Changes In Surface Circulation Research Articles

Variability of Arabian Sea surface circulation and chlorophyll distribution: a remote sensing estimation

High resolution mean surface velocity field of the Arabian Sea was derived by combining sea level anomalies based on satellite altimetry data with the surface drifter data. The mean current field exhibits the western boundary Somali Current, weak westward North Equatorial Current, weak West Indian Coastal Current. Besides strong currents, the Arabian Sea exhibits strong mesoscale eddy activity in the western side. The seasonal surface circulation changes of the Arabian Sea and variability associated with the Indian Ocean Dipole (IOD) events were determined. The Somali Current is northward during summer, with a speed above 1.8 m/s in July and it reverses towards south in winter. Significant changes were found in the flow pattern during IOD events. An anticyclonic gyre near the equator was a conspicuous feature during the positive IOD. Further, the Great Whirl shows an earlier formation and dissipation during the positive IOD. Notably, a delay of southward reversal of the Somali Current is found during the positive IOD. The southward flow of West Indian Coastal Current during summer monsoon is much stronger during the positive IOD and the northward flow in fall is prominent during negative IOD. The eastward Equatorial Jet is absent in fall and instead an opposite (westward) flow prevails during the positive IOD. Enhanced Chl-a concentration occurs during the negative IOD, compared to the positive IOD, especially in the western Arabian Sea. The wind-driven upwelling and mesoscale eddies are significantly modifying the Chl-a distribution along the Somali Coast.

Exceptional 20th Century Shifts in Deep-Sea Ecosystems Are Spatially Heterogeneous and Associated With Local Surface Ocean Variability

Traditionally, deep-sea ecosystems have been considered to be insulated from the effects of modern climate change, but with the recognition of the importance of food supply from the surface ocean and deep-sea currents to sustaining these systems, the potential for rapid response of benthic systems to climate change is gaining increasing attention. However, very few ecological time-series exist for the deep ocean covering the twentieth century. Benthic responses to past climate change have been well-documented using marine sediment cores on glacial-interglacial timescales, and ocean sediments have also begun to reveal that planktic species assemblages are already being influenced by global warming. Here, we use benthic foraminifera found in mid-latitude and subpolar North Atlantic sediment cores to show that, in locations beneath areas of major surface water change, benthic ecosystems have also changed significantly over the last ∼150 years. The maximum benthic response occurs in areas which have seen large changes in surface circulation, temperature, and/or productivity. We infer that the observed surface-deep ocean coupling is due to changes in the supply of organic matter exported from the surface ocean and delivered to the seafloor. The local-to-regional scale nature of these changes highlights that accurate projections of changes in deep-sea ecosystems will require (1) increased spatial coverage of deep-sea proxy records, and (2) models capable of adequately resolving these relatively small-scale oceanographic features.

The Cyclonic Mode of Arctic Ocean Circulation

AbstractArctic Ocean surface circulation change should not be viewed as the strength of the anticyclonic Beaufort Gyre. While the Beaufort Gyre is a dominant feature of average Arctic Ocean surface circulation, empirical orthogonal function analysis of dynamic height (1950–89) and satellite altimetry–derived dynamic ocean topography (2004–19) show the primary pattern of variability in its cyclonic mode is dominated by a depression of the sea surface and cyclonic surface circulation on the Russian side of the Arctic Ocean. Changes in surface circulation after Arctic Oscillation (AO) maxima in 1989 and 2007–08 and after an AO minimum in 2010 indicate the cyclonic mode is forced by the AO with a lag of about 1 year. Associated with a one standard deviation increase in the average AO starting in the early 1990s, Arctic Ocean surface circulation underwent a cyclonic shift evidenced by increased spatial-average vorticity. Under increased AO, the cyclonic mode complex also includes increased export of sea ice and near-surface freshwater, a changed path of Eurasian runoff, a freshened Beaufort Sea, and weakened cold halocline layer that insulates sea ice from Atlantic water heat, an impact compounded by increased Atlantic Water inflow and cyclonic circulation at depth. The cyclonic mode’s connection with the AO is important because the AO is a major global scale climate index predicted to increase with global warming. Given the present bias in concentration of in situ measurements in the Beaufort Gyre and Transpolar Drift, a coordinated effort should be made to better observe the cyclonic mode.

