Oceanic Polar Front Research Articles

The Long-term variability of Global Typical Oceanic Fronts

Using high-resolution NOAA Optimum Interpolation sea surface temperature (OISST) data, the temporal and spatial characteristics of typical oceanic fronts in the past 39 years, including Oyashio front, Gulf Stream front, Peru current front and southern Indian Ocean polar front, are analyzed. The results show that the intensities of the four typical fronts have all strengthened, among which the Peru current is the most significant and the South Indian Ocean polar front is the weakest. The movement of the fronts are quite different: the Oyashio front has a significant poleward movement, and the Gulf Stream front extending into the open sea part has a marginal poleward movement; while the south Indian Ocean polar front and Peru current front have no obvious movements.

A climatology of open and closed mesoscale cellular convection over the Southern Ocean derived from Himawari-8 observations

Abstract. Marine atmospheric boundary layer clouds cover vast areas of the Southern Ocean (SO), where they are commonly organized into mesoscale cellular convection (MCC). Using 3 years of Himawari-8 geostationary satellite observations, open and closed MCC structures are identified using a hybrid convolutional neural network. The results of the climatology show that open MCC clouds are roughly uniformly distributed over the SO storm track across midlatitudes, while closed MCC clouds are most predominant in the southeast Indian Ocean, with a second maximum along the storm track. The ocean polar front, derived from ECMWF-ERA5 sea surface temperature gradients, is found to be aligned with the southern boundaries for both MCC types. Along the storm track, both closed and open MCCs are commonly located in post-frontal, cold air masses. The hourly classification of closed MCC reveals a pronounced daily cycle, with a peak occurring late night/early morning. Seasonally, the diurnal cycle of closed MCC is most intense during the summer months (December–February; DJF). Conversely, almost no diurnal cycle is evident for open MCC.

Observations of Clouds, Aerosols, Precipitation, and Surface Radiation over the Southern Ocean: An Overview of CAPRICORN, MARCUS, MICRE, and SOCRATES

AbstractWeather and climate models are challenged by uncertainties and biases in simulating Southern Ocean (SO) radiative fluxes that trace to a poor understanding of cloud, aerosol, precipitation, and radiative processes, and their interactions. Projects between 2016 and 2018 used in situ probes, radar, lidar, and other instruments to make comprehensive measurements of thermodynamics, surface radiation, cloud, precipitation, aerosol, cloud condensation nuclei (CCN), and ice nucleating particles over the SO cold waters, and in ubiquitous liquid and mixed-phase clouds common to this pristine environment. Data including soundings were collected from the NSF–NCAR G-V aircraft flying north–south gradients south of Tasmania, at Macquarie Island, and on the R/V Investigator and RSV Aurora Australis. Synergistically these data characterize boundary layer and free troposphere environmental properties, and represent the most comprehensive data of this type available south of the oceanic polar front, in the cold sector of SO cyclones, and across seasons. Results show largely pristine environments with numerous small and few large aerosols above cloud, suggesting new particle formation and limited long-range transport from continents, high variability in CCN and cloud droplet concentrations, and ubiquitous supercooled water in thin, multilayered clouds, often with small-scale generating cells near cloud top. These observations demonstrate how cloud properties depend on aerosols while highlighting the importance of dynamics and turbulence that likely drive heterogeneity of cloud phase. Satellite retrievals confirmed low clouds were responsible for radiation biases. The combination of models and observations is examining how aerosols and meteorology couple to control SO water and energy budgets.

A Climatology of the Marine Atmospheric Boundary Layer Over the Southern Ocean From Four Field Campaigns During 2016–2018

AbstractA climatology of the marine atmospheric boundary layer (MABL) and the lower free troposphere over the Southern Ocean (SO) is constructed using 2,186 high‐resolution atmospheric soundings from four recent campaigns conducted in the period of 2016–2018. Relationships between the synoptic meteorology and MABL thermodynamic structure are examined using a k‐means cluster analysis, complemented by front and cyclone composite analyses. Seven distinct clusters are identified, five of which are consistent with an established climatology over the SO storm track. Two new clusters (C1 and C2) are introduced over the high‐latitude SO. C1 is commonly located poleward of the ocean polar front near mesocyclones, while C2 is located along the Antarctic coastline. A multilayer cloud structure is frequently present in clusters in the vicinity of fronts and cyclones, while a single‐layer coverage is more common in a suppressed environment, particularly at lower latitudes. A cloud‐free, multilevel inversion is frequently observed in cluster C2, possibly linked to the descending, dry, katabatic winds off the Antarctic coast. A strong, primary inversion is typically present in clusters at lower latitudes with high mean sea level pressure. Across the SO storm track and higher latitudes (cluster C1), a multilevel inversion structure is also commonly observed. A preliminary analysis of two case studies suggests that upper level advection and detrainment of convection associated with mesocyclones are potential drivers of the multilayer cloud coverage over the high‐latitude SO rather than the decoupling mechanisms common in the subtropics.

