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

The fine spatial resolution of laser altimeters makes them potentially valuable to oceanography studying features at mesoscale, close to land, and in the marginal ice zone. To fulfill this promise, we must understand laser sea state bias (SSB). SSB occurs in the measurement of sea surface height in the presence of waves when the altimeter observations are preferentially influenced by particular parts (e.g., wave troughs) of the wave-covered surface. Radar altimeters have received considerable attention relating radar SSB to wave properties and wind speed. Comparatively, little attention has been devoted to the SSB of laser altimeters, and the studies of laser SSB which have been done have led to indeterminate or ambiguous results even as to sign. Here, we find that to make changes in satellite dynamic ocean topography (DOT) from the Ice, Clouds, and Land Elevation Satellite (ICESat) period, 2004–2009, to the CryoSat-2 period, 2011–2015, consistent with hydrography plus ocean bottom pressure in the subarctic Greenland and Norwegian seas, we need to correct the ICESat DOT for SSB. On average, ICESat SSB is −18% of significant wave height in excess of 1.7 m.

A budget of meteoric water (MW = river runoff, net precipitation minus evaporation, and glacial meltwater) over four regions of the Arctic Ocean is constructed using a simple box model, regional precipitation-evaporation estimates from reanalysis data sets, and estimates of import and export fluxes derived from the literature with a focus on the 2003–2008 period. The budget indicates an approximate/slightly positive balance between MW imports and exports (i.e., no change in storage); thus, the observed total freshwater increase observed during this time period likely resulted primarily from changes in non-MW freshwater components (i.e., increases in sea ice melt or Pacific water and/or a decrease in ice export). Further, our analysis indicates that the MW increase observed in the Canada Basin resulted from a spatial redistribution of MW over the Arctic Ocean. Mean residence times for MW were estimated for the Western Arctic (5–7 years), Eastern Arctic (3–4 years), and Lincoln Sea (1–2 years). The MW content over the Siberian shelves was estimated (∼14,000 km3) based on a residence time of 3.5 years. The MW content over the entire Arctic Ocean was estimated to be ≥44,000 km3. The MW export through Fram Strait consisted mostly of water from the Eastern Arctic (3,237 ± 1,370 km3 yr−1) whereas the export through the Canadian Archipelago was nearly equally derived from both the Western Arctic (1,182 ± 534 km3 yr−1) and Lincoln Sea (972 ± 391 km3 yr−1).

As part of the 2013 and 2014 North Pole Environmental Observatories (NPEO) in the Amundsen Basin of the Arctic Ocean, two similar temperature microstructure experiments were performed with different results. In 2013, vertical fluxes were through a thermohaline staircase, and in 2014 the thermohaline staircase was largely absent. Here we investigate the reasons for this difference. The 2013 data set was characterized by an extensive thermohaline staircase, indicative of the diffusive convective type of double diffusion (DC), from 120-250 m depths. The staircase was absent above 200 m in 2014, even though analysis of density ratio, Rρ, still shows high susceptibility to DDC. In the depth range of interest, survey averaged Rρ = 3.8 in 2013 and Rρ = 3.6 in 2014, indicating that the temperature-salinity structure in the pycnocline was not the cause of the lack of a staircase in 2014. We propose that exceptionally weak turbulent mixing, even for the typically quiescent Arctic Ocean, allowed formation of the staircase in 2013. Average thermal diffusivity, KT, between 50 – 120 m is elevated in 2014, 2 x 10−5 m2s−1, compared to 2013, 1 x 10−6 m2s−1. However, vertical Atlantic Water (AW) DC heat fluxes in 2013 are remarkably consistent with turbulent heat fluxes in 2014. Similar data sets collected in 2007 and 2008 both resemble 2014, showing consistently higher mixing values compared to 2013. The suppression of turbulence during NPEO 2013 resulted from increased near-surface stratification, possibly caused by a different large-scale circulation pattern that year.

To understand the factors causing the interannual variations in the summer retreat of the Beaufort Sea ice edge, Seasonal Ice Zone Reconnaissance Surveys (SIZRS) aboard U.S. Coast Guard Arctic Domain Awareness flights were made monthly from June to October in 2012, 2013, and 2014. The seasonal ice zone (SIZ) is where sea ice melts and reforms annually and encompasses the nominally narrower marginal ice zone (MIZ) where a mix of open-ocean and ice pack processes prevail. Thus, SIZRS provides a regional context for the smaller-scale MIZ processes. Observations with aircraft expendable conductivity–temperature–depth probes reveal a salinity pattern associated with large-scale gyre circulation and the seasonal formation of a shallow (~20 m) fresh layer moving with the ice edge position. Repeat occupations of the SIZRS lines from 72° to 76°N on 140° and 150°W allow a comparison of observed hydrography to atmospheric indices. Using this relationship, the basinwide salinity signals are separated from the fresh layer associated with the ice edge. While this layer extends northward under the ice edge as the melt season progresses, low salinities and warm temperatures appear south of the edge. Within this fresh layer, average salinity is correlated with distance from the ice edge. The salinity observations suggest that the upper-ocean freshening over the summer is dominated by local sea ice melt and vertical mixing. A Price–Weller–Pinkel model analysis reveals that observed changes in heat content and density structure are also consistent with a 1D mixing process.

As sources of reactive halogens, snowpacks in sea ice regions control the oxidative capacity of the Arctic atmosphere. However, measurements of snowpack halide concentrations remain sparse, particularly in the high Arctic, limiting our understanding of and ability to parameterize snowpack participation in tropospheric halogen chemistry. To address this gap, we measured concentrations of chloride, bromide, and sodium in snow samples collected during polar spring above remote multi-year sea ice (MYI) and first-year­ sea ­ice­(FYI) ­north ­of ­Greenland­ and ­Alask, ­as­ well­ as ­in ­the ­central ­Arctic, ­and ­compared these measurements to a larger dataset collected in the Alaskan coastal Arctic by Krnavek et al. (2012). Regardless of sea ice region, these surface snow samples generally featured lower salinities, compared to­ coastal ­snow. ­­Surface­ snow­ in ­FYI­ regions ­was ­typically­ enriched ­in bromide ­and­ chloride ­compared ­to seawater, indicating snowpack deposition of bromine and chlorine-containing trace gases and an ability of the snowpack to participate further in bromine and chlorine activation processes. In contrast, surface snow in MYI regions was more often depleted in bromide, indicating it served as a source of bromine-containing trace gases to the atmosphere prior to sampling. Measurements at various snow depths indicate that the deposition of sea salt aerosols and halogen-containing trace gases to the snowpack surface played a larger role in determining surface snow halide concentrations compared to upward brine migration from sea ice. Calculated enrichment factors for bromide and chloride, relative to sodium, in the MYI snow­ samples ­suggests ­that ­MYI­ regions, ­in addition ­to ­FYI­ regions, ­have ­the ­potential ­to ­play ­an ­active role in Arctic boundary layer bromine and chlorine chemistry. The ability of MYI regions to participate in springtime atmospheric halogen chemistry should be considered in regional modeling of halogen activation and interpretation of satellite-based tropospheric bromine monoxide column measurements.

