Modification and pathways of Southern Ocean Deep Waters in the Scotia Sea

An unprecedented high-quality, quasi-synoptic hydrographic data set collected during the ALBATROSS cruise along the rim of the Scotia Sea is examined to describe the pathways of the deep water masses flowing through the region, and to quantify changes in their properties as they cross the sea. Owing to sparse sampling of the northern and southern boundaries of the basin, the modification and pathways of deep water masses in the Scotia Sea had remained poorly documented despite their global significance. Weddell Sea Deep Water (WSDW) of two distinct types is observed spilling over the South Scotia Ridge to the west and east of the western edge of the Orkney Passage. The colder and fresher type in the west, recently ventilated in the northern Antarctic Peninsula, flows westward to Drake Passage along the southern margin of the Scotia Sea while mixing intensely with eastward-flowing Circumpolar Deep Water (CDW) of the antarctic circumpolar current (ACC). Although a small fraction of the other WSDW type also spreads westward to Drake Passage, the greater part escapes the Scotia Sea eastward through the Georgia Passage and flows into the Malvinas Chasm via a deep gap northeast of South Georgia. A more saline WSDW variety from the South Sandwich Trench may leak into the eastern Scotia Sea through Georgia Passage, but mainly flows around the Northeast Georgia Rise to the northern Georgia Basin. In Drake Passage, the inflowing CDW displays a previously unreported bimodal property distribution, with CDW at the Subantarctic Front receiving a contribution of deep water from the subtropical Pacific. This bimodality is eroded away in the Scotia Sea by vigorous mixing with WSDW and CDW from the Weddell Gyre. The extent of ventilation follows a zonation that can be related to the CDW pathways and the frontal anatomy of the ACC. Between the Southern Boundary of the ACC and the Southern ACC Front, CDW cools by 0.15°C and freshens by 0.015 along isopycnals. The body of CDW in the region of the Polar Front splits after overflowing the North Scotia Ridge, with a fraction following the front south of the Falkland Plateau and another spilling over the plateau near 49.5°W. Its cooling (by 0.07°C) and freshening (by 0.008) in crossing the Scotia Sea is counteracted locally by NADW entraining southward near the Maurice Ewing Bank. CDW also overflows the North Scotia Ridge by following the Subantarctic Front through a passage just east of Burdwood Bank, and spills over the Falkland Plateau near 53°W with decreased potential temperature (by 0.03°C) and salinity (by 0.004). As a result of ventilation by Weddell Sea waters, the signature of the Southeast Pacific Deep Water (SPDW) fraction of CDW is largely erased in the Scotia Sea. A modified form of SPDW is detected escaping the sea via two distinct routes only: following the Southern ACC Front through Georgia Passage; and skirting the eastern end of the Falkland Plateau after flowing through Shag Rocks Passage

Circulation at the western boundary of the South and Equatorial Atlantic: Exchanges with the ocean interior.

International audienceData from a hydrographic section carried out in January-March 1994 offshore from the eastern coast of South America from 50S to 10N, are used to quantify the full-depth exchanges of water between the western boundary currents and the ocean interior. In the upper and intermediate layers, the westward transport associated with the southern branch of the South Equatorial Current was 49 Sv at the time of the cruise. The transports of the central and northern branches in the upper 200 m were 17 Sv and 12 Sv, respectively. After subtraction of the parts that recirculate in the subtropical, subequatorial, and equatorial domains, the fraction of the South Equatorial Current that effectively contributes to the warm water export to the North Atlantic is estimated at 18 Sv. The poleward boundary of the current southern branch is at 31S through the whole thickness of the subtropical gyre, but the latitude of the northern boundary varies from 7°30primeS at the surface to 27S at 1400 m depth. The estimated latitude of its bifurcation into the Brazil Current and North Brazil Undercurrent also varies downward from about 14S at the surface to 28S at a depth of 600 m. In the North Atlantic Deep Water, eastward flows exceeding 10 Sv are observed at 3°-4° of latitude in both hemispheres, at 10S, and at 34S-30S. Between 4S and 17S, a net westward flow with an estimated transport of 19 Sv reinforces the southward deep western boundary current. Cyclonic circulations of Antarctic Bottom Water along the western boundaries of the Argentine and Brazil basins have amplitudes of 15 Sv and 13 Sv, respectively, exceeding those of the interbasin exchanges. The net alongshore transport of this water mass between the hydrographic section and the continental slope reverses to a southward direction from 13S to 27S, probably in relation with an eastward shift of the equatorward near-bottom boundary current at these latitudes

