Arctic Ice Dynamics Joint Experiment Research Articles

Pacific Waters Pathways and Vertical Mixing in the CESM1‐LE: Implication for Mixed Layer Depth Evolution and Sea Ice Mass Balance in the Canada Basin

AbstractWe compare the vertical hydrography of the Community Earth System Model Large Ensemble (CESM1‐LE) with observations from two specific periods: the Arctic Ice Dynamics Joint Experiment (AIDJEX; 1975–1976) and Ice‐Tethered Profilers (ITP; 2004–2018). A comparison between simulated and observed salinity and potential temperature profiles highlights two key model biases in all ensemble members: (a) an absence of Pacific Waters in the water column and (b) a slight deepening of the May mixed layer contrary to observations, which show a large reduction in the mixed‐layer depth and an increase in stratification over the same time period. We examine processes controlling the sea ice mass balance using a one‐dimensional vertical heat budget in the light of the model limitations implied by these two biases. Results indicate that remnant solar heat trapped beneath the halocline is mostly ventilated to the surface by mixing before the following melt season. Furthermore, we find that vertical advection associated with Ekman pumping has only a small effect on the vertical heat transport, even in early fall when the winds are strong and the pack ice is weak. Lastly, we estimate the impact of the missing Pacific Waters at 0.40 m of reduced winter ice growth.

Surface Salinity Under Transitioning Ice Cover in the Canada Basin: Climate Model Biases Linked to Vertical Distribution of Fresh Water

AbstractThe Canada Basin has exhibited a significant trend toward a fresher surface layer and thus a more stratified upper‐ocean over the past three decades. State‐of‐the‐art ice‐ocean models, by contrast, tend to simulate a surface layer that is saltier and less stratified than observed. Here, we examine decadal changes to seasonal processes that may contribute to this wide‐reaching model bias using climate model simulations from the Community Earth System Model and below‐ice observations from the Arctic Ice Dynamics Joint Experiment in 1975 and Ice Tethered Profilers in 2006–2012. In contrast to the observations, the models simulate salinity profiles that show relatively little variation between 1975 and 2012. We demonstrate that this bias can be mainly attributed to unrealistically deep vertical mixing in the model, creating a surface layer that is saltier than observed. The results provide insight for climate model improvement with broad implications for Arctic sea ice and ecosystem dynamics.

The Future of Sea Ice Modeling: Where Do We Go from Here?

Earth System Models (ESMs) include a sea ice component to physically represent sea ice changes and impacts on planetary albedo and ocean circulation (Manabe & Stouffer, 1980). Most contemporary sea ice models describe the sea ice pack as a continuum material, a principle laid by the AIDJEX (Arctic Ice Dynamics Joint EXperiment) group in the 1970s (Pritchard, 1980). Initially intended for climate studies, the sea ice components in ESMs are now used across a wide range of resolutions, including very high resolutions more than 100 times finer than those they were designed for, in an increasingly wide range of applications that challenge the AIDJEX model foundations (Coon et al., 2007), including operational weather and marine forecasts. It is therefore sensible to question the applicability of contemporary sea ice models to these applications. Are there better alternatives available? Large advances in high performance computing (HPC) have been made over the last few decades and this trend will continue. What constraints and opportunities will these HPC changes provide for contemporary sea ice models? Can continuum models scale well for use in exascale computing? To address these important questions, members of the sea ice modelling community met in September 2019 for a workshop in Laugarvatn, Iceland. Thirty-two sea ice modelling scientists from 11 countries across Europe and North America attended, spanning 3 key areas: (i) developers of sea-ice models; (ii) users of sea-ice models in an ESM context; (iii) users of sea64 ice models for operational forecasting and (re)analyses. The workshop was structured around 2 key themes: 1. Scientific and technical validity and limitations of the physics and numerical approaches used in the current models 2. Physical processes and complexity: bridging the gap between weather and climate requirements For each theme, 5 keynote speakers were invited to address the motivating questions and stimulate debate. Further details can be found in the Supplementary Material.

Sea-Ice Morphology Change in the Canada Basin Summer: 2006–2015 Ship Observations Compared to Observations From the 1960s to the Early 1990s

An ice watch, encompassing visual observation of sea ice conducted from the ship’s bridge, is typically performed on sea-ice oriented research cruises. Ice watch data collected during a decade, 2006-2015, of annual cruises to the Beaufort Sea and Canadian Basin characterizes sea-ice morphology during mid to late summer. Observations not otherwise available on extensive regional scales with remote sensing are highlighted, including stage of melt, floe size and surface topography in summer. The summer ice pack in the Canada Basin is found to be of smaller floe size than found in 1973-1975. A disintegrating ice pack of mainly small floe size covered much on the Canada Basin in 2006-2009 and 2011-2012. Since 2007 the ice in the Canada Basin has experienced more extensive melt, and more advanced stages of melt than in 1973-1975, during the Arctic Ice Dynamics Joint Experiment (AIDJEX), and early 1990s. Since 2007, Summer ridge sail heights are lower and ridges less extensive compared to the AIDJEX period.

