Showing papers on "Ocean current published in 1984"

Rapid atmospheric CO2 variations and ocean circulation

Abstract: Studies on air trapped in old polar ice1,2 have shown that during the last ice age, the atmospheric carbon dioxide concentration was probably significantly lower than during the Holocene—about 200 p.p.m. rather than 270 p.p.m. Also, Stauffer et al.3 recently showed by detailed analyses of Greenland ice cores, that during the ice age, between about 30,000 and 40,000 yr BP, the atmospheric CO2 level probably varied between 200 and 260 p.p.m. These variations occurred parallel to climatic variations as indicated by δ18O of the ice; astonishingly, the changes took place within rather short times, no more than a few centuries or even less. Here we examine the hypothesis4 that CO2 variations arose from changes in ocean circulation that affected the distribution of chemical properties and thus of P CO2 in the surface waters of the world ocean. Such changes can take place in a rather short time, in contrast to changes of whole ocean properties.

A review of the Southern Oscillation: oceanic-atmospheric circulation changes and related rainfall anomalies

Abstract: In this review of the Southern Oscillation we consider oceanic and atmospheric circulation changes and related rainfall anomalies with special emphasis on the region of South America. The climate anomalies associated with Southern Oscillation—El Nino (ENSO) events are highly persistent and nearly global in extent. The persistent nature of these events derives from strong coupling between atmosphere and ocean. Although the initial causes of the oscillation are unclear, once initiated the Southern Oscillation (SO)follows a certain sequence of events with well-defined effects on rainfall over a large portion of the tropics and subtropics. Drought in Australia, Indonesia, India, West Africa and Northeast Brazil as well as excessive rainfall in the central and eastern Pacific, Peru, Ecuador and Southern Brazil are all related to the SO. ENSO events are also associated with dramatic changes in the tropospheric flow pattern over a broad area of both hemispheres. Wintertime upper tropospheric subtropical jets are especially pronounced as are changes in the low level trade wind regime of both the South Pacific and South Atlantic Oceans. Mid-latitude blocking patterns are also more frequent in certain regions during ENSO events. DOI: 10.1111/j.1600-0870.1984.tb00264.x

Average Seasonal Variation of the Atlantic Equatorial Currents from Historical Ship Drifts

Abstract: Surface currents in the tropical Atlantic were studied using historical ship-drift data. These are the only available data capable of resolving the long-term seasonal fluctuations of currents over a broad geographical region. The North Equatorial Countercurrent was found to extend as a continuous eastward-flowing current across the Atlantic in the band 5–8°N from July to December. The maximum average eastward velocity was ∼30 cm s−1 during July-September. East of 20°W the countercurrent continued throughout the year. West of 20°W and from January to June the countercurrent disappeared and westward currents were observed. Average westward velocity reached 20 cm β−1 in April and May, centered near 40°W. The annual cycle in surface velocity (20 cm s−1 amplitude) agrees closely with that of geostrophic velocity inferred from variations in thermocline depth across the counter current jets, centered near 2°N and 4°S, separated by a minimum near 1°S. The northern jet has a large (14 cm s−1) semiannual v...

The circulation of the Mozambique channel

Abstract: Based on hydrographic data from 1977 to 1980 off the coast of Mozambique and historical data from the Mozambique Channel, the general circulation pattern of the area is described. The circulation pattern is characterized by the influence of three anticyclonic gyres covering the northern, the central, and the southern parts of the channel. Additionally, smaller cyclonic eddies are observed, of which some probably are topographically induced. The results strongly indicate that in the upper 1000 m the role of the Mozambique Current as one of the tributaries to the Agulhas Current is of minor significance and draw into question the concept of the Mozambique Current as a continuous one.

Thermohaline circulation below the Ross Ice Shelf: A consequence of tidally induced vertical mixing and basal melting

Abstract: The warmest water below parts of the Ross Ice Shelf resides in the lowest portion of the water column because of its high salinity. Vertical mixing caused by tidal stirring can thus induce ablation by lifting the warm but dense water into contact with the ice shelf. A numerical tidal simulation indicates that vertically well-mixed conditions predominate in the southeastern part of the sub-ice shelf cavity, where the water column thickness is small. Basal melting in this region is expected to be between 0.05 and 0.5 m/yr and will drive a thermohaline circulation having the following characteristics: high salinity shelf water (at - 1.8 C), formed by winter sea ice production in the open Ross Sea, flows along the seabed toward the tidal mixing fronts below the ice shelf; and meltwater (at -2.2 C), produced in the well-mixed region, flows out of the sub-ice shelf cavity along the ice shelf bottom. Sensitivity of this ablation process to climatic change is expected to be small because high salinity shelf water is constrained to have the sea surface freezing temperature.