Modelling the future biogeography of North Atlantic zooplankton communities in response to climate change

Advances in habitat and climate modelling allow us to reduce uncertainties of climate change impacts on species distribution. We evaluated the impacts of future climate change on community structure, diversity, distribution and phenology of 14 copepod species in the North Atlantic. We developed and validated habitat models for key zooplankton species using continuous plankton recorder (CPR) survey data collected at mid latitudes of the North Atlantic. Generalized additive models (GAMs) were applied to relate the occurrence of species to environmental variables. Models were projected to future (2080–2099) environmental conditions using coupled hydroclimatix–biogeochemical models under the Intergovernmental Panel on Climate Change (IPCC) A1B climate scenario, and compared to present (2001–2020) conditions. Our projections indicated that the copepod community is expected to respond substantially to climate change: a mean poleward latitudinal shift of 8.7 km per decade for the overall community with an important species range variation (–15 to 18 km per decade); the species seasonal peak is expected to occur 12–13 d earlier for Calanus finmarchicus and C. hyperboreus; and important changes in community structure are also expected (high species turnover of 43–79% south of the Oceanic Polar Front). The impacts of the change expected by the end of the century under IPCC global warming scenarios on copepods highlight poleward shifts, earlier seasonal peak and changes in biodiversity spatial patterns that might lead to alterations of the future North Atlantic pelagic ecosystem. Our model and projections are supported by a temporal validation undertaken using the North Atlantic climate regime shift that occurred in the 1980s: the habitat model built in the cold period (1970–1986) has been validated in the warm period (1987–2004).

Polar front associated variation in prokaryotic community structure in Arctic shelf seafloor.

Spatial variations in composition of marine microbial communities and its causes have largely been disclosed in studies comprising rather large environmental and spatial differences. In the present study, we explored if a moderate but temporally permanent climatic division within a contiguous arctic shelf seafloor was traceable in the diversity patterns of its bacterial and archaeal communities. Soft bottom sediment samples were collected at 10 geographical locations, spanning spatial distances of up to 640 km, transecting the oceanic polar front in the Barents Sea. The northern sampling sites were generally colder, less saline, shallower, and showed higher concentrations of freshly sedimented phytopigments compared to the southern study locations. Sampling sites depicted low variation in relative abundances of taxa at class level, with persistent numerical dominance by lineages of Gamma- and Deltaproteobacteria (57–66% of bacterial sequence reads). The Archaea, which constituted 0.7–1.8% of 16S rRNA gene copy numbers in the sediment, were overwhelmingly (85.8%) affiliated with the Thaumarchaeota. Beta-diversity analyses showed the environmental variations throughout the sampling range to have a stronger impact on the structuring of both the bacterial and archaeal communities than spatial effects. While bacterial communities were significantly influenced by the combined effect of several weakly selective environmental differences, including temperature, archaeal communities appeared to be more uniquely structured by the level of freshly sedimented phytopigments.

Annually resolved δ13C shell chronologies of long-lived bivalve mollusks ( Arctica islandica) reveal oceanic carbon dynamics in the temperate North Atlantic during recent centuries

