On the interannual variability of arctic sea‐level pressure and sea ice

Abstract

Monthly mean sea‐level pressure (SLP) data from the Northern Hemisphere for the period January 1952‐December 1987 are analysed. Fluctuations in this field over the Arctic on interannual time‐scales and their statistical association with fluctuations farther south are determined. The standard deviation of the interannual variability is largest compared with that of the annual cycle along the seaboards of the major land masses. The SLP anomalies are generally in phase over the entire Arctic Basin and extend south over the northern Russia and Canada, but tend to be out of phase with fluctuations at mid‐latitudes. The anomalies are most closely associated with fluctuations over the North Atlantic and Europe except near the Chukchi Sea to the north of Bering Strait. The associations with the North Pacific fluctuations become increasingly more prominent at most Arctic sites (e.g. the Canadian Arctic Archipelago) as the time‐scale increases.

Associations between the SLP fluctuations and atmospheric indices that represent processes affecting sea‐ice drift (wind stress and wind stress curl) are determined. In every case local associations dominate, but some remote ones are also evident. For example, changes in the magnitude of the wind stress curl over the Beaufort Sea are increased if the atmospheric circulation over the North Pacific is intensified; wind stress over the region where sea ice is exchanged between the Beaufort Gyre and the Transpolar Drift Stream is modulated by both the Southern and North Atlantic Oscillations.

Severe sea‐ice conditions in the Greenland Sea (as measured by the Koch Ice Index) coincide with a weakened atmospheric circulation over the North Atlantic.     

On the relationship between arctic sea‐ice anomalies and fluctuations in Northern Canadian air temperature and river discharge

Abstract

A lagged cross‐correlation analysis of climatic data from the period 1953–1984 was carried out for three regions of Northern Canada (Beaufort Sea, Hudson Bay, Baffin Bay/Labrador Sea) to determine the relationships between sea‐ice anomalies and surface air temperature and river discharge anomalies. Significant negative correlations at the 95% level were found between sea‐ice and temperature anomalies. A significant correlation at the 95% level was found between sea‐ice and river discharge anomalies in only one of two subregions studied.     

Decadal‐to‐interdecadal fluctuations of arctic sea‐ice cover and the atmospheric circulation during 1954–1994

Abstract

The relationship between the Arctic and subarctic sea‐ice concentration (SIC) anomalies, particularly those associated with the decadal‐scale Greenland and Labrador Seas “Ice and Salinity Anomalies (ISAs) “, and the overlying atmospheric circulation fluctuations is investigated using the singular value decomposition (SVD) and composite map analysis methods. The data analyzed are monthly SIC and sea level pressure (SLP) anomalies, which cover the northern hemisphere poleward of 45°N and extend over the 41‐year period 1954–1994.

The SVD₁ (first) mode of the coupled variability, which accounts for 57% of the square covariance, is for the most part an atmosphere‐to‐ice forcing mode characterized by the decadal timescale. The aforementioned ISA anomalies are clearly captured by this mode whose SIC anomalies are dominated by a strong dipole across Greenland. However, as part of the same mode, there is also a weaker SIC dipole in the northern North Pacific which has opposite‐signed anomalies in the Sea of Okhotsk and the Bering Sea. It is also shown that there exists a significant negative correlation between the decadal SIC variability in the Greenland‐Barents Seas region associated with this mode and the North Atlantic Oscillation, whose spectrum also exhibits a quasi‐decadal signal.

The SVD₂ mode accounts for 12% of the square covariance and shows no evidence of a dominant forcing field of either SIC or SLP. This SVD mode exhibits very low frequency (interdecadal) variability, and its co‐variability is mainly concentrated in the northern North Pacific. It appears to be a high‐latitude extension of the recently investigated interdecadal North Pacific Oscillation. The spatial structure of the second mode complements the case of the first SVD mode whose co‐variability mainly occurs in the northern North Atlantic.     

