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To study the role of the Atlantic meridional overturning circulation (AMOC) in climate change, we perform an abrupt CO2-doubling experiment using a coupled atmosphere-ocean-ice model with a simple geometry that separates the ocean into small and large basins. As in observations and high-end climate models, the small basin exhibits a MOC and warms at a faster rate than the large basin. In our set-up, this contrast in heat storage rates is 0.6 ± 0.1 W m2, and we argue that this is due to the small basin MOC. However, the MOC weakens significantly, yet this has little impact on the small basin’s heat storage rate. We find this is due to the effects of both compensating warming pat-\ud 22 terns and interbasin heat transports. Thus, although the presence of a MOC is important for enhanced heat storage, MOC weakening is surprisingly unimportant.

Changes in ocean gateway configuration can induce basin‐scale rearrangements in ocean current characteristics. However, there is large uncertainty in the relative timing of the Oligocene/Miocene subsidence histories of the Greenland‐Scotland Ridge (GSR) and the Fram Strait (FS). By using a climate model, we investigate the temperature and salinity changes in response to the subsidence of these two key ocean gateways during early to middle Miocene. For a singular subsidence of the GSR, we detect warming and a salinity increase in the Nordic Seas and the Arctic Ocean. As convection sites shift to the north of Iceland, North Atlantic Deep Water (NADW) is formed at cooler temperatures. The associated deep ocean cooling and upwelling of deep waters to the Southern Ocean surface can cause a cooling in the southern high latitudes. These characteristic responses to the GSR deepening are independent of the FS being shallow or deep. An isolated subsidence of the FS gateway for a deep GSR shows less pronounced warming and salinity increase in the Nordic Seas. Arctic temperatures remain unaltered, but a stronger salinity increase is detected, which further increases the density of NADW. The increase in salinity enhances the contribution of NADW to the abyssal ocean at the expense of the colder southern source water component. These relative changes largely counteract each other and cause a negligible warming in the upwelling regions of the Southern Ocean.

The time mean Indian Ocean (IO) deep meridional overturning circulation (MOC) is compared across three ocean reanalysis products (ORAS4, GECCO2, and GFDL). The MOC stream functions obtained by vertically integrating the mass flux across a latitude-depth section in three products are found to be significantly different from each other. Detailed analysis suggests that ORAS4 delivers the best depiction of IO MOC. The inferred IO deep MOC consists of two deep and strong counterclockwise cells located south of 30°S and around 10°S, respectively. The geostrophic component along with the barotropic or external mode dominates the former, and a combination of Ekman and geostrophic components dominates the latter. GECCO2 depicts a steady decline in the northward meridional transport in the bottom layer and a consequent reduction in the MOC strength. The tropical thermocline in GECCO2 responds to this MOC variability leading to rapid and monotonic warming of the tropical IO.

A climatically-induced acceleration in ocean-driven melting of Antarctic ice shelves would have consequences for both the discharge of continental ice into the ocean and thus global sea level, and for the formation of Antarctic Bottom Water and the oceanic meridional overturning circulation. Using a novel gas-tight in-situ water sampler, noble gas samples have been collected from six locations beneath the Filchner Ice Shelf, the first such samples from beneath an Antarctic Ice shelf. Helium and neon are uniquely suited as tracers of glacial meltwater in the ocean. Basal meltwater fractions range from 3.6% near the ice shelf base to 0.5% near the sea floor, with distinct regional differences. We estimate an average basal melt rate for the Filchner-Ronne Ice Shelf of 177 ± 95 Gt/year, independently confirming previous results. We calculate that up to 2.7% of the meltwater has been refrozen, and we identify a local source of crustal helium.

The Filchner‐Ronne Ice Shelf, the ocean cavity beneath it, and the Weddell Sea that bounds it, form an important part of the global climate system by modulating ice discharge from the Antarctic Ice Sheet and producing cold dense water masses that feed the global thermohaline circulation. A prerequisite for modeling the ice sheet and oceanographic processes within the cavity is an accurate knowledge of the sub‐ice sheet bedrock elevation, but beneath the ice shelf where airborne radar cannot penetrate, bathymetric data are sparse. This paper presents new seismic point measurements of cavity geometry from a particularly poorly sampled region south of Berkner Island that connects the Filchner and Ronne ice shelves. An updated bathymetric grid formed by combining the new data with existing data sets reveals several new features. In particular, a sill running between Berkner Island and the mainland could alter ocean circulation within the cavity and change our understanding of paleo‐ice stream flow in the region. Also revealed are deep troughs near the grounding lines of Foundation and Support Force ice streams, which provide access for seawater with melting potential. Running an ocean tidal model with the new bathymetry reveals large differences in tidal current velocities, both within the new gridded region and further afield, potentially affecting sub‐ice shelf melt rates.