The ability of the ocean to absorb carbon dioxide is likely to be adversely affected by recent climate change. However, relatively little is known about the spatiotemporal variability in the oceanic carbon cycle due to the lack of long-term, high-resolution dissolved inorganic carbon isotope ( δ 13C DIC) data, especially for the temperate North Atlantic, which is the major oceanic sink for anthropogenic CO 2. Here, we report shell carbon isotope values ( δ 13C shell), a potential proxy for δ 13C DIC, of old-grown specimens of the long-lived bivalve mollusk, Arctica islandica. This paper presents the first absolutely dated, annually resolved δ 13C shell record from surface waters of the North Atlantic (Iceland, Gulf of Maine) covering the time interval between 1753 and 2003. According to our results, the δ 13C shell data were unaffected by trends related to ontogenetic age. However, the shell carbonate was precipitated with a constant offset from expected equilibrium by − 1.54 to − 2.7 ± 0.2‰ corresponding to a 6.2 to 10.8 ± 0.8% contribution of respiratory CO 2 (− 25‰). The offset did not appear to vary through the lifetime of individual specimens and among specimens. Therefore, the δ 13C shell data of this species can very likely be used as a measure of δ 13C DIC. Furthermore, shell stable carbon isotope chronologies exhibited habitat-specific differences and a significant inter-annual and decadal variability related to the natural carbon cycle. In addition, a distinct negative δ 13C shell shift was found reflecting the oceanic Suess effect, i.e. the admixture of anthropogenic CO 2. However, this shift only occurred after the early 1920s when a major climate regime shift led to a northward movement of the oceanic Polar Front in the Nordic Seas and a large-scale reorganization of atmospheric and oceanic currents in the North Atlantic. This likely resulted in a reduced admixture of cold Polar water onto the North Icelandic shelf (through the East Iceland Current) and the Gulf of Maine (through the Labrador Current) with an increased volume of warmer, isotopically well-equilibrated Atlantic waters. Our shell-based δ 13C DIC proxy record provides the basis to quantitatively assess natural and anthropogenically induced patterns of carbon uptake in the North Atlantic.

The Role of Eddies in the Southern Ocean Temperature Response to the Southern Annular Mode

Abstract The role of eddies in modulating the Southern Ocean response to the southern annular mode (SAM) is examined, using an ocean model run at multiple resolutions from coarse to eddy resolving. The high-resolution versions of the model show an increase in eddy kinetic energy that peaks 2–3 yr after a positive anomaly in the SAM index. Previous work has shown that the instantaneous temperature response to the SAM is characterized by predominant cooling south of 45°S and warming to the north. At all resolutions the model captures this temperature response. This response is also evident in the coarse-resolution implementation of the model with no eddy mixing parameterization, showing that eddies do not play an important role in the instantaneous response. On the longer time scales, an intensification of the mesoscale eddy field occurs, which causes enhanced poleward heat flux and drives warming south of the oceanic Polar Front. This warming is of greater magnitude and occurs for a longer period than the initial cooling response. The results demonstrate that this warming is surface intensified and strongest in the mixed layer. Non-eddy-resolving models are unable to capture the delayed eddy-driven temperature response to the SAM. The authors therefore question the ability of coarse-resolution models, such as those commonly used in climate simulations, to accurately represent the full impacts of the SAM on the Southern Ocean.

Location of the Antarctic Polar Front from AMSR-E Satellite Sea Surface Temperature Measurements

Abstract The location of the Southern Ocean polar front (PF) is mapped from the first 3 yr of remotely sensed Advanced Microwave Scanning Radiometer for the Earth Observing System (AMSR-E) sea surface temperature (SST) measurements. In agreement with previous studies, the mean path of the Antarctic PF and its standard deviation are strongly influenced by bottom topography. However, the mean PF path diverges slightly from previous studies in several regions where there is high mesoscale variability. Although the SST and SST gradient at the PF show spatially coherent seasonal variations, with the highest temperature and the lowest temperature gradient during summer, the seasonal variations in the location of the PF are not spatially coherent. The temporal mean SST at the PF corresponds well to the mean PF path: the temperature is high in the Atlantic and Indian Ocean sections and is low in the Pacific Ocean section where the PF has a more southerly position. The relationship of the wind field with the Antarctic PF location and proxies for the zonal and meridional PF transports are examined statistically. Coherence analysis suggests that the zonal wind stress accelerates the zonal transport of the PF. The analysis presented herein also suggests that the meridional shifts of the Antarctic PF path correspond to the meridional shifts of the wind field.

