North Atlantic Surface Research Articles

Enhanced Mediterranean‐Atlantic exchange during Atlantic freshening phases

The Atlantic‐Mediterranean exchange of water at Gibraltar represents a significant heat and freshwater sink for the North Atlantic and is a major control on the heat, salt and freshwater budgets of the Mediterranean Sea. Consequently, an understanding of the response of the exchange system to external changes is vital to a full comprehension of the hydrographic responses in both ocean basins. Here, we use a synthesis of empirical (oxygen isotope, planktonic foraminiferal assemblage) and modeling (analytical and general circulation) approaches to investigate the response of the Gibraltar Exchange system to Atlantic freshening during Heinrich Stadials (HSs). HSs display relatively flat W–E surface hydrographic gradients more comparable to the Late Holocene than the Last Glacial Maximum. This is significant, as it implies a similar state of surface circulation during these periods and a different state during the Last Glacial Maximum. During HS1, the gradient may have collapsed altogether, implying very strong water column stratification and a single thermal and δ18Owater condition in surface water extending from southern Portugal to the eastern Alboran Sea. Together, these observations imply that inflow of Atlantic water into the Mediterranean was significantly increased during HS periods compared to background glacial conditions. Modeling efforts confirm that this is a predictable consequence of freshening North Atlantic surface water with iceberg meltwater and indicate that the enhanced exchange condition would last until the cessation of anomalous freshwater supply into to the northern North Atlantic. The close coupling of dynamics at Gibraltar Exchange with the Atlantic freshwater system provides an explanation for observations of increased Mediterranean Outflow activity during HS periods and also during the last deglaciation. This coupling is also significant to global ocean dynamics, as it causes density enhancement of the Atlantic water column via the Gibraltar Exchange to be inversely related to North Atlantic surface salinity. Consequently, Mediterranean enhancement of the Atlantic Meridional Overturning Circulation will be greatest when the overturning itself is at its weakest, a potentially critical negative feedback to Atlantic buoyancy change during times of ice sheet collapse.

North Atlantic sea surface temperatures and their relation to the North Atlantic Oscillation during the last 230 years

August Sea Surface Temperatures (aSSTs) based on fossil diatom assemblages are generated with 2 year average resolution from a 230-year-long sediment core (Rapid 21-12B), from the Reykjanes Ridge in the subpolar North Atlantic. The results indicate a warming trend of ~0.5°C of the surface waters at the Reykjanes Ridge for the last 230 years. Superimposed on this warming trend there is a multidecadal to decadal aSST variability of up to 1°C. The interval from the 1770s to the 1830s represents the coldest period, whereas ~1860–1880 represents the warmest period during the last 230 years. The last 25 years is characterized by a warming trend showing strong decadal aSST variability with several warm years, but also the coldest years since the 1820s. The time of these cold years in the mid-1970s, -1980s and -1990s correspond with the documented great salinity anomalies (GSA) in the North Atlantic suggesting increased fluxes of cold, low-salinity waters from the Arctic during the last decades. The aSST record and the August North Atlantic Oscillation (aNAO) index show similar multidecadal-scale variability indicating a close coupling between the oceanic and atmospheric patterns. The aSST record shows a negative correlation with the aNAO indicating cold aSST during the positive aNAO trend and vice versa. Results suggest that the wind driven variation in volume fluxes of the North Atlantic surface waters could be the major mechanism behind the observed relationship.

The feeding ecology of little auks raises questions about winter zooplankton stocks in North Atlantic surface waters

Copepods are essential components of marine food webs worldwide. In the North Atlantic, they are thought to perform vertical migration and to remain at depths more than 500 m during winter. We challenge this concept through a study of the winter feeding ecology of little auks (Alle alle), a highly abundant planktivorous seabird from the North Atlantic. By combining stable isotope and behavioural analyses, we strongly suggest that swarms of copepods are still available to their predators in water surface layers (less than 50 m) during winter, even during short daylight periods. Using a new bioenergetic model, we estimate that the huge number (20-40 million birds) of little auks wintering off southwest Greenland consume 3600-7200 tonnes of copepods daily, strongly suggesting substantial zooplankton stocks in surface waters of the North Atlantic in the middle of the boreal winter.