A model-observation synthesis is conducted to investigate changes in the upper ocean circulation and stratification in the Canada Basin [CB] of the Arctic Ocean. Results show that the Beaufort Gyre [BG] has been generally intensifying during 1992–2015 in conjunction with changes in sea ice and the upper ocean including increasing sea surface height [SSH], sea ice and ocean speed, Ekman transport convergence and downwelling, and freshwater content, decreasing ice thickness and upper ocean salinity, shoaling summer halocline and mixed layer, and deepening winter halocline and mixed layer. Increasing Ekman transport convergence draws more water from surrounding areas into the CB, thus lowering SSH in those areas and raising SSH in the CB. The rate of change in the CB began to decrease in 2008 and the BG circulation appears to be stabilizing, if not relaxing slightly. This is reflected in the general plateauing of SSH, the intensity of the sea ice and ocean circulation, and various measures of the CB thermohaline stratification. The BG intensification and subsequent stabilization appear to have been strongly controlled by atmospheric changes in the CB characterized by generally increasing anticyclonic wind circulation and sea level pressure (SLP) before 2008 and falling wind strength and SLP to below-average levels in some years after 2008. Changes in SLP are highly correlated with changes in ocean surface stress curl and downwelling. Since 2008, the magnitude of the stress curl and downwelling in much of the CB has declined, contributing to BG stabilization. The general leveling-off of sea ice thickness also contributes to the stabilization by limiting melt water input to the CB that increases freshwater content. Temperatures in the Near Surface Temperature Maximum layer trended upward slightly over 1992–2015, which is closely correlated with decreasing sea ice thickness. Upper ocean heat content increased over the study period mainly due to strong temperature increases in the summer Pacific Water layer.

Using time-varying ocean bottom pressure (OBP) from the Gravity Recovery and Climate Experiment (GRACE), a 9-year in situ OBP record at the North Pole, and wind reanalysis products, we perform a linear regression analysis to identify primary predictor time series that enable us to create a proxy representation of the Arctic time-varying OBP that explains the largest fraction of the observed Arctic OBP variability. After cross-validation, two predictors – North Pole OBP record, and wind-OBP coupling from maximum covariance analysis – explain 50% of the total variance of the Arctic OBP. This work provides a means for bridging existing short gaps in GRACE measurements, and potentially longer future gaps that may result if GRACE and its follow-on mission do not overlap. The technique may be applicable to bridge gaps in GRACE measurements in other oceanic regions.

The low levels of mechanically driven mixing in many regions of the Arctic Ocean make double diffusive convection virtually the only mechanism for moving heat up from the core of Atlantic Water towards the surface. In an attempt to quantify double diffusive heat fluxes in the Arctic Ocean, a temperature microstructure experiment was performed as a part of the North Pole Environmental Observatory (NPEO) 2013 field season from the drifting ice station Barneo located in the Amundsen Basin near the Lomonosov Ridge (89.58N, 758W). A diffusive convective thermohaline staircase was present between 150 and 250 m in nearly all of the profiles. Typical vertical heat fluxes across the high-gradient interfaces were consistently small, O(1021 )Wm22 . Our experiment was designed to resolve the staircase and differed from earlier Arctic studies that utilized inadequate instrumentation or sampling. Our measured fluxes from temperature microstructure agree well with the laboratory derived flux laws compared to previous studies, which could find agreement only to within a factor of two to four. Correlations between measured and parameterized heat fluxes are slightly higher when using the more recent Flanagan et al. [2013] laboratory derivation than the more commonly used derivation presented in Kelley [1990]. Nusselt versus Rayleigh number scaling reveals the convective exponent, g, to be closer to 0.29 as predicted by recent numerical simulations of single-component convection rather than the canonical 1/3 assumed for double diffusion. However, the exponent appears to be sensitive to how convective layer height is defined. publishedVersion

Fourteen years (2000–2014) of bottle chemistry data collected during the North Pole Environmental Observatory were compiled to examine variations in the composition of freshwater (meteoric water, net sea-ice meltwater, and Pacific water) over mixed layer of the Central Arctic Ocean. In addition to significant spatial and interannual variability, there was a general decrease in meteoric water (MW) fractions at the majority of stations reoccupied over the duration of the program that was approximately balanced by a concomitant increase in freshwater from sea-ice melt (SIM FW) between 2000 and 2012. Inventories (0–120 m) of MW and SIM FW computed using available data between 2005 and 2012 exhibited similar variations over the study area, allowing for first-order estimates of the mean annual changes in MW (−389 ± 194 km3 yr−1) and SIM FW (292 ± 97 km3 yr−1) for the Central Arctic region. These mean annual changes were attributed to the diversion of Siberian river runoff to the Beaufort Gyre and the overall reduction of sea ice volume across the Arctic, respectively. In addition to this lower-frequency variability, spatial gradients and interannual variations in MW, SIM FW, and Pacific water contributions to specific locations were attributed to shifts in the Transpolar Drift that advects waters of eastern and western Arctic origin through the study area.

High-precision deep Arctic Bottom Pressure Recorders (ABPRs) were developed to measure ocean bottom pressure variations in the perennial ice-covered Arctic Ocean. The ABPRs use the tsunami detection DART acoustic modem technology and have been programmed to store and transmit the data acoustically without the need to recover the instrument. ABPRs have been deployed near the North Pole, where the ice cover is a year-round challenge for access with a ship. Instead, the ABPRs have been built as light-weight mechanical systems that we can install using aircraft landing on the ice. ABPRs have provided the first records of uninterrupted pressure data over continuous years ever made in the central Arctic. The ABPR data have allowed us to identify and understand modes of Arctic Ocean bottom pressure variability that were unknown before the ABPR records and have offered new means of investigating and understanding the rapidly changing Arctic system. The ABPR records have also shown outstanding agreement with the satellite-sensed ocean bottom pressure anomalies from GRACE, providing ground truth data for validation of the satellite system. Due to the successful science findings as well as the ABPRs' capability to fulfill the upcoming potential gaps of pressure measurements between GRACE and a GRACE follow-on mission, we highlight the urgent need to develop and maintain an Arctic observing network using ABPRs.