Turbulent mixing produced by breaking of internal waves plays an important role in setting the patterns of downwelling and upwelling of deep dense waters and thereby helps sustain the global deep ocean overturning circulation. A key parameter used to characterize turbulent mixing is its efficiency, defined here as the fraction of the energy available to turbulence that is invested in mixing. Efficiency is conventionally approximated by a constant value near one sixth. Here we show that efficiency varies significantly in the abyssal ocean and can be as large as approximately one third in density stratified regions near topographic features. Our results indicate that variations in efficiency exert a first-order control over the rate of overturning of the lower branch of the meridional overturning circulation.

International audience; Atmospheric CO2 was ∼90 ppmv lower at the Last Glacial Maximum (LGM) compared to the late Holocene, but the mechanisms responsible for this change remain elusive. Here we employ a carbon isotope‐enabled Earth System Model to investigate the role of ocean circulation in setting the LGM oceanic δ13C distribution, thereby improving our understanding of glacial/interglacial atmospheric CO2 variations. We find that the mean ocean δ13C change can be explained by a 378 ± 88 Gt C(2σ) smaller LGM terrestrial carbon reservoir compared to the Holocene. Critically, in this model, differences in the oceanic δ13C spatial pattern can only be reconciled with a LGM ocean circulation state characterized by a weak (10–15 Sv) and relatively shallow (2000–2500 m) North Atlantic Deep Water cell, reduced Antarctic Bottom Water transport (≤10 Sv globally integrated), and relatively weak (6–8 Sv) and shallow (1000–1500 m) North Pacific Intermediate Water formation. This oceanic circulation state is corroborated by results from the isotope‐enabled Bern3D ocean model and further confirmed by high LGM ventilation ages in the deep ocean, particularly in the deep South Atlantic and South Pacific. This suggests a poorly ventilated glacial deep ocean which would have facilitated the sequestration of carbon lost from the terrestrial biosphere and atmosphere.

Observed and predicted changes in the strength of the westerly winds blowing over the Southern Ocean have motivated a number of studies of the response of the Antarctic Circumpolar Current and Southern Ocean Meridional Overturning Circulation (MOC) to wind perturbations and led to the discovery of the``eddy-compensation" regime, wherein the MOC becomes insensitive to wind changes. In addition to the MOC, tracer transport also depends on mixing processes. Here we show, in a high-resolution process model, that isopycnal mixing by mesoscale eddies is strongly dependent on the wind strength. This dependence can be explained by mixing-length theory and is driven by increases in eddy kinetic energy; the mixing length does not change strongly in our simulation. Simulation of a passive ventilation tracer (analogous to CFCs or anthropogenic CO$_2$) demonstrates that variations in tracer uptake across experiments are dominated by changes in isopycnal mixing, rather than changes in the MOC. We argue that, to properly understand tracer uptake under different wind-forcing scenarios, the sensitivity of isopycnal mixing to winds must be accounted for.

There is growing evidence that the interocean exchange south of Africa is an important link in the global overturning circulation of the ocean, the so-called ocean conveyer belt. At this location, warm and salty Indian Ocean waters enter the South Atlantic and are pulled by currents that eventually reach the North Atlantic, where water cools and sinks. A major contributor to the exchange is the frequent shedding of ring eddies from the termination of the Agulhas Current south of the tip of Africa.This shedding is controlled by developments far upstream in the Indian Ocean, and variations in this ‘Agulhas Leakage’ can lead to changes in the rate and stability of the Atlantic overturning, with possible associated global climate variations [Weijer et al., 1999]. Regional climate variations in the tropical and subtropical Indian Ocean are known to affect the whole system of the Agulhas Current, including the interocean exchanges. This article reports on some of the seminal results of ongoing multinational, multidisciplinary projects that explore these issues.

