A Model Study of the Relationship between Sea-Ice Variability and Surface and Intermediate Water Mass Properties in the Labrador Sea

Sea ice plays an important role in the variability of the Labrador Sea especially in its most western part adjacent to an important region of deep convection. Winter-to-winter re-emergence and propagation of both sea-ice concentration (SIC) and sea surface temperature anomalies have been observed following years of high SIC in this region. They have potentially important links to water mass properties and freshwater and heat transports in the subpolar North Atlantic. This article builds on the results of two precursor papers and presents results from a coupled sea-ice–ocean model study of the interannual variability of sea ice in the Labrador Sea. The relationships between SIC and water column properties in the subpolar North Atlantic are assessed. Winters with high SIC and strong surface cooling are found to be conducive to intensified convection. Surface and mid-depth temperature and salinity anomalies are observed in the Labrador Sea and the northwestern North Atlantic during winters with anomalous Labrador Sea SIC. These anomalies are found to propagate along the major circulation patterns in the subpolar North Atlantic and to persist for up to three years.     

Convection induced long term freshening of the subpolar North Atlantic Ocean

This paper is mainly concerned with the understanding and attribution of the recent observed freshening trend in the subpolar North Atlantic Ocean. From previous coupled model studies and an analysis of the long HadCM3 control simulation, it seems unlikely that this freshening trend is a direct consequence of anthropogenically forced climate change. It is shown in this paper that the subpolar North Atlantic can be freshened to the observed degree without invoking substantial large-scale surface freshwater flux changes. The source of freshening can come from a freshwater redistribution within the Arctic/subpolar North Atlantic. The redistribution (involving both liquid water and sea ice) is carried by a perturbed ocean circulation change in the subpolar seas and triggered by deep convection in the Labrador Sea. The freshening can be widespread but mainly in the north and northwest of the subpolar North Atlantic. A sustained 30–40 years freshening trend can be easily identified in specific locations such as the Labrador Sea or in the basin wide integral of freshwater storage. At the peak, the model subpolar North Atlantic can hold around 10,000 km³ of extra freshwater. An analysis of 1,400 years HadCM3 control simulation also reveals a good correlation between freshwater content anomalies and gyre transport in the subpolar North Atlantic on decadal timescales. A general mechanism involving circulation regime changes and freshwater redistribution between the subpolar North Atlantic and the Arctic/Nordic Seas is proposed, which can resolve a number of seemingly contradictory observed changes in the North Atlantic and contributes to the longer term goal of a full understanding of recent North Atlantic fresh water changes.     

Simulated decadal oscillations of the Atlantic meridional overturning circulation in a cold climate state

The significance of the Atlantic meridional overturning circulation (MOC) for regional and hemispheric climate change requires a complete understanding using fully coupled climate models. Here we present a persistent, decadal oscillation in a coupled atmosphere–ocean general circulation model. While the present study is limited by the lack of comparisons with paleo-proxy records, the purpose is to reveal a new theoretically interesting solution found in the fully-coupled climate model. The model exhibits two multi-century-long stable states with one dominated by decadal MOC oscillations. The oscillations involve an interaction between anomalous advective transport of salt and surface density in the North Atlantic subpolar gyre. Their time scale is fundamentally determined by the advection. In addition, there is a link between the MOC oscillations and North Atlantic Oscillation (NAO)-like sea level pressure anomalies. The analysis suggests an interaction between the NAO and an anomalous subpolar gyre circulation in which sea ice near and south of the Labrador Sea plays an important role in generating a large local thermal anomaly and a meridional temperature gradient. The latter induces a positive feedback via synoptic eddy activity in the atmosphere. In addition, the oscillation only appears when the Nordic Sea is completely covered by sea ice in winter, and deep convection is active only near the Irminger Sea. Such conditions are provided by a substantially colder North Atlantic climate than today.     

Sea ice induced changes in ocean circulation during the Eemian

We argue that Arctic sea ice played an important role during early stages of the last glacial inception. Two simulations of the Institut Pierre Simon Laplace coupled model 4 are analyzed, one for the time of maximum high latitude summer insolation during the last interglacial, the Eemian, and a second one for the subsequent summer insolation minimum, at the last glacial inception. During the inception, increased Arctic freshwater export by sea ice shuts down Labrador Sea convection and weakens overturning circulation and oceanic heat transport by 27 and 15%, respectively. A positive feedback of the Atlantic subpolar gyre enhances the initial freshening by sea ice. The reorganization of the subpolar surface circulation, however, makes the Atlantic inflow more saline and thereby maintains deep convection in the Nordic Seas. These results highlight the importance of an accurate representation of dynamic sea ice for the study of past and future climate changes.     

