Showing papers on "Thermohaline circulation published in 1999"

A Dipole Mode in the Tropical Indian Ocean

Abstract: For the tropical Pacific and Atlantic oceans, internal modes of variability that lead to climatic oscillations have been recognized1,2, but in the Indian Ocean region a similar ocean–atmosphere interaction causing interannual climate variability has not yet been found3. Here we report an analysis of observational data over the past 40 years, showing a dipole mode in the Indian Ocean: a pattern of internal variability with anomalously low sea surface temperatures off Sumatra and high sea surface temperatures in the western Indian Ocean, with accompanying wind and precipitation anomalies. The spatio-temporal links between sea surface temperatures and winds reveal a strong coupling through the precipitation field and ocean dynamics. This air–sea interaction process is unique and inherent in the Indian Ocean, and is shown to be independent of the El Nino/Southern Oscillation. The discovery of this dipole mode that accounts for about 12% of the sea surface temperature variability in the Indian Ocean—and, in its active years, also causes severe rainfall in eastern Africa and droughts in Indonesia—brightens the prospects for a long-term forecast of rainfall anomalies in the affected countries.

Coupled ocean–atmosphere dynamics in the Indian Ocean during 1997–98

Abstract: Climate variability in the Indian Ocean region seems to be, in some aspects, independent of forcing by external phenomena such as the El Nino/Southern Oscillation1,2,3,4 But the extent to which, and how, internal coupled ocean–atmosphere dynamics determine the state of the Indian Ocean system have not been resolved Here we present a detailed analysis of the strong seasonal anomalies in sea surface temperatures, sea surface heights, precipitation and winds that occurred in the Indian Ocean region in 1997–98, and compare the results with the record of Indian Ocean climate variability over the past 40 years We conclude that the 1997–98 anomalies—in spite of the coincidence with the strong El Nino/Southern Oscillation event—may primarily be an expression of internal dynamics, rather than a direct response to external influences We propose a mechanism of ocean–atmosphere interaction governing the 1997–98 event that may represent a characteristic internal mode of the Indian Ocean climate system In the Pacific Ocean, the identification of such a mode has led to successful predictions of El Nino5; if the proposed Indian Ocean internal mode proves to be robust, there may be a similar potential for predictability of climate in the Indian Ocean region

Remote Sea Surface Temperature Variations during ENSO: Evidence for a Tropical Atmospheric Bridge

Abstract: In an El Nino event, positive SST anomalies usually appear in remote ocean basins such as the South China Sea, the Indian Ocean, and the tropical North Atlantic approximately 3 to 6 months after SST anomalies peak in the tropical Pacific. Ship data from 1952 to 1992 and satellite data from the 1980s both demonstrate that changes in atmospheric circulation accompanying El Nino induce changes in cloud cover and evaporation which, in turn, increase the net heat flux entering these remote oceans. It is postulated that this increased heat flux is responsible for the surface warming of these oceans. Specifically, over the eastern Indian Ocean and South China Sea, enhanced subsidence during El Nino reduces cloud cover and increases the solar radiation absorbed by the ocean, thereby leading to enhanced SSTs. In the tropical North Atlantic, a weakening of the trade winds during El Nino reduces surface evaporation and increases SSTs. These relationships fit the concept of an “atmospheric bridge” that conne...

Forcing of the cold event of 8,200 years ago by catastrophic drainage of Laurentide lakes

Abstract: The sensitivity of oceanic thermohaline circulation to freshwater perturbations is a critical issue for understanding abrupt climate change1 Abrupt climate fluctuations that occurred during both Holocene and Late Pleistocene times have been linked to changes in ocean circulation2,3,4,5,6, but their causes remain uncertain One of the largest such events in the Holocene occurred between 8,400 and 8,000 calendar years ago2,7,8 (7,650–7,200 14C years ago), when the temperature dropped by 4–8 °C in central Greenland2 and 15–3 °C at marine4,7 and terrestrial7,8 sites around the northeastern North Atlantic Ocean The pattern of cooling implies that heat transfer from the ocean to the atmosphere was reduced in the North Atlantic Here we argue that this cooling event was forced by a massive outflow of fresh water from the Hudson Strait This conclusion is based on our estimates of the marine 14C reservoir for Hudson Bay which, in combination with other regional data, indicate that the glacial lakes Agassiz and Ojibway9,10,11,12, (originally dammed by a remnant of the Laurentide ice sheet) drained catastrophically ∼8,470 calendar years ago; this would have released >1014 m3 of fresh water into the Labrador Sea This finding supports the hypothesis2,7,8 that a sudden increase in freshwater flux from the waning Laurentide ice sheet reduced sea surface salinity and altered ocean circulation, thereby initiating the most abrupt and widespread cold event to have occurred in the past 10,000 years

Oceanic forcing of the wintertime North Atlantic Oscillation and European climate

Abstract: The weather over the North Atlantic Ocean, particularly in winter, is often characterized by strong eastward air-flow between the ‘Icelandic low’ and the ‘Azores high’, and by a ‘stormtrack’ of weather systems which move towards western Europe. The North Atlantic Oscillation — an index of which can be defined as the difference in atmospheric pressure at sea level between the Azores and Iceland — is an important mode of variability in the global atmosphere1,2 and is intimately related to the position and strength of the North Atlantic stormtrack owing to dynamic processes internal to the atmosphere3,4. Here we use a general circulation model of the atmosphere to investigate the ocean's role in forcing North Atlantic and European climate. Our simulations indicate that much of the multiannual to multidecadal variability of the winter North Atlantic Oscillation over the past half century may be reconstructed from a knowledge of North Atlantic sea surface temperature. We argue that sea surface temperature characteristics are ‘communicated’ to the atmosphere through evaporation, precipitation and atmospheric-heating processes, leading to changes in temperature, precipitation and storminess over Europe. As it has recently been proposed that there may be significant multiannual predictability of North Atlantic sea surface temperature patterns5, our results are encouraging for the prediction of European winter climate up to several years in advance.