Changes in synoptic weather patterns and Greenland precipitation in the 20th and 21st centuries: 2. Analysis of 21st century atmospheric changes using self‐organizing maps

Using a three‐model ensemble, predictions of increasing Greenland precipitation over the 21st century are analyzed using self‐organizing maps (SOMs). The models that make up the three‐model ensemble (CCCMA‐CGCM3.1(T63), MIROC3.2(hires), and MPI‐ECHAM5), which are all atmosphere‐ocean global circulation models used in the Intergovernmental Panel on Climate Change Fourth Assessment Report, were chosen due to their ability to best reproduce North Atlantic surface synoptic climatology and Greenland precipitation from ERA‐40. Daily sea level pressure and precipitation data from model simulations for years 1961–1999, 2046–2065, and 2081–2100 are compared, where future simulations are based on the SRES A1B emissions scenario. Results indicate that the North Atlantic storm track is predicted to shift northward through the 21st century and Greenland precipitation is predicted to increase from 35.8 cm yr−1 to 45.8 cm yr−1 by the end of the 21st century, a 27.8% increase. The precipitation change is attributed to changes in atmospheric circulation, which are due to changes in synoptic pattern frequency of occurrence, and changes in the amount of precipitation that occurs when a given synoptic circulation pattern occurs, termed intrapattern variability, during the future time periods. The northward shift in storm track results in less precipitation being produced dynamically over the southeast coast of Greenland, but an increase in precipitation over the remainder of the ice sheet, with largest increases over the southwest coast of Greenland and the eastern region. Intrapattern variability changes, however, dominate the future precipitation changes, accounting for 82.5% of the total change. This is due to an increase in precipitable water in the atmosphere in response to rising temperatures. Changes in sea ice and ocean temperature are also thought to contribute to this change.

Comparison of diatom records of the Heinrich event 1 in the Western North Atlantic

Heinrich event 1 (H1) is a climate event resulting from the release into the North Atlantic of a huge volume of sea ice and icebergs from the northern hemisphere ice sheets. We present here high-resolution diatom records from the Bermuda Rise (Sargasso Sea) and the Laurentian Fan (South of Newfoundland) to assess its impacts on North Atlantic surface circulation and its timing. The event is composed of three phases: the two first correspond to major pulses of Ice Rafted Debris (IRD) and the third phase relates to its immediate aftermath. At both sites, diatom abundances start to rise at 16.8 ka. This increase is marked by diatom species thriving in cold environments over the Laurentian Fan, while brackish and fresh water diatom species characterize this increase over the Bermuda Rise. This last record implies icebergs migration to subtropical latitudes and nutrient-rich meltwater to support such diatom productivity. During the second phase of the event, both sites record maximum diatom abundances, when sea ice conditions are dominant over the Laurentian Fan and the contribution of brackish-fresh water diatoms culminates over the Bermuda Rise. The persistence of lower salinity related diatom species over the Bermuda Rise suggests a continuous injection of cold fresh water by cold-core rings, as observed nowadays. The last phase witnesses the persistence of low salinity water over the Laurentian Fan, while a decrease towards the disappearance of diatoms in the sediment occurs over the Bermuda Rise. Regarding the relationship between IRD and diatom abundance, it appears that a critical amount of icebergs is necessary to stimulate diatom productivity. The disturbances induced by H1 appear to end ~14.6 ka over the Bermuda Rise, while over the Laurentian Fan, the high diatom production persists until 14.1 ka and the salinity anomaly until 13.8 ka.

Causes of tropical Atlantic paleo‐salinity variation during periods of reduced AMOC

During periods of reduced Atlantic meridional overturning circulation (AMOC) associated with a freshening of northern North Atlantic surface water, paleo proxy records indicate a corresponding surface salinity increase over the entire tropical Atlantic. Although latitudinal‐shifts in the mean position of the Atlantic Intertropical Convergence Zone (ITCZ) can explain certain features of the paleo salinity reconstructions, this mechanism does not offer an explanation for the reconstructed basin‐wide paleo‐salinity response to AMOC change. Here, we present new results from general circulation model simulations that suggest the sea surface salinity (SSS) increase in the tropical north Atlantic during periods of weakened AMOC is mainly controlled by the atmospheric response to the North Atlantic cooling, while the oceanic teleconnection contributes to increased SSS over the equatorial and south tropical Atlantic Ocean.

