also important in those regions.

Abstract

In September 1988 six sections were occupied across the Mediterranean outflow plume in the Gulf of Cadiz within 100 km of the Strait of Gibraltar. Vertical profiles of temperature and salinity were collected at CTD stations. Velocity and temperature profiles were collected with expendable current profilers at a subset of these stations. At the channel base, the plume undergoes geostrophic adjustment and turns northwest to flow along the continental slope. There it decelerates and spreads gradually down the slope as friction slows the current and allows it to cross isobaths. Within the plume, downstream velocity and density increase rapidly in the interfacial layer with depth to the velocity maximum, or nose, 5–150 m above the bottom. Below the nose, in the bottom layer, downstream velocity decreases rapidly toward the bottom, but the stratification is weak. Ekman-like veering occurs in the interfacial layer. Local bottom stresses on the plume are estimated by fitting the near-bottom velocity profiles to a log-layer model. These stresses are compared with bulk estimates of total stresses from momentum budget residuals and of interfacial stresses from combining the mean vertical shear with bulk turbulent dissipation estimates. The downstream pattern of the sum of the local bottom stresses and the bulk interfacial stresses agrees well in magnitude and distribution with that of the bulk total stresses. The largest stresses reach a mean of 5 Pa where the plume is flowing rapidly westward down a channel after exiting the strait, thinning, and accelerating. These stresses are an order of magnitude larger than mean wind stress values over the ocean gyres and exceed most bottom stress estimates in other regions.

Abstract

The Mediterranean Sea can be viewed as a “barometer” of the North Atlantic Ocean, because its sea level responds to oceanic-gyre-scale changes in atmospheric pressure and wind forcing, related to the North Atlantic Oscillation (NAO). The climate of the North Atlantic is influenced by the Atlantic meridional overturning circulation (AMOC) as it transports heat from the South Atlantic toward the subpolar North Atlantic. This study reports on a teleconnection between the AMOC transport measured at 26.5°N and the Mediterranean Sea level during 2004–17: a reduced/increased AMOC transport is associated with a higher/lower sea level in the Mediterranean. Processes responsible for this teleconnection are analyzed in detail using available satellite and in situ observations and an atmospheric reanalysis. First, it is shown that on monthly to interannual time scales the AMOC and sea level are both driven by similar NAO-like atmospheric circulation patterns. During a positive/negative NAO state, stronger/weaker trade winds (i) drive northward/southward anomalies of Ekman transport across 26.5°N that directly affect the AMOC and (ii) are associated with westward/eastward winds over the Strait of Gibraltar that force water to flow out of/into the Mediterranean Sea and thus change its average sea level. Second, it is demonstrated that interannual changes in the AMOC transport can lead to thermosteric sea level anomalies near the North Atlantic eastern boundary. These anomalies can (i) reach the Strait of Gibraltar and cause sea level changes in the Mediterranean Sea and (ii) represent a mechanism for negative feedback on the AMOC.

Abstract

The role of wind stress curl (WSC) forcing in the observed interannual variability of the Florida Current (FC) transport is investigated. Evidence is provided for baroclinic adjustment as a physical mechanism linking interannual changes in WSC forcing and changes in the circulation of the North Atlantic subtropical gyre. A continuous monthly time series of FC transport is constructed using daily transports estimated from undersea telephone cables near 27°N in the Straits of Florida. This 25-yr-long time series is linearly regressed against interannual WSC variability derived from the NCEP–NCAR reanalysis. The results indicate that a substantial fraction of the FC transport variability at 3–12-yr periods is explained by low-frequency WSC variations. A lagged regression analysis is performed to explore hypothetical adjustment times of the wind-driven circulation. The estimated lag times are at least 2 times faster than those predicted by linear beta-plane planetary wave theory. Possible reasons for this discrepancy are discussed within the context of recent observational and theoretical developments. The results are then linked with earlier findings of a low-frequency anticorrelation between FC transport and the North Atlantic Oscillation (NAO) index, showing that this relationship could result from the positive (negative) WSC anomalies that develop between 20° and 30°N in the western North Atlantic during high (low) NAO phases. Ultimately, the observed role of wind forcing on the interannual variability of the FC could represent a benchmark for current efforts to monitor and predict the North Atlantic circulation.

Abstract

Credible tropical cyclone (TC) intensity prediction by coupled models requires accurate forecasts of enthalpy flux from ocean to atmosphere, which in turn requires accurate forecasts of sea surface temperature cooling beneath storms. Initial ocean fields must accurately represent ocean mesoscale features and the associated thermal and density structure. Observing system simulation experiments (OSSEs) are performed to quantitatively assess the impact of assimilating profiles collected from multiple underwater gliders deployed over the western North Atlantic Ocean TC region, emphasizing advantages gained by profiling from moving versus stationary platforms. Assimilating ocean profiles collected repeatedly at fixed locations produces large root-mean-square error reduction only within ~50 km of each profiler for two primary reasons. First, corrections performed during individual update cycles tend to introduce unphysical eddy structure resulting from smoothing properties of the background error covariance matrix and the tapering of innovations by a localization radius function. Second, advection produces rapid nonlinear error growth at larger distances from profiler locations. The ability of each individual moving glider to cross gradients and map mesoscale structure in its vicinity substantially reduces this nonlinear error growth. Glider arrays can be deployed with horizontal separation distances that are 50%–100% larger than those of fixed-location profilers to achieve similar mesoscale error reduction. By contrast, substantial larger-scale bias reduction in upper-ocean heat content can be achieved by deploying profiler arrays with separation distances up to several hundred kilometers, with moving gliders providing only modest additional improvement. Expected sensitivity of results to study region and data assimilation method is discussed.

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).