Holocene periodicity in North Atlantic climate and deep-ocean flow south of Iceland

Abstract: Climate fluctuations during the past millennium are relatively well documented1 On a longer timescale, there is growing evidence of millennial-scale variability of Holocene climate, at periodicities of ∼2,500 and 950 years (possibly caused by changes in solar flux)2,3 and ∼1,500 years (maybe related to an internal oscillation of the climate system)4,5,6 But the involvement of deep water masses in these Holocene climate changes has yet to be established Here we use sediment grain-size data from the Iceland basin to reconstruct past changes in the speed of deep-water flow The study site is under the influence of Iceland–Scotland Overflow Water (ISOW), the flow of which is an important component of the ‘thermohaline’ circulation that modulates European climate Flow changes coincide with some known climate events (the Little Ice Age and the Mediaeval Warm Period), and extend over the entire Holocene epoch with aquasi-periodicity of ∼1,500 years The grain-size data indicate afaster ISOW flow when the climate of northern Europe is warmer However, a second mode of operation is observed in the early Holocene, when warm climate intervals are associated with slower ISOW flow At that time the melting remnant of land-based, glacial-age ice may have provided a sufficient source of fresh water to the ocean to reduce ISOW flow south of Iceland

On the water masses and mean circulation of the South Atlantic Ocean

Abstract: We examine recent observations of water mass distribution and circulation schemes at different depths of the South Atlantic Ocean to propose a layered, qualitative representation of the mean distribution of flow in this region This furthers the simple upper layer geostrophic flow estimates of Peterson and Stramma [1991] In addition, we assess how well ocean general circulation models (GCMs) capture the overall structure of flow in the South Atlantic in this regard The South Atlantic Central Water (SACW) is of South Atlantic origin in the subtropical gyre, while the SACW in the tropical region in part originates from the South Indian Ocean The Antarctic Intermediate Water in the South Atlantic originates from a surface region of the circumpolar layer, especially in the northern Drake Passage and the Falkland Current loop, but also receives some water from the Indian Ocean The subtropical South Atlantic above the North Atlantic Deep Water and north of the Antarctic Circumpolar Current (ACC) is dominated by the anticyclonic subtropical gyre In the eastern tropical South Atlantic the cyclonic Angola Gyre exists, embedded in a large tropical cyclonic gyre The equatorial part of the South Atlantic shows several depth-dependent zonal current bands besides the Angola Gyre Ocean GCMs have difficulty capturing this detailed zonal circulation structure, even at eddy-permitting resolution The northward extent of the subtropical gyre reduces with increasing depth, located near Brazil at 16°S in the near-surface layer and at 26°S in the Antarctic Intermediate Water layer, while the tropical cyclonic gyre progresses southward The southward shift of the northern part of the subtropical gyre is well resolved in global ocean GCMs However, high horizontal resolution is required to capture the South Atlantic Current north of the ACC The North Atlantic Deep Water in the South Atlantic progresses mainly southward in the Deep Western Boundary Current, but some water also moves southward at the eastern boundary

Paleoclimatology: Reconstructing Climates of the Quaternary

Abstract: Preface. Acknowledgments. Paleoclimatic Reconstruction. Introduction. Sources Of Paleoclimatic Information. Levels Of Paleoclimatic Analysis. Modeling In Paleoclimatic Research. Climate And Climatic Variation. The Nature Of Climate And Climatic Variation. The Climate System. Feedback Mechanisms. Energy Balance Of The Earth And Its Atmosphere. Timescales Of Climatic Variation. Variations Of The Earth's Orbital Parameters. Dating Methods I. Introduction And Overview. Radioisotopic Methods. Dating Methods II. Paleomagnetism. Dating Methods Involving Chemical Changes. Biological Dating Methods. Ice Cores. Introduction. Stable Isotope Analysis. Dating Ice Cores. Paleoclimatic Reconstruction From Ice Cores. Marine Sediments And Corals. Introduction. Paleoclimatic Information From Biological Material In Ocean Cores. Oxygen Isotope Studies Of Calcareous Marine Fauna. Relative Abundance Studies. Paleotemperature Records From Alkenones. Dissolution Of Deep-Sea Carbonates. Paleoclimatic Information From Inorganic Material In Ocean Cores. Coral Records Of Past Climate. Thermohaline Circulation Of The Oceans. Ocean Circulation Changes And Climate Over The Last Glacial-Interglacial Cycle. Changes In Atmospheric Carbon Dioxide: The Role of the Oceans. Orbital Forcing: Evidence From The Marine Record. Non-Marine Geological Evidence. Introduction. Loess. Periglacial Features. Snowlines And Glaciation Thresholds. Mountain Glacier Fluctuations. Lake-Level Fluctuations. Lake Sediments. Speleothems. Non-Marine Biological Evidence. Introduction. Former Vegetation Distribution From Plant Macrofossils. Insects. Pollen Analysis. Introduction. The Basis Of Pollen Analysis. How Rapidly Does Vegetation Respond To Changes In Climate? Pollen Analysis Of A Site: The Pollen Diagram. Mapping Vegetation Change: Isopolls And Isochrones. Quantitative Paleoclimatic Reconstructions Based On Pollen Analysis. Paleoclimatic Reconstruction From Long Quaternary Pollen Records. Dendroclimatology. Introduction. Fundamentals Of Dendroclimatology. Dendroclimatic Reconstructions. Isotope Dendroclimatology. Documentary Data: Introduction. Historical Records And Their Interpretation. Regional Studies Based On Historical Records. Records Climate Forcing Factors. Paleoclimate Models Introduction. Types Of Models. Sensitivity Experiments Using General Circulation Models. Model Simulations: 18ka B.P. To The Present. Coupled Ocean-Atmosphere Model Experiments Of The Thermohaline Circulation. GCM Paleoclimate Simulations And The Paleo Record. Appendix A: Further Considerations On Radiocarbon Dating Calculation Of Radiocarbon Age And Standardization Procedure. Fractionation Effects. World Wide Web-Based Resources In Paleoclimatology References. Index.