A 20-year coupled ocean-sea ice-atmosphere variability mode in the North Atlantic in an AOGCM

In order to understand potential predictability of the ocean and climate at the decadal time scales, it is crucial to improve our understanding of internal variability at this time scale. Here, we describe a 20-year mode of variability found in the North Atlantic in a 1,000-year pre-industrial simulation of the IPSL-CM5A-LR climate model. This mode involves the propagation of near-surface temperature and salinity anomalies along the southern branch of the subpolar gyre, leading to anomalous sea-ice melting in the Nordic Seas, which then forces sea-level pressure anomalies through anomalous surface atmospheric temperatures. The wind stress associated to this atmospheric structure influences the strength of the East Greenland Current across the Denmark Strait, which, in turn, induces near-surface temperature and salinity anomalies of opposite sign at the entrance of the Labrador Sea. This starts the second half of the cycle after approximatively 10 years. The time scale of the cycle is thus essentially set by advection of tracers along the southern branch of the subpolar gyre, and by the time needed for anomalous East Greenland Current to accumulate heat and freshwater anomalies at the entrance of the Labrador Sea. The Atlantic meridional overturning circulation (AMOC) does not play a dominant role in the mode that is confined in the subpolar North Atlantic, but it also has a 20-year preferred timescale. This is due to the influence of the propagating salinity anomalies on the oceanic deep convection. The existence of this preferred timescale has important implications in terms of potential predictability of the North Atlantic climate in the model, although its realism remains questionable and is discussed.     

The role of salinity in the decadal variability of the North Atlantic meridional overturning circulation

An OGCM hindcast is used to investigate the linkages between North Atlantic Ocean salinity and circulation changes during 1963–2003. The focus is on the eastern subpolar region consisting of the Irminger Sea and the eastern North Atlantic where a careful assessment shows that the simulated interannual to decadal salinity changes in the upper 1,500 m reproduce well those derived from the available record of hydrographic measurements. In the model, the variability of the Atlantic meridional overturning circulation (MOC) is primarily driven by changes in deep water formation taking place in the Irminger Sea and, to a lesser extent, the Labrador Sea. Both are strongly influenced by the North Atlantic Oscillation (NAO). The modeled interannual to decadal salinity changes in the subpolar basins are mostly controlled by circulation-driven anomalies of freshwater flux convergence, although surface salinity restoring to climatology and other boundary fluxes each account for approximately 25% of the variance. The NAO plays an important role: a positive NAO phase is associated with increased precipitation, reduced northward salt transport by the wind-driven intergyre gyre, and increased southward flows of freshwater across the Greenland–Scotland ridge. Since the NAO largely controlled deep convection in the subpolar gyre, fresher waters are found near the sinking region during convective events. This markedly differs from the active influence on the MOC that salinity exerts at decadal and longer timescales in most coupled models. The intensification of the MOC that follows a positive NAO phase by about 2 years does not lead to an increase in the northward salt transport into the subpolar domain at low frequencies because it is cancelled by the concomitant intensification of the subpolar gyre which shifts the subpolar front eastward and reduces the northward salt transport by the North Atlantic Current waters. This differs again from most coupled models, where the gyre intensification precedes that of the MOC by several years.     

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.     

一个耦合气候模式中的副极地涡旋指数与北大西洋经向翻转流

The subpolar gyre index (SPG), derived from the analysis of sea surface height (SSH), is proposed to be a potential indicator for the North Atlantic Meridional Overturning Circulation (AMOC) based on observation as well as the Ocean General Circulation Model (OGCM). We investigated the correspondence between the SPG and the AMOC in a coupled climate model. Our results confirm that the SPG can be used as an early indicator for the AMOC in the subtropical North Atlantic. Changes in the SPG are closely related to variations in the air-sea heat exchange in the Labrador Sea, and variations in deep water formation and southward dense water transport with the deep western boundary current (DWBC) in the North Atlantic. Citation: Gao, Y. Q., and L. Yu, 2008: Subpolar gyre index and the North Atlantic meridional overturning circulation in a coupled climate model, Atmos. Oceanic Sci. Lett., 1, 29-32     

Decadal predictions of the cooling and freshening of the North Atlantic in the 1960s and the role of ocean circulation