Influence of Bering Strait flow and North Atlantic circulation on glacial sea-level changes

Sea-level fluctuations of about 20-30m occurred throughout the last glacial period. These fluctuations seem to have been derived primarily from changes in the volume of Northern Hemisphere ice sheets(1-3), and cannot be attributed solely to ice melt caused by varying solar radiation(4). Here we use a fully coupled climate model to show that the transport of relatively fresh Pacific water into the North Atlantic Ocean was limited when lower sea level restricted or closed the Bering Strait, resulting in saltier North Atlantic surface waters. This invigorated deep convection in the North Atlantic Ocean, strengthening meridional overturning circulation and northward heat transport in our model, which consequently promoted melting of ice sheets in North America and Europe. Our simulations show that the associated sea-level rise led to a reopening of the Bering Strait; the flux of relatively fresh water into the North Atlantic Ocean muted meridional overturning circulation and led to cooling and ice-sheet advance in the Northern Hemisphere. We conclude that the repetition of this cycle could produce the sea-level changes that have been observed throughout the last glacial cycle.

Ocean heat transport in Simple Ocean Data Assimilation: Structure and mechanisms

The trend and variability of global ocean heat transport for the period 1958–2004 are investigated using the Simple Ocean Data Assimilation (SODA) analysis. The ocean model is forced with the European Center for Medium Range Weather Forecast (ECMWF) ERA‐40 atmospheric reanalysis winds from 1958 to 2001 and with QuikSCAT winds from 2002 to 2004. The assimilation is based on a sequential estimation algorithm, with observations from the historical archive of hydrographic profiles supplemented by ship intake measurements, moored hydrographic observations and remotely sensed sea surface temperature. Heat transport is calculated using temperature and velocity from the ocean analysis. Mean heat transport from the analysis generally agrees with previously published estimates from observational and modeling studies. Trends of heat transport show a range of behaviors. In the Atlantic and Pacific Oceans there is mostly increasing poleward heat transport with two important exceptions. In the Atlantic Ocean there is decreasing heat transport around 50°N and 60°N, and in both the Atlantic and Pacific Oceans there is decreasing heat transport near 10°S. There is also prominent interannual and decadal variability in all of the ocean basins. The results suggest that ocean heat transport variability is primarily determined by the strength of the meridional overturning circulation (MOC), which is controlled by complex processes governing fresh water flux in the northern North Atlantic and surface wind stress. However, the role of temperature variability increases at high latitude, particularly in the northern North Atlantic Ocean. Eddies play an important role in heat transport in the Gulf Stream and its extension in the Atlantic Ocean, and the Kuroshio and its extension in the Pacific Ocean and enhanced Subtropical cells (STCs) affect heat transport estimates in the tropics. In the northern North Atlantic Ocean, a small increase in meridional heat transport and a slight weakening of MOC are detected. Weakening in the northern North Atlantic MOC mainly arises from a freshening in the Labrador Sea and slowdown of the overflows from the Nordic Seas into the northern North Atlantic Ocean. Trends in North Atlantic surface momentum forcing are uniform across several atmospheric reanalyses, however there is less agreement in the role of precipitation in forcing trends of MOC and this exists as a primary source of uncertainty in our analysis.