Rapid changes in North Atlantic climate over the last millennia were driven by coupled sea surface/atmospheric processes and rates of deep water formation. Holocene climate changes, however, remain poorly documented due to a lack of high-resolution paleoclimate records, and their impacts on marine ecosystems remain unknown. We present a 4500 year absolute-dated sea surface radiocarbon record from northeast Atlantic cold-water corals. In contrast to the current view that surface ocean changes occurred on millennial-scale cycles, our record shows more abrupt changes in surface circulation. Changes were centered at 3.4, 2.7, 1.7, and 1.2 kyr B.P. and associated with atmospheric reorganization. Solar irradiance may have influenced these anomalies but changes in North Atlantic deep water convection are likely to have amplified these signals. Critically, we provide the first evidence that these perturbations in Atlantic Meridional Overturning Circulation led to the decline of cold-water coral ecosystems from 1.2 to ~ 0.1 kyr B.P.

The sensitivity of the neodymium isotopic composition (ϵNd) to tectonic rearrangements of seaways is investigated using an Earth System Model of Intermediate Complexity. The shoaling and closure of the Central American Seaway (CAS) is simulated, as well as the opening and deepening of Drake Passage (DP). Multiple series of equilibrium simulations with various intermediate depths are performed for both seaways, providing insight into ϵNd and circulation responses to progressive throughflow evolutions. Furthermore, the sensitivity of these responses to the Atlantic Meridional Overturning Circulation (AMOC) and the neodymium boundary source is examined. Modeled ϵNd changes are compared to sediment core and ferromanganese (Fe-Mn) crust data. The model results indicate that the North Atlantic ϵNd response to the CAS shoaling is highly dependent on the AMOC state, i.e., on the AMOC strength before the shoaling to shallow depths (preclosure). Three scenarios based on different AMOC forcings are discussed, of which the model-data agreement favors a shallow preclosure (Miocene) AMOC (∼6 Sv). The DP opening causes a rather complex circulation response, resulting in an initial South Atlantic ϵNd decrease preceding a larger increase. This feature may be specific to our model setup, which induces a vigorous CAS throughflow that is strongly anticorrelated to the DP throughflow. In freshwater experiments following the DP deepening, ODP Site 1090 is mainly influenced by AMOC and DP throughflow changes, while ODP Site 689 is more strongly influenced by Southern Ocean Meridional Overturning Circulation and CAS throughflow changes. The boundary source uncertainty is largest for shallow seaways and at shallow sites.

It has been well documented that there is an anticyclonic anomaly over the western North Pacific (WNPAC, hereafter) during El Nino decaying summer. This El Nino-WNPAC relationship is greatly useful for the seasonal prediction of summer climate in the WNP and East Asia. In this study, we investigate the modification of the El Nino-WNPAC relationship induced by a weakened Atlantic thermohaline circulation (THC) in a water-hosing experiment. The results suggest that the WNPAC during the El Nino decaying summer, as well as the associated precipitation anomaly over the WNP, is intensified under the weakened THC. On the one hand, this intensification is in response to the increased amplitude and frequency of El Nino events in the water-hosing experiment. On the other hand, this intensification is also because of greater climatological humidity over the western to central North Pacific under the weakened THC. We suggest that the increase of climatological humidity over the western to central North Pacific during summer under the weakened THC is favorable for enhanced interannual variability of precipitation, and therefore favorable for the intensification of the WNPAC during El Nino decaying summer. This study suggests a possible modulation of the El Nino–Southern Oscillation-WNP summer monsoon relationship by the low-frequency fluctuation of Atlantic sea surface temperature. The results offer an explanation for the observed modification of the multidecadal fluctuation of El Nino-WNPAC relationship by the Atlantic multidecadal oscillation.

Ocean and atmosphere circulation and continental weathering regimes have undergone great changes over thousands of years as well as tens of millions of years. During the glacial stages of the Pleistocene, ocean circulation was generally more sluggish and deep water circulation in the Atlantic had a shallower flow. At the same time, weathering on the continents was enhanced by glacial erosion, particularly in high northern latitudes, which increased the input of erosional detritus into the ocean. In addition, atmospheric pressure gradients were larger, leading to higher wind speeds and increased supply of aeolian dust to the ocean. Prior to the onset of Northern Hemisphere glaciation and pronounced glacial/interglacial cyclicity at ∼3 m.ya., global climate was warmer than at present. There is also evidence for a more vigorous thermohaline circulation during the early Pliocene.