In the 1960s North Atlantic sea surface temperatures (SST) cooled rapidly. The magnitude of the cooling was largest in the North Atlantic subpolar gyre (SPG), and was coincident with a rapid freshening of the SPG. Here we analyze hindcasts of the 1960s North Atlantic cooling made with the UK Met Office’s decadal prediction system (DePreSys), which is initialised using observations. It is shown that DePreSys captures—with a lead time of several years—the observed cooling and freshening of the North Atlantic SPG. DePreSys also captures changes in SST over the wider North Atlantic and surface climate impacts over the wider region, such as changes in atmospheric circulation in winter and sea ice extent. We show that initialisation of an anomalously weak Atlantic Meridional Overturning Circulation (AMOC), and hence weak northward heat transport, is crucial for DePreSys to predict the magnitude of the observed cooling. Such an anomalously weak AMOC is not captured when ocean observations are not assimilated (i.e. it is not a forced response in this model). The freshening of the SPG is also dominated by ocean salt transport changes in DePreSys; in particular, the simulation of advective freshwater anomalies analogous to the Great Salinity Anomaly were key. Therefore, DePreSys suggests that ocean dynamics played an important role in the cooling of the North Atlantic in the 1960s, and that this event was predictable.     

Interdecadal North-Atlantic meridional overturning circulation variability in EC-EARTH

The Atlantic meridional overturning circulation (AMOC) in a 600?years pre-industrial run of the newly developed EC-EARTH model features marked interdecadal variability with a dominant time-scale of 50–60?years. An oscillation of approximately 2 Sverdrup (1?Sv?=?10⁶?m³?s^?1) is identified, which manifests itself as a monopole causing the overturning to simultaneously strengthen (/weaken) and deepen (/shallow) as a whole. Eight years before the AMOC peaks, density in the Labrador-Irminger Sea region reaches a maximum, triggering deep water formation. This density change is caused by a counterclockwise advection of temperature and salinity anomalies at lower latitudes, which we relate to the north-south excursions of the subpolar-subtropical gyre boundary and variations in strength and position of the subpolar gyre and the North Atlantic Current. The AMOC fluctuations are not directly forced by the atmosphere, but occur in a delayed response of the ocean to forcing by the North Atlantic Oscillation, which initiates “intergyre”-gyre fluctuations. Associated with the AMOC is a 60-year sea surface temperature variability in the Atlantic, with a pattern and timescale showing similarities with the real-world Atlantic Multidecadal Variability. This good agreement with observations lends a certain degree of credibility that the mechanism that is described in this article could be seen as representative of the real climate system.     

Anomalies of hydrometeorological fields in the Black Sea area associated with sea surface temperature gradients in the North Atlantic

To describe the spatial structure of hydrometeorological fields in the North Atlantic and in the Black Sea, the temperature indices (sea surface temperature gradients) representing the difference in sea surface temperature anomalies averaged throughout the water area and normalized by standard deviations are introduced. Temperature gradients between the Sargasso Sea and the tropics (STI), the Sargasso Sea and the subpolar cyclonic gyre (SNI), the Sargasso Sea and the Black Sea (SBI) are considered. The spatial structure of the sea surface temperature field anomalies in the North Atlantic and in the Black Sea at extreme values of STI and SBI is discussed. A sign-variable structure of air temperature and precipitation anomalies in the Black Sea area at extreme values of SBI is shown. Time scales of sign-variable variability of anomalies of hydrometeorological fields in the Black Sea area are estimated.     

北极海冰变化的时间和空间型

利用 4 4a(195 1～ 1994年 )北极海冰密度逐月资料 ,分析提出了一种与北极冰自然季节变化相吻合的分季法 ,并根据这种分季法 ,使用EOF分解 ,揭示了北极各季海冰面积异常的特征空间型及其对应的时间变化尺度。结果表明 :(1)北极冰面积异常变化的关键区 ,冬季 (2～ 4月 )主要位于北大西洋一侧的格陵兰海、巴伦支海和戴维斯海峡以及北太平洋一侧的鄂霍次克海和白令海 ,夏季 (8～ 10月 )则主要限于从喀拉海、东西伯利亚海、楚科奇海到波佛特海的纬向带状区域内 ,格陵兰海和巴伦支海是北极海冰面积异常变化的最重要区域 ;(2 )春 (5～ 7月 )、秋 (11月～次年 1月 )季各主要海区海冰面积异常基本呈同相变化 ,夏季东西伯利亚海、楚科奇海、波佛特海一带海冰面积异常和喀拉海呈反相变化 ,而冬季巴伦支海、格陵兰海海冰面积异常和戴维斯海峡、拉布拉多海、白令海、鄂霍次克海的海冰变化呈反相变化 ;(3)北极冰总面积过去 4 4a来确实经历了一种趋势性的减少 ,并且叠加在这种趋势变化之上的是年代尺度变化 ,其中春季 (5～ 7月 )海冰面积异常变化对年平均北极冰总面积异常变化作出了主要贡献 ;(4)位于北太平洋一侧极冰面积异常型基本具有半年的持续性 ,而位于北大西洋一侧极冰面积异常型具有半年至一年的持续性     