Imprint of North-Atlantic abrupt climate changes on western European loess deposits as viewed in a dust emission model

Western European loess sequences of the last glaciation (∼100,000–15,000 years BP) exhibit strong, cyclic variations of the sedimentation rate, which are coeval to the Greenland stadial/interstadial cycles and the Heinrich events. These North-Atlantic rapid climate changes appear, thus, as a potential cause for the sedimentation variations, via changes in dust intensity cycle. Here we make a first step in testing this hypothesis, by modelling the impact of the North-Atlantic abrupt climate variations on dust emission. Our dust emission calculations use meteorological fields generated by the LMDZ atmospheric general circulation model at a resolution down to 60 km over Western Europe. Three numerical experiments are run, representing a Greenland stadial, an interstadial and a Heinrich event. Orbital parameters and ice-sheet configuration correspond to conditions from Marine Isotope Stage 3 (∼60,000–25,000 years BP), a period characterized by strong millennial-scale climate variability. The only differences we impose in the boundary conditions regard the North-Atlantic surface temperature and sea-ice cover in the latitudinal band 30°–63°N. The changes in wind, precipitation, soil moisture and snow cover from one simulated state to another result in small differences in dust emission intensity. In contrast, when the inhibition of the aeolian erosion by vegetation is taken into account, the dust fluxes for the cold climate states (Greenland stadial and Heinrich event) become generally more than twice higher than those for the relatively warmer Greenland interstadial, in agreement with the loess data. These results support the hypothesis that the North-Atlantic millennial-scale variability is imprinted in Western European loess profiles, and point to vegetation changes as the main factor responsible for millennial-scale sedimentation variations. An analysis for the English Channel and southern North Sea areas, major potential dust sources, shows that the seasonality of dust emission is not controlled by the wind speed, as in modern large deserts, but by the surface conditions. Consequently, the dusty season lasts from late winter to early summer, with maximum activity in April–May, and is shifted towards summer when the climate is colder.

On the Source of Mediterranean Overflow Water Property Changes

Abstract A recent study of the eastern North Atlantic detailed significant increases in the temperature and salinity of the Mediterranean Overflow Water (MOW) from 1950 to 2000. To examine the degree to which the source waters, which spill over the sill at the Strait of Gibraltar, could be responsible for these observations in the open Atlantic, a box model of water mass transformation by marginal seas was employed. Time series for the salinity of the inflowing North Atlantic surface waters, freshwater fluxes in the Mediterranean (evaporation and precipitation and river runoff), and the volumetric flow rates for the inflow and outflow across the Strait of Gibraltar were used to predict the salinity of the source waters to the North Atlantic from 1950 to 2000. Results from this calculation reveal that source water changes have minimal impact on MOW property changes on interannual and decadal time scales. It is suggested instead that circulation changes within the open Atlantic alter the advective–diffusive pathways of MOW such that property changes within the MOW reservoir are created.

The Role of Aerosols in the Evolution of Tropical North Atlantic Ocean Temperature Anomalies

Observations and models show that northern tropical Atlantic surface temperatures are sensitive to regional changes in stratospheric volcanic and tropospheric mineral aerosols. However, it is unknown whether the temporal variability of these aerosols is a key factor in the evolution of ocean temperature anomalies. We used a simple physical model, incorporating 26 years of satellite data, to estimate the temperature response of the ocean mixed layer to changes in aerosol loadings. Our results suggest that the mixed layer's response to regional variability in aerosols accounts for 69% of the recent upward trend, and 67% of the detrended and 5-year low pass-filtered variance, in northern tropical Atlantic Ocean temperatures.

Neural network‐based estimates of North Atlantic surface pCO2 from satellite data: A methodological study

A new method is proposed to estimate ocean surface pCO2 from remotely sensed surface temperature and chlorophyll data. The method is applied to synthetic observations provided by an eddy‐resolving biogeochemical model of the North Atlantic. The same model also provides a perfectly known simulated pCO2 “ground truth” used to quantitatively assess the success of the estimation method. Model output is first sampled according to realistic voluntary observing ship (VOS) and satellite coverage. The model‐generated VOS “observations” are then used to train a self‐organizing neural network that is subsequently applied to model‐generated “satellite data” of surface temperature and surface chlorophyll in order to derive basin‐wide monthly maps of surface pCO2. The accuracy of the estimated pCO2 maps is analyzed with respect to the “true” surface pCO2 fields simulated by the biogeochemical circulation model. We also investigate the accuracy of the estimated pCO2 maps as a function of VOS line coverage, remote sensing errors, and the interpolation of missing remote sensing data due to cloud cover and low solar irradiation in winter. For a simulated “sampling” corresponding to VOS lines and patterns of optical satellite coverage of the year 2005, the neural net can successfully reproduce pCO2 from model‐generated “remote sensing data” of SST and Chl. Basin‐wide RMS errors amount to 19.0 μatm for a hypothetical perfect interpolation scheme for remote sensing data gaps and 21.1 μatm when climatological surface temperature and chlorophyll values are used to fill in areas lacking optical satellite coverage.