Alpine Evidence for Atmospheric Circulation Patterns in Europe during the Last Glacial Maximum

The configuration of Alpine accumulation areas during the last glacial maximum (LGM) has been reconstructed using glacial–geological mapping. The results indicate that the LGM ice surface consisted of at least three major ice domes, all located south of the principal weather divide of the Alps. This implies that the buildup of the main Alpine ice cover during oxygen isotope stage (OIS) 2 was related to precipitation by dominant southerly atmospheric circulation, in contrast to today's prevalent westerly airflow. Such a reorganization of the atmospheric circulation is consistent with a southward displacement of the Oceanic Polar Front in the North Atlantic and of the associated storm track to the south of the Alps. These results, combined with additional paleoclimate records from western and southern Europe, allow an interpretation of the asynchronous evolution of the different European ice caps during the last glaciation. δ18O stages (OIS) 4 and 3 were characterized by location of the Polar Front north of 46°N (Gulf of Biscay). This affected prevailing westerly circulation and, thus, ice buildup in western Scandinavia, the Pyrénées, Vosges, and northern Alps. At the LGM, however, the Polar Front lay at ∼44°N, causing dominating southerly circulation and reduced precipitation in central and northern Europe.

Biomass and productivity distributions and their variability in the Barents Sea

On the basis of results that largely have been derived from the Norwegian research programme Pro Mare (1984-1989), an overview of the distribution of biomass and productivity at different trophic levels of the pelagic Barents Sea ecosystem is presented, with comments on year-to-year and horizontal variations. Average biomasses for the whole Barents Sea and several years range from 2000 kg of carbon km −2 for each of Calanus spp. and phytoplankton stocks, down to 0.1 kg carbon km −2 for the polar bear population.Phytoplankton blooms that deplete the winter nutrients give rise locally to a “new” productivity of 40-50 g C m −2 . Areal differences, however, are pronounced in terms of annual productivity: the “new” fraction is more than twice as high in the southern (Atlantic) part of the Barents Sea as in the areas north of the oceanic Polar Front (90 vs. <40 g C m −2 yr −1 ), Wind-generated vertical mixing (in association with passing atmospheric depressions) and turbulence generated on and around banks in the southern part are crucial in accounting for this difference. In the northern half of the Barents Sea a pronounced upper layer stability caused by the supply of meltwater from seasonal ice retards mixing so that “new” production is small following the depletion of winter nutrients - high productivity is restricted to the 20-50 km wide ice edge bloom.Year-to-year variations in fish stocks are pronounced and have always been so - there exists no “ecological balance” in any meaningful sense. These variations are clearly related to the influx of Atlantic water to the Barents Sea (i.e. “warm” years are high-productive years) and, thus, ultimately to the same forcing factors that determine climate variations in coastal Europe. Thus, sound management may diminish the annual fluctuations in fish stock sizes but certainly cannot eradicate them.

Seasonal heat and freshwater budgets of the upper ocean in the North‐east Atlantic

AbstractHeat‐ and freshwater‐content changes were calculated from two repeated sections at 48°N, 18°W and 48°N, 26°W, surveyed during the Vivaldi cruise of the research ship RRS Charles Darwin in May 1991. Best estimates of the air‐sea heat‐fluxes agreed within error with the observed heat‐content changes, suggesting that diabatic forcing dominated the heat budget. This was confirmed by a numerical model which showed that Ekman and geostrophic advection and eddy diffusion together contributed less than 20 W m−2 to the heat budget.However, the model also showed that, near the oceanic polar front under westerly winds, Ekman advection of cold, fresh water southwards over the front can cause ‐40 W m−2 and 4 mm d−1 heat‐ and freshwater‐content changes in the upper 53 m, compared to air‐sea fluxes of typically 67 W m−2 and less than 1 mm d−1. the Ekman advection contributed little to the model's density‐budget as the advective cooling and freshening had opposing effects. However, an air‐sea interaction model showed that the advective surface‐cooling may be reduced because it triggers an extra air‐sea heat flux and the remaining advective freshening then makes an important contribution to the density budget.

Late-glacial (Allerød – Younger Dryas) Coleoptera from central Cape Breton Island, Nova Scotia, Canada

Coleoptera from three late-glacial sites on Cape Breton Island, Nova Scotia, Canada, provide proxy climate data from 12 600 to 10 400 BP. Samples older than about 11 800 BP contain tundra to tree-line species. Between 11 800 and about 10 800 BP, beetle assemblages become typical of the modern boreal forest. Beetles younger than 10 800 BP responded to climatic deterioration during the Younger Dryas. Although coleopteran evidence for the Younger Dryas is not as strong as that presented by palynological and stratigraphic studies, the appearance of Olophrum boreale and other species found today in the northern boreal forest and on tundra or coastal tundra corresponds with the decline of shrub birch and the rise in willow and alder pollen. Beetles responded just as likely to changes in vegetation and ground cover as to changes in temperature. The extent to which Coleoptera were affected may lie somewhere between a moderated continental response seen in the Great Lakes region in North America and more extreme changes recorded in Europe. This response is consistent with shifts in the North Atlantic oceanic Polar Front in the Ruddiman and McIntyre model.