A mid-european decadal isotope-climate record from 15,500 to 5000 years B.P

Abstract: Oxygen-isotope ratios of precipitation (delta18OP) inferred from deep-lake ostracods from the Ammersee (southern Germany) provide a climate record with decadal resolution. The record in detail shows many of the rapid climate shifts seen in central Greenland ice cores between 15,000 and 5000 years before the present (B.P.). Negative excursions in the estimated delta18OP from both of these records likely reflect short weakenings of the thermohaline circulation caused by episodic discharges of continental freshwater into the North Atlantic. Deviating millennial-scale trends, however, indicate that climate gradients between Europe and Greenland changed systematically, reflecting a gradual rearrangement of North Atlantic circulation during deglaciation.

Alternative global Cretaceous paleogeography

Abstract: Plate tectonic reconstructions for the Cretaceous have assumed that the major continental blocks—Eurasia, Greenland, North America, South America, Africa, India, Australia, and Antarctica—had separated from one another by the end of the Early Cretaceous, and that deep ocean passages connected the Pacific, Tethyan, Atlantic, and Indian Ocean basins. North America, Eurasia, and Africa were crossed by shallow meridional seaways. This classic view of Cretaceous paleogeography may be incorrect. The revised view of the Early Cretaceous is one of three large continental blocks— North America–Eurasia, South America–Antarctica-India-Madagascar-Australia; and Africa—with large contiguous land areas surrounded by shallow epicontinental seas. There was a large open Pacific basin, a wide eastern Tethys, and a circum- African Seaway extending from the western Tethys (“Mediterranean”) region through the North and South Atlantic into the juvenile Indian Ocean between Madagascar-India and Africa. During the Early Cretaceous the deep passage from the Central Atlantic to the Pacific was blocked by blocks of northern Central America and by the Caribbean plate. There were no deep-water passages to the Arctic. Until the Late Cretaceous the Atlantic-Indian Ocean complex was a long, narrow, sinuous ocean basin extending off the Tethys and around Africa. Deep passages connecting the western Tethys with the Central Atlantic, the Central Atlantic with the Pacific, and the South Atlantic with the developing Indian Ocean appeared in the Late Cretaceous. There were many island land areas surrounded by shallow epicontinental seas at high sea-level stands.

Use of proxies in paleoceanography : examples from the South Atlantic

Abstract: Clues to Ocean History: a Brief Overview of Proxies.- Surface Water Circulation.- Sea-Surface Temperature Estimations Using a Modern Analog Technique with Foraminiferal Assemblages from Western Atlantic Quaternary Sediments.- The Distribution of Living Planktic Foraminifera in Relation to Southeast Atlantic Oceanography.- Coccolithophores as Indicators of Ocean Water Masses, Surface-Water Temperature, and Paleoproductivity - Examples from the South Atlantic.- Calcareous Dinoflagellate Cysts as Paleo-Environmental Tools.- Oxygen Isotope Values of Planktic Foraminifera: A Tool for the Reconstruction of Surface Water Stratification.- Stable Isotopes of Pteropod Shells as Recorders of Sub-Surface Water Conditions: Comparison to the Record of G. ruber and to Measured Values.- On the Reconstruction of Paleosalinities.- Bottom- and Deep Water Circulation.- Stable Carbon Isotopes in Benthic Foraminifera: Proxies for Deep and Bottom Water Circulation and New Production.- Carbonate Dissolution in the Deep-Sea: Methods, Quantification and Paleoceanographic Application.- Kaolinite and Chlorite as Tracers of Modern and Late Quaternary Deep Water Circulation in the South Atlantic and the Adjoining Southern Ocean.- Paleoproductivity and Nutrients.- Organic Carbon and Carbonate as Paleoproductivity Proxies: Examples from High and Low Productivity Areas of the Tropical Atlantic.- Biogenic Barium as a Proxy for Paleoproductivity: Methods and Limitations of Application.- Variability in Export Production Documented by Downward Fluxes and Species Composition of Marine Planktic Diatoms: Observations from the Tropical and Equatorial Atlantic.- Reliability of the 231Pa/230 Th Activity Ratio as a Tracer for Bioproductivity of the Ocean.- Sediment Redistribution, 230Thex - Normalization and Implications for the Reconstruction of Particle Flux and Export Paleoproductivity.- The South Atlantic Carbon Isotope Record of Planktic Foraminifera.- Reconstruction of Surface Ocean Nitrate Utilization Using Stable Nitrogen Isotopes in Sinking Particles and Sediments.- CO2 in Oceans and Atmosphere.- Alkenone ?13C as a Proxy for Past PCO2 in Surface Waters: Results from the Late Quaternary Angola Current.- Reassessing Foraminiferal Stable Isotope Geochemistry: Impact of the Oceanic Carbonate System (Experimental Results).- Implications of a Carbonate Ion Effect on Shell Carbon and Oxygen Isotopes for Glacial Ocean Conditions.- Atmospherical Circulation.- Pollen and Spores in Marine Sediments from the East Atlantic - A View from the Ocean into the African Continent.- Terrestrial Organic Matter in Marine Sediments: Analytical Approaches and Eolian-Marine Records in the Central Equatorial Atlantic.- Environmental Magnetism.- The Magnetic View on the Marine Paleoenvironment: Parameters, Techniques, and Potentials of Rock Magnetic Studies as a Key to Paleoclimatic and Paleoceanographic Changes.- Using Rock Magnetic Proxy Records for Orbital Tuning and Extended Time Series Analyses into the Super- and Sub-Milankovitch Bands.- Geomagnetic Events and Relative Paleointensity Records - Clues to High-Resolution Paleomagnetic Chronostratigraphies of Late Quaternary Marine Sediments?.- Modelling.- Simulation of Oxygen Isotopes in a Global Ocean Model.- Reconstructing and Modelling the Last Glacial Maximum: Beyond CLIMAP.- Data Management.- Data Management of Proxy Parameters with PANGAEA.