Sea level responses to climatic variability and change in northern British Columbia

Abstract

Sea level responses to climatic variability (CV) and change (CC) signals at multiple temporal scales (interdecadal to monthly) are statistically examined using long‐term water level records from Prince Rupert (PR) on the north coast of British Columbia. Analysis of observed sea level data from PR, the longest available record in the region, indicates an annual average mean sea level (MSL) trend of +1.4±0.6 mm yr^?1 for the period (1939–2003), as opposed to the longer term trend of 1±0.4 mm yr^?1 (1909–2003). This suggests a possible acceleration in MSL trends during the latter half of the twentieth century. According to the results of this study, the causes behind this acceleration can be attributed not only to the effects of global warming but also to cyclic climate variability patterns such as the strong positive Pacific Decadal Oscillation (PDO) phase that has been present since the mid‐1970s. The linear regression model based on highest sea levels (MAXSL) of each calendar year showed a trend exceeding twice that (3.4 mm yr^?1) of MSL. Previous work shows that the influence of vertical crustal motions on relative sea level are negligible at PR.

Relations between sea levels and known CV indices (e.g., the Multivariate ENSO Index (MEI), PDO, Northern Oscillation Index (NOI), and Aleutian Low Pressure Index (ALPI)) are explored to identify potential controls of CV phenomena (e.g., the El Niño Southern Oscillation (ENSO), PDO) on regional MSL and MAXSL. Linear and non‐linear statistical methods including correlation analyses, multiple regression, Cumulative Sum (CumSum) analysis, and Superposed Epoch Analysis (SEA) are used. Results suggest that ENSO forcing (as shown by the MEI and NOI indices) exerts significant influence on winter sea level fluctuations, while the PDO dominates summer sea level variability. The observational evidence at PR also shows that, during the period 1939–2003, these cyclic shorter temporal scale sea level fluctuations in response to CV were significantly greater than the longer term sea‐level rise trend by as much as an order of magnitude and with trends over twice that of MSL. Such extreme sea level fluctuations related to CV events should be the immediate priority for the development of coastal adaptation strategies, as they are superimposed on long‐term MSL trends, resulting in greater hazard than longer term MSL rise trends alone.     

Sea‐ice dynamics and CO2 sensitivity in a global climate model

Abstract

Present‐day results and CO₂ sensitivity are described for two versions of a global climate model (genesis) with and without sea‐ice dynamics. Sea‐ice dynamics is modelled using the cavitating‐fluid method of Flato and Hibler (1990, 1992). The atmospheric general circulation model originated from the NCAR Community Climate Model version 1, but is heavily modified to include new treatments of clouds, penetrative convection, planetary boundary‐layer mixing, solar radiation, the diurnal cycle and the semi‐Lagrangian transport of water vapour. The surface models include an explicit model of vegetation (similar to BATS and SiB), multilayer models of soil, snow and sea ice, and a slab ocean mixed layer.

When sea‐ice dynamics is turned off, the CO₂‐induced warming increases drastically around ～60–80°S in winter and spring. This is due to the much greater (and unrealistic) compactness of the Antarctic ice cover without dynamics, which is reduced considerably when CO₂ is doubled and exposes more open ocean to the atmosphere. With dynamics, the winter ice is already quite dispersed for 1 × CO₂ so that its compactness does not decrease as much when CO₂ is doubled.     

Arctic Sea‐Ice extent and anomalies, 1953–1984

Abstract

A study is presented of the seasonal and interannual variability of Arctic sea‐ice extent over the 32‐year period 1953–84. The data set used consists of monthly sea‐ice concentration values given on a 1°‐latitude grid and represents a 7‐year extension of the 25‐year data set analysed by Walsh and Johnson (1979). By focussing attention on the variability in seven distinct subregions that circumscribe the polar region, a number of interesting spatial patterns emerge in the regional seasonal cycles and anomalies of ice coverage. For example, the time‐scale of the smoothed anomaly fluctuations varies from a 4–6 year cycle in the western Arctic (e.g. the Beaufort Sea) to a decadal one in the eastern Arctic (e.g. the Barents Sea). Also, in agreement with earlier studies, a significant out‐of‐phase relationship was found between the 25‐month smoothed anomalies in the Beaufort and Chukchi Sea region and the Greenland Sea. It is proposed that this behaviour is related to atmospheric pressure anomalies associated with the see‐saw in winter air temperature between northern Europe and western Greenland. Finally, a particularly large 9‐year ice anomaly in the Greenland Sea that was centred on 1968 appears to have evolved into a substantial 4‐year Labrador Sea anomaly that peaked in 1972. Both of these anomalies coincided with the passage of the “ Great Salinity Anomaly”, which traversed cyclonically around the subpolar gyre in the northern North Atlantic during the period 1968–82.     