Surface-circulation change in the southwest Pacific Ocean across the Middle Eocene Climatic Optimum: inferences from dinoflagellate cysts and biomarker paleothermometry

Abstract. Global climate cooled from the early Eocene hothouse (∼52–50 Ma) to the latest Eocene (∼34 Ma). At the same time, the tectonic evolution of the Southern Ocean was characterized by the opening and deepening of circum-Antarctic gateways, which affected both surface- and deep-ocean circulation. The Tasmanian Gateway played a key role in regulating ocean throughflow between Australia and Antarctica. Southern Ocean surface currents through and around the Tasmanian Gateway have left recognizable tracers in the spatiotemporal distribution of plankton fossils, including organic-walled dinoflagellate cysts. This spatiotemporal distribution depends on both the physicochemical properties of the water masses and the path of surface-ocean currents. The extent to which climate and tectonics have influenced the distribution and composition of surface currents and thus fossil assemblages has, however, remained unclear. In particular, the contribution of climate change to oceanographic changes, superimposed on long-term and gradual changes induced by tectonics, is still poorly understood. To disentangle the effects of tectonism and climate in the southwest Pacific Ocean, we target a climatic deviation from the long-term Eocene cooling trend: the Middle Eocene Climatic Optimum (MECO; ∼40 Ma). This 500 kyr phase of global warming was unrelated to regional tectonism, and thus provides a test case to investigate the ocean's physicochemical response to climate change alone. We reconstruct changes in surface-water circulation and temperature in and around the Tasmanian Gateway during the MECO through new palynological and organic geochemical records from the central Tasmanian Gateway (Ocean Drilling Program Site 1170), the Otway Basin (southeastern Australia), and the Hampden Beach section (New Zealand). Our results confirm that dinocyst communities track specific surface-ocean currents, yet the variability within the communities can be driven by superimposed temperature change. Together with published results from the east of the Tasmanian Gateway, our new results suggest a shift in surface-ocean circulation during the peak of MECO warmth. Simultaneous with high sea-surface temperatures in the Tasmanian Gateway area, pollen assemblages indicate warm temperate rainforests with paratropical elements along the southeastern margin of Australia. Finally, based on new age constraints, we suggest that a regional southeast Australian transgression might have been coincident with the MECO.

Interannual Variation of Surface Circulation in the Japan/East Sea due to External Forcings and Intrinsic Variability

The interannual variation of surface ocean currents can be as large as seasonal variation in the Japan/East Sea (JES). To identify the major factors that cause such interannual variability of surface ocean circulation in the JES, surface circulation was simulated from 1998 to 2009 using a three-dimensional model. Contributions of atmospheric forcing (ATM), open boundary data (OBC), and intrinsic variability (ITV) of the surface flow in the JES on the interannual variability of surface ocean circulation were separately examined using numerical simulations. Variability in surface circulation was quantified in terms of variance in sea surface height, 100-m depth water temperature, and surface currents. ITV was found to be the dominant factor that induced interannual variabilities of surface circulation, the main path of the East Korea Warm Current (EKWC), and surface kinetic energy on a time scale of 2–4 years. OBC and ATM were secondary factors contributing to the interannual variation of surface circulation. Interannual variation of ATM changed the separation latitude of EKWC and increased the variability of surface circulation in the Ulleung Basin. Interannual variation of OBC enhanced low-frequency changes in surface circulation and eddies in the Yamato Basin. It also modulated basin-wide uniform oscillations of sea level. This study suggests that precise estimation of initial conditions using data assimilation is essential for long-term prediction of surface circulation in the JES.