Importance of ice-ocean interactions for the global ocean circulation: A model study

Abstract: Numerical experiments are conducted with a coarse-resolution global ice-ocean model in order to determine to what degree the sea ice-ocean exchanges of heat, salt/freshwater, and momentum control the general circulation of the world ocean on long timescales. These experiments reveal that the formation of North Atlantic Deep Water (NADW) in the model results from the strong heat losses that occur at the oceanic surface in the high-latitude North Atlantic. The large-scale ice-ocean interactions have nearly no influence on this process. In particular, neglecting the freshwater flux associated with the southward ice transport at Fram Strait does not impact seriously on the salinity of the Greenland and Norwegian Seas. At equilibrium the absence of this freshwater flux is balanced by an enhanced oceanic freshwater transport from the Arctic. Furthermore, it appears that the model NADW formation does not critically depend on the media (ice or ocean) transporting the freshwater. Besides, both the salt/freshwater and heat exchanges between sea ice and ocean are crucial in the Southern Ocean for the deep water production, properties, and export. The large amount of brine released during ice formation on the model Antarctic continental shelf leads to very high salinities there. The resulting dense shelf waters are then transported toward great depths after some mixing with ambient waters, finally forming the Antarctic Bottom Water body. On the other hand, the net ice melting associated with ice convergence in some areas, such as the southwestern Pacific, stabilizes the water column and forbids deep mixing in these regions. Furthermore, the contact with the ice imposes that the polar surface waters must be maintained very close to their freezing point temperature. Our results suggest that this process takes an important part in increasing the density of the Antarctic Bottom Water. We also show that the modifications of the stress at the ocean surface induced by the internal ice stress have only a regional effect.

The mean flow field of the tropical Atlantic Ocean

Abstract: The mean horizontal sow Þeld of the tropical Atlantic Ocean is described between 20iN and 20iS from observations and literature results for three layers of the upper ocean, Tropical Surface Water, Central Water, and Antarctic Intermediate Water. Compared to the subtropical gyres the tropical circulation shows several zonal current and countercurrent bands of smaller meridional and vertical extent. The wind-driven Ekman layer in the upper tens of meters of the ocean masks at some places the sow structure of the Tropical Surface Water layer as is the case for the Angola Gyre in the eastern tropical South Atlantic. Although there are regions with a strong seasonal cycle of the Tropical Surface Water circulation, such as the North Equatorial Countercurrent, large regions of the tropics do not show a signiÞcant seasonal cycle. In the Central Water layer below, the eastward North and South Equatorial undercurrents appear imbedded in the westward-sowing South Equatorial Current. The Antarcic Intermediate Water layer contains several zonal current bands south of 3iN, but only weak sow exists north of 3iN. The sparse available data suggest that the Equatorial Intermediate Current as well as the Southern and Northern Intermediate Countercurrents extend zonally across the entire equatorial basin. Due to the convergence of northern and southern water masses, the western tropical Atlantic north of the equator is an important site for the mixture of water masses, but more work is needed to better understand the role of the various zonal under- and countercurrents in cross-equatorial water mass transfer. ( 1999 Elsevier Science Ltd. All rights reserved

The large deep water transient in the Eastern Mediterranean

Abstract: The recent changes in the thermohaline circulation of the Eastern Mediteranean caused by a transition from a system with a single source of deep water in the Adriatic to one with an additional source in the Aegean are described and assessed in detail. The name Cretan Sea Overflow Water (CSOW) is proposed for the new deep water mass. CSOW is warmer (θ>13.6°C) and more saline (S>38.80) than the previously dominating Eastern Mediterranean Deep Water (EMDW), causing temperatures and salinities to rise towards the bottom. All major water masses of the Eastern Mediterranean, including the Levantine Intermediate Water (LIW), have been strongly affected by the change. The stronger inflow into the bottom layer caused by the discharge of CSOW into the Ionian and Levantine Basins induced compensatory flows further up in the water column, affecting the circulation at intermediate depth. In the northeastern Ionian Sea the saline intermediate layer consisting of Levantine Intermediate Water and Cretan Intermediate Water (CIW) is found to be less pronounced. The layer thickness has been reduced by factor of about two, concurrently with a reduction of the maximum salinity, reducing advection of saline waters into the Adriatic. As a consequence, a salinity decrease is observed in the Adriatic Deep Water. Outside the Aegean the upwelling of mid-depth waters reaches depths shallow enough so that these waters are advected into the Aegean and form a mid-depth salinity-minimum layer. Notable changes have been found in the nutrient distributions. On the basin-scale the nutrient levels in the upper water column have been elevated by the uplifting of nutrient-rich deeper waters. Nutrient-rich water is now found closer to the euphotic zone than previously, which might induce enhanced biological activity. The observed salinity redistribution, i.e. decreasing values in the upper 500–1400 m and increasing values in the bottom layer, suggests that at least part of the transition is due to an internal redistribution of salt. An initiation of the event by a local enhancement of salinity in the Aegean through a strong change in the fresh water flux is conceivable and is supported by observations.