Evaluation of ocean and climate models using present‐day observations and forcing

Abstract

The most common method used to evaluate climate models involves spinning them up under perpetual present‐day forcing and comparing the model results with present‐day observations. This approach clearly ignores any potential long‐term memory of the model ocean to past climatic conditions. Here we examine the validity of this approach through the 6000‐year integration of a coupled atmosphere–ocean–sea‐ice model. The coupled model is initially spun‐up with atmospheric CO₂ concentrations and orbital parameters applicable for 6KBP. The model is then integrated forward in time to 2100. Results from this transient coupled model simulation are compared with the results from two additional simulations, in which the model is spun up with perpetual 1850 (preindustrial) and 1998 (present‐day) atmospheric CO₂ concentrations and orbital parameters. This comparison leads to substantial differences between the equilibrium climatologies and the transient simulation, even at 1850 (in weakly ventilated regions), prior to any significant changes in atmospheric CO₂. When compared to the present‐day equilibrium climatology, differences are very large: the global mean surface air and sea surface temperatures are ,0.5°C and ,0.4°C colder, respectively, deep ocean temperatures are substantially cooler, Southern Hemisphere sea‐ice cover is 38% larger, and the North Atlantic conveyor 16% weaker in the transient case. These differences are due to the long timescale memory of the deep ocean to climatic conditions which prevailed throughout the late Holocene, as well as to its large thermal inertia. It is also demonstrated that a ‘cold start’ global warming simulation (one that starts from a 1998 equilibrium climatology) underestimates the global temperature increase at 2100 by ,10%. Our results question the accuracy of current techniques for climate model evaluation and underline the importance of using paleoclimatic simulations in parallel with present‐day simulations in this evaluation process.     

Sea‐ice interaction and the stability of the thermohaline circulation

Abstract

The role of sea‐ice in affecting the stability and long‐term variability of the oceanic thermohaline circulation (THC) is studied in this paper. The emphasis is placed on studying how sea‐ice might affect the stability and the long‐term variability of the THC through modulations of the surface heat and freshwater fluxes. A simple box model is analyzed to elucidate qualitatively the distinct physical meanings of these two processes. The analytical solution of this simple model indicates that, for the long timescales considered here, the thermal insulation stabilizes the THC while the freshwater feedback increases the effective inertia of the coupled ice‐ocean system. Sea‐ice insulation lessens the negative feedback between heat flux and the SST, and therefore, allows the SST to play a greater role in counteracting changes of the THC and high latitude salinity field. The freshwater feedback effectively links the surface heat flux to a freshwater reservoir, and thus, increases the effective inertia of the coupled ocean‐ice system. A two‐dimensional ocean model coupled with a thermodynamic sea‐ice model is used to estimate quantitatively the magnitudes of these two feedbacks. The numerical experiments involve the model's responses both to initial anomalies and to changes of forcing fields. For the free response cases (model responses to initial anomalies without changing the forcing fields), the model shows that the decay rate of an initial anomaly is greater when sea‐ice is included. For small perturbations the thermal insulation effect dominates over the freshwater feedback. The latter becomes increasingly more important for larger perturbations. In response to a change of external forcing, the presence of sea‐ice reduces the magnitude and the pace of the model's response. The numerical results are qualitatively consistent with the analytical solution of the box model.     

Validation of synoptic circulation patterns simulated by the Canadian climate centre general circulation model for western north America: Research note

Abstract

In order to assess the ability of a GCM to simulate regional to synoptic scale atmospheric structures, a correlation‐based computer‐assisted gridded map typing procedure is used to compare daily pressure (MSL) and geopotential height fields (500 hPa) from a GCM simulation of the present climate to a decade of NMC analyses. The model is able to reproduce the entire range of synoptic circulation types. However, statistically significant differences in the seasonal frequencies and variances of the main circulation types are evident. These differences, which are most pronounced in the winter (at 500 hPa) and in spring and autumn in the MSL fields, are consistent with subtle errors in the predicted fields at the hemispheric scale. The lack of agreement between the NMC climatology and the “control” simulation precludes extension of this approach to investigation of climate change impacts in western north America, and to more meteorologically dynamic extra‐tropical regions. The map‐typing procedure is shown to be an appropriate GCM synoptic‐scale validation tool that permits direct comparison of GCM output and observed fields. As such, it has the potential to elucidate the regional‐scale impacts of global climatic change through established synoptic circulation environment relationships.     