Influence of Oceanic Heat Transport Upon the Sensitivity of a Model Climate

Abstract: The influence of oceanic heat transport on the sensitivity of climate to an increase of the atmospheric CO 2 concentration is studied by comparing the CO2-induced changes of two mathematical models. The first model is a general circulation model of the coupled ocean-atmosphere system which includes ocean currents. In the second model the oceanic component of the first model is replaced by a simple mixed layer without ocean currents. Both models have limited computational domain with idealized geography and annual mean insolation. For each model, the sensitivity of climate is evaluated from the difference between the equilibrium climates of the normal CO2 and 4 times the normal COconcentrations. The results indicate that the presence of ocean currents reduces the sensitivity of surface air temperature because of the difference in magnitude of the surface albedo feedback effect. The poleward transport of heat by ocean currents raises the surface temperature at high latitudes, shifts poleward the margins of snow and sea ice, decreases the contribution of the albedo feedback effect, and reduces the sensitivity of climate. The equilibrium response of climate is compared with the transient response of climate to a sudden increase of atmospheric COcontent. According to this comparison, the latitudinal dependence of the equilibrium response of zonally averaged surface temperature is qualitatively similar to the tran- sient response approximately 25 years after the time of the sudden CO 2 increase. This result suggests that  the distribution of the zonally averaged temperature change in response to a gradual increase of atmo- spheric carbon dioxide also resembles the distribution of the equilibrium response provided that the characteristic time scale of the COincrease is longer than 25 years. Several studies have been made of the climatic effect of increasing the atmospheric CO2 concentration in general circulation models of the atmosphere (e.g., Manabe and We- theraid, 1975, 1980). However, none of these studies takes into consideration the effect of oceanic heat transport, with the exception of the recent study by Bryan et al. (1982) which investigates the transient response of climate to a sudden in- crease of CO2 in a coupled ocean-atmosphere model. The present study is an outgrowth of their study. We investigate the influence of oceanic heat transport upon the equilibrium response of climate to an increase in the atmospheric CO2 concentration. Very little is known about the effects of ocean currents upon the sensitivity of the climate. Held and $uarez (1974) specu- lated that the poleward transport of heat by ocean currents reduces the meridional temperature gradient, increases the latitudinal shift of the margin of snow-covered area re- sponding to a given change in surface temperature, and thus enhances climate sensitivity by increasing the contribution of the albedo feedback process. Their argument applies to the comparison of two climates with the same amount of ice but with different meridional temperature gradients. However, we also expect oceanic heat transport to shift the margins of snow cover and sea ice poleward. In one-dimensional energy bal- ance models of the atmosphere (e.g., Budyko, 1969), the equi- librium response of surface temperature to a given change in the solar constant typically decreases as the ice margin re- treats toward the poles. If this effect dominates, oceanic heat transport should reduce the sensitivity of climate. In view of these counteracting influences of oceanic heat transport, we decided to investigate whether ocean currents enhance or reduce the sensitivity of climate. For this purpose,

An Introduction to the World's Oceans

Abstract: 1 The History of Oceanography 2 The Water Planet 3 Plate Tectonics 4 The Sea Floor and Its Sediments 5 The Physical Properties of Water 6 The Chemistry of Seawater 7 The Structure and Motion of the Atmosphere 8 Circulation and Ocean Structure 9 The Surface Currents 10 The Waves 11 The Tides 12 Coasts, Beaches, and Estuaries 13 Environmental Issues and Concerns 14 The Living Ocean 15 Production and Life 16 The Plankton: Drifters of the Open Ocean 17 The Nekton: Free Swimmers of the Sea 18 The Benthos: Dwellers of the Sea Floor Appendixes Glossary Credits Index

Apparent changes in the climatic state of the deep North Atlantic Ocean

Abstract: Determination of any long-term changes in the large-scale characteristics of the deep ocean circulation would be an important clue in understanding the climatic interactions of the ocean and atmosphere. In the summer of 1981, the RV Atlantis II reoccupied two transatlantic sections at nominal latitudes of 24°30′ N and 36°16′ N with a conductivity–temperature-depth instrument (CTD). One purpose of the work was to make a comparison with previous surveys conducted during the International Geophysical Year (IGY)1, when sections were obtained in October 1957 and April–May 1959. We report here that significant warming occurred in an ocean-wide band from 700 m to 3,000 m with a maximum temperature difference of 0.2 °C. These changes are sufficient to expand the water column by several centimetres. The historical temperature–salinity curve was apparently unchanged. Interannual changes in local water mass characteristics have been proposed previously2,3. Perhaps it would be most surprising if no changes were seen to occur. What remains obscure is the significance of these changes and the extent to which they represent long-term climate trends, or merely the minor and random fluctuations to be expected in any complex fluid system.