Late glacial sea level and ocean margin environmental changes interpreted from biostratigraphic and lithostratigraphic studies of isolation basins in northwest Scotland

Abstract Detailed biostratigraphic and lithostratigraphic analyses from isolation basins at Ardtoe, Rumach, Loch nan Eala and Fearnbeg, northwest Scotland, allow an interpretation of sea level movements and environmental changes within the Lateglacial Interstadial and Younger Dryas (13-10 ka BP ). Microfossil analyses illustrate the gradual transition of the isolation basins from marine basins to freshwater lakes within an area of isostatic uplift. These data provide evidence for a fall in relative sea level in northwest Scotland from the mid-Lateglacial Interstadial to the early Holocene. Pollen, diatom and dinoflagellate cyst (dinocyst) analyses link the oceanic record of major climate and oceanic circulation changes with the terrestrial biostratigraphic record. Movement of the oceanic Polar Front west and north during a period of relative climatic amelioration within the Lateglacial Interstadial and a corresponding change in the position of the North Atlantic Current may be inferred from the dinocyst record. This is followed by a relative cooling.

Climatic changes in areas adjacent to the North Atlantic during the last glacial‐interglacial transition (14‐9 ka BP): A contribution to IGCP‐253

AbstractThis paper presents a summary of the evidence for climatic changes during the last glacial‐interglacial transition (14‐9 ka BP) in land areas adjacent to the North Atlantic. It is a synthesis of the results of the 12 regional summaries compiled by contributors to the North Atlantic Seaboard Programme of IGCP‐253 that are published separately in this issue of Journal of Quaternary Science. Eighteen palaeotemperature curves are compared and arranged in three transects, one from southern Europe to Spitsbergen, a second from Ireland to Poland, and the third from southern New England to the Labrador‐Baffin Shelf area. Ten maps are presented that summarise the synoptic climatic conditions of the region in 500‐year time periods. The purpose of the exercise is to examine the extent to which any emerging patterns support the Ruddiman and Mclntyre model of large‐scale shifts in the position of the oceanice Polar Front during the last glacial‐interglacial transition. Some broad agreement emerges, although the influence of oceanic changes is diminished in areas that lay in close proximity to the Laurentide and Fennoscandian ice sheets. Attention is drawn to limitations in the compilations and to the potential for improved models in the future.

Recent calcium carbonate dissolution in the Barents Sea: Paleoceanographic applications

Studies of benthic foraminifera in surface sediments of the Arctic, epicontinental Barents Sea, show several indications of calcium carbonate dissolution. Low values of calcium carbonate correspond to low ratios of calcareous/agglutinating foraminifera, planktic/benthic foraminifera, high ratios of living/dead foraminifera, corroded calcareous foraminifera, and high numbers of exposed organic linings of foraminifera. Dissolution increases eastwards and northwards in the study area. We suggest that the dissolution is caused by the presence of dense, cold, saline and CO2-rich bottom water, which is linked to sea-ice production and the position of the Oceanic Polar Front. We also think calcium carbonate dissolution in this area indicates a sink for atmospheric CO2.