Interannual variability of sea‐ice cover in Hudson bay,Baffin bay and the Labrador sea

Abstract

The spatial and temporal relationships between subarctic Canadian sea‐ice cover and atmospheric forcing are investigated by analysing sea‐ice concentration, sea‐level pressure and surface air temperature data from 1953 to 1988. The sea‐ice anomalies in Hudson Bay, Baffin Bay and the Labrador Sea are found to be related to the North Atlantic Oscillation (NAO) and the Southern Oscillation (SO). Through a spatial Student's i‐test and a Monte Carlo simulation, it is found that sea‐ice cover in both Hudson Bay and the Baffin Bay‐Labrador Sea region responds to a Low/Wet episode of the SO (defined as the period when the SO index becomes negative) mainly in summer. In this case, the sea‐ice cover has a large positive anomaly that starts in summer and continues through to autumn. The ice anomaly is attributed to the negative anomalies in the regional surface air temperature record during the summer and autumn when the Low/Wet episode is developing. During strong winter westerly wind events of the NAO, the Baffin Bay‐Labrador Sea ice cover in winter and spring has a positive anomaly due to the associated negative anomaly in surface air temperature. During the years in which strong westerly NAO and Low/Wet SO events occur simultaneously (as in 1972/73 and 1982/83), the sea ice is found to have large positive anomalies in the study region; in particular, such anomalies occurred for a major portion of one of the two years. A spectral analysis shows that sea‐ice fluctuations in the Baffin Bay‐Labrador Sea region respond to the SO and surface air temperature at about 1.7‐, 5‐ and 10‐year periods. In addition, a noticeable sea‐ice change was found (i.e. more polynyas occurred) around the time of the so‐called “climate jump” during the early 1960s. Data on ice thickness and on ice‐melt dates from Hudson Bay are also used to verify some of the above findings.     

Simulation of sea ice and ocean variability in the arctic during 1955–2002 with an intermediate complexity model

Abstract

A number of recent sea‐ice and ocean changes in the Arctic and subarctic regions are simulated using the global University of Victoria (UVic) Earth System Climate Model version 2.6. This is an intermediate complexity model which includes a three‐dimensional ocean model (MOM 2.2), an energy‐moisture balance model for the atmosphere with heat and moisture transport, and a dynamic‐thermodynamic sea‐ice model with elastic‐viscous‐plastic rheology. The model is first spun up for 1800 years with monthly wind stress forcing derived from the National Centers for Environmental Prediction (NCEP) climatology winds and a pre‐industrial atmospheric CO₂ concentration of 280 ppm. After a second spin‐up for the period 1800–1947 with daily climatology winds‐tress forcing, and a linearly increasing atmospheric CO₂ concentration, the model is run with interannually varying wind stresses for the period 1948–2002 with an average forcing interval of 2.5 days and an exponentially increasing atmospheric CO₂ concentration varying from 315 to 365 ppm. However, the analysis of the model output is only carried out for the years 1955–2002.

The simulated maximum and minimum sea‐ice areas for the Arctic are within 6% of the observed climatologies for the years 1978–2001. The model output also shows a small downward trend in sea‐ice extent, which, however, is smaller than has been observed during the past few decades. In addition, the model simulates a decrease in sea‐ice thickness in the SCICEX (SCientific ICe EXpeditions) measurement area in the central Arctic that is consistent with, but smaller than, that observed from submarine sonar profiling data.

The observed variability and magnitude of the export of sea ice through Fram Strait is quite well captured in the simulation. The change in correlation between the North Atlantic Oscillation (NAO) index and the sea‐ice export around 1977 as found in a data study by Hilmer and Jung (2000) is also reproduced. Within the Arctic basin the model simulates well the patterns and the timing of the two major regimes of wind‐forced sea‐ice drift circulation (cyclonic and anticyclonic) as found earlier by Proshutinsky and Johnson (1997). The influence of variations in the Fram Strait ice export on the strength of the North Atlantic thermohaline circulation and surface air temperature are also determined. In particular, it is shown that 3–4 years after a large ice export, the maximum meridional overturning streamfunction decreases by more than 10%.