Surface and deepwater paleoclimate records in Irminger Sea core SO82-5 (59°N, 31°W) and Icelandic Sea core PS2644 (68°N, 22°W) exhibit large fluctuations in thermohaline circulation (THC) from 60 to 18 calendar kyr B.P., with a dominant periodicity of 1460 years from 46 to 22 calendar kyr B.P., matching the Dansgaard-Oeschger (D-O) cycles in the Greenland Ice Sheet Project 2 (GISP2) temperature record [Grootes and Stuiver, 1997]. During interstadials, summer sea surface temperatures (SSTsu) in the Irminger Sea averaged to 8°C, and sea surface salinities (SSS) averaged to ∼36.5, recording a strong Irminger Current and Atlantic THC. During stadials, SSTsu dropped to 2°-4°C, in phase with SSS drops by ∼1-2. They reveal major meltwater injections along with the East Greenland Current, which turned off the North Atlantic deepwater convection and hence the heat advection to the north, in harmony with various ocean circulation and ice models. On the basis of the IRD composition, icebergs came from Iceland, east Greenland, and perhaps Svalbard and other northern ice sheets. However, the southward drifting icebergs were initially jammed in the Denmark Strait, reaching the Irminger Sea only with a lag of 155-195 years. We also conclude that the abrupt stadial terminations, the D-O warming events, were tied to iceberg melt via abundant seasonal sea ice and brine water formation in the meltwater-covered northwestern North Atlantic. In the 1/1460-year frequency band, benthic δ18O brine water spikes led the temperature maxima above Greenland and in the Irminger Sea by as little as 95 years. Thus abundant brine formation, which was induced by seasonal freezing of large parts of the northwestern Atlantic, may have finally entrained a current of warm surface water from the subtropics and thereby triggered the sudden reactivation of the THC. In summary, the internal dynamics of the east Greenland ice sheet may have formed the ultimate pacemaker of D-O cycles.

14 pages, 11 figures, This work explores a method to recover temperature, salinity, and potential density of the ocean using acoustic reflectivity data and time and space coincident expendable bathythermographs (XBT). The acoustically derived (vertical frequency >10 Hz) and the XBT-derived (vertical frequency, The first author is funded by a Marie Curie IOF Fellowship (EU) with the project FP7-PEOPLE-2010-IOF-271936-OCEANSEIS. This work was also supported by the project POSEIDON (CTM2010-25169) and complementary action APOGEO (CTM2011-16001-E) from the Spanish Ministry MINECO

During the GasEx-98 field campaign, observations of the upper ocean structure were performed to identify relationships between the fine thermohaline structure, turbulence, and gas exchange in the near-surface layer of the ocean. The upper ocean dynamics were then simulated using a 1-D mixed layer model with the mixing parameterization developed during TOGA Coupled Ocean-Atmosphere Response Experiment (COARE). The model was initialized with the temperature, salinity, and velocity profiles in the upper 50 m thick layer of the ocean obtained from the Conductivity-Temperature-Depth (CTD) and Acoustic Doppler Current Profiler (ADCP) measurements and was forced with the air-sea heat and momentum fluxes measured by Edson et al. [1999]. The model produced a set of parameters including the time and depth dependent mixing coefficient and the depth of the mixed layer. The simulated mixed layer depth is consistent with the depth of the actively mixed layer determined from the turbulence profiles taken occasionally during GasEx-98 leg 2 with a free-rising profiler. Moderate wind speed conditions prevailed during GasEx-98 leg 2 with several storms and a few periods of calm weather. Both the modeling and experimental results demonstrate that under conditions of low wind speed, the surface-generated turbulence is constrained within a relatively thin surface layer of the ocean. In the near-surface layer, appreciable temperature, salinity, and gas concentration differences are formed because of diurnal warming or precipitation effects. These results are applied to the estimation of the effect of mixed layer processes on the bulk-flux formulation for the air-sea exchange of gases.