The Control of Polar Haloclines by Along-Isopycnal Diffusion in Climate Models

Abstract Increasing the value of along-isopycnal diffusivity in a coupled model is shown to lead to enhanced stability of North Atlantic Deep Water (NADW) formation with respect to freshwater (FW) perturbations. This is because the North Atlantic (NA) surface salinity budget is dominated by upward salt fluxes resulting from winter convection for low values of along-isopycnal diffusivity, whereas along-isopycnal diffusion exerts a strong control on NA surface salinity at higher diffusivity values. Shutdown of wintertime convection in response to a FW pulse allows the development of a halocline responsible for the suppression of deep sinking. In contrast to convection, isopycnal salt diffusion proves a more robust mechanism for preventing the formation of a halocline, as surface freshening leads only to a flattening of isopycnals, leaving at least some diffusive removal of anomalous surface FW in place. As a result, multiple equilibria are altogether absent for sufficiently high values of isopycnal diffusivity. Furthermore, the surface salinity budget of the North Pacific is also dominated by along-isopycnal diffusion when diffusivity values are sufficiently high, leading to a breakdown of the permanent halocline there and the associated onset of deep-water formation.

New Insights into North European and North Atlantic Surface Pressure Variability, Storminess, and Related Climatic Change since 1830

Abstract The authors present initial results of a new pan-European and international storminess since 1800 as interpreted from European and North Atlantic barometric pressure variability (SENABAR) project. This first stage analyzes results of a new daily pressure variability index, dp(abs)24, from long-running meteorological stations in Denmark, the Faroe Islands, Greenland, Iceland, the United Kingdom, and Ireland, some with data from as far back as the 1830s. It is shown that dp(abs)24 is significantly related to wind speed and is therefore a good measure of Atlantic and Northwest European storminess and climatic variations. The authors investigate the temporal and spatial consistency of dp(abs)24, the connection between annual and seasonal dp(abs)24 and the North Atlantic Oscillation Index (NAOI), as well as dp(abs)24 links with historical storm records. The results show periods of relatively high dp(abs)24 and enhanced storminess around 1900 and the early to mid-1990s, and a relatively quiescent period from about 1930 to the early 1960s, in keeping with earlier studies. There is little evidence that the mid- to late nineteenth century was less stormy than the present, and there is no sign of a sustained enhanced storminess signal associated with “global warming.” The results mark the first step of a project intending to improve on earlier work by linking barometric pressure data from a wide network of stations with new gridded pressure and reanalysis datasets, GCMs, and the NAOI. This work aims to provide much improved spatial and temporal coverage of changes in European, Atlantic, and global storminess.

Evidence for the linked biogeochemical cycling of zinc, cobalt, and phosphorus in the western North Atlantic Ocean

Many trace metals such as iron, copper, and manganese have lower concentrations in the surface waters of the North Pacific Ocean than in North Atlantic surface waters. However, cobalt and zinc concentrations in North Atlantic surface waters are often as low as those reported in the North Pacific. We studied the relationship between the distribution of cobalt, zinc, and phosphorus in surface waters of the western North Atlantic Ocean. Both metals show strong depletion in the southern Sargasso Sea, a region characterized by exceedingly low dissolved inorganic phosphorus (generally <4 nmol L−1) and measurable alkaline phosphatase activity. Alkaline phosphatase is a metalloenzyme (typically containing zinc) that cleaves phosphate monoesters and is a diagnostic indicator of phosphorus stress in phytoplankton. In contrast to the North Pacific Ocean, cobalt and zinc appear to be drawn down to their lowest values only when inorganic phosphorus is below 10 nmol L−1 in the North Atlantic Ocean. Lower levels of phosphorus in the Atlantic may contribute to these differences, possibly through an increased biological demand for zinc and cobalt associated with dissolved organic phosphorus acquisition. This hypothesis is consistent with results of a culture study where alkaline phosphatase activity decreased in the model coccolithophore Emiliania huxleyi upon zinc and cobalt limitation.