The temperature and salinity increase at depths of 200–300 m, as observed in the eastern Arctic by Morison et al. (1998), between the USS Pargo cruise in 1993 and the Environmental Working Group (EWG) Joint USRussian Arctic Atlas climatology for the years 1948–87, are just visible in the model simulation. The increases are more noticeable, however, when the ocean model data are averaged over the pentade 1995–2000 and compared with model data averaged over the pentade 1955–60. The fact that these, and some of the other modelled changes, are smaller than the observed changes can likely be attributed to the relatively coarse resolution of the UVic Earth System Climate Model (3.6°E‐W and 1.8°N‐S). Nevertheless, the fact that the model captures qualitatively many of the recent sea‐ice and ocean changes in the Arctic suggests that it can be successfully used to investigate other Arctic‐North Atlantic Ocean climate interactions during past and future eras.     

Sea level variability in the Bras d'Or Lakes

Abstract

An analysis of sea level variability during summer indicates that semi‐diurnal and diurnal tidal components are rapidly attenuated as they propagate into the Bras d'Or Lakes. On the other hand, low frequency sea level fluctuations originating in Sydney Bight, immediately outside the Lakes, are only moderately affected. The fluctuations in Sydney Bight are predominantly the result of the isostatic adjustment of sea level to atmospheric pressure changes. The resulting sea level gradient drives flow into or out of the Lakes. A simple numerical model consisting of a balance of acceleration, pressure gradient and friction accounts for most of the observations.     

Impact of Hudson Bay on the energy balance in the Hudson Bay Lowlands and the potential for climatic modification

Abstract

Three sites were instrumented to measure all components of the energy balance. The sites were located in the Churchill, Manitoba region and comprised a Sea Site on a sand spit 1 km seaward from the mainland, a Nearcoast Site 2 km inland from the coast and an Inland Site 65 km inland. Measurements were made continuously over a 90‐day period from 19 May to 16 August 1984. This period encompassed the bulk of the growing season.

The measurements were stratified into onshore and offshore wind directions and were compared for 10‐day periods. The comparisons show very significant differences attributable to the cold summer conditions promoted by the sea ice in Hudson Bay. The ground heat flux and latent heat flux were much greater during offshore winds but the sensible heat flux was greatest for onshore winds. Air temperatures averaged 7°C warmer for offshore than for onshore winds. The reasons for these differences are detailed and the climatic modifications that would probably result from earlier sea‐ice melt are discussed. Some implications of climatic modification are also noted.     

A simulation of sea‐ice motion and distribution off Newfoundland during LIMEX,March 1987

Abstract

A coupled ice‐ocean dynamical model is applied to the simulation of sea‐ice motion and distribution off Newfoundland during the Labrador Ice Margin Experiment (LIMEX), March 1987. In the model, the ice is coupled to a barotropic ocean through an Ekman layer that deepens with increasing wind speed. A 6‐hourly gridded wind dataset was used as input to drive the ice and the ocean. The results show that ice velocities with ice‐ocean coupling are appreciably higher than those without coupling because of the generation of wind‐driven coastal currents. This suggests that coupled ice‐ocean dynamics should always be considered in short‐term sea‐ice models. The model gives reasonable agreement with the observed ice edge except in the southern boundary where ice‐melt has a strong influence on the ice‐edge position. Ocean currents, sea level and ice velocities computed from the model are in qualitative agreement with limited current‐meter, tide‐gauge, and ice drifter trajectory data.     

The UVic earth system climate model: Model description,climatology, and applications to past,present and future climates