The interaction of the ocean and atmosphere plays an important role in shaping the climate and its variations. This chapter reviews the current state of knowledge of air–sea interaction and climate variations over the global ocean. The largest source of climate variability in the instrumental record is El Nino–Southern Oscillation (ENSO), which extends its reach globally through the ability of the atmosphere to bridge ocean basins. The growth of ENSO owes its existence to a positive ocean–atmosphere feedback mechanism (originally envisioned by J. Bjerknes) that involves the interaction of ocean dynamics, atmospheric convection, and winds in the equatorial Pacific. The Bjerknes feedback and the resultant equatorial zonal mode of climate variability are a common feature to all three tropical oceans despite differences in dimension, geometry and mean climate. In addition to this zonal mode, the tropics also support a meridional mode, whose growth is due to a thermodynamic feedback mechanism involving the interaction of the cross–equatorial gradient of properties such as sea surface temperature and displacements of the seasonal intertropical convergence zone. This meridional mode is observed in the tropical Atlantic, with some evidence of its existence in the Pacific and Indian Oceans. In the extratropics, in contrast, the sources of climate variability are more distributed. Much of climate variability may be explained by the presence of white noise due to synoptic weather disturbances whose impact on climate at longer timescales is due to the integrating effect of the ocean’s ability to store and release heat. Still, there is some evidence of a more active role for the mid–latitude ocean in climate variability, especially near major ocean currents/fronts. Finally, various atmospheric and oceanic bridges that link different ocean basins are discussed, along with their implications for paleoclimate changes and the current global warming.

An ocean data assimilation product from the Geophysical Fluid Dynamics Laboratory (GFDL) was used to investigate the seasonal variability of the ocean circulation in the northern tropical Atlantic. Analyses indicate that the seasonal variation of the circulation plays an important role in mediating meridional exchange between the subtropics and equatorial zone. In particular, seasonal changes in the Northern Equatorial Countercurrent (NECC) and in the cyclonic recirculation of the Guinea dome directly affect the trajectory of a subducted water parcel. Furthermore, seasonal variation of the zonal slope of the thermocline in the interior of the basin controls, to a large extent, the amount of water participating in the equatorial circulation. These changes can be explained in terms of a simple dynamical model where local Ekman pumping dominates thermocline variation in the western part of the basin, and Rossby wave adjustment comes into playing in the eastern basin.

The origins of tropical Atlantic variability (TAV) are investigated using a coupled modeling surgery approach. The coupled global circulation model captures the major TAV modes in the observations: the Atlantic Nino (ATL-Nino), the North Tropical Atlantic (NTA) and the South Tropical Atlantic (STA) modes. With the modeling surgery, it is found that these variability modes predominantly originate from local tropical Atlantic climate system, while remote ENSO and NAO forcing can enhance their variance and modulate their temporal evolution. Specifically, the interannual evolution of these modes is dictated predominantly by the remote ENSO, while the decadal evolution of these modes is dictated by extratropical-tropical interactions. Local ocean-atmosphere coupling is critical for the full development of the NTA and the ATL-Nino, but in general is not necessary for the STA mode. In the north tropical Atlantic and the subtropical south Atlantic (poleward of 20°S), the model simulated decadal SST anomalies are primarily generated by the surface heat flux and damped by the oceanic heat transport, but at the equator and in the south tropical Atlantic (equatorward of 20°S), they are primarily associated with oceanic heat transport through the anomalous vertical heat advection, and damped by the surface heat flux.

The main water masses in the northern Barents Sea are surface water, Arctic water, transformed Atlantic water, and cold bottom water. Using summer data from 1981 and 1982, the formation, distribution, modification and circulation of these water masses are discussed. Recent estimates show that about 2 Sv of Atlantic water enters the Barents Sea by the North Cape Current, balanced by a similar outflow through the strait between Novaya Zemlya and Frans Josef Land. Passing through the Barents Sea, Atlantic-derived water is modified by interaction with other water masses as well as with the atmosphere, and the end products are believed to be important contributors to the hydrographic structure of the Arctic Ocean.

A 100 year integration of an eddy resolving numerical model of the tropical Atlantic is analyzed to quantify the interannual variability caused by internal variability of ocean dynamics. It is found that, except for the spring position of the SST maximum, the strength of internal variability in the tropical Atlantic is comparable to published mid-latitude values but is dwarfed by the strength of the seasonal cycle. During spring however, the equatorial meridional SST gradient is very weak, and internal oceanic variability causes a variability in the position of the SST maximum that is comparable to its observed variability. It is shown that these variations in the SST are due to tropical instability waves whose strength varies from year to year, even under climatological forcing. The results suggest that in winter, the predictability of the location of the tropical SST maximum is limited to the persistence time of SST anomalies which is approximately 100 days.