Routine Reversal of the AMOC in an Ocean Model Ensemble

We describe a form of AMOC variability that we believe has not previously appeared in observations or models. It is found in an ensemble of eddy-resolving North Atlantic simulations that the AMOC frequently reverses in sign at ∼35N with gyre-wide anomalies in size and that reach throughout the water column. The duration of each reversal is roughly one month. The reversals are part of the annual AMOC cycle occurring in boreal winter, although not all years feature an actual reversal in sign. The occurrence of the reversals appears in our ensemble mean, suggesting it is a forced feature of the circulation. A partial explanation is found in an Ekman response to wind stress anomalies. Model ensemble simulations run with different combinations of climatological and realistic forcings argue that it is the atmospheric forcing specifically that results in the reversals, despite the signals extending into the deep ocean. Key Points The AMOC can reverse in sign Reversals appear in the annual cycle The reversals are the result of atmospheric forcing Plain Language Summary The Atlantic Meridional Overturning Circulation (AMOC) is a climatically important component of the ocean circulation. It is routinely thought to flow northward at the surface and southward at depths of 2000-3000m. Here we show that a significant component of the annual AMOC cycles are intervals during which it actually reverses this sense of flow and argue further that this is a response of the AMOC to the atmosphere. The AMOC anomaly is basin scale in size and extends over the full depth. These results have implications for annual heat storage in the North Atlantic

Spatio-temporal patterns of Chaos in the Atlantic Overturning Circulation

International audienc

Formation of subantarctic mode water in the southeastern Indian Ocean

Subantarctic Mode Water (SAMW) is the name given to the relatively deep surface mixed layers found directly north of the Subantarctic Front in the Southern Ocean, and their extension into the thermocline as weakly stratified or low potential vorticity water masses. The objective of this study is to begin an investigation into the mechanisms controlling SAMW formation, through a heat budget calculation. ARGO profiling floats provide estimates of temperature and salinity typically in the upper 2,000 m and the horizontal velocity at various parking depths. These data are used to estimate terms in the mode water heat budget; in addition, mode water circulation is determined with ARGO data and earlier ALACE float data, and climatological hydrography. We find a rapid transition to thicker layers in the central South Indian Ocean, at about 70°S, associated with a reversal of the horizontal eddy heat diffusion in the surface layer and the meridional expansion of the ACC as it rounds the Kerguelen Plateau. These effects are ultimately related to the bathymetry of the region, leading to the seat of formation in the region southwest of Australia. Upstream of this region, the dominant terms in the heat budget are the air–sea flux, eddy diffusion, and Ekman heat transport, all having approximately equal importance. Within the formation area, the Ekman contribution dominates and leads to a downstream evolution of mode water properties

Fast Warming of the Surface Ocean Under a Climatological Scenario

International audienceKey Points: 6 • Weakly varying climatological winds reduce upper ocean vertical mixing, affect-7 ing the redistribution of air-sea fluxes 8 • Coupled to an atmospheric boundary layer, the modeled ocean response to clima-9 tological winds is to warm up considerably at the surface 10 • Results illustrate the pivotal improvements in air-sea interactions achieved by driv-11 ing an ocean model with an atmospheric boundary layer 12 Abstract 13 We examine various strategies for forcing ocean-only models, including an atmospheric 14 boundary layer model. This surface forcing allows air-sea exchanges to affect atmospheric 15 temperature and relative humidity, thus removing the assumption of an infinite atmo-16 spheric heat capacity associated with the prescription of these variables. When exposed 17 to climatological winds, the simulated North Atlantic oceanic temperature warms con-18 siderably at the surface as compared to a model with full atmospheric variability. This 19 warming is mainly explained by a weakened upper ocean vertical mixing in response to 20 the weakly varying climatological winds. Specifying the atmospheric temperatures in-21 hibits this warming, but depends on the unrealistic large atmospheric heat capacity. We 22 thus interpret the simulated warmer ocean as a more physically consistent ocean response. 23 We conclude the use of an atmospheric boundary layer model provides many benefits 24 for ocean only modeling, although a 'normal' year strategy is required for maintaining 25 high frequency winds. 2