An Eclectic Atlantic Ocean Circulation Model. Part I: The Meridional Flux of Heat

Abstract: A model of the Atlantic has been formulated that combines ordinary quasi-geostrophic constraints (based upon the dynamic method and Ekman layer) with a great variety of additional information available about the time-average ocean circulation. The goal is to combine very diverse data types and beliefs and to be able to test for compatibility and incremental usefulness as a way around the paucity of conventional data, a lack of which otherwise greatly hinders determination of the circulation. The approach is axiomatic. Such a model is based here upon the use of linear inequality constraints, which permit the combination of the dynamic method with “core layer”-like constraints, as well as observations of deep water velocities, overflow transports and the like. The model is then exploited to find absolute bounds (maxima and minima) upon the annual mean and seasonal meridional fluxes of heat and the maximum rate of tropical near-surface upwelling. Some latitudes of nearly vanishing mean meridional he...

Instability waves observed on the Equator in the Atlantic Ocean during 1983

Abstract: A packet of surface confined westward propagating waves apparently generated by barotropic instability of the surface currents was observed in the equatorial Atlantic during 1983. Initial wavenumber analyses and energetics calculations are presented. Local working by the horizontal Reynolds stress was large enough and at the correct time to account for the wave packet's existence. The amplitude of the waves was comparable to the mean currents implying that the waves are of primary importance in the momentum balance of the seasonally varying equatorial surface currents.

Circulation On the Continental Shelf of the Southeastern United States. Part I: Subtidal Response to Wind and Gulf Stream Forcing During Winter

Abstract: Subtidal current and sea level response to wind and Gulf Stream forcing are investigated for the South Atlantic Bight shelf during winter conditions. Low-frequency flow variability in the outer shelf results primarily from wavelike meanders and eddies in the Gulf Stream front that occur in a 2-day to 2-week period band. Current meter derived vertically integrated momentum balances indicated that these large amplitude flow events are in approximate geostrophic balance with baroclinic pressure gradients induced by northward propagating Gulf Stream disturbances. Low-frequency flow at midshelf is primarily a local Ekman response to wind forcing. Cross-shelf momentum balance for the total water column is between the along-shelf geostrophic current and the cross-shelf barotropic pressure gradient resulting from wind induced sea level changes at the coast. This balance holds for both mean and fluctuating parts of the flow, with the along-shelf barotropic current lagging sea level by 6 to 12 hours and al...

Fluctuations of monthly sea level as related to the intensity of the Gulf Stream from Key West to Norfolk

Abstract: Tide gauge data from Key West to Norfolk were used to identify a monthly signal in sea level that is uncorrelated with local shelf-trapped processes. Time series (1955–1975) of local winds, sea level slope, and river runoff were used in a regression model of sea level to separate a local response and a residual signal. The monthly means of the residual contribution were investigated for their relationship to the seasonal fluctuations of the Gulf Stream. In the Florida Channel, lower sea level is found to correspond to increased flow of the Florida Current. During July and August a marked fall in residual sea level, unrelated to the coastal winds, is found from Key West to Charleston, suggesting that the transport of the Gulf Stream increases both in and north of the Florida Channel during this time. Measured long-term monthly surface currents at Diamond Shoals, Cape Hatteras, which markedly increase in summer to high velocities, tend to substantiate this claim. An additional wintertime low in residual sea level occurs north of the Florida Channel. The wintertime low does not result from steric heating within the upper 100–150 m of water, nor does it, in contrast to the summertime low, appear to coincide with increased northward surface flow. Monthly mean Sverdrup transport was computed across the North Atlantic between 15°N and 35°N. When compared at the same latitude, residual sea level and Sverdrup transport, both interpreted as indices of Gulf Stream transport, generally disagree in phase during summer. However, north of the Florida Channel they are consistent during winter, assuming that lower sea level at this time reflects increased flow in the stream.

Atmospheric chlorofluoromethanes as tracers of ocean circulation and mixing: measurement and calibration techniques and studies in the Greenland and Norwegian seas

Abstract: EXTRACT (SEE PDF FOR FULL ABSTRACT): An analytical system was designed and constructed for the rapid and accurate shipboard measurement of anthropogenic chlorofluoromethanes in seawater and in air, using electron capture gas chrometography. The distribution of these compounds in the marine atmosphere and the water column in the Greenland and Norwegian seas were studied during February and March, 1982. The compounds, dissolved in the ocean from the atmosphere, can be used as tracers of subsurface ocean circulation and mixing processes.

