North Pole Environmental Observatory Research Articles

Sea ice mass balance during the MOSAiC drift experiment: Results from manual ice and snow thickness gauges

Precise measurements of Arctic sea ice mass balance are necessary to understand the rapidly changing sea ice cover and its representation in climate models. During the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition, we made repeat point measurements of snow and ice thickness on primarily level first- and second-year ice (FYI, SYI) using ablation stakes and ice thickness gauges. This technique enabled us to distinguish surface and bottom (basal) melt and characterize the importance of oceanic versus atmospheric forcing. We also evaluated the time series of ice growth and melt in the context of other MOSAiC observations and historical mass balance observations from the Surface Heat Budget of the Arctic (SHEBA) campaign and the North Pole Environmental Observatory (NPEO). Despite similar freezing degree days, average ice growth at MOSAiC was greater on FYI (1.67 m) and SYI (1.23 m) than at SHEBA (1.45 m, 0.53 m), due in part to initially thinner ice and snow conditions on MOSAiC. Our estimates of effective snow thermal conductivity, which agree with SHEBA results and other MOSAiC observations, are unlikely to explain the difference. On MOSAiC, FYI grew more and faster than SYI, demonstrating a feedback loop that acts to increase ice production after multi-year ice loss. Surface melt on MOSAiC (mean of 0.50 m) was greater than at NPEO (0.18 m), with considerable spatial variability that correlated with surface albedo variability. Basal melt was relatively small (mean of 0.12 m), and higher than NPEO observations (0.07 m). Finally, we present observations showing that false bottoms reduced basal melt rates in some FYI cases, in agreement with other observations at MOSAiC. These detailed mass balance observations will allow further investigation into connections between the carefully observed surface energy budget, ocean heat fluxes, sea ice, and ecosystem at MOSAiC and during other campaigns.

Observational validation of the diffusive convection flux laws in the Amundsen Basin, Arctic Ocean

AbstractThe 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 (89.5°N, 75°W). 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(10−1) W m−2. 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, , 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.

Variability in the meteoric water, sea‐ice melt, and Pacific water contributions to the central Arctic Ocean, 2000–2014

AbstractFourteen 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.

Sea ice mass balance observations from the North Pole Environmental Observatory

In recent years the periphery of the Arctic sea ice cover has undergone significant changes, with a reduction in summer ice extent, a thinning of the ice, and a shift from multiyear to first year ice. Here we examine sea ice conditions during nine summers between 2000 and 2013 in the interior of the ice pack, using autonomous measurements of sea ice mass balance deployed near the North Pole. Results exhibit no definitive trends. There is large interannual variability, with surface melt ranging from 0.02 m to 0.50 m and bottom melt from 0.10 m to 0.57 m. The largest amounts of bottom melt have occurred in the past few years. For all 9 years the ice at the end of the melt season was at least 1.2 m thick.

Revisiting internal waves and mixing in the Arctic Ocean

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‐Väisälä 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.

Sensor-based profiles of the NO parameter in the central Arctic and southern Canada Basin: New insights regarding the cold halocline

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, O 2-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 recent Arctic warm period

Arctic winter, spring and autumn surface air temperature (SAT) anomalies and associated sea level pressure (SLP) fields have decidedly different spatial patterns at the beginning of the 21st century (2000–2007) compared to most of the 20th century; we suggest calling this recent interval the Arctic warm period. For example, spring melt date as measured at the North Pole Environmental Observatory (2002–2007) is 7 d earlier than the records from the Russian North Pole stations (1937–1987) and statistically different at the 0.05 level. The 20th centurywas dominated by the two main climate patterns, the Arctic Oscillation/Northern Annular Mode (AO/NAM)and the Pacific North American-like (PNA*) pattern. The predominately zonal winds associated with the positive phases of these patterns contribute to warm anomalies in the Arctic primarily over their respective Eastern andWestern Hemisphere land areas, as in 1989–1995 and 1977–1987. In contrast, SAT in winter (DJF) and spring (MAM) for 2000–2007 show an Arctic-wide SAT anomaly of greater than +1.0°C and regional hot spots over the central Arctic of greater than +3.0°C. Unlike the AO and PNA*, anomalous geostrophic winds for 2000–2007 often tended to blow toward the central Arctic, a meridional wind circulation pattern. In spring 2000–2005, these winds were from the Bering Sea toward the North Pole, whereas in 2006–2007 they were mostly from the eastern Barents Sea. A meridional pattern was also seen in the late 1930s with anomalous winter (DJFM) SAT, at Spitzbergen, of greater than +4°C. Both periods suggest natural atmospheric advective contributions to the hot spots with regional loss of sea ice. Recent warm SAT anomalies in autumn are consistent with climate model projections in response to summer reductions in sea ice extent. The recent dramatic loss of Arctic sea ice appears to be due to a combination of a global warming signal and fortuitous phasing of intrinsic climate patterns.