Impact of freshwater discharge from the Greenland ice sheet on North Atlantic climate variability

Using a coupled ocean–atmosphere general circulation model, we investigated the impact of Greenland ice sheet melting on North Atlantic climate variability. The positive-degree day (PDD) method was incorporated into the model to control continental ice melting (PDD run). Models with and without the PDD method produce a realistic pattern of North Atlantic sea surface temperature (SST) variability that fluctuates from decadal to multidecadal periods. However, the interdecadal variability in PDD run is significantly dominated in the longer time scale compared to that in the run without PDD method. The main oscillatory feature in these experiments likely resembles the density-driven oscillatory mode. A reduction in the ocean density over the subpolar Atlantic results in suppression of the Atlantic Meridional Overturning Circulation (AMOC), leading to a cold SST due to a weakening of northward heat transport. The decreased surface evaporation associated with the cold SST further reduces the ocean density and thus, simultaneously acts as a positive feedback mechanism. The southward meridional current associated with the suppressed AMOC causes a positive tendency in the ocean density through density advection, which accounts for the phase transition of this oscillatory mode. The Greenland ice melting process reduces the mean meridional current and meridional density gradient because of additional fresh water flux, which suppress the delayed negative feedback due to meridional density advection. As a result, the oscillation period becomes longer and the transition is more delayed.     

Simulation of the Atlantic meridional overturning circulation in an atmosphere–ocean global coupled model. Part I: a mechanism governing the variability of ocean convection in a preindustrial experiment

A preindustrial climate experiment was conducted with the third version of the CNRM global atmosphere–ocean–sea ice coupled model (CNRM-CM3) for the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4). This experiment is used to investigate the main physical processes involved in the variability of the North Atlantic ocean convection and the induced variability of the Atlantic meridional overturning circulation (MOC). Three ocean convection sites are simulated, in the Labrador, Irminger and Greenland–Iceland–Norwegian (GIN) Seas in agreement with observations. A mechanism linking the variability of the Arctic sea ice cover and convection in the GIN Seas is highlighted. Contrary to previous suggested mechanisms, in CNRM-CM3 the latter is not modulated by the variability of freshwater export through Fram Strait. Instead, the variability of convection is mainly driven by the variability of the sea ice edge position in the Greenland Sea. In this area, the surface freshwater balance is dominated by the freshwater input due to the melting of sea ice. The ice edge position is modulated either by northwestward geostrophic current anomalies or by an intensification of northerly winds. In the model, stronger than average northerly winds force simultaneous intense convective events in the Irminger and GIN Seas. Convection interacts with the thermohaline circulation on timescales of 5–10 years, which translates into MOC anomalies propagating southward from the convection sites.     

Variability of Fram Strait sea ice export: causes, impacts and feedbacks in a coupled climate model

Analyses of a 500-year control integration of the global coupled atmosphere–sea ice–ocean model ECHAM5.0/MPI-OM show a high variability in the ice export through Fram Strait on interannual to decadal timescales. This variability is mainly determined by variations in the sea level pressure gradient across Fram Strait and thus geostrophic wind stress. Ice thickness anomalies, formed at the Siberian coast and in the Chukchi Sea, propagate across the Arctic to Fram Strait and contribute to the variability of the ice export on a timescale of about 9 years. Large anomalies of the ice export through Fram Strait cause fresh water signals, which reach the Labrador Sea after 1–2 years and lead to significant changes in the deep convection. The associated anomalies in ice cover and ocean heat release have a significant impact on air temperature in the Labrador Sea and on the large-scale atmospheric circulation. This affects the sea ice transport and distribution in the Arctic again. Sensitivity studies, simulating the effect of large ice exports through Fram Strait, show that the isolated effect of a prescribed ice/fresh water anomaly is very important for the climate variability in the Labrador Sea. Thus, the ice export through Fram Strait can be used for predictability of Labrador Sea climate up to 2 years in advance.     