Warming of the tropical Atlantic Ocean and slowdown of thermohaline circulation during the last deglaciation

Abstract: Evidence for abrupt climate changes on millennial and shorter timescales is widespread in marine and terrestrial climate records1,2,3,4. Rapid reorganization of ocean circulation is considered to exert some control over these changes5, as are shifts in the concentrations of atmospheric greenhouse gases6. The response of the climate system to these two influences is fundamentally different: slowing of thermohaline overturn in the North Atlantic Ocean is expected to decrease northward heat transport by the ocean and to induce warming of the tropical Atlantic7,8, whereas atmospheric greenhouse forcing should cause roughly synchronous global temperature changes9. So these two mechanisms of climate change should be distinguishable by the timing of surface-water temperature variations relative to changes in deep-water circulation. Here we present a high-temporal-resolution record of sea surface temperatures from the western tropical North Atlantic Ocean which spans the past 29,000 years, derived from measurements of temperature-sensitive alkenone unsaturation in sedimentary organic matter. We find significant warming is documented for Heinrich event H1 (16,900–15,400 calendar years bp) and the Younger Dryas event (12,900–11,600 cal. yr bp), which were periods of intense cooling in the northern North Atlantic. Temperature changes in the tropical and high-latitude North Atlantic are out of phase, suggesting that the thermohaline circulation was the important trigger for these rapid climate changes.

Indian‐Atlantic interocean exchange: Dynamics, estimation and impact

Abstract: Interocean exchange of heat and salt around South Africa is thought to be a key link in the maintenance of the global overturning circulation of the ocean. It takes place at the Agulhas Retroflection, largely by the intermittent shedding of enormous rings that penetrate into the South Atlantic Ocean. This makes it extremely hard to estimate the inter ocean fluxes. Estimates of direct Agulhas leakage from hydrographic and tracer data range between 2 and 10 Sv (1 Sv = 106 m3 s−1). The average ring shedding frequency, determined from satellite information, is approximately six rings per year. Their associated interocean volume transport is between 0.5 and 1.5 Sv per ring. A number of Agulhas rings have been observed to cross the South Atlantic. They decay exponentially to less than half their initial size (measured by their available potential energy) within 1000 km from the shedding region. Consequently, most of their properties mix into the surroundings of the Benguela region, probably feeding directly into the upper (warm) limb of the global thermohaline circulation. The most recent observations suggest that in the present situation Agulhas water and Antarctic Intermediate Water are about equally important sources for the Benguela Current. Variations in the strength of these may lead to anomalous stratification and stability of the Atlantic at decadal and longer timescales. Modeling studies suggest that the Indian-Atlantic interocean exchange is strongly related to the structure of the wind field over the South Indian Ocean. This leads in the mean to a subtropical supergyre wrapping around the subtropical gyres of the South Indian and Atlantic Oceans. However, local dynamical processes in the highly nonlinear regime around South Africa play a crucial role in inhibiting the connection between the two oceans. The regional bottom topography also seems to play an important role in locking the Agulhas Currents' retroflection. State-of-the-art global and regional “eddy-permitting” models show a reasonably realistic representation of the mean Agulhas system; but the mesoscale variability and the local geometrical and topographic features that determine largely the interocean fluxes still need considerable improvement. In this article we present a review of the above mentioned aspects of the interocean exchange around South Africa: the estimation of the fluxes into the South Atlantic from different types of observations, our present level of understanding of the exchanges dynamics and forcing, its representation in state-of-the-art models, and, finally, the impact of the Indian-Atlantic fluxes on regional and global scale both within the Atlantic Ocean and in interaction with the overlying atmosphere.

Rapid changes in the mechanism of ocean convection during the last glacial period

Abstract: High-amplitude, rapid climate fluctuations are common features of glacial times The prominent changes in air temperature recorded in the Greenland ice cores1,2 are coherent with shifts in the magnitude of the northward heat flux carried by the North Atlantic surface ocean3,4; changes in the ocean's thermohaline circulation are a key component in many explanations of this climate flickering5 Here we use stable-isotope and other sedimentological data to reveal specific oceanic reorganizations during these rapid climate-change events Deep water was generated more or less continuously in the Nordic Seas during the latter part of the last glacial period (60 to 10 thousand years ago), but by two different mechanisms The deep-water formation occurred by convection in the open ocean during warmer periods (interstadials) But during colder phases (stadials), a freshening of the surface ocean reduced or stopped open-ocean convection, and deep-water formation was instead driven by brine-release during sea-ice freezing These shifting magnitudes and modes nested within the overall continuity of deep-water formation were probably important for the structuring and rapidity of the prevailing climate changes

Seasonal variability of the global ocean wind and wave climate

Abstract: A data set spanning a period of 10 years and obtained from a combination of satellite remote sensing and model predictions is used to construct a global climatology of ocean wind and wave conditions. Results are presented for: significant wave height, peak and mean wave period and wave direction as well as wind speed and direction. The results are presented in terms of mean monthly statistics. The processed data set provides global resolution of 2°. The climatology clearly shows the zonal variation in both wind speed and wave height, with extreme conditions occurring at high latitudes. The important role played by the intense wave generation systems of the Southern Ocean is clear. Swell generated from storms in the Southern Ocean penetrates throughout the Indian, South Pacific and South Atlantic Oceans. During the Southern Hemisphere winter, this swell even penetrates into the North Pacific. The results confirm visual observations that the Southern Ocean is consistently the roughest ocean on earth. It is shown, however, that this is mainly caused by consistent high wind speeds, rather than the extended westerly fetch which exists. The west coasts of most continents have noticeably rougher wave climates than their respective east coasts, as a result of the longer generation fetches which exist on the west coasts. Copyright © 1999 Royal Meteorological Society.