The sole instrument on the upcoming Ice, Cloud, and Land Elevation Satellite (ICESat-2) altimetry mission is a micropulse lidar that measures the time of flight of individual photons from laser pulses transmitted at 532 nm. Prior to launch, the Multiple Altimeter Beam Experimental Lidar (MABEL) serves as an airborne implementation for testing and development. This paper provides a first examination of MABEL data acquired on two flights over sea ice in April 2012: one north of the Arctic coast of Greenland and the other in the east Greenland Sea. The phenomenology of photon distributions in the sea ice returns is investigated. An approach to locate the surface and estimate its elevation in the distributions is described, and its achievable precision is assessed. Retrieved surface elevations over relatively flat leads in the ice cover suggest that precisions of several centimeters are attainable. Restricting the width of the elevation window used in the surface analysis can mitigate potential biases in the elevation estimates due to subsurface returns at 532 nm. Comparisons of nearly coincident elevation profiles from MABEL with those acquired by an analog lidar show good agreement. Discrimination of ice and open water, a crucial step in the determination of sea ice freeboard and the estimation of ice thickness, is facilitated by contrasts in the observed signal–background photon statistics. Future flight paths will sample a broader range of seasonal ice conditions for further evaluation of the year-round profiling capabilities and limitations of the MABEL instrument.

Measurements of ocean bottom pressure (OBP) anomalies from the satellite mission Gravity Recovery and Climate Experiment (GRACE), complemented by information from two ocean models, are used to investigate the variations and distribution of the Arctic Ocean mass from 2002 through 2011. The forcing and dynamics associated with the observed OBP changes are explored. Major findings are the identification of three primary temporal–spatial modes of OBP variability at monthly-to-interannual time scales with the following characteristics. Mode 1 (50% of the variance) is a wintertime basin-coherent Arctic mass change forced by southerly winds through Fram Strait, and to a lesser extent through Bering Strait. These winds generate northward geostrophic current anomalies that increase the mass in the Arctic Ocean. Mode 2 (20%) reveals a mass change along the Siberian shelves, driven by surface Ekman transport and associated with the Arctic Oscillation. Mode 3 (10%) reveals a mass dipole, with mass decreasing in the Chukchi, East Siberian, and Laptev Seas, and mass increasing in the Barents and Kara Seas. During the summer, the mass decrease on the East Siberian shelves is due to the basin-scale anticyclonic atmospheric circulation that removes mass from the shelves via Ekman transport. During the winter, the forcing mechanisms include a large-scale cyclonic atmospheric circulation in the eastern-central Arctic that produces mass divergence into the Canada Basin and the Barents Sea. In addition, strengthening of the Beaufort high tends to remove mass from the East Siberian and Chukchi Seas. Supporting previous modeling results, the month-to-month variability in OBP associated with each mode is predominantly of barotropic character.

[1] Observational studies have shown that an unprecedented warm anomaly has recently affected the temperature of the Atlantic Water (AW) layer lying at intermediate depth in the Arctic Ocean. Using observations from four profiling moorings, deployed in the interior of the Canada Basin between 2003 and 2011, the upward diffusive vertical heat flux from this layer is quantified. Vertical diffusivity is first estimated from a fine-scale parameterization method based on CTD and velocity profiles. Resulting diffusive vertical heat fluxes from the AW are in the range 0.1–0.2 W m−2 on average. Although large over the period considered, the variations of the AW temperature maximum yields small variations for the temperature gradient and thus the vertical diffusive heat flux. In most areas, variations in upward diffusive vertical heat flux from the AW have only a limited effect on temperature variations of the overlying layer. However, the presence of eddies might be an effective mechanism to enhance vertical heat transfer, although the small number of eddies sampled by the moorings suggest that this mechanism remains limited and intermittent in space and time. Finally, our results suggest that computing diffusive vertical heat flux with a constant vertical diffusivity of ∼2 × 10−6 m2 s−1 provides a reasonable estimate of the upward diffusive heat transfer from the AW layer, although this approximation breaks down in the presence of eddies.

Hydrographic data from the Arctic Ocean show that freshwater content in the Lincoln Sea, north of Greenland, increased significantly from 2007 to 2010, slightly lagging changes in the eastern and central Arctic. The anomaly was primarily caused by a decrease in the upper ocean salinity. In 2011 upper ocean salinities in the Lincoln Sea returned to values similar to those prior to 2007. Throughout 2008–2010, the freshest surface waters in the western Lincoln Sea show water mass properties similar to fresh Canada Basin waters north of the Canadian Arctic Archipelago. In the northeastern Lincoln Sea fresh surface waters showed a strong link with those observed in the Makarov Basin near the North Pole. The freshening in the Lincoln Sea was associated with a return of a subsurface Pacific Water temperature signal although this was not as strong as observed in the early 1990s. Comparison of repeat stations from the 2000s with the data from the 1990s at inline image showed an increase of the Atlantic temperature maximum which was associated with the arrival of warmer Atlantic water from the Eurasian Basin. Satellite-derived dynamic ocean topography of winter 2009 showed a ridge extending parallel to the Canadian Archipelago shelf as far as the Lincoln Sea, causing a strong flow toward Nares Strait and likely Fram Strait. The total volume of anomalous freshwater observed in the Lincoln Sea and exported by 2011 was close to inline image, approximately 13% of the total estimated FW increase in the Arctic in 2008.

[1] To determine whether deep background mixing has increased with the diminishment of the Arctic sea ice, we compare recent internal wave energy and mixing observations with historical measurements. Since 2007, the North Pole Environmental Observatory has launched expendable current probes (XCPs) as a part of annual airborne hydrographic surveys in the central Arctic Ocean. Mixing in the upper 500 m is estimated from XCP shear variance and Conductivity-Temperature-Depth (CTD) derived Brunt-Vaisala frequency. Internal wave energy levels vary by an order of magnitude between surveys, although all surveys are less energetic and show more vertical modes than typical midlatitude Garrett-Munk (GM) model spectra. Survey-averaged mixing estimates also vary by an order of magnitude among recent surveys. Comparisons between modern and historical data, reanalyzed in identical fashion, reveal no trend evident over the 30 year period in spite of drastic diminution of the sea ice. Turbulent heat fluxes are consistent with recent double-diffusive estimates. Both mixing and internal wave energy in the Beaufort Sea are lower when compared to both the central and eastern Arctic Ocean, and expanding the analysis to mooring data from the Beaufort Sea reveals little change in that area compared to historical results from Arctic Internal Wave Experiment. We hypothesize that internal wave energy remains lowest in the Beaufort Sea in spite of dramatic declines in sea ice there, because increased stratification amplifies the negative effect of boundary layer dissipation on internal wave energy.