CLIMATIC INFLUENCES ON DEEP‐SEA OSTRACODE (CRUSTACEA) DIVERSITY FOR THE LAST THREE MILLION YEARS

Ostracodes are small, bivalved crustaceans with the finest-scale fossil resolution of any metazoan, rivaled only by the fossil record of the protistan Foraminifera. This article presents a synthesis of the patterns and possible causes of alpha species diversity variation in benthic deep-sea ostracodes at drilling sites in the North Atlantic and Arctic Oceans. Taken together, these sites represent a period of great climatic variability covering the past three million years. Sediment cores taken from the Mid-Atlantic Ridge show a positive correlation between warm temperatures and high species diversity. These Mid-Atlantic Ridge cores, at the same latitude as northern Spain, show the same positive correlation during the last two glacial-interglacial cycles (200-0 ka [thousands of years ago]) as they do during the pre-glacial Pliocene 2.85-2.4 Ma (millions of years ago). This positive correlation is also found in Pliocene cores from the Rockall Plateau, at the same latitude as Ireland. During the last 200 thousand years, however, this correlation is reversed in cores taken from both the Rockall and Iceland Plateaus. The discovery of high diversity during colder periods in recent high-latitude Rockall and Iceland cores seems to be explained by spikes in diversity caused by ice-rafting events, which would not affect the lower-latitude Mid-Atlantic Ridge. The Heinrich ice-rafting events reduce North Atlantic surface temperatures and salinity every approximately 6-12 ka, dramatically decreasing surface productivity. This increase in diversity during Heinrich events may be explained either by a negative correlation between surface productivity and benthic diversity or by increase in diversity caused by moderate disturbance when ice rafted debris fall to the bottom of the ocean.

Episodic reductions in bottom-water currents since the last ice age

Although North Atlantic deep-water formation was greatly reduced during the last glacial maximum, bottom-water currents were as vigorous as at present. However, they were weakened during periods of North Atlantic surface freshening. A strong correlation can be seen between bottom-water-current strength and Greenland air-temperature records, thus confirming a close connection between ocean circulation and abrupt climate change. Past changes in the freshwater balance of the surface North Atlantic Ocean are thought to have influenced the rate of deep-water formation, and consequently climate1,2. Although water-mass proxies are generally consistent with an impact of freshwater input on meridional overturning circulation3, there has been little dynamic evidence to support this linkage. Here we present a 25,000 year record of variations in sediment grain size from south of Iceland, which indicates vigorous bottom-water currents during both the last glacial maximum and the Holocene period. Together with reconstructions of North Atlantic water-mass distribution, vigorous bottom currents suggest a shorter residence time of northern-source waters during the last glacial maximum, relative to the Holocene period. The most significant reductions in flow strength occur during periods that have been associated with freshening of the surface North Atlantic. The short-term deglacial oscillations in bottom current strength are closely coupled to changes in Greenland air temperature, with a minimum during the Younger Dryas cold reversal and a maximum at the time of rapid warming at the onset of the Holocene. Our results support a strong connection between ocean circulation and rapid climate change.

Freshwater transport estimates and the global overturning circulation: Shallow, deep and throughflow components