Long-term global warming scenarios computed with an efficient coupled climate model

Abstract: We present global warming scenarios computed with an intermediate-complexity atmosphere-ocean-sea ice model which has been extensively validated for a range of past climates (e.g., the Last Glacial Maximum). Our simulations extend to the year 3000, beyond the expected peak of CO2 concentrations. The thermohaline ocean circulation declines strongly in all our scenarios over the next 50 years due to a thermal effect. Changes in the hydrological cycle determine whether the circulation recovers or collapses in the long run. Both outcomes are possible within present uncertainty limits. In case of a collapse, a substantial long-lasting cooling over the North Atlantic and a drying of Europe is simulated.

Observation of α‐stable noise induced millennial climate changes from an ice‐core record

Abstract: The last glacial period showed millennium scale climatic shifts between two dieren t stable climate states. The state of thermohaline ocean circulation probably gov- erns the climate, and the triggering mechanism for climate changes is random uctuation s of the atmospheric forcing on the ocean circulation. The high temporal resolution paleo- climatic data from ice-cores are consistent with this picture and a bi-stable climate pseudo-potential can be derived. It is found that the fast time scale noise forcing the climate contains a component with an -stable distribution. As a consequence the abrupt climatic changes observed could be triggered by single extreme events. These events are re- lated to ocean-atmosphere dynamics on annual or shorter time scales and could indicate a fundamental limitation in predictability of climate changes. Paleoclimatic records from ice-cores (Dansgaard et al.,

Oceanic sinks for atmospheric CO2

Abstract: There is approximately 50 times more inorganic carbon in the global ocean than in the atmosphere. On time scales of decades to millions of years, the interaction between these two geophysical fluids determines atmospheric CO2 levels. During glacial periods, for example, the ocean serves as the major sink for atmospheric CO2, while during glacial–interglacial transitions, it is a source of CO2 to the atmosphere. The mechanisms responsible for determining the sign of the net exchange of CO2 between the ocean and the atmosphere remain unresolved. There is evidence that during glacial periods, phytoplankton primary productivity increased, leading to an enhanced sedimentation of particulate organic carbon into the ocean interior. The stimulation of primary production in glacial episodes can be correlated with increased inputs of nutrients limiting productivity, especially aeolian iron. Iron directly enhances primary production in high nutrient (nitrate and phosphate) regions of the ocean, of which the Southern Ocean is the most important. This trace element can also enhance nitrogen fixation, and thereby indirectly stimulate primary production throughout the low nutrient regions of the central ocean basins. While the export flux of organic carbon to the ocean interior was enhanced during glacial periods, this process does not fully account for the sequestration of atmospheric CO2. Heterotrophic oxidation of the newly formed organic carbon, forming weak acids, would have hydrolyzed CaCO3 in the sediments, increasing thereby oceanic alkalinity which, in turn, would have promoted the drawdown of atmospheric CO2. This latter mechanism is consistent with the stable carbon isotope pattern derived from air trapped in ice cores. The oceans have also played a major role as a sink for up to 30% of the anthropogenic CO2 produced during the industrial revolution. In large part this is due to CO2 solution in the surface ocean; however, some, poorly quantified fraction is a result of increased new production due to anthropogenic inputs of combined N, P and Fe. Based on ‘circulation as usual’, models predict that future anthropogenic CO2 inputs to the atmosphere will, in part, continue to be sequestered in the ocean. Human intervention (large-scale Fe fertilization; direct CO2 burial in the deep ocean) could increase carbon sequestration in the oceans, but could also result in unpredicted environmental perturbations. Changes in the oceanic thermohaline circulation as a result of global climate change would greatly alter the predictions of C sequestration that are possible on a ‘circulation as usual’ basis.

Unusual ocean-atmosphere conditions in the tropical Indian Ocean during 1994

Abstract: The southeastern tropical Indian Ocean (SETIO) was characterized by unusually cold sea surface temperature (SST) and strong northwestward alongshore surface winds during 1994. Using multi-source data sets including ocean model simulation, two key processes are identified for the SETIO cooling. Entrainment cooling produced most of the negative SST anomaly near the coast whereas evaporative cooling dominated the process away from the coast. Convection was anomalously suppressed over SETIO and the divergence of moist air from the region helped the local evaporative process. This also led to anomalous moisture transports that explain the enhanced convection over the central equatorial Indian Ocean, India and East Asia. The positive feedback between the enhanced and suppressed convection regions in turn helped maintain the surface wind anomalies. These evidences clearly indicate the existence of an ocean-atmosphere coupled phenomenon in the Indian Ocean during 1994.