Potential role of Atlantic Warm Pool-induced freshwater forcing in the Atlantic Meridional Overturning Circulation: ocean–sea ice model simulations

Recent studies have indicated that the multidecadal variations of the Atlantic Warm Pool (AWP) can induce a significant freshwater change in the tropical North Atlantic Ocean. In this paper, the potential effect of the AWP-induced freshwater flux on the Atlantic Meridional Overturning Circulation (AMOC) is studied by performing a series of ocean–sea ice model experiments. Our model experiments demonstrate that ocean response to the anomalous AWP-induced freshwater flux is primarily dominated by the basin-scale gyre circulation adjustments with a time scale of about two decades. The positive (negative) freshwater anomaly leads to an anticyclonic (cyclonic) circulation overlapping the subtropical gyre. This strengthens (weakens) the Gulf Stream and the recirculation in the interior ocean, thus increases warm (cold) water advection to the north and decreases cold (warm) water advection to the south, producing an upper ocean temperature dipole in the midlatitude. As the freshwater (salty water) is advected to the North Atlantic deep convection region, the AMOC and its associated northward heat transport gradually decreases (increases), which in turn lead to an inter-hemispheric SST seesaw. In the equilibrium state, a comma-shaped SST anomaly pattern develops in the extratropical region, with the largest amplitude over the subpolar region and an extension along the east side of the basin and into the subtropical North Atlantic. Based on our model experiments, we argue that the multidecadal AWP-induced freshwater flux can affect the AMOC, which plays a negative feedback role that acts to recover the AMOC after it is weakened or strengthened. The sensitivity of AMOC response to the AWP-induced freshwater forcing amplitude is also examined and discussed.     

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.     

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.     

Sea-ice effects on climate model sensitivity and low frequency variability

A change in a sea-ice parameter in a global coupled climate model results in a reduction in amplitude (of about 60%) and a shortening of the predominant period of decadal low frequency variability in the time series of globally averaged surface air temperature. These changes are global in extent and also are reflected in time series of area-averaged SSTs in the equatorial eastern Pacific Ocean, the principal components of the first EOFs of global surface air temperature and sea level pressure, Asian monsoon precipitations and other quantities. Coupled ocean-atmosphere-sea ice processes acting on a global scale are modified to produce these changes. Global climate sensitivity is reduced when ice albedo feedback is weakened due to the change in sea ice that makes it more difficult to melt. The changes in the amplitude and time scale of the low frequency variability in the model are traced to changes in the base state of the climate simulations as affected by modifications associated with the changes in sea ice. Making sea ice more difficult to melt results in increased sea-ice area, colder high latitudes, increased meridional surface temperature gradients, and, to a first order, stronger surface winds in most regions which strengthen near-surface currents, particularly in the Northern Hemisphere, and decreases the advection time scale in the upper ocean gyres. Additionally, in the North Atlantic there is enhanced meridional overturning due to increased density mainly in the Greenland Sea region. This also contributes to an intensified North Atlantic gyre. The changes in base state due to the sea ice change result in a more predominant decadal time scale of near 14 years and significantly reduced contributions from lower frequencies in the range of 15–40 year periods. Received: 11 December 1998 / Accepted: 4 October 1999     

Modeled winter sea ice variability and the North Atlantic Oscillation: a multi-century perspective

The relationship between winter sea ice variability and the North Atlantic Oscillation (NAO) is examined for the time period 1860–2300. This study uses model output to extend recently reported observational results to multi-century time scales. Nine ensemble members are used in two Global Climate Models with forcing evolving from pre-industrial conditions through the so-called A1B scenario in which carbon dioxide stabilizes at 720 ppm by 2100. Throughout, the NAO generates an east-west dipole pattern of sea ice concentration (SIC) anomalies with oppositely signed centers of action over the Labrador and Barents Seas. During the positive polarity of the NAO, SIC increases over the Labrador Sea due to wind-driven equatorward advection of ice, and SIC decreases over the Barents Sea due to wind-driven poleward transport of heat within the mixed layer of the ocean. Although this NAO-driven SIC variability pattern can always be detected, it accounts for a markedly varying fraction of the total sea ice variability depending on the strength of the forced sea ice extent trend. For the first half of the 20th century or 1990 control conditions, the NAO-driven SIC pattern accounts for almost a third of the total SIC variance. In the context of the long term winter sea ice retreat from 1860 to 2300, the NAO-driven SIC pattern is robustly observable, but accounts for only 2% of the total SIC variance. The NAO-driven SIC dipole retreats poleward with the retreating marginal ice zone, and its Barents Sea center of action weakens. Results presented here underscore the idea that the NAO’s influence on Arctic climate is robustly observable, but time dependent in its form and statistical importance.