Abstract

A new earth system climate model of intermediate complexity has been developed and its climatology compared to observations. The UVic Earth System Climate Model consists of a three‐dimensional ocean general circulation model coupled to a thermodynamic/dynamic sea‐ice model, an energy‐moisture balance atmospheric model with dynamical feedbacks, and a thermomechanical land‐ice model. In order to keep the model computationally efficient a reduced complexity atmosphere model is used. Atmospheric heat and freshwater transports are parametrized through Fickian diffusion, and precipitation is assumed to occur when the relative humidity is greater than 85%. Moisture transport can also be accomplished through advection if desired. Precipitation over land is assumed to return instantaneously to the ocean via one of 33 observed river drainage basins. Ice and snow albedo feedbacks are included in the coupled model by locally increasing the prescribed latitudinal profile of the planetary albedo. The atmospheric model includes a parametrization of water vapour/planetary longwave feedbacks, although the radiative forcing associated with changes in atmospheric CO₂ is prescribed as a modification of the planetary longwave radiative flux. A specified lapse rate is used to reduce the surface temperature over land where there is topography. The model uses prescribed present‐day winds in its climatology, although a dynamical wind feedback is included which exploits a latitudinally‐varying empirical relationship between atmospheric surface temperature and density. The ocean component of the coupled model is based on the Geophysical Fluid Dynamics Laboratory (GFDL) Modular Ocean Model 2.2, with a global resolution of 3.6° (zonal) by 1.8° (meridional) and 19 vertical levels, and includes an option for brine‐rejection parametrization. The sea‐ice component incorporates an elastic‐viscous‐plastic rheology to represent sea‐ice dynamics and various options for the representation of sea‐ice thermodynamics and thickness distribution. The systematic comparison of the coupled model with observations reveals good agreement, especially when moisture transport is accomplished through advection.

Global warming simulations conducted using the model to explore the role of moisture advection reveal a climate sensitivity of 3.0°C for a doubling of CO₂, in line with other more comprehensive coupled models. Moisture advection, together with the wind feedback, leads to a transient simulation in which the meridional overturning in the North Atlantic initially weakens, but is eventually re‐established to its initial strength once the radiative forcing is held fixed, as found in many coupled atmosphere General Circulation Models (GCMs). This is in contrast to experiments in which moisture transport is accomplished through diffusion whereby the overturning is reestablished to a strength that is greater than its initial condition.

When applied to the climate of the Last Glacial Maximum (LGM), the model obtains tropical cooling (30°N‐30°S), relative to the present, of about 2.1°C over the ocean and 3.6°C over the land. These are generally cooler than CLIMAP estimates, but not as cool as some other reconstructions. This moderate cooling is consistent with alkenone reconstructions and a low to medium climate sensitivity to perturbations in radiative forcing. An amplification of the cooling occurs in the North Atlantic due to the weakening of North Atlantic Deep Water formation. Concurrent with this weakening is a shallowing of, and a more northward penetration of, Antarctic Bottom Water.

Climate models are usually evaluated by spinning them up under perpetual present‐day forcing and comparing the model results with present‐day observations. Implicit in this approach is the assumption that the present‐day observations are in equilibrium with the present‐day radiative forcing. The comparison of a long transient integration (starting at 6 KBP), forced by changing radiative forcing (solar, CO₂, orbital), with an equilibrium integration reveals substantial differences. Relative to the climatology from the present‐day equilibrium integration, the global mean surface air and sea surface temperatures (SSTs) are 0.74°C and 0.55°C colder, respectively. Deep ocean temperatures are substantially cooler and southern hemisphere sea‐ice cover is 22% greater, although the North Atlantic conveyor remains remarkably stable in all cases. The differences are due to the long timescale memory of the deep ocean to climatic conditions which prevailed throughout the late Holocene. It is also demonstrated that a global warming simulation that starts from an equilibrium present‐day climate (cold start) underestimates the global temperature increase at 2100 by 13% when compared to a transient simulation, under historical solar, CO₂ and orbital forcing, that is also extended out to 2100. This is larger (13% compared to 9.8%) than the difference from an analogous transient experiment which does not include historical changes in solar forcing. These results suggest that those groups that do not account for solar forcing changes over the twentieth century may slightly underestimate (～3% in our model) the projected warming by the year 2100.     

Cross‐equatorial error propagation: Research note

Abstract

Numerical simulation experiments published in 1974 by Daley have been repeated with a much higher resolution, spectral, shallow water model. With a forecast period extending toll d, it is shown that a global model in which only the largest scales are used at initial time in the Southern Hemisphere yields a more accurate forecast for the Northern Hemisphere than a hemispheric model does. Compared with a uniform high‐resolution, global model, the error in the Northern Hemisphere forecast is high in the ultra‐long waves but decreases rather rapidly while the resolution of the initial Southern Hemispheric data is increased.     