Reproducibility in the Random Settling Method for Quantitative Diatom Analysis

Seven lower Pliocene sediment samples from Ocean Drilling Program (ODP) Hole 642B on the outer V0ring Plateau were analyzed in order to test the reproducibility of Moore's random settling method in quantitative diatom analysis. The results indicate that the absolute abundance of diatom valves per gram dry sediment are obtained within an error of ?5% at the 95% confidence level after a count of 400 valves. The results also indicate that relative abundance percentages for abundant species (approximately 20% of the total assemblage) are determined with a maximum error of ?3.5%, whereas relative abundance percentages for less abundant species are determined with a maximum error of ?1.5%. Species having relative abundances of approximately two percent or less may be overlooked. This study demonstrates that absolute abundance data are required in order to understand the cause of change in the relative abundance. INTRODUCTION The usefulness of quantitative methods in diatom biostratigraphy and paleoceanography has been demonstrated by, for example, Burckle and Trainer (1979). They were using relative abundances to define Pliocene diatom datums not only by first and last appearances, but also by recurrences and by changes in dominance of two closely related species (see also Baldauf 1985; Barron 1985). Counts of total diatom abundance changes in marine sediments provide valuable paleoceanographic information. For example, Stabell (1986) analyzed absolute abundances of diatoms in Quaternary sediments for recognition of high productivity events that were caused by movements of the oceanic polar front (see also Pokras and Ruddiman 1989; Stabell 1989). Among described methods that are used to calculate diatom abundances per weight or volume of unit sediment are the evaporation technique (Batterbee 1973), the pipetting technique (Schrader and Gersonde 1978), and the use of marker grains (Kaland and Stabell 1981). The random settling method described by Moore (1973) was developed for studies of radiolarian abundances. Laws (1983) tested the accuracy of the method by using a known mixture of microspheres, and showed that this method produced accurate strewn mounts within the limits of error relative to the original sample. In contrast, pipetted mounts were biased toward larger particles (i.e. larger particles were obtained at frequencies higher than the occurrence within the original sample). Moore's method can also be adopted to estimate absolute abundances of diatoms. The major purpose of this study is to test empirically the reproducibility of Moore's method for use in quantitative diatom analysis, using both sample splitting and settling containers. Another purpose is to evaluate how absolute abundance changes are reflected in a relative abundance analysis. In order to achieve these goals, both absolute and relative abundances were analyzed in seven samples, and counts were performed on multiple slides from each sample. This allows comparison between absolute and relative abundance changes, and the reliability of the obtained values can be evaluated.

The Holocene glacial history of Lyngshalvöya, northern Norway: chronology and climatic implications

Up to four nested Neoglacial moraines occur in front of glaciers on Lyngshalvöya. Lichenometric measurements at 21 glaciers demonstrate that these represent five episodes of glacier expansion, one of which predated the Little Ice Age. Lichenometric, dendrochronological and historical evidence indicates that the oldest Little Ice Age moraines date to the mid‐18th century, and the youngest to A.D. 1910‐30. At nine small glaciers the A.D. 1910‐30 moraine represents the Neoglacial maximum; only larger glaciers were more extensive in the 18th century. It is inferred that conditions for glacier growth were less favourable in the 18th century than in A.D. 1880–1910 because of low winter snowfall. Comparison of the relative magnitude of 18th‐ and 20th‐century advances on Lyngshalvöya with those of southern Norway suggests that the diminished winter precipitation was due to the southerly location of the North Atlantic oceanic polar front in the 18th century, which resulted in a reduction in winter cyclonic activity in northern Scandinavia but in an increase in snowfall farther south.

Secular change of the seasonal variation in sea level and of the pole tide in the Baltic Sea

Regular sea level observations have been performed in Stockholm since 1825, yielding the longest unbroken series of monthly mean sea levels in the world. Applying Fourier analysis to these data we have investigated the annual and semiannual sea level variations as well as the pole tide. Whereas the semiannual variation has a fairly constant amplitude of about 4 cm, the annual variation shows a secular increase in amplitude from 8 to 10 cm. This increase is statistically significant at the 99% level. It is suggested that this change might be connected to secular changes of the oceanographic conditions that have been observed to occur in the northeastern North Atlantic, especially a movement of the oceanic polar front. The pole tide also shows a secular change. The amplitude for the 14.3‐month Chandler period was less than 2 cm (below the noise level) during the previous century. During our own century the amplitude is 3 cm, 6 times the equilibrium tide. This might indicate that a meteorological forcing component close to the Chandler period has changed and added variability to the sea level at this period.

The diatom flora in the Fram Strait. A comparison with the Pleistocene/Holocene boundary flora in the Norwegian Sea

High abundances of diatoms were recorded near the Pleistocene/Holocene boundary in sediment cores from the Norwegian-Greenland Sea (Stabell 1986 and references therein). Diatom maxima of similar age were recently reported from the northeast Norwegian Sea (Pedersen 1987). Stabell (1986) suggested that the diatom maxima reflect the passage of the Subarctic Convergence (oceanic polar front) which moved from the southern North Atlantic (about 45N) at about l8.000B.P. (glacial maximum) to its present position south and east of Greenland (Ruddiman & Mclntyre 1981). The northward movement of the polar front resulted in cold Arctic water being replaced by warm, saline Atlantic water of the North Atlantic Current. Mixing of polar and subpolar Atlantic surface water masses was expected by our models to result in more nutrient-enriched surface water and thereby increased diatom productivity.