HIGH-RESOLUTION SURFACE CIRCULATION OF THE BAY OF BENGAL DERIVED FROM SATELLITE OBSERVATION DATA

High-resolution surface circulation and its variability of the Bay of Bengal are derived by combining surface drifter and satellite observation data. The satellite altimetry data, satellitetracked surface drifter data and ocean surface winds from satellite scatterometers during 1993-2012 are used. The estimated velocities show good agreement with in-situ acoustic Doppler current profiler observations. The estimated velocity components are significantly correlated with monthly mean velocity components from Research Moored Array for African- Asian-Australian Monsoon Analysis and Prediction buoy data. The mean circulation exhibits the strong western boundary current, zonal currents and weak eastern boundary flow. Large spatial and temporal variations are found in the western boundary current and intense mesoscale eddy activity in the western Bay of Bengal. Significant changes in surface circulation during positive and negative Indian Ocean Dipole (IOD) events are evident. During positive IOD, the eastward equatorial jet is reversed and the western boundary current is much weakened. Meanwhile, the western boundary current is the prominent flow during negative IOD events. High eddy kinetic energy is found during strong IOD events.

North Atlantic ecosystem sensitivity to Holocene shifts in Meridional Overturning Circulation

AbstractRapid 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.

Can large scale surface circulation changes modulate the sea surface warming pattern in the Tropical Indian Ocean?

The increased rate of Tropical Indian Ocean (TIO) surface warming has gained a lot of attention in the recent years mainly due to its regional climatic impacts. The processes associated with this increased surface warming is highly complex and none of the mechanisms in the past studies could comprehend the important features associated with this warming such as the negative trends in surface net heat fluxes and the decreasing temperature trends at thermocline level. In this work we studied a previously unexplored aspect, the changes in large scale surface circulation pattern modulating the surface warming pattern over TIO. We use ocean reanalysis datasets and a suit of Ocean General Circulation Model (OGCM) experiments to address this problem. Both reanalysis and OGCM reveal strengthening large scale surface circulation pattern in the recent years. The most striking feature is the intensification of cyclonic gyre circulation around the thermocline ridge in the southwestern TIO. The surface circulation change in TIO is mainly associated with the surface wind changes and the geostrophic response to sea surface height decrease in the western/southwestern TIO. The surface wind trends closely correspond to SST warming pattern. The strengthening mean westerlies over the equatorial region are conducive to convergence in the central and divergence in the western equatorial Indian Ocean (IO) resulting central warming and western cooling. The resulting east west SST gradient further enhances the equatorial westerlies. This positive feedback mechanism supports strengthening of the observed SST trends in the equatorial Indian Ocean. The cooling induced by the enhanced upwelling in the west is compensated to a large extent by warming due to reduction in mixed layer depth, thereby keeping the surface temperature trends in the west to weak positive values. The OGCM experiments showed that the wind induced circulation changes redistribute the excess heat received in the western TIO to central and east thereby enhancing warming in the central equatorial IO. The increased surface warming in central TIO increases the latent heat loss, and keeps the net heat flux trends negative. Model sensitivity experiments reveal that the subsurface cooling at thermocline level in TIO is contributed by variability in Pacific via Indonesian Through Flow whereas the surface warming trend is mainly induced by the changes in the local forcing. The long term changes in IO Rossby waves are not induced by local atmospheric forcing but are forced by Pacific. The thermocline shoaling in the west is therefore amplified by the remote influence of Pacific via wave transmission through Indonesian archipelago.