The role of local heat sources in synoptic activity within the polar basin

Abstract

A quasi‐geostrophic model of the atmosphere is used to determine the significance of the surface enthalpy flux in synoptic activity within the Polar Basin. Of primary interest is whether the enthalpy flux from open water in the seasonal sea‐ice zone is the predominant contributing mechanism or whether the advective fields of vorticity and thickness are controlling factors. This is of importance in discussions of the feedback processes between the atmosphere, cryosphere and ocean.

For a case selected in the Laptev Sea near the end of the fall period of ice growth, the surface enthalpy flux is as significant a contribution to synoptic activity as the vorticity advection is. The enthalpy flux is a relatively insignificant factor at this time in the Beaufort Sea, however, because of the smaller area of open water and the lower wind speeds associated with the weaker synoptic systems in this region. It is also relatively insignificant at both locations at the beginning of the fall freeze‐up interval and in June, during the melt period.     

Atmosphere‐ocean heat fluxes and stresses in general circulation models

Abstract

The coupling of atmospheric general circulation models (AGCMs) to oceanic general circulation models (OGCMs) requires that each behaves appropriately in the uncoupled mode. The lower boundary conditions for uncoupled AGCMs are particularly simple over the oceans and consist of the specified climatological sea surface temperatures and sea‐ice extents. AGCMs develop fluxes of energy, momentum and moisture in response to these specified sea surface temperatures while they interact with their internal dynamics and parametrized physics.

The atmosphere‐ocean fluxes of energy and momentum developed in a collection of twelve AGCMs are compared with the climatological estimates of these terms. For the snapshot provided by this particular collection of models, the fluxes developed in the AGCMs are qualitatively similar to the climatological estimates, but there may be quantitative differences of considerable magnitude for some models as well as scatter among model values. Both the observation‐based estimates and the model‐generated values of these basic climatological quantities deserve attention, and efforts in this area are briefly noted.     

Application of a coupled ice‐ocean model to the Labrador Sea

Abstract

The steady, coupled ice‐ocean circulation model of Willmott and Mysak (1989) for a meridional channel is applied to the Labrador Sea for the winter season. The model consists of a thermodynamic reduced‐gravity ocean combined with a variable thickness ice cover that is in thermal equilibrium. Upon specifying the forcing fields of surface air temperature, wind stress and water temperature along the open southern boundary, the winter climatological ice‐edge position, ice thickness, ocean circulation and temperature fields are determined in the channel domain. The sensitivity of the results to the various model parameters is examined. In particular, the optimum heat exchange coefficients for the interfaces of air‐water, ice‐water and air‐ice are found.

The model ice‐edge position compares favourably with the 50% winter climatological ice concentration isoline obtained from an analysis of 32 years (1953–84) of sea‐ice concentration data. The simulations of the ocean temperature and ice thickness are also quite realistic according to the observed records available. The model is also applied to two specific winters (1981 and 1983) during which anomalous sea‐ice and weather conditions prevailed in the Labrador Sea.     

Linking Arctic sea‐ice and atmospheric circulation anomalies on interannual and decadal timescales

Abstract

The relationship between Arctic sea‐ice concentration anomalies, particularly those associated with the “Great Salinity Anomaly” of 1968–1982, and atmospheric circulation anomalies north of 45°N is investigated. Empirical orthogonal function (EOF) analyses are performed on winter Arctic ice concentration from 1954 to 1990, sea level pressure and 500‐hPa heights from 1947 to 1994, and 850‐hPa temperatures from 1963 to 1994. Variability on both interannual and decadal timescales is apparent in the time series of the leading winter EOFs of all variables. The first EOF of winter sea‐ice concentration was found to characterize the patterns of ice variability associated with the Great Salinity Anomaly in the northern North Atlantic from 1968–82. Spatial maps of temporal correlation coefficients between the time series of the first EOF of winter sea‐ice concentration and the winter atmospheric anomaly fields are calculated at lags of 0 and ±7 year. Maximum correlations were found to exist when the time‐series of this ice EOF 1 leads the atmospheric anomaly fields by one year. A particularly interesting result is the connection between the presence of ice anomalies in the Greenland and Barents Seas and subsequent pressure anomalies of the same sign over the Irminger Basin and the Canadian Arctic. The main emphasis of the paper is to identify connections between Arctic sea‐ice and atmospheric circulation anomalies at interannual time‐scales.