Meridional ocean freshwater transports and convergences are calculated from absolute geostrophic velocities and Ekman transports. The freshwater transports are analyzed in terms of mass-balanced contributions from the shallow, ventilated circulation of the subtropical gyres, intermediate and deep water overturns, and Indonesian Throughflow and Bering Strait components. The following are the major conclusions: 1. Excess freshwater in high latitudes must be transported to the evaporative lower latitudes, as is well known. The calculations here show that the northern hemisphere transports most of its high latitude freshwater equatorward through North Atlantic Deep Water (NADW) formation (as in [Rahmstorf, S., 1996. On the freshwater forcing and transport of the Atlantic thermohaline circulation. Climate Dynamics 12, 799–811]), in which saline subtropical surface waters absorb the freshened Arctic and subpolar North Atlantic surface waters (0.45 ± 0.15 Sv for a 15 Sv overturn), plus a small contribution from the high latitude North Pacific through Bering Strait (0.06 ± 0.02 Sv). In the North Pacific, formation of 2.4 Sv of North Pacific Intermediate Water (NPIW) transports 0.07 ± 0.02 Sv of freshwater equatorward. In complete contrast, almost all of the 0.61 ± 0.13 Sv of freshwater gained in the Southern Ocean is transported equatorward in the upper ocean, in roughly equal magnitudes of about 0.2 Sv each in the three subtropical gyres, with a smaller contribution of <0.1 Sv from the Indonesian Throughflow loop through the Southern Ocean. The large Southern Ocean deep water formation (27 Sv) exports almost no freshwater (0.01 ± 0.03 Sv) or actually imports freshwater if deep overturns in each ocean are considered separately (−0.06 ± 0.04 Sv). This northern–southern hemisphere asymmetry is likely a consequence of the “Drake Passage” effect, which limits the southward transport of warm, saline surface waters into the Antarctic [Toggweiler, J.R., Samuels, B., 1995a. Effect of Drake Passage on the global thermohaline circulation. Deep-Sea Research I 42(4), 477–500]. The salinity contrast between the deep Atlantic, Pacific and Indian source waters and the denser new Antarctic waters is limited by their small temperature contrast, resulting in small freshwater transports. No such constraint applies to NADW formation, which draws on warm, saline subtropical surface waters . 2. The Atlantic/Arctic and Indian Oceans are net evaporative basins, hence import freshwater via ocean circulation. For the Atlantic/Arctic north of 32°S, freshwater import (0.28 ± 0.04 Sv) comes from the Pacific through Bering Strait (0.06 ± 0.02 Sv), from the Southern Ocean via the shallow gyre circulation (0.20 ± 0.02 Sv), and from three nearly canceling conversions to the NADW layer (0.02 ± 0.02 Sv): from saline Benguela Current surface water (−0.05 ± 0.01 Sv), fresh AAIW (0.06 ± 0.01 Sv) and fresh AABW/LCDW (0.01 ± 0.01 Sv). Thus, the NADW freshwater balance is nearly closed within the Atlantic/Arctic Ocean and the freshwater transport associated with export of NADW to the Southern Ocean is only a small component of the Atlantic freshwater budget. For the Indian Ocean north of 32°S, import of the required 0.37 ± 0.10 Sv of freshwater comes from the Pacific through the Indonesian Throughflow (0.23 ± 0.05 Sv) and the Southern Ocean via the shallow gyre circulation (0.18 ± 0.02 Sv), with a small export southward due to freshening of bottom waters as they upwell into deep and intermediate waters (−0.04 ± 0.03 Sv). The Pacific north of 28°S is essentially neutral with respect to freshwater, −0.04 ± 0.09 Sv. This is the nearly balancing sum of export to the Atlantic through Bering Strait (−0.07 ± 0.02 Sv), export to the Indian through the Indonesian Throughflow (−0.17 ± 0.05 Sv), a negligible export due to freshening of upwelled bottom waters (−0.03 ± 0.03 Sv), and import of 0.23 ± 0.04 Sv from the Southern Ocean via the shallow gyre circulation. 3. Bering Strait’ssmall freshwater transport of <0.1 Sv helps maintains the Atlantic–Pacific salinity difference. However, proportionally large variations in the small Bering Strait transport would only marginally impact NADW salinity, whose freshening relative to saline surface water is mainly due to air-sea/runoff fluxes in the subpolar North Atlantic and Arctic. In contrast, in the Pacific, because the total overturning rate is much smaller than in the Atlantic, Bering Strait freshwater export has proportionally much greater impact on North Pacific salinity balances, including NPIW salinity.