Recent changes in Arctic Ocean circulation revealed by iodine‐129 observations and modeling

Anthropogenic radionuclides released into European coastal waters from nuclear fuel reprocessing plants at Sellafield (UK) and La Hague (France) flow northward through the Nordic Seas and label Atlantic Water (AW) entering the Arctic Ocean. Transport of the soluble radionuclide 129I through the Arctic Ocean has been simulated using a numerical model for the period from 1970 to 2010. The simulated tracer distributions closely conform to 129I measurements made across the Arctic Ocean during the mid‐1990s and 2000s and clearly illustrate the dramatic changes in oceanic circulation which occurred during this time. The largest changes in surface circulation were associated with the transition from a negative to a positive phase of the Arctic Oscillation in the early 1990s and the subsequent return to a weak positive phase in the late 1990s and early 2000s. Model and experimental results indicate that a new circulation regime evolved after 2004 when a period of intense, anti‐cyclonic surface stress led to a strengthening of the Beaufort Gyre. We submit that this resulted in a suppression of the cyclonic boundary current of mid‐depth Atlantic Water (AW) below the Beaufort Gyre, with upper AW in the Canada Basin showing signs of a reversal from cyclonic to anti‐cyclonic flow. These results are consistent with the development of a new AW circulation scheme involving a separation between flow at intermediate depths in the Eurasian and Canada Basins which could eventually result in modification of the Arctic intermediate water which feeds the overflows.

Spatial and temporal changes in sea surface temperature, circulation and subpolar front of the East Sea (Japan Sea) during the last 130,000 years

In order to reconstruct spatial and temporal changes in sea surface temperature (SST) of the East Sea (Japan Sea) on glacial–interglacial time scale, alkenone temperature records from the both Korean and Japan margins have been investigated for the period over the last 130 kyr. Results show that the temperature gradient between the Korean and Japan margins was relatively small (~ 2 °C) during the warm interval of the interglacial period (MIS 5e). But it was large (~ 10 °C) during the transitional period from the interglacial to the glacial (MIS 5a and b). The temperature of the Korean margin was colder than that of the Japan margin during the transitional period. Based on the reconstructed alkenone temperature, temporal changes in surface circulation and subpolar front (SPF) have been investigated. During the interglacial period when sea level was higher than today (MIS 5e), the influx of the Tsushima Warm Current (TWC) was enhanced and the both Korean and Japan margins were influenced by the warm water. At that time, the SPF appears to be located in the northern part compared to today. During the transitional period when the sea level was lower than today (MIS 5a and b), the TWC volume transport was reduced contributing to the lower SST. Its effect on the surface temperature appears to be larger along the Korean margin than the Japan margin, indicating that the strength of the East Korea Warm Current appears to be strongly correlated with the TWC volume transport. The SPF would displace southward orienting from southwest to northeast at that time. During the glacial period when the sea level was very lower (~ 100 m deep), the influx of warm water was almost completely restricted and the cold water expanded into the southern area. The SPF would disappear at that time. Model simulation results also support the relationship between the TWC volume transport and surface circulation and the SST.

Albian calcareous nannofossils from ODP Site 1258, Demerara Rise

Site 1258, drilled during Ocean Drilling Program (ODP) Leg 207 on Demerara Rise off the northern coast of South America, recovered organic-rich Albian sediments that yield abundant, moderately to well preserved calcareous nannofossils. Biostratigraphic analysis shows the section primarily spans Roth’s (Initial Reports DSDP 44. US Government Printing Office, Washington (1978) 731) middle to late Albian Zone NC9. A disconformity separates these sediments from overlying uppermost Albian laminated shales from Zone NC10. The presence of Seribiscutum primitivum within the Albian section represents the first known occurrences of this species at such low latitudes, as Demerara Rise was located within 15° of the equator during the mid-Cretaceous. This species exhibits a bipolar distribution and is considered a cool-water, high-latitude species. Its presence on Demerara Rise indicates cooler water incursions either through changes in surface circulation or upwelling conditions during the opening of the Equatorial Atlantic.