AIDJEX Revisited: A Look Back at the U.S.-Canadian Arctic Ice Dynamics Joint Experiment 1970–78

AIDJEX Revisited: A Look Back at the U.S.-Canadian Arctic Ice Dynamics Joint Experiment 1970–78

Arctic Ice Dynamics Joint Experiment (AIDJEX) assumptions revisited and found inadequate

This paper revisits the Arctic Ice Dynamics Joint Experiment (AIDJEX) assumptions about pack ice behavior with an eye to modeling sea ice dynamics. The AIDJEX assumptions were that (1) enough leads were present in a 100 km by 100 km region to make the ice isotropic on that scale; (2) the ice had no tensile strength; and (3) the ice behavior could be approximated by an isotropic yield surface. These assumptions were made during the development of the AIDJEX model in the 1970s, and are now found inadequate. The assumptions were made in part because of insufficient large‐scale (10 km) deformation and stress data, and in part because of computer capability limitations. Upon reviewing deformation and stress data, it is clear that a model including deformation on discontinuities and an anisotropic failure surface with tension would better describe the behavior of pack ice. A model based on these assumptions is needed to represent the deformation and stress in pack ice on scales from 10 to 100 km, and would need to explicitly resolve discontinuities. Such a model would require a different class of metrics to validate discontinuities against observations.

On the aggregate‐scale partitioning of solar radiation in Arctic sea ice during the Surface Heat Budget of the Arctic Ocean (SHEBA) field experiment

The partitioning of the incident solar irradiance among reflection to the atmosphere, absorption in the snow and ice, and transmission to the ocean is a critical component of the summer melt cycle of Arctic sea ice. Observations from a year‐long field experiment (Surface Heat Budget of the Arctic Ocean (SHEBA)) showed that of the solar radiation incident between 1 April and 5 October 1998, 68% was reflected to the atmosphere, 24% was absorbed in the snow and ice, and 8% was transmitted to the ocean. The amount of energy reflected to the atmosphere was greatest in May, when the surface albedo and the incident irradiance were large. The energy absorbed in the snow and ice increased slowly in April and May in conjunction with the increase in incident solar irradiance, followed by a sharp increase in June associated with the onset of melt. In spring, virtually none of the solar energy incident on snow‐covered sea ice is transmitted to the ocean compared to over 90% of that incident on leads. The energy transmitted to the ocean reached a maximum near the end of the melt season in mid‐August when the albedo and ice thickness were at minima and lead and pond fractions were at maxima. While much of the solar energy transmitted to the ocean was through leads, substantial portions were also transmitted through bare ice (23%) and ponded ice (16%). During the melt season, the combined energy transmitted through bare ice and ponds was equivalent to that through leads. This marked a significant change in the interaction of solar energy with the sea ice cover, with substantial amounts of solar energy transmitted to the ocean through the thin summer ice cover of SHEBA compared to the thicker ice studied in 1975 (Arctic Ice Dynamics Joint Experiment (AIDJEX)).

Modeling the evolution of the Arctic mixed layer during the fall 1997 Surface Heat Budget of the Arctic Ocean (SHEBA) Project using measurements of 7Be

During the Surface Heat Budget of the Arctic Ocean (SHEBA) project in October 1997 the first measurements of 7Be ever made within the Arctic Ocean were used to reconstruct the evolution of the mixed layer over the previous season and confirmed that the reservoir of heat beneath the fall mixed layer was emplaced in the summer, rather than input cumulatively over several seasons or advected in from distant sources. As suggested by McPhee et al. [1998], several times as much heat was emplaced at SHEBA than at Arctic Ice Dynamics Joint Experiment 20 years earlier. A likely mechanism for this would be substantially greater lead opening coupled with the positive feedback loop between increased open water, increased heat absorption, and further ice melting. In addition, a rate of net primary production ≥13 gC m−2 for the preceding spring‐summer period was derived using this mixed layer history with an oxygen profile from the fall.

Freshening of the upper ocean in the Arctic: Is perennial sea ice disappearing?

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.

Modeling the thermodynamics of a sea ice thickness distribution: 2. Sea ice/ocean interactions

In this paper we examine the coupling of an ice thickness distribution model with an ocean mixed layer model. The annual cycle of the modeled mixed layer temperature, salinity, and depth are compared to observations from the drifting ice stations of the Arctic Ice Dynamics Joint Experiment (AIDJEX). The role of the ice thickness distribution in determining the ice/ocean coupling is examined. We find that the ice thickness distribution is important for the exchange of heat, salt, and fresh water between the ice and the ocean, especially with regard to the effects of thin, first‐year ice. Several different parameterizations of the ice/ocean turbulent heat exchange are compared. The results indicate that the storage of heat in the ocean mixed layer is an important physical process for determining the annual average ice thickness. Double diffusion and the stabilizing effects of meltwater also impact the exchange of heat between the ocean and ice cover. Questions remain as to the relative importance of these processes and their accurate parameterization in ice/ocean coupled models.

The relation between ice deformation, oceanic heat flux, and the ice thickness distribution in the Arctic Ocean

A one‐dimensional ocean model, coupled with an ice model where the ice cover is partitioned into a number of thickness categories, is used to investigate how the ice thickness distribution in the Arctic Ocean depends on the two deformation processes, export (net divergence) and ridging. The model standard case generates vertical profiles of temperature, salinity and an ice thickness distribution similar to the observations. The standard case oceanic heat flux agrees well with recently deduced heat fluxes from Arctic Ice