The SailBuoy remotely-controlled unmanned vessel: Measurements of near surface temperature, salinity and oxygen concentration in the Northern Gulf of Mexico

An experimental deployment of a new type of unmanned vessel is presented. The Christian Michelsen Research SailBuoy, a remotely-controlled surface vehicle, sampled near-surface properties during a two-month mission in the northern Gulf of Mexico in March–May, 2013. Averaged over the entire deployment, the vessel speed over ground was View the MathML source42±30cm s⎻¹ (± one standard deviation) with a maximum of View the MathML source180cm s⎻¹. During the 62 days of the mission, the SailBuoy covered a total range of approximately 400 km in both meridional and zonal directions, with a cumulative total distance of approximately 2400 km. Three parameters were recorded: sea surface temperature, conductivity, and dissolved oxygen. Observed surface temperature and salinity records are compared with remote sensing data and the salinity fields from a regional ocean modeling system, respectively. The absolute difference between remote sensing data to surface temperature is on an average approximately 0.5 °C. The comparison with the full Gulf of Mexico and the nested Northern Gulf of Mexico HYCOM models demonstrates the validity and usefulness of SailBuoy measurements and the instrument’s utility in evaluating fields produced by ocean models having different attributes. The potential of the SailBuoy for mapping a large-scale river plume, which would be challenging or costly with conventional ship surveys and/or remote sensing, is demonstrated

Wavelet-based wavenumber spectral estimate of eddy kinetic energy: Application to the North Atlantic

An ensemble of eddy-rich North Atlantic simulations is analyzed, providing estimates of eddy kinetic energy (EKE) wavenumber spectra and spectral budgets below the mixed layer where energy input from surface convection and wind stress are negligible. A wavelet transform technique is used to estimate a spatially localized ‘pseudo-Fourier’ spectrum (Uchidaet al., 2023b), permitting comparisons to be made between spectra at different locations in a highly inhomogeneous and anisotropic environment. The EKE spectra tend to be stable in time but the spectral budgets are highly time dependent. We find evidence of a Gulf Stream imprint on the near Gulf Stream eddy field appearing as enhanced levels of EKE in the (nominally) North-South direction relative to the East-West direction. Surprisingly, this signature of anisotropy holds into the quiescent interior with a tendency of the orientation aligned with maximum EKE being associated with shallower spectral slopes and elevated levels of inverse EKE cascade. Conversely, the angle associated with minimum EKE is aligned with a steeper spectral slope and forward cascade of EKE. Our results also indicate that vertical motion non-negligibly affects the direction of EKE cascade. A summary conclusion is that the spectral characteristics of eddies in the wind-driven gyre below the mixed layer where submesoscale dynamics are expected to be weak tend to diverge from expectations built on inertial-range assumptions, which are stationary in time and horizontally isotropic in space

Shelf break Exchange Events near the De Soto Canyon

Observations of currents, temperature, sea-surface height, sea-surface temperature and ocean color, derived from moorings, surface and deep drifters, hydrographic surveys, and satellites, are used to characterize shelf-slope exchange events near the apex of the De Soto Canyon in the northeast Gulf of Mexico. During the winter of 2012–2013, shelf-break time series showed a number of events where cold shelf water extruded over the slope. These events were largely consistent with slope eddies of both signs influencing shelf break currents. Larger-scale circulations, derived from the Loop Current and a separating Loop Current eddy, strongly influenced circulation over the De Soto slope during summer 2012, with flow patterns consistent with potential vorticity conservation over shoaling topography. Statistical investigation into shelf-slope exchange using large numbers of surface drifters indicated that export from the shelf is larger than vice-versa, and is more uniformly distributed along the shelf break. Import onto the shelf appears to favor a region just east of the Mississippi Delta, which is also consistent with the observed onshore transport of surface oil from the Deepwater Horizon disaster