Coccolith evidence for instabilities in surface circulation south of Iceland during Holocene times

The coccolith-based micropaleontological investigation of an exceptionally thick deep-sea Holocene sediment core was conducted in order to document the pattern and timing of surface circulation changes south of Iceland, over the Gardar contour drift, during the last 12 000 years. Fluctuations in bulk carbonate content at the core location are primarily driven by the coccolith fraction. The observed overall correlation between bottom flow speed, as given by the ‘sortable silt’ mean size index, and the bulk coccolith concentration (abundance/g dry sediment) suggests that the accumulation of this fine carbonate fraction is, as a first order, controlled by processes of sediment redistribution by bottom current. The down-core variations in coccolith assemblage structure (species %) indicate that changes in properties of surface waters south of Iceland occurred as two distinct steps at ca. 11.2 and 6 ka. The onset of North Atlantic Drift water influence over the Gardar Drift after 11.2 ka was associated with excess export flux of coccolith carbonate in the vicinity of an active frontal system. The period from 10 to 6 ka saw the progressive warming of the study area, which culminated between 6 and 7 ka. Cooling of the surface waters after 6 ka, as indicated changes in the relative abundances of the dominant coccolith species, took place in two phases, the present hydrological regime being only established after a last cooling step between 3.5 and 2.8 ka. These long-term reorganisations of the surface hydrology are interpreted as the response of the North Atlantic to the combined force of the solar insolation and the waning Laurentide ice sheet. Millennial-scale perturbations of the surface hydrology are documented by changes in accumulation of the species Emiliania huxleyi. These successive decreases in the export fluxes of E. huxleyi exhibit a distinct millennial pacing, in phase with previously recorded Holocene advection of cool, ice-bearing waters from the Greenland–Iceland seas to the British islands. From a clear analogy with the most recent extreme variation in global ocean climate, we argue that mechanisms responsible for these observed millennial-scale perturbations might be found in long-term modulations of the atmospheric processes associated with the North Atlantic Oscillation (NAO).

Late Quaternary palaeo-oceanography of the Denmark Strait overflow pathway, South-East Greenland margin

Direct interaction between the atmosphere and the deep ocean basins takes place today only in the Southern Ocean near the Antarctic continent and in the northern extremity of the North Atlantic Ocean, notably in the Norwegian–Greenland Sea and Labrador Sea. Cooling and evaporation cause surface waters in the latter region to become dense and sink. At depth, further mixing occurs with Arctic water masses from adjacent polar shelves. Export of these water masses from the Norwegian–Greenland Sea (Norwegian Sea Overflow Water) to the North Atlantic basin occurs via two major gateways, the Denmark Strait system and the Faeroe– Shetland Channel and Faeroe Bank Channel system (e.g. Dickson et al. 1990; Fig.1). Deep convection in the Labrador Sea produces intermediate waters (Labrador Sea Water), which spreads across the North Atlantic. Deep waters thus formed in the North Atlantic (North Atlantic Deep Water) constitute an essential component of a global ‘conveyor’ belt extending from the North Atlantic via the Southern and Indian Oceans to the Pacific. Water masses return as a (warm) surface water flow. In the North Atlantic this is the Gulf Stream and the relatively warm and saline North Atlantic Current. Numerous palaeo-oceanographic studies have indicated that climatic changes in the North Atlantic region are closely related to changes in surface circulation and in the production of North Atlantic Deep Water. Abrupt shut-down of the ocean-overturning and subsequently of the conveyor belt is believed to represent a potential explanation for rapid climate deterioration at high latitudes, such as those that caused the Quaternary ice ages. Here it should be noted, that significant changes in deep convection in Greenland waters have also recently occurred. While in the Greenland Sea deep water formation over the last decade has drastically decreased, a strong increase of deep convection has simultaneously been observed in the Labrador Sea (Sy et al. 1997).

Late Quaternary palaeo-oceanography of the Denmark Strait overflow pathway, South-East Greenland margin