Losing its character The eastern Eurasian Basin of the Arctic Ocean is on the far side of the North Pole from the Atlantic, but it is becoming more like its larger neighbor as the climate warms. Polyakov et al. show that this region is also evolving toward a state of weakened stratification with increased vertical mixing, release of oceanic heat, and less sea ice. These changes could have considerable impacts on other geophysical and biogeochemical aspects of the Arctic Ocean system and presage a fundamentally new Arctic climate state. Science , this issue p. 285

International audience; In the central Arctic Ocean, autonomous observations of the ocean mixed layer and ice documented the transition from cold spring to early summer in 2011. Ice-motion measurements using GPS drifters captured three events of lead opening and ice ridge formation in May and June. Satellite sea ice concentration observations suggest that locally observed lead openings were part of a larger-scale pattern. We clarify how these ice deformation events are linked with the onset of basal sea ice melt, which preceded surface melt by 20 days. Observed basal melt and ocean warming are consistent with the available input of solar radiation into leads, once the advent of mild atmospheric conditions prevents lead refreezing. We use a one-dimensional numerical simulation incorporating a Local Turbulence Closure scheme to investigate the mechanisms controlling basal melt and upper ocean warming. According to the simulation, a combination of rapid ice motion and increased solar energy input at leads promotes basal ice melt, through enhanced mixing in the upper mixed layer, while slow ice motion during a large lead opening in mid-June produced a thin, low-density surface layer. This enhanced stratification near the surface facilitates storage of solar radiation within the thin layer, instead of exchange with deeper layers, leading to further basal ice melt preceding the upper surface melt.

Melting of the Greenland Ice Sheet represents a major uncertainty in projecting future rates of global sea level rise. Much of this uncertainty is related to a lack of knowledge about subsurface ocean hydrographic properties, particularly heat content, how these properties are modified across the continental shelf, and about the extent to which the ocean interacts with glaciers. Early results from NASA’s five-year Oceans Melting Greenland (OMG) mission, based on extensive hydrographic and bathymetric surveys, suggest that many glaciers terminate in deep water and are hence vulnerable to increased melting due to ocean-ice interaction. OMG will track ocean conditions and ice loss at glaciers around Greenland through the year 2020, providing critical information about ocean-driven Greenland ice mass loss in a warming climate.

During the summer of 2010 ice concentration in the Eurasian Basin, Arctic Ocean was unusually low. This study examines the sea-ice reduction in the Eurasian Basin using ice-based autonomous buoy systems that collect temperature and salinity of seawater under the ice along the course of buoy drift. An array of GPS drifters was deployed with 10 miles radius around an ice-based profiler, enabling the quantitative discussion for mechanical ice divergence/convergence and its contribution to the sea-ice reduction. Oceanic heat fluxes to the ice estimated using buoy motion and mixed-layer (ML) temperature suggest significant spatial difference between fluxes under first-year and multi-year ice. In the former, the ML temperature reached 0.6 K above freezing temperature, providing >60–70 W m −2 of heat flux to the overlying ice, equivalent to about 1.5 m of ice melt over three months. In contrast, the multiyear ice region indicates nearly 40 W m −2 at most and cumulatively produced 0.8 m ice melt. The ice concentration was found to be reduced in association with an extensive low pressure system that persisted over the central Eurasian Basin. SSM/I indicates that ice concentration was reduced by 30–40% while the low pressure persisted. The low ice concentration persisted for 30 days even after the low dissipated. It appears that the wind-forced ice divergence led to enhanced absorption of incident solar energy in the expanded areas of open water and thus to increased ice melt.

Here we report the first optical, sensor-based profiles of nitrate from the central Makarov and Amundsen and southern Canada Basins of the Arctic Ocean. These profiles were obtained as part of the International Polar Year program during spring 2007 and 2008 field seasons of the North Pole Environmental Observatory (NPEO) and Beaufort Gyre Exploration Program (BGEP). These nitrate data were combined with in-situ, sensor-based profiles of dissolved oxygen to derive the first high-resolution vertical NO profiles to be reported for the Arctic Ocean. The focus of this paper is on the halocline layer that insulates sea ice from Atlantic water heat and is an important source of nutrients for marine ecosystems within and downstream of the Arctic. Previous reports based on bottle data have identified a distinct lower halocline layer associated with an NO minimum at about S=34.2 that was proposed to be formed initially in the Nansen Basin and then advected downstream. Greater resolution afforded by our data reveal an even more pronounced NO minimum within the upper, cold halocline of the Makarov Basin. Thus a distinct lower salinity source ventilated the Makarov and not the Amundsen Basin. In addition, a larger Eurasian River water influence overlies this halocline source in the Makarov. Observations in the southern Canada Basin corroborate previous studies confirming multiple lower halocline influences including diapycnal mixing between Pacific winter waters and Atlantic-derived lower halocline waters, ventilation via brine formation induced in persistent openings in the ice, and cold, O2-rich lower halocline waters originating in the Eurasian Basin. These findings demonstrate that continuous sensing of chemical properties promises to significantly advance understanding of the maintenance and circulation of the halocline.

The first-ever observed horizontal profiles of summertime ice-ocean boundary layer fluxes were obtained using vertical water velocity, temperature, and salinity collected by an Autonomous Underwater Vehicle during the Surface Heat Balance of the Arctic Ocean (SHEBA) experiment of 1998. Scalars and their vertical fluxes, as well as vertical stability, varied in the horizontal direction with correspondence to changes in the overlying surface. In early summer, fresh meltwater was trapped at the upper ice surface and only entered the ocean through leads. A highly stable fresh layer was formed in the SHEBA lead, which eventually grew to depths greater than the mean draft of the local first-year ice. Near the end of July, a storm removed this layer via shear-generated turbulence, supercritical hydraulic flow speeds, and ice divergence. The mixed layer freshened and deepened at this time. Particularly strong fluxes were observed under and downstream of rough, ridged ice, and properties changed rapidly with distance downstream of leads. The location and signs of the fluxes are suggestive of a mechanism of instability in which fresh surface water is forced under salty water downstream of leads and/or ridges. Simulations from a two-dimensional unsteady model suggest that both mechanical forcing from ice topography and a dynamic instability near downstream lead edges may enhance vertical mixing, particularly when ice velocity is large. The horizontal variability in interfacial fluxes observed at SHEBA may explain the difference between the observed melt rates and those calculated using a bulk relationship because this relationship may not adequately parameterize the large lateral heat fluxes at lead edges and basal heat fluxes under ridge keels.

Temperature, salinity, and chemical measurements, including the nutrients silicic acid (Si), nitrate (NO3), nitrite (NO2), ammonium (NH4), and phosphate (PO4 or P), the oxygen isotopic composition of seawater (d 18 O), and barium (Ba) concentrations were obtained from the central Arctic Ocean along transects radiating from the North Pole in early spring, 2000–2006. Stations that were reoccupied over this time period were grouped into five regions: from Ellesmere Island, (1) north along 701W and (2) northwest along 901W; near the North Pole, (3) on the Amundsen Basin flank and (4) directly over the Lomonosov Ridge; (5) through the Makarov Basin along 170–1801W. These regions had been shown by others to have undergone marked changes in water-mass assemblies in the early 1990s, but our time series tracer hydrographic data indicate a partial return of Pacific origin water within the mixed layer and the upper halocline layers beginning in 2003–2004. Back-trajectories derived from satellite-tracked ice buoys for these stations indicate that the upper levels of Pacific water in the central Arctic in 2004–2006 transited westward from the Bering Strait along the Siberian continental slope into the East Siberian Sea before entering the Transpolar Drift Stream (TPD). By 2004, the TPD shifted back from an alignment over the Alpha-Mendeleev Ridge toward the Lomonosov Ridge, as was characteristic prior to the early 1990s. At most stations occupied in 2006, a decrease in the Pacific influence was observed, both in the mixed layer and in the upper halocline, which suggests the Canadian branch of the TPD was shifting back toward North America. Clearly the system is more variable than has been previously appreciated. r 2007 Elsevier Ltd. All rights reserved.

Dissolved oxygen (O2) profiling by new generation sensors was conducted in the Arctic Ocean via aircraft during May 2003 as part of the North Pole Environmental Observatory (NPEO) and Freshwater Switchyard (SWYD) projects. At stations extending from the North Pole to the shelf off Ellesmere Island, such profiles display what appear to be various O2 maxima (with concentrations 70% of saturation or less) over depths of 70–110 m in the halocline, corresponding to salinity and temperature ranges of 33.3–33.9 and � 1.7 to � 1.51C. The features appear to be widely distributed: Similar features based on bottle data were recently reported for a subset of the 1997–1998 SHEBA stations in the southern Canada Basin and in recent Beaufort Sea sensor profiles. Oxygen sensor data from August 2002 Chukchi Borderlands (CBL) and 1994 Arctic Ocean Section (AOS) projects suggest that such features arise from interleaving of shelf-derived, O2-depleted waters. This generates apparent oxygen maxima in Arctic Basin profiles that would otherwise trend more smoothly from near-saturation at the surface to lower concentrations at depth. For example, in the Eurasian Basin, relatively low O2 concentrations are observed at salinities of about 34.2 and 34.7. The less saline variant is identified as part of the lower halocline, a layer originally identified by a Eurasian Basin minimum in ‘‘NO,’’ which, in the Canadian Basin, is reinforced by additional inputs. The more saline and thus denser variant appears to arise from transformations of Atlantic source waters over the Barents and/or Kara shelves. Additional lowoxygen waters are generated in the vicinity of the Chukchi Borderlands, from Pacific shelf water outflows that interleave with Eurasian waters that flow over the Lomonosov Ridge into the Makarov Basin and then into the Canada Basin. One such input is associated with the well-known silicate maximum that historically has been associated with a salinity of E33.1. Above that (32oSo33), there is a layer moderately elevated in temperature (summer Bering Sea water) that we show is also O2-depleted. We propose that these low O2 waters influence the NPEO and SWYD profiles to varying extents in a manner reflective of the large-scale circulation. The patterns of halocline circulation we infer from the intrusive features defy a simple boundary-following cyclonic flow. These results demonstrate the value of the improved

Although certain regions of the Arctic have experienced periods of decadal warming during the last 100 years, recent decades show an ongoing suite of Arctic-wide, interrelated atmospheric, oceanic, terrestrial, and human dimension changes. A major workshop conclusion stresses the necessity for coordination between arctic disciplines to form a continuing and enhanced dataset of arctic change. A major workshop recommendation is to increase the use of past and evolving datasets for understanding arctic change through, for example, an arctic change protocol, an aaaarctic system reanalysis, timely intercalibration of satellite products, and multidisciplinary, multiregional analyses. Understanding of the Arctic can improve change detection due to the roles of vegetation type, sea ice, and other feedbacks in providing a multiyear memory for the climate system.

The authors show that vertical turbulent fluxes in the upper ocean can be measured directly with an autonomous underwater vehicle (AUV). A horizontal profile of vertical water velocity is obtained by applying a Kalman smoother to AUV motion data. The smoother uses a linearized model for vehicle motion and vehicle data such as depth, pitch, and pitch rate to produce an optimal estimate of the state of the system, which includes other vehicle variables and the vertical water velocity. Vertical water velocity estimated by applying the smoother to data from the autonomous microconductivity temperature vehicle (AMTV) is accurate at horizontal scales from three to several hundred meters, encompassing the energy-containing scales of most oceanic turbulence. The zero-lag covariances between vertical water velocity and concurrent measurements of temperature or salinity represent the heat and salt fluxes, respectively. The authors have measured horizontal profiles of turbulent fluxes with two different AUVs in three separate polar ocean experiments using this technique. Flux magnitudes and directions are reasonable and in general agreement with fixed turbulence sensors. With this technique, one can gather boundary layer data in inaccessible regions without disturbing or affecting the surface.

A summary is presented of the Surface Heat Budget of the Arctic Ocean (SHEBA) project, with a focus on the field experiment that was conducted from October 1997 to October 1998. The primary objective of the field work was to collect ocean, ice, and atmospheric datasets over a full annual cycle that could be used to understand the processes controlling surface heat exchanges—in particular, the ice–albedo feedback and cloud–radiation feedback. This information is being used to improve formulations of arctic ice–ocean–atmosphere processes in climate models and thereby improve simulations of present and future arctic climate. The experiment was deployed from an ice breaker that was frozen into the ice pack and allowed to drift for the duration of the experiment. This research platform allowed the use of an extensive suite of instruments that directly measured ocean, atmosphere, and ice properties from both the ship and the ice pack in the immediate vicinity of the ship. This summary describes the project goal...

The accuracy of state-of-the-art global barotropic tide models is assessed using bottom pressure data, coastal tide gauges, satellite altimetry, various geodetic data on Antarctic ice shelves, and independent tracked satellite orbit perturbations. Tide models under review include empirical, purely hydrodynamic ("forward"), and assimilative dynamical, i.e., constrained by observations. Ten dominant tidal constituents in the diurnal, semidiurnal, and quarter-diurnal bands are considered. Since the last major model comparison project in 1997, models have improved markedly, especially in shallow-water regions and also in the deep ocean. The root-sum-square differences between tide observations and the best models for eight major constituents are approximately 0.9, 5.0, and 6.5 cm for pelagic, shelf, and coastal conditions, respectively. Large intermodel discrepancies occur in high latitudes, but testing in those regions is impeded by the paucity of high-quality in situ tide records. Long-wavelength components of models tested by analyzing satellite laser ranging measurements suggest that several models are comparably accurate for use in precise orbit determination, but analyses of GRACE intersatellite ranging data show that all models are still imperfect on basin and subbasin scales, especially near Antarctica. For the M-2 constituent, errors in purely hydrodynamic models are now almost comparable to the 1980-era Schwiderski empirical solution, indicating marked advancement in dynamical modeling. Assessing model accuracy using tidal currents remains problematic owing to uncertainties in in situ current meter estimates and the inability to isolate the barotropic mode. Velocity tests against both acoustic tomography and current meters do confirm that assimilative models perform better than purely hydrodynamic models.

An oceanographic field program called the Antarctic Zone Flux experiment was carried out in the eastern Weddell Sea during austral winter (July–September) 1994. Data from a drift buoy array were used in concert with shipboard observations to provide exceptionally high horizontal resolution of upper ocean hydrographic parameters near Maud Rise. Chemical and tracer data were obtained from the ship. We identify a “warm pool” southwest of the rise as a dynamically necessary region of positive (cyclonic) vorticity that is associated with a Taylor column over the rise. Both a warm “halo” surrounding the Taylor column and the warm pool are associated with thermocline shoaling that is a necessary condition for high upward heat fluxes to occur. These features extend the influence of Maud Rise bottom topography on upper ocean heat flux over a region that is larger, by a factor of at least 2, than the area directly overlying the rise. Areal mean upward heat fluxes of about 25 W m−2 are derived using both upper ocean T (“instantaneous”) values and tracer data (“integrated”) values. Fluxes derived over the warm halo and pool regions using only upper ocean T exceeded 100 W m−2 at specific sites. Elsewhere in the region, the T-derived heat fluxes varied widely from 50 W m−2, whereas the tracer-derived heat fluxes showed a considerably more uniform distribution. Our mean values are similar to those that have been previously reported. Historical ice cover data have shown that the geographical region encompassed by Maud Rise and the warm pool area to the southwest is a preferred site for polynya formation, consistent with these findings. Time series analyses of the historical upper ocean data set suggest that conditions conducive to polynya formation are correlated with climate processes remote from the Southern Ocean.

Studies from a variety of disciplines documentrecentchange in the northern high-latitude environment.Prompted by predictions of an amplified response oftheArctic to enhanced greenhouse forcing, we present asynthesis of these observations. Pronounced winter andspring warming over northern continents since about 1970ispartly compensated by cooling over the northern NorthAtlantic. Warming is also evident over the centralArcticOcean. There is a downward tendency in sea ice extent,attended by warming and increased areal extent of theArctic Ocean's Atlantic layer. Negative snow coveranomalies have dominated over both continents sincethelate 1980s and terrestrial precipitation has increasedsince 1900. Small Arctic glaciers have exhibitedgenerally negative mass balances. While permafrost haswarmed in Alaska and Russia, it has cooled in easternCanada. There is evidence of increased plant growth,attended by greater shrub abundance and northwardmigration of the tree line. Evidence also suggeststhatthe tundra has changed from a net sink to a net sourceofatmospheric carbon dioxide.Taken together, these results paint a reasonablycoherent picture of change, but their interpretationassignals of enhanced greenhouse warming is open todebate.Many of the environmental records are either short,areof uncertain quality, or provide limited spatialcoverage. The recent high-latitude warming is also nolarger than the interdecadal temperature range duringthis century. Nevertheless, the general patterns ofchange broadly agree with model predictions. Roughlyhalfof the pronounced recent rise in Northern Hemispherewinter temperatures reflects shifts in atmosphericcirculation. However, such changes are notinconsistentwith anthropogenic forcing and include generallypositive phases of the North Atlantic and ArcticOscillations and extratropical responses to theEl-NinoSouthern Oscillation. An anthropogenic effect is alsosuggested from interpretation of the paleoclimaterecord,which indicates that the 20th century Arctic is thewarmest of the past 400 years.

Seasonal sea ice, which plays a pivotal role in air–sea interaction in the Weddell Sea (a region of large deep-water formation with potential impact on climate), depends critically on heat flux from the deep ocean. During the austral winter of 1994, an intensive process-oriented field program named the Antarctic Zone Flux Experiment measured upper-ocean turbulent fluxes during two short manned ice-drift station experiments near the Maud Rise seamount region of the Weddell Sea. Unmanned data buoys left at the site of the first manned drift provided a season-long time series of ice motion, mixed layer temperature and salinity, plus a (truncated) high-resolution record of temperature within the ice column. Direct turbulence flux measurements made in the ocean boundary layer during the manned drift stations were extended to the ice–ocean interface with a “mixing length” model and were used to evaluate parameters in bulk expressions for interfacial stress (a “Rossby similarity” drag law) and ocean-to-...

During the Surface Heat Budget of the Arctic (SHEBA) deployment in October, 1997, multiyear ice near the center of the Beaufort Gyre was anomalously thin. The upper ocean was both warmer and less saline than in previous years. The salinity deficit in the upper 100 m, compared with the same region during the Arctic Ice Dynamics Joint Experiment (AIDJEX) in 1975, is equivalent to surface input of about 2.4 m of fresh water. Heat content has increased by 67 MJ m−2. During AIDJEX the change in salinity over the melt season implied melt equivalent to about 0.8 m of fresh water. As much as 2 m of freshwater input may have occurred during the 1997 summer, possibly resulting from decreased ice concentration from changes in atmospheric circulation early in the summer , in the classic albedo-feedback scenario. Unchecked, the pattern could lead to a significantly different sea-ice regime in the central Arctic.

The winter Lead Experiment (LeadEx) was conducted during 1992 in the Beaufort Sea. There the Autonomous Conductivity Temperature Vehicle (ACTV) was operated under and around leads. These are the first measurements of this kind. Measurements of temperature, salinity, and turbulence parameters were also made at lead edges with sensors fixed to rigid masts. By combining information from the fixed measurements and the ACTV an inertial dissipation method is used to derive horizontal profiles of salt flux from ACTV salinity measurements. Comparisons also show that turbulent vertical velocity perturbations can be estimated from the vertical motion of the ACTV. With these velocity estimates, horizontal profiles of the turbulent fluxes of heat, w′T′, and salt, w′S′, are also computed directly. Data from a wide (1000-m-wide) lead with rapid ice motion indicate stronger turbulence under the lead than downstream, but the character of the flow is that of a convective boundary layer modified by a change in surface buoyancy flux. The convection is characterized by multiple, intermittent plumes that scale in size with the mixed layer depth. At a medium-sized (100-m-wide) lead with slower ice motion the convective circulation set up by the lead is more apparent. Convective plumes are stronger and more prevalent at the lead edges, particularly the downstream edge. The comparison between the horizontal profiles from the ACTV and time series from fixed sensors suggests that lead convection is a combination of a near surface convective boundary layer and a deeper quasi-stationary convection pattern.

The distribution of dense brine under leads in sea ice is associated with convective sinking of individual plumes and near-surface mixing associated with ice-water momentum flux. The processes have been studied in the recent Lead Experiment (LeadEx) field program and previously modeled using a two-dimensional hydrostatic numerical model. In this study these processes are reexamined using a nonhydrostatic model. It is shown that nonhydrostatic effects give rise to pressure distributions which counter hydrostatic pressure gradients in the fluid. These effects are largest in sinking plume events and can be substantial when plumes are free to sink to depths of 100 m. In the presence of the Arctic halocline (typically at 30–40 m), however, vertical accelerations of convective plumes are limited spatially and temporally, and nonhydrostatic effects are relatively small. It is the purpose of this paper to demonstrate that for realistic Arctic stratification, convective circulations under leads in sea ice are predominantly hydrostatic.

In 1993, the USS Pargo made the first civilian oceanographic submarine cruise in the Arctic Ocean. The hydrographic measurements provide a unique synoptic description of the upper Arctic Ocean. The results indicate that the influence of Atlantic Water has increased relative to existing climatologies. The front between waters with an Atlantic versus Pacific character has shifted from over the Lomonosov Ridge to roughly over the Alpha and Mendeleyev Ridges. Warm cores of Atlantic Water are observed over the Lomonosov and Mendeleyev Ridges. These data indicate a fundamental change in the circulation of the Arctic Ocean beginning in the early 1990s.

The upper ocean current and temperature fields in the western Weddell Sea were measured from the drifting pack ice at Ice Station Weddell 1 (ISW) and nearby sites using a vertical profiler and an array of moored sensors in January–June 1992. These data document the structure and variability of the internal gravity wave field and tidal currents in this remote region. The variance of the internal wave continuum (ƒ < frequency < N) at ISW was 0.2–0.6 of the Garrett-Munk (GM) universal level for the first 60 days, increasing to near GM levels during the final 10 days of the deployment. In contrast, the energy density at site C, 50 km west of ISW and farther up the continental slope, was always near GM levels. Variations may be due to a combination of spatial and temporal gradients of the internal wave field. At ISW, coherence between vertically separated sensors was used to estimate vertical wave number bandwidth. Energy and bandwidth estimates are compared with previous studies in both ice-covered and temperate oceans. Using our measurements of the internal wave field and existing parameterizations of mixing, we estimate the vertical heat flux from the Warm Deep Water toward the surface. At ISW the upward heat flux due to mixing associated with the internal waves was about 1 W m−2, much less than the 20 W m−2 average flux required to balance the heat budget for the Weddell Gyre. Tidal currents contributed significantly to the total measured horizontal velocity variance. The tides were primarily barotropic and increased toward the west in both the semidiurnal and diurnal frequency bands. It is suggested that the stronger tidal currents to the west, over the shallower water of the upper continental slope, are indirectly responsible for the higher internal wave energy at site C relative to ISW.

In winter the eastern Weddell Sea in the Atlantic sector of the Southern Ocean hosts some of the most dynamic air–ice–sea interactions found on earth. Sea ice in the region is kept relatively thin by heat flux from below, maintained by upper-ocean stirring associated with the passage of intense, fast-moving cyclones. Ocean stratification is so weak that the possibility of deep convection exists, and indeed, satellite imagery from the Weddell Sea in the 1970s shows a large expanse of open water (the Weddell Polynya) that persisted through several seasons and may have significantly altered global deep-water production. Understanding what environmental conditions could again trigger widespread oceanic overturn may thus be an important key in determining the role of high latitudes in deep-ocean ventilation and global atmospheric warming. During the Antarctic Zone Flux Experiment in July and August 1994, response of the upper ocean and its ice cover to a series of storms was measured at two drifting stations s...

During the LeadEx main field experiment, held in April 1992 in the Alaskan Beaufort Sea, a number of large ice stalactites were observed growing under young lead ice. Formation of the stalactites was associated with rafting of the thin, highly saline ice. The rafting caused the brine to drain rapidly from the ice at a temperature well below the freezing point of the surrounding water, which in turn caused ice to form in a hollow cylinder around the brine plume. Within a 15 h period after the rafting event, the stalactites, which were located approximately 10 m apart in a line along the upwind edge of a 150 m wide lead, had grown to a length of 2 m. A detailed structural analysis of the upper part of one of these stalactites revealed that the interior channel, down which the brine flowed, was bounded by a zone of frazil ice that developed into a shell of columnar ice. The growth of the columnar ice was directed radially outward and the c axes of these crystals were oriented perpendicular to their growth direction. Development of the stalactites illustrates the impact ice deformation can have on the process of brine rejection in freezing leads and potentially on the thermohaline structure of the upper ocean in the immediate vicinity of the lead.

[1] The carbon system of the western Arctic Ocean is undergoing a rapid transition as sea ice extent and thickness decline. These processes are dynamically forcing the region, with unknown consequences for CO2 fluxes and carbonate mineral saturation states, particularly in the coastal regions where sensitive ecosystems are already under threat from multiple stressors. In October 2011, persistent wind-driven upwelling occurred in open water along the continental shelf of the Beaufort Sea in the western Arctic Ocean. During this time, cold ( 32.4) halocline water—supersaturated with respect to atmospheric CO2 (pCO2 > 550 μatm) and undersaturated in aragonite (Ωaragonite < 1.0) was transported onto the Beaufort shelf. A single 10-day event led to the outgassing of 0.18–0.54 Tg-C and caused aragonite undersaturations throughout the water column over the shelf. If we assume a conservative estimate of four such upwelling events each year, then the annual flux to the atmosphere would be 0.72–2.16 Tg-C, which is approximately the total annual sink of CO2 in the Beaufort Sea from primary production. Although a natural process, these upwelling events have likely been exacerbated in recent years by declining sea ice cover and changing atmospheric conditions in the region, and could have significant impacts on regional carbon budgets. As sea ice retreat continues and storms increase in frequency and intensity, further outgassing events and the expansion of waters that are undersaturated in carbonate minerals over the shelf are probable.

A practical method for determining the CTD thermal-lag correction amplitude α and time constant τ is presented. The method is based upon minimizing the salinity separation of temperature-salinity curves from upcasts and downcasts of a yo-yo sequence of CTD profiles. For the Sea-Bird 9 CTD operated at the 1989 Coordinated Eastern Arctic Experiment O Camp with a 1.75 m s−1 water velocity through the conductivity cell, the optimum coefficients are α = 0.0245 and τ = 9.5 s. These results combined with those of Lueck and Picklo and results obtained from other Sea-Bird CTDs operating at lower flow rates confirm the flow dependence of α and τ predicted by Lueck but indicate that the theoretical constants are too high. Based on the empirical results, the formulas for α and τ as a function of the average velocity V through the cell are found to be α = 0.0264 V−1 + 0.0135 and τ = 2.7858 V−1/2 + 7.1499. where V is in units of meters per second.

[1] A sub-monthly mode of non-tidal variability of ocean bottom pressure (OBP) is observed in a 5-year record of deep-sea bottom pressure at the North Pole. OBP records from other regions in the Arctic show that the North Pole non-tidal mass fluctuation is part of a non-propagating basin-coherent variation that is well represented by the ice-ocean model PIOMAS, with a basin-averaged winter-only RMS of 3.3 cm. Wavelet analysis of the modeled OBP shows that the basin-averaged mass variations are non-stationary and only significant during the winter. The basin-averaged OBP is strongly related to the meridional wind component over the Nordic Seas. The ocean response is consistent with episodic wind forcing driving a northward geostrophic slope current. The mass transport anomaly associated with the mode is significant relative to the annual net mean flow.

The Arctic Oscillation, rather than the Beaufort High, is the main factor affecting the freshening of the Arctic Ocean since the 1990s. Changes in Arctic Ocean freshwater distribution are a major influence on climate through their effect on global heat exchange. However, they have been difficult to measure and understand. Recent advances in satellite and in situ observations now make it possible to monitor changes in the freshwater content of the Arctic Ocean, and to analyse the underlying mechanisms controlling recent changes in salinity. Earlier work suggested that an acceleration of the Beaufort Gyre controlled the freshening. Now James Morison and colleagues present maps of freshwater changes across the Arctic Ocean, showing that the pathway of Russian runoff shifted in 2005–08. The Canadian Basin greatly freshened and the Eurasian Basin became saltier, with a net overall freshening equal to 40% of runoff. This was caused by cyclonic circulation in the Eurasian Basin forced by a strengthened Arctic Oscillation, which leads to a strengthening of Northern Hemisphere westerly winds. Freshening in the Canada basin of the Arctic Ocean began in the 1990s1,2 and continued3 to at least the end of 2008. By then, the Arctic Ocean might have gained four times as much fresh water as comprised the Great Salinity Anomaly4,5of the 1970s, raising the spectre of slowing global ocean circulation6. Freshening has been attributed to increased sea ice melting1 and contributions from runoff7, but a leading explanation has been a strengthening of the Beaufort High—a characteristic peak in sea level atmospheric pressure2,8—which tends to accelerate an anticyclonic (clockwise) wind pattern causing convergence of fresh surface water. Limited observations have made this explanation difficult to verify, and observations of increasing freshwater content under a weakened Beaufort High suggest that other factors2 must be affecting freshwater content. Here we use observations to show that during a time of record reductions in ice extent from 2005 to 2008, the dominant freshwater content changes were an increase in the Canada basin balanced by a decrease in the Eurasian basin. Observations are drawn from satellite data (sea surface height and ocean-bottom pressure) and in situ data. The freshwater changes were due to a cyclonic (anticlockwise) shift in the ocean pathway of Eurasian runoff forced by strengthening of the west-to-east Northern Hemisphere atmospheric circulation characterized by an increased Arctic Oscillation9 index. Our results confirm that runoff is an important influence on the Arctic Ocean and establish that the spatial and temporal manifestations of the runoff pathways are modulated by the Arctic Oscillation, rather than the strength of the wind-driven Beaufort Gyre circulation.

[1] We construct the dynamic ocean topography (DOT) of the Arctic Ocean, for five ICESat campaigns (winter of 2004–2008), using sea surface height estimates in open leads. Results show that the mean winter DOT over the Arctic Ocean varies by ∼1 m and features a distinct dome of ∼40 cm over the Beaufort Sea. Standard deviation of the mean field is ∼20 cm. Spatial coherence between the five winter DOTs is consistently high (>0.9), whereas the coherence between the DOTs and the winter (DJFM) sea-level pressure fields over the Arctic Basin is variable. This suggests persistence of the underlying hydrodynamic processes at interannual time-scales compared to seasonal atmospheric forcing. Comparison of dynamic heights (DH) from hydrographic surveys and the DOT in 2008 shows a remarkable correlation of 0.92. The geostrophic velocity fields computed from the DOT and interpolated DH fields highlight the smaller scale oceanographic features in the satellite estimates.

[1] Ocean bottom pressure (OBP) observations in the Arctic from in situ pressure recorders and the Gravity Recovery and Climate Experiment (GRACE) satellite mission, averaged over the basin, reveal annual oscillations of about 2 cm. The maximum occurs in late summer to early fall and the minimum in late winter to early spring. We derive a simple model of OBP response to runoff and precipitation minus evaporation (P-E) that agrees in phase with the observations and is 10% larger.

[1] Sea ice plays a crucial role in the exchange of heat between the ocean and the atmosphere, and areas of intense air-sea-ice interaction are important sites for water mass modification. The Weddell Sea is one of these sites where a relatively thin first-year ice cover is constantly being changed by mixing of heat from below and stress exerted from the rapidly changing and intense winds. This study presents mixed layer turbulence measurements obtained during two wintertime drift stations in August 2005 in the eastern Weddell Sea, close to the Maud Rise seamount. Turbulence in the boundary layer is found to be controlled by the drifting ice. Directly measured heat fluxes compare well with previous studies and are well estimated from the mixed layer temperatures and mixing. Heat fluxes are also found to roughly balance the conductive heat flux in the ice; hence, little freezing/melting was observed. The under-ice topography is estimated to be hydraulically very smooth; comparison with a steady 1-D model shows that these estimates are made too close to the ice-ocean interface to be representative for the entire ice floe. The main source and sink of turbulent kinetic energy are shear production and dissipation. Observations indicate that the dynamics of the under-ice boundary layer are influenced by a horizontal variability in mixed layer density and an increasing amount of open leads in the area.