Major flux of terrigenous dissolved organic matter through the Arctic Ocean

Abstract: High-latitude rivers supply the Arctic Ocean with a disproportionately large share of global riverine discharge and terrigenous dissolved organic matter (DOM). We used the abundance of lignin, a macromolecule unique to vascular plants, and stable carbon isotope ratios (δ13C) to trace the high molecular weight fraction of terrigenous DOM in major water masses of the Arctic Ocean. Lignin oxidation products in ultrafiltered DOM (UDOM; >1,000 Da) from Arctic rivers were depleted in syringyl relative to vanillyl phenols (S/V = 0.3–0.5) compared to UDOM in temperate and tropical rivers (S/V = 0.5–1.2), indicating that gymnosperm vegetation is a major source of terrigenous UDOM to the Arctic Ocean. High concentrations of lignin oxidation products (83–320 ng L−1) and a depletion of 13C (δ13C = −23.0 to −21.9) in UDOM throughout the surface Arctic Ocean indicate that terrigenous UDOM accounts for a much greater fraction of the UDOM in the surface Arctic (5–33%) than in the Pacific and Atlantic oceans (0.7–2.4%). In contrast, UDOM in deep water from the Arctic Ocean as well as waters from throughout the Greenland Gyre had relatively low concentrations of lignin oxidation products (24–45 ng L−1 ) and was enriched in 13C (δ13C = −21.0 to −20.8). Terrigenous UDOM has a relatively short residence (∼1–6 yr) in surface polar waters prior to export to the north Atlantic Ocean. Assuming that the bulk of Arctic-derived DOM is compositionally similar to the UDOM fraction, we estimate that 12–41% of terrigenous DOM (2.9–10.3 Tg C yr−1 ) discharged by rivers to the Arctic Ocean is exported to the North Atlantic via surface waters of the East Greenland Current. It appears very little terrigenous DOM from the Arctic is incorporated into North Atlantic Deep Water and distributed globally via deep thermohaline circulation.

Global warming and marine carbon cycle feedbacks on future atmospheric CO2

Abstract: A low-order physical-biogeochemical climate model was used to project atmospheric carbon dioxide and global warming for scenarios developed by the Intergovernmental Panel on Climate Change. The North Atlantic thermohaline circulation weakens in all global warming simulations and collapses at high levels of carbon dioxide. Projected changes in the marine carbon cycle have a modest impact on atmospheric carbon dioxide. Compared with the control, atmospheric carbon dioxide increased by 4 percent at year 2100 and 20 percent at year 2500. The reduction in ocean carbon uptake can be mainly explained by sea surface warming. The projected changes of the marine biological cycle compensate the reduction in downward mixing of anthropogenic carbon, except when the North Atlantic thermohaline circulation collapses.

Interhemispheric linkage of paleoclimate during the last glaciation

Abstract: Combined glacial geologic and palynologic data from the southern Lake District, Seno Reloncavi, and Isla Grande de Chiloe in middle latitudes (40°35’–42°25’S) of the Southern Hemisphere Andes suggest (1) that full-glacial or near-full-glacial climate conditions persisted from about 29,400 to 14,550 14C yr BP in late Llanquihue time, (2) that within this late Llanquihue interval mean summer temperature was depressed 6°–8°C compared to modern values during major glacier advances into the outer moraine belt at 29,400, 26,760, 22,295–22,570, and 14,550–14,805 14C yr BP, (3) that summer temperature depression was as great during early Llanquihue as during late Llanquihue time, (4) that climate deteriorated from warmer conditions during the early part to colder conditions during the later part of middle Llanquihue time, (5) that superimposed on long-term climate deterioration are Gramineae peaks on Isla Grande de Chiloe that represent cooling at 44,520–47,110 14C yr BP (T-11), 32,105–35,764 14C yr BP (T-9), 24,895–26,019 14C yr BP (T-7), 21,430–22,774 14C yr BP (T-5), and 13,040–15,200 14C yr BP (T-3), (6) that the initial phase of the glacial/interglacial transition of the last termination involved at least two major steps, one beginning at 14,600 14C yr BP and another at 12,700–13,000 14 C yr BP, and (7) that a late-glacial climate reversal of ≥2–3° C set in close to 12,200 14C yr BP, after an interval of near-interglacial warmth, and continued into Younger Dryas time. The late-glacial climate signal from the southern Chilean Lake District ties into that from proglacial Lago Mascardi in the nearby Argentine Andes, which shows rapid ice recession peaking at 12,400 14C yr BP, followed by a reversal of trend that culminated in Younger-Dryas-age glacier readvance at 11,400–10,200 14C yr BP. Many full- and late-glacial climate shifts in the southern Lake District match those from New Zealand at nearly the same Southern Hemisphere middle latitudes. At the last glacial maximum (LGM), snowline lowering relative to present-day values was nearly the same in the Southern Alps (875 m) and the Chilean Andes (1000 m). Particularly noteworthy are the new Younger-Dryas-age exposure dates of the Lake Misery moraines in Arthur's Pass in the Southern Alps. Moreover, pollen records from the Waikato lowlands on North Island show that a major vegetation shift at close to 14,700 14C yr BP marked the beginning of the last glacial/interglacial transition (Newnham et al. 1989). The synchronous and nearly uniform lowering of snowlines in Southern Hemisphere middle-latitude mountains compared with Northern Hemisphere values suggests global cooling of about the same magnitude in both hemispheres at the LGM. When compared with paleoclimate records from the North Atlantic region, the middle-latitude Southern Hemisphere terrestrial data imply interhemispheric symmetry of the structure and timing of the last glacial/interglacial transition. In both regions atmospheric warming pulses are implicated near the beginning of Oldest Dryas time (∼14,600 14C yr BP) and near the Oldest Dryas/Bolling transition (∼12,700–13,000 14 C yr BP). The second of these warming pulses was coincident with resumption of North Atlantic thermohaline circulation similar to that of the modern mode, with strong formation of Lower North Atlantic Deep Water in the Nordic Seas. In both regions, the maximum Bolling-age warmth was achieved at 12,200–12,500 14 C yr BP, and was followed by a reversal in climate trend. In the North Atlantic region, and possibly in middle latitudes of the Southern Hemisphere, this reversal culminated in a Younger-Dryas-age cold pulse. Although changes in ocean circulation can redistribute heat between the hemispheres, they cannot alone account either for the synchronous planetary cooling of the LGM or for the synchronous interhemispheric warming steps of the abrupt glacial-to-interglacial transition. Instead, the dominant interhemispheric climate linkage must feature a global atmospheric signal. The most likely source of this signal is a change in the greenhouse content of the atmosphere. We speculate that the Oldest Dryas warming pulse originated from an increase in atmospheric water-vapor production by half-precession forcing in the tropics. The major thermohaline switch near the Oldest Dryas/Bolling transition then couldhave triggered another increase in tropical water-vapor production to near-interglacial values.