Subtropical Atlantic salinity variability and Atlantic meridional circulation during the last deglaciation

Research Article| December 01, 2008 Subtropical Atlantic salinity variability and Atlantic meridional circulation during the last deglaciation Anders E. Carlson; Anders E. Carlson 11Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA *Current address: Department of Geology and Geophysics, University of Wisconsin, Madison, WI, 53706, USA; acarlson@geology.wisc.edu. Search for other works by this author on: GSW Google Scholar Delia W. Oppo; Delia W. Oppo 11Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA Search for other works by this author on: GSW Google Scholar Rosemarie E. Came; Rosemarie E. Came 22Geology and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA †Current address: Department of Geological Sciences, Jackson School of Geoscience, University of Texas, Austin, TX, 78713, USA. Search for other works by this author on: GSW Google Scholar Allegra N. LeGrande; Allegra N. LeGrande 33National Aeronautics and Space Administration, Goddard Institute for Space Studies and Center for Climate Systems Research, Columbia University, New York, New York 10025, USA Search for other works by this author on: GSW Google Scholar Lloyd D. Keigwin; Lloyd D. Keigwin 11Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA Search for other works by this author on: GSW Google Scholar William B. Curry William B. Curry 11Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA Search for other works by this author on: GSW Google Scholar Geology (2008) 36 (12): 991–994. https://doi.org/10.1130/G25080A.1 Article history received: 23 Apr 2008 rev-recd: 19 Aug 2008 accepted: 03 Sep 2008 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Anders E. Carlson, Delia W. Oppo, Rosemarie E. Came, Allegra N. LeGrande, Lloyd D. Keigwin, William B. Curry; Subtropical Atlantic salinity variability and Atlantic meridional circulation during the last deglaciation. Geology 2008;; 36 (12): 991–994. doi: https://doi.org/10.1130/G25080A.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract During the last deglaciation (ca. 21–10 ka), freshening of the North Atlantic surface likely caused reductions in Atlantic meridional overturning circulation (AMOC); the mechanisms related to AMOC recovery remain poorly understood. Here we present three new deglacial surface temperature and δ18Oseawater (δ18Osw) reconstructions from the western subtropical North and South Atlantic. Similarities to tropical Caribbean and western Atlantic δ18Osw records suggest that a salty surface water mass accumulated in the western Atlantic from 27°S to 33°N during periods of reduced AMOC. However, δ18Osw decreases led deep AMOC resumption by hundreds of years. We suggest that the northward export of salt previously trapped in the western Atlantic resulted in the early establishment of a shallow overturning circulation that eventually culminated in deep AMOC resumption, implying that AMOC may constitute a self-limiting system. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Distribution and redox speciation of dissolved iron on the European continental margin

To investigate the biogeochemistry of iron in the waters of the European continental margin, we determined the dissolved iron distribution and redox speciation in filtered (&lt;0.2 µm) open‐ocean and shelf waters. Depth profiles were sampled over the shelf slope southeast of the Chapelle Bank area (47.61°N, 4.24°W to 46.00°N, 8.01°W) and a horizontal surface‐water transect over the shelf and through the English Channel (la Manche) and the southern North Sea (46°N, 8°W to 52°N, 4°E). An abrupt trace‐metal front was found near the shelf slope, indicated by a horizontal gradient of dissolved iron (DFe) and aluminium (DAl), which correlated with changing salinities (r2 = 0.572 and 0.528, respectively, n = 92). Labile Fe(II) concentrations varied from &lt;12 pmol L−1 in North Atlantic surface waters to &gt;200 pmol L−1 in the near bottom waters of the shelf break. Labile Fe(II) accounted for ~5% of the dissolved iron species in surface shelf waters (mean 5.0 ± 2.7%), whereas higher Fe(II) fractions (i.e., &gt;8%) were observed near the sea bottom on the shelf break and during a midday solar maximum in surface waters in the vicinity of the Scheldt river plume. Benthic processes (resuspension and diagenesis) constituted important sources of Fe(II) and DFe in this region, and photoreduction of Fe(III) species in shelf waters caused enhanced labile Fe(II) concentrations. These processes increased the lability of iron and its potential availability to marine organisms in the shelf ecosystem.