Direct interaction between the atmosphere and the deep ocean basins takes place today only in the Southern Ocean near the Antarctic continent and in the northern extremity of the North Atlantic Ocean, notably in the Norwegian–Greenland Sea and Labrador Sea. Cooling and evaporation cause surface waters in the latter region to become dense and sink. At depth, further mixing occurs with Arctic water masses from adjacent polar shelves. Export of these water masses from the Norwegian–Greenland Sea (Norwegian Sea Overflow Water) to the North Atlantic basin occurs via two major gateways, the Denmark Strait system and the Faeroe– Shetland Channel and Faeroe Bank Channel system (e.g. Dickson et al. 1990; Fig.1). Deep convection in the Labrador Sea produces intermediate waters (Labrador Sea Water), which spreads across the North Atlantic. Deep waters thus formed in the North Atlantic (North Atlantic Deep Water) constitute an essential component of a global ‘conveyor’ belt extending from the North Atlantic via the Southern and Indian Oceans to the Pacific. Water masses return as a (warm) surface water flow. In the North Atlantic this is the Gulf Stream and the relatively warm and saline North Atlantic Current. Numerous palaeo-oceanographic studies have indicated that climatic changes in the North Atlantic region are closely related to changes in surface circulation and in the production of North Atlantic Deep Water. Abrupt shut-down of the ocean-overturning and subsequently of the conveyor belt is believed to represent a potential explanation for rapid climate deterioration at high latitudes, such as those that caused the Quaternary ice ages. Here it should be noted, that significant changes in deep convection in Greenland waters have also recently occurred. While in the Greenland Sea deep water formation over the last decade has drastically decreased, a strong increase of deep convection has simultaneously been observed in the Labrador Sea (Sy et al. 1997).

Deglacial surface circulation changes in the northeastern Atlantic: Temperature and salinity records off NW Scotland on a century scale

Sea surface temperature and salinity estimates reconstructed from a core collected on the Barra Fan, northwest Scotland (56°43′N, 09°19′W; water depth 1320 m), show a series of rapid oscillations during the last deglacial period that are very similar to those observed in the δ18O records from Greenland ice cores. These records indicate that the transport of heat and salt toward the Nordic Seas was highest during the Bølling period. This “superconveyor” weakened after the Bølling, probably as a consequence of increased meltwater flux reducing the oceanic salt content, as suggested by the Barbados sea‐level record. Evidence for a phase of ice rafting during the Allerød is presented for the first time from this latitude in the northeast Atlantic. The Younger Dryas stadial, resolved here at a century/decadal scale, is characterized by very rapid oscillations in temperature and salinity, indicating that warm, relatively saline waters repeatedly displaced cool polar waters at this latitude. These observations attest to the inherent instability of the deglacial climate system.

Molluscan and foraminiferal biostratigraphy of an Eemian‐Early Weichselian section on Karmøy, southwestern Norway

Sejrup, Hans Petter 1987 03 01: Molluscan and foraminiferal biostratigraphy of an Eemian‐Early Weichselian section on Karmøy, southwestern Norway. Boreas, Vol. 16, pp. 27–42. Oslo. ISSN 0300–9483.At Karmøy, southwestern Norway, a section with marine sediments from the last interglacial (the Avaldsnes Interglacial) and from two ice‐free periods (the Torvastad and Bø Interstadial) in the Weichselian have been examined for molluscs and foraminifera. The following conclusions concerning the depositional environments of these sediments can be drawn: (1) The Avaldsnes Interglacial was a high‐energy environment with a sea level 20 to 50 m higher than at present, regressing towards the end of the interglacial. Sea temperatures were as in the area today or slightly warmer. (2) During the Torvastad Interstadial (71–85 ka) the sea level was between 0 and c. 20 m higher than at present, and sea temperatures were as between Svalbard and northern Norway today. (3) The Bø Interstadial (40–64 ka) shows a complete interstadial cycle, with changing sea level and temperatures. Its optimum was close to the conditions prevailing in North Norway today or slightly colder. By comparison with other sites, a total of at least four interstadial episodes through the Weichselian in southwestern Norway is proposed. These date to c. 30 ka, 40–64 ka, 71–85 ka and 87–101 ka. The episodes and the glacial advances between them do not directly correlate with published interpretations of changes in surface circulation in the Norwegian Sea through the Weichselian. It is suggested that the nourishment of the southern part of the Scandinavian ice sheet might be more related to sea surface conditions in the North Atlantic than to those of the Norwegian Sea.