Changing spatial structure of the thermohaline circulation in response to atmospheric CO2 forcing in a climate model

Abstract: The heat transported northwards by the North Atlantic thermohaline circulation warms the climate of western Europe1,2,3. Previous model studies4,5,6 have suggested that the circulation is sensitive to increases in atmospheric greenhouse-gas concentrations, but such models have been criticised for the use of unphysical ‘flux adjustments’7,8,9 (artificial corrections that keep the model from drifting to unrealistic states), and for their inability to simulate deep-water formation both north and south of the Greenland–Iceland–Scotland ridge, as seen in observations10,11. Here we present simulations of today's thermohaline circulation using a coupled ocean–atmosphere general circulation model without flux adjustments. These simulations compare well with the observed thermohaline circulation, including the formation of deep water on each side of the Greenland–Iceland–Scotland ridge. The model responds to forcing with increasing atmospheric greenhouse-gas concentrations by a collapse of the circulation and convection in the Labrador Sea, while the deep-water formation north of the ridge remains stable. These changes are similar intwo simulations with different rates of increase of CO2 concentrations. The effects of increasing atmospheric greenhouse-gas concentrations that we simulate are potentially observable, suggesting that it is possible to set up an oceanic monitoring system for the detection of anthropogenic influence on ocean circulation.

Mixing and Energetics of the Oceanic Thermohaline Circulation

Abstract: Using an idealized tube model and scaling analysis, the physics supporting the oceanic thermohaline circulation is examined. Thermal circulation in the tube model can be classified into two categories. When the cooling source is at a level higher than that of the heating source, the thermal circulation is friction-controlled; thus, mixing is not important in determining the circulation rate. When the cooling source is at a level lower than that of the heating source, the circulation is mixing controlled; thus, weak (strong) mixing will lead to weak (strong) thermal circulation. Within realistic parameter regimes the thermohaline circulation requires external sources of mechanical energy to support mixing in order to maintain the basic stratification. Thus, the oceanic circulation is only a heat conveyor belt, not a heat engine. Simple scaling shows that the meridional mass and heat fluxes are linearly proportional to the energy supplied to mixing. The rate of tidal dissipation in the open oceans (excluding the shallow marginal seas) is about 0.9‐1.3 (31012 W); the rate of potential energy generated by geothermal heating is estimated to be 0.5 3 1012 W. Accordingly, the global-mean rate of mixing inferred from oceanic climatological data is about 0.22 3 1024 m2 s21. Using a primitive equation model, numerical experiments based on a fixed energy source for mixing have been carried out in order to test the scaling law. In comparison with models under fixed rate of mixing, a model under a fixed energy for mixing is less sensitive to changes in the forcing conditions due to climatic changes. Under a surface relaxation condition for temperature and standard parameters, the model is well within the region of Hopf bifurcation, so decadal variability is expected.

Hydrography and circulation of the West Antarctic Peninsula Continental Shelf

Abstract: The water mass structure and circulation of the continental shelf waters west of the Antarctic Peninsula are described from hydrographic observations made in March—May 1993. The observations cover an area that extends 900 km alongshore and 200 km o⁄shore and represent the most extensive hydrographic data set currently available for this region. Waters above 100—150 m are composed of Antarctic Surface Water and its end member Winter Water. Below the permanent pycnocline is a modified version of Circumpolar Deep Water, which is a cooled and freshened version of Upper Circumpolar Deep Water. The distinctive signature of cold and salty water from the Bransfield Strait is found at some inshore locations, but there is little indication of significant exchange between Bransfield Strait and the west Antarctic Peninsula shelf. Dynamic topography at 200 m relative to 400 m indicates that the baroclinic circulation on the shelf is composed of a large, weak, cyclonic gyre, with sub-gyres at the northeastern and southwestern ends of the shelf. The total transport of the shelf gyre is 0.15 Sv, with geostrophic currents of order 0.01 m s~1. A simple model that balances across-shelf di⁄usion of heat and salt from o⁄shore Upper Circumpolar Deep Water with vertical di⁄usion of heat and salt across the permanent pycnocline into Winter Water is used to explain the formation of the modified Circumpolar Deep Water that is found on the shelf. Model results show that the observed thermohaline distributions across the shelf can be maintained with a coeƒcient of vertical di⁄usion of 10~4 m2 s~1 and horizontal di⁄usion coeƒcients for heat and salt of 200 and 1200 m2 s~1, respectively. When the e⁄ects of double di⁄usion are included in the model, the required horizontal di⁄usion coeƒcients for heat and salt are 200 and 400 m2 s~1, respectively. ( 1999 Elsevier Science Ltd. All rights reserved.