International audience

The deep waters of the Caribbean Sea are renewed by sporadic flow over the two deep sills connecting with the open Atlantic. In turn, the deep waters of the Gulf of Mexico are renewed by flow from the northwest Caribbean over the sill in Yucatan Channel, between Mexico and Cuba. So we examine the obvious question: if all this renewal water is coming in, what is going out'? The view that emerges is a three-layer mean deep flow in the Caribbean and the Gulf. There is strong vertical shear in the water above 800 m associated with the primary upper-layer flow. Surprisingly, this shear continues to at least 1400 m. In the three-layer scenario described here, the upper-most deep layer flows into the Gulf at depths of ∼800-1100 m as an extension of the upper-layer circulation. The deepest inflow is sporadic, just above sill depth. The third layer, containing the required mean return flow to achieve mass balance, is found between these two mean flows at depths of ∼1100 to ∼1900 m. There is little information about the horizontal pattern of this deep return flow. The transport in these deep flows into the Gulf and in the return flow is slightly less than 1 Sv. These conclusions are inferred from the computed mean vertical shear, conservation of potential vorticity in the deep layers, the observed salinity gradients, and the assumption that there can be no net flow between 800 and 2000 m. The deep return flow of water from the Gulf of Mexico, having salinity ∼34.96 psu, appears to flow from Yucatan Channel all the way back to Windward and Jungfern Passages, leaving the Caribbean above the entering deep renewal water.

Model simulations targeting the ocean circulation response to changes in surface salinity in the high latitudes of both Northern and Southern Hemispheres demonstrate that meltwater impacts in one hemisphere may lead to a strengthening of the thermohaline conveyor driven by the source in the opposite hemisphere. This, in turn, leads to significant changes in poleward heat transport. Further, meltwater events caused largely by sea ice melting can lead to deep-sea warming and thermal expansion of abyssal water, that in turn can cause a substantial sea level change even without a major ice sheet melting. Experiments with a glacial ocean circulation regime prone to northern and southern meltwater events imply that glacial cycles may have been influenced by both northern and southern deepwater sources. Importantly, the experiments suggest that the southern source can be a more powerful modulator of the meridional deep-ocean conveyor that the northern source, which challenges our current vision of the North Atlantic Deep Water as an ultimate driver of deep-ocean circulation. Our experiments show that the southern impact can overpower northern ones. Even in the experiment in which the amplitude of the perturbation in the North Atlantic was as high as -3 psu, and the amplitude in the Southern Ocean was only-1 psu, the deep-water regime was qualitatively the same as in the pure Southern Ocean scenario, with somewhat less deep-ocean warming, yet still global and substantial.

A sectorially averaged, ocean-atmosphere-sea-ice model is used to compare the climatic impacts of a freshwater input (FI) in the North Atlantic ocean in pre-industrial and glacial climates. The FI is designed in order to be compatible with paleoceanographic evidence about Heinrich Events (HE) 4 and 5. In agreement with previous studies performed in interglacial conditions, it is shown that the THC turnoff and its resumption along with HE were likely to be responsible for the Antarctic warmings observed during some of them, as well as the subsequent DO abrupt warmings recorded in Greenland ice cores. However, when the FI is applied in glacial climate, the thermohaline circulation recovers in two steps separated in time by about 1470 years. Both steps correspond to abrupt shifts in Greenland temperature by about 4-5 °C induced by a massive release of the heat accumulated at intermediate depth in the Atlantic ocean. This 1470-year time lag, that is here physically linked to the turnover time of the ocean, turns out to be remarkably unsensitive to the duration and the time distribution of the FI, provided that the amplitude of the latter is large enough to induce a complete turnoff of the THC. The implication of these results are discussed by taking into account, in particular, the interpretation limits inherent to the sectorial structure of the model used here.