Excess 227 Ac in deep ocean water

Abstract: Understanding the nature and rates of processes by which materials are transported and mixed in the deep ocean is essential for evaluating environmental effects and risks involved in human uses of the ocean, such as mining of manganese nodules and deep ocean dumping of wastes. I report here the first measurements of 227 Ac (half life, t1/2, 21.8 yr) in seawater. The results clearly show the presence of excess 227 Ac relative to its progenitor 231Pa (see Fig. 1) in deep waters below 3 km, suggesting that the 227 Ac is supplied by diffusion from sediments. Two other natural radionuclides, 222Rn (ref. 1) and 228Ra (ref. 2) which have bottom sources, have proved to be useful for determining rates of mixing from days to decades. The excess 227 Ac can be used as a novel tracer for basin-wide circulation and mixing on time scales up to 100 yr.

The Three-Dimensional Circulation near the Eastern North pacific Subtropical Front

Abstract: The purpose of our study is to describe and compute the large-scale, three-dimensional circulation near the Subtropical Front in the eastern North Pacific along 31°N. This was accomplished through the use of four extensive hydrographic surveys, historical wind-stress data and also the movement of surface drifters. Our results indicate that, in wintertime, surface water sinks on the north side of the front and rises on its south side. During the summer, however, the subtropical salty surface water overflows the frontal area to the north. Potential vorticity and heat are best conserved in a vertical flow pattern where the annual mean Ekman convergence sinks to a depth of 300 m and water upwells throughout the main thermocline. The computed horizontal flow below 700 m amounts to less than 0.6 cm s−1; both strength and direction depend greatly on the treatment of noise within the data set and also on the conservation statement that is specified in addition to geostrophic and hydrostatic dynamics. A q...

A three-dimensional numerical shelf-sea front model with variable eddy viscosity and diffusivity

Abstract: A three-dimensional numerical model of circulation and eddy development in shelf-sea fronts is applied to three frontal structures, with two parameterization schemes for vertical eddy viscosity and diffusivity. The three fronts resemble those in the German Bight (a front between relatively fresh coastal water and saltier water offshore, with an interface extending from surface to bottom), the Norwegian Coastal Current (also formed by fresh coastal water but with a thermocline on one side), and the Celtic Sea (a front between water which is stratified in summer and water which is well mixed throughout the year). The two mixing assumptions, modelling the reduction of turbulence in stratified zones, are based on the Munk-Anderson scheme and the turbulent energy equation. Many features of frontal dynamics are common to all the results: strong surface currents along the front, cross-frontal circulation cells, a considerable enhancement of vertical velocities when eddies are formed, and development of eddies into cyclonic-anticyclonic vortex pairs. Cross-frontal circulation and frontal sharpening are the variables most sensitive to the different mixing assumptions. The German Bight front is the one most affected by changing these assumptions. The comparisons suggest that realistic results may be obtained from models despite the present uncertainty about vertical mixing in stratified shelf seas.

The Potential for Ocean Prediction and the Role of Altimeter Data

Abstract: : A skillful ocean forecasting capability will depend on adequate data input and computing power, and properly designed and adequately validated ocean models for data assimilation and forecasting . Once these conditions are satisfied, forecasts of meandering currents and eddies up to a few months appear to be a reasonable expectation. Simulation studies suggest that prediction of meandering currents and eddies requires high horizontal resolution. High vertical resolution can be obtained by using the circulation model to provide horizontal and vertical advection to a grid of one-dimensional mixed-layer models. Circulation models on subdomains of major ocean basins with extensive open ocean boundaries require accurate boundary information at all levels in the vertical and for the duration of forecasts longer than a few days. Basin-scale models appear to be the most promising source for this information. To provide useful boundary conditions, the basin-scale models must also resolve the meandering currents and eddies. Class 7 computers approx. expected on the market in the mid to late 1980s, are required for eddy-solving models of major ocean basins.

A View from the Ocean

Abstract: Migration is defined as a coming and a going with the seasons on a regular basis and a distinction is made between local and more extensive movements in terms of fish-body lengths (103 L and 105 L, respectively), the former being likely to be associated with scalar and the latter with vector quantities. It is suggested that clue and cue be used, relative to migration, in the sense of information as to where and when respectively. The open ocean is considered as the sea area where oceanic currents are stronger than tidal currents and the importance to migration of the main circulatory features of the ocean, at different scales, is discussed. Boundaries (at the surface, between water masses, and near the bottom) are identified as sites where migrants might obtain directional clues.