The recent Arctic warm period

Arctic winter, spring and autumn surface air temperature (SAT) anomalies and associated sea level pressure (SLP) fields have decidedly different spatial patterns at the beginning of the 21st century (2000–2007) compared to most of the 20th century; we suggest calling this recent interval the Arctic warm period. For example, spring melt date as measured at the North Pole Environmental Observatory (2002–2007) is 7 d earlier than the records from the Russian North Pole stations (1937–1987) and statistically different at the 0.05 level. The 20th centurywas dominated by the two main climate patterns, the Arctic Oscillation/Northern Annular Mode (AO/NAM)and the Pacific North American-like (PNA*) pattern. The predominately zonal winds associated with the positive phases of these patterns contribute to warm anomalies in the Arctic primarily over their respective Eastern andWestern Hemisphere land areas, as in 1989–1995 and 1977–1987. In contrast, SAT in winter (DJF) and spring (MAM) for 2000–2007 show an Arctic-wide SAT anomaly of greater than +1.0◦C and regional hot spots over the central Arctic of greater than +3.0◦C. Unlike the AO and PNA*, anomalous geostrophic winds for 2000–2007 often tended to blow toward the central Arctic, a meridional wind circulation pattern. In spring 2000–2005, these winds were from the Bering Sea toward the North Pole, whereas in 2006–2007 they were mostly from the eastern Barents Sea. A meridional pattern was also seen in the late 1930s with anomalous winter (DJFM) SAT, at Spitzbergen, of greater than +4◦C. Both periods suggest natural atmospheric advective contributions to the hot spots with regional loss of sea ice. Recent warm SAT anomalies in autumn are consistent with climate model projections in response to summer reductions in sea ice extent. The recent dramatic loss of Arctic sea ice appears to be due to a combination of a global warming signal and fortuitous phasing of intrinsic climate patterns.

Ice mass-balance buoys: a tool for measuring and attributing changes in the thickness of the Arctic sea-ice cover

AbstractRecent observational and modeling studies indicate that the Arctic sea-ice cover is undergoing significant climate-induced changes, affecting both its extent and thickness. The thickness or, more precisely, the mass balance of the ice cover is a key climate-change indicator since it is an integrator of both the surface heat budget and the ocean heat flux. Accordingly, efforts are underway to develop and deploy in situ observing systems which, when combined with satellite remote-sensing information and numerical models, can effectively monitor and attribute changes in the mass balance of the Arctic sea-ice cover. As part of this effort, we have developed an autonomous ice mass-balance buoy (IMB), which is equipped with sensors to measure snow accumulation and ablation, ice growth and melt, and internal ice temperature, plus a satellite transmitter. The IMB is unique in its ability to determine whether changes in the thickness of the ice cover occur at the top or bottom of the ice cover, and hence provide insight into the driving forces behind the change. Since 2000, IMBs have been deployed each spring from the North Pole Environmental Observatory and in several other areas, including a few in the Beaufort Sea and Central Basin. At this point, the collective time series is too short to draw significant and specific conclusions regarding interannual and regional variability in ice mass balance. Comparisons of available data indicate that ice surface ablation is greater in the Beaufort region (67–80 cm), relative to the North Pole (0–30 cm), consistent with a longer period of melt in the more southerly location. Ablation at the bottom of the ice (22 cm), maximum ice thickness (235 cm) and maximum snow depth (28 cm) were comparable in the two regions.

A drop in mid‐summer shortwave radiation induced by changes in the ice‐surface condition in the central Arctic

Ice‐surface changes during summer and effects on solar heat input in the Arctic were analyzed using incoming shortwave radiation data from drifting buoys deployed in 2002, 2003, and 2004 as part of the North Pole Environmental Observatory project. Observed shortwave radiation was about half of the incoming shortwave radiation at the top of the atmosphere during early summer, suggesting cloudy skies. In each year, events occurred after mid‐summer during which time the shortwave radiation decreased 33% to one third of shortwave radiation at the top of the atmosphere. Snow‐depth data, aerial photos, and date of melt onset at the buoy site suggest that a decrease in snow/ice albedo induced by snow melting and melt‐pond formation modified the amount of shortwave radiation through multiple reflections. A simplified calculation of the ice‐albedo feedback revealed that the drop in shortwave radiation after mid‐summer was self damping.

Dissolved oxygen extrema in the Arctic Ocean halocline from the North Pole to the Lincoln Sea

Dissolved oxygen (O 2) 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 O 2 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 −1.5 °C. 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, O 2-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 O 2 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 low-oxygen 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 ≈33.1. Above that ( 32 < S < 33 ), there is a layer moderately elevated in temperature (summer Bering Sea water) that we show is also O 2-depleted. We propose that these low O 2 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 resolution made feasible with continuous O 2 profiling. In the drive to better understand variability and change in the Arctic Ocean, deployment of appropriately calibrated CTD-O 2 packages offers the promise of important new insights into circulation and ecosystem function.

Sea ice surface temperature product from MODIS

Global sea ice products are produced from the Earth Observing System (EOS) Moderate Resolution Imaging Spectroradiometer (MODIS) onboard both the Terra and Aqua satellites. Daily sea ice extent and ice surface temperature (IST) products are available at 1- and 4-km resolution. Validation activities during the "cold period" (when meltwater is generally not present) in the Northern Hemisphere, defined here as October through May, have been undertaken to assess the accuracy of the 1-km resolution MODIS IST algorithm and product. Validation was also done at the South Pole station in Antarctica. In the Arctic Ocean, near-surface air temperatures from the National Oceanic and Atmospheric Administration (NOAA) National Ocean Service (NOS) Center for Operational Oceanographic Products and Services (CO-OPS) Alaska tide stations and from drifting buoys from the North Pole Environmental Observatory (NPEO) buoy program were compared with MODIS-derived ISTs. Using the standard MODIS sea ice product, which utilizes the MODIS cloud mask, results show a bias (mean error) of -2.1 K and a root mean square (RMS) error of 3.7 K. The negative bias means that the satellite retrieval is less than the air temperature. With the bias removed, the RMS error is 3.0 K. When additional visual cloud screening is performed to eliminate MODIS pixels thought to be contaminated by fog, results improved, with a subset of the larger dataset showing a bias of -0.9 K and an RMS error of 1.6 K. Uncertainties would be reduced in the Arctic Ocean dataset if the skin temperature of the sea ice were reported instead of the near-surface air temperatures. With the bias removed, the RMS error for the Arctic Ocean dataset is 1.3 K. Results from the South Pole station in Antarctica show that under clear skies as determined using lidar measurements, the MODIS ISTs are also very close to those of the near-surface air temperatures with a bias of -1.2 K and an RMS error of 1.7 K. With the bias removed, the RMS error for the South Pole dataset is 1.2 K. Thus, the accuracy (RMS error) of the IST measurement is 1.2-1.3 K. It is not possible to obtain an accurate IST from MODIS in the presence of even very thin clouds or fog, and this is the main limitation of the MODIS ice surface temperature product. MODIS sea ice products may be ordered from the National Snow and Ice Data Center in Boulder, CO.

Ocean‐to‐ice heat flux at the North Pole environmental observatory

Data from drifting buoys deployed in April, 2002, as part of the North Pole Environmental Observatory project have been analysed to estimate ocean heat flux in the time period from 1 May 2002 to 11 Mar 2003. Prior to late January, the observatory remained in deep water, but subsequently drifted directly over the Yermak Plateau, a relatively shallow feature north of Svalbard. While over deep water, heat flux was dominated by storage and release of solar energy in the ocean boundary layer during summer. The most likely annual average value for 2002 was 2.6 W m−2, less than previous determinations in the western Arctic. Over Yermak Plateau, heat flux at the interface came from mixing of warmer water into the boundary layer from below. When the observatory was in water with depths less than 1200 m, the average heat flux was around 22 W m−2.

North Pole Environmental Observatory delivers early results

Scientists have argued for a number of years that the Arctic may be a sensitive indicator of global change, but prior to the 1990s, conditions there were believed to be largely static. This has changed in the last 10 years. Decadal‐scale changes have occurred in the atmosphere, in the ocean, and on land [Serreze et al., 2000]. Surface atmospheric pressure has shown a declining trend over the Arctic, resulting in a clockwise spin‐up of the atmospheric polar vortex. In the 1990s, the Arctic Ocean circulation took on a more cyclonic character, and the temperature of Atlantic water in the Arctic Ocean was found to be the highest in 50 years of observation [Morison et al., 2000]. Sea‐ice thickness over much of the Arctic decreased 43% in 1958–1976 and 1993–1997 [Rothrock et al., 1999].