Twentieth-century relative sea level rise shows considerable variability along the U.S. East and Gulf coasts. Local rates of rise lie in the range of about 1.5 to more than 4 mm per year for records from Key West, Florida, to New York City. Rates of sea level rise in the Gulf of Mexico can be much higher. In Texas and Louisiana, long-term water levels are rising up to about 10 mm per year. This is having disastrous consequences in the form of wetlands loss in the region, estimated to be as much as 65 km 2 per year in the Mississippi Delta area of Louisiana alone. Beach erosion is also significant along both the Gulf and Atlantic coasts, resulting in ever-increasing exposure of fixed structures to the damaging impacts of storms. The especially high rates of sea level rise in Louisiana and Texas are a result of their particular geomorphology, and anthropogenic alterations in the form of sediment diversion and withdrawal of underground fluids. The average long-term local rate of sea level rise on the rest of the U.S. East and Gulf coasts when corrected for glacial isostatic adjustment is about 2 mm per year, in conformity with 20 th century global sea level rise. U.S. East and Gulf coast tide gauge records also have regionally coherent low frequency (decadal and longer) variations that need to be understood because of their impact on wetlands loss, and to enable accurate determination of long-term trends of sea level rise.

We investigate the influences of the diffusion coefficients for heat and salt in the deep ocean upon the stability of the Atlantic thermohaline circulation through a parameter space investigation that employs the GFDL Modular Ocean Model (MOM). The ocean general circulation model is configured to represent an idealized Atlantic basin and steadily (including an annual cycle) forced under the assumption of mixed boundary conditions. The impact on the results of the use of mixed boundary conditions is examined through additional sensitivity experiments to demonstrate that this choice of boundary conditions does not affect the main conclusions. The two primary sets of the experiments that we describe consist of sensitivity analyses for both horizontal and vertical diffusion coefficients; Bryan-Lewis type diffusion coefficient profiles are applied in such a way that one diffusion coefficient profile is varied while the other diffusion coefficient profile is fixed. We are thereby able to demonstrate that the choice of the horizontal diffusion coefficient plays a crucial role in determining the stability of the thermohaline circulation which is such that the thermohaline circulation exhibits very intense oscillations on a multi-millennium timescale when the horizontal diffusion coefficient of the deep ocean is set to a sufficiently small value. We demonstrate that the critical value required to realize an oscillatory solution of the thermohaline circulation lies well within the range of observed large-scale ocean diffusion coefficients, whereas the value of vertical diffusion coefficients affect not the bifurcation structure but the period of the millennial timescale oscillation.

Low-frequency circulation over the Texas-Louisiana continental shelf is examined. Currents over the inner shelf are upcoast (Rio Grande to Mississippi River) in summer and downcoast in nonsummer and are driven by an annual cycle of winds. This results in an annual signal for salinity, with lowest salinity waters occurring (a) in late spring along the inner portion of the western shelf when downcoast flows carry the high discharges from the Mississippi-Atchafalya and other rivers to the Mexican border, and (b) in summer over the inner and outer eastern shelf when the upcoast flow causes a pooling of the discharges from the Mississippi-Atchafalya rivers over that shelf. Upcoast winds during summer also result in high salinities over the western shelf due to advection from off Mexico and upwelling. Currents over the outer shelf are variable, but predominantly upcoast throughout the year, probably a result of the integrated effects of anticy-clonic eddies impinging on the shelf edge. Comparison of currents in the weather band (2-10 d) with the mesoscale band (10-100 d) suggests the shelf is divided at approximately the 50-m isobath. The weather band predominates over the inner shelf, reflecting frequent passage of fronts over the region. The mesoscale band predominates over the outer shelf, indicating the presence of offshelf eddies that frequent this region.

We study the dependence of the volume and transport of the Antarctic Bottom Water (AABW) in the Atlantic Ocean on vertical mixing. Numerical results from a set of OGCM runs show that, while the vertical extent of the AABW cell decreases with intensifying vertical mixing, the cell's transport increases. An analytical model of the deep boundary layers is then used to interpret the results. The decrease in the AABW thickness is explained by the downward expansion of the upper, North Atlantic Deep Water cell. The intensification of the AABW transport is attributed to the increase in the deep meridional pressure gradient, which drives the flow. An estimate of the AABW transport is then derived from the density balance in the deep western boundary layer and compared with the OGCM results.

Evidence from paleoclimatic archives suggests that the ocean atmosphere system has undergone dramatic and abrupt changes with widespread consequences in the past. Climatic changes are most pronounced in the North Atlantic region where annual mean temperature can change by 10° C and more within a few decades. Climate models are capable of simulating some features of abrupt climate change. These same models also indicate that changes of this type may be triggered by global warming. Here we summarize what is known about such future changes and discuss the state of our knowledge about these potential threats to the stability of the Earth System.