Wind-driven circulation on the northern Great Barrier Reef continental shelf in summer

Abstract: Observations of wind, atmospheric pressure, sea levels and current are presented for the northern (9°14′S) Great Barrier Reef continental shelf for November 1981 to May 1982. Strong low-frequency non-tidal oceanic fluctuations were observed, resulting in alternating northward and southward transport and a weak ‘mean’ circulation, and are the result of long ‘arrested’ topographic waves driven by a quasi-steady longshore wind stress and damped by turbulent bottom friction. The coefficient of friction for low-frequency currents is found to be proportional to the tidal velocities. The coefficient is also found to be much larger than that in the central (15–19°S) region of the Great Barrier Reef continental shelf, and this difference is attributed to the greatly enhanced energy dissipation by secondary circulation around coral reefs in the reef-studded northern region. Sough of Cape York (10·5°S), the primary effect of the cross-shelf wind is to complement the geostrophic set-up due to the longshore current. Intense tidal currents in the vicinity of Cape York combined with a dissipative western boundary in Torres Strait appear to prevent long wave propagation north of Cape York where a steady-state analytical model is used to show that both cross- and longshore wind components generate the reversing currents observed through Bligh Entrance.

Observations of the California Countercurrent.

Abstract: : This thesis describes results from moored current meters, 150-350 m, for a region over the continental slope off Cape San Martin, California, from January 1979 to April 1980 Current vector time series were constructed from the data and compared to a local coastal upwelling index Progressive vector diagrams were also constructed, and spectrum analysis was performed for alongshore and cross-slope currents The California Countercurrent was found to be present in the study area during the entire period Seasonally, the countercurrent was substantially stronger during the spring Frequent current reversals and oscillations occurred between equatorward and poleward flow, less often at the nearshore station Preferred low frequency energy peaks were found at periods of about 10 days The intensity of the countercurrent increased with increasing coastal upwelling index, and the cross-slope flow also appeared to be related to the local coastal upwelling index Keywords include: California undercurrent; Davidson current; California current; Eastern boundary currents; and metered currents

Temporal variability of the Antarctic Circumpolar Current observed from satellite altimetry

Abstract: Sea level measurements by the Seasat altimeter were used to study the temporal variability of the Antarctic Circumpolar Current between July and October 1978. Large-scale zonal coherence in the cross-stream sea level difference was observed, indicating a general increase in the surface geostrophic velocity of the current around the Southern Ocean. The result demonstrates the power of satellite altimetry to monitor the variability of large-scale ocean currents.

Circulation in the Cretaceous Western Interior Seaway of North America, a Review

Abstract: Geologists have used several approaches in modeling circulation of the Cretaceous Western Interior seaway of North America. These include qualitative and numerical models of atmospheric circulation, surface currents, rainfall, storm tracks and storm-driven currents, salinity stratification, and tides. Some of the models are theoretical, others are derived empirically from the information in the rock record. Although the predictions of various models agree in many respects, a number of discrepancies among the models also exists. Integration of data from the rock record and model predictions shows great promise for an eventual understanding of circulation in the Western Interior seaway in particular, as well as epicontinental seas in general.

Possible gyre–gyre interaction in the Pacific Ocean

Abstract: The subtropical anticyclonic gyres of the North and South Pacific Ocean are among the largest circulation systems found in the ocean. The gyres contain the warm waters of the warm water sphere1,2, and the rotation of these water masses is subject to low-frequency variations3. The presence in these gyres of baroclinic Rossby waves with periods of many years has been documented4,5 and this seems to be sufficient proof that the gyres undergo vacillations. Using sea-level data we demonstrate here that the subtropical gyres of the North and South Pacific oscillate with a period of about 4 years and that these oscillations are probably induced by the major El Nino events.

A model of estuarine circulation in the Pliocene Mediterranean based on new ostracod evidence

Abstract: The recent discovery of the psychrospheric ostracod Agrenocythere pliocenica (Seguenza) in Zanclean marls of eastern Crete1 expands the reach of early Pliocene deep-oceanic circulation far into the eastern Mediterranean Current models of Pliocene water-mass circulation, though allowing for the Atlantic psychrosphere to flow freely into the western basin, consider the eastern basin as being largely stagnant2,3 I propose here a new model of estuarine circulation affecting both west and east Mediterranean This circulation was associated with increased atmospheric humidity and driven by upwelling The model is compatible with organic-rich sedimentation in parts of the eastern Mediterranean and does not imply an excessively deep or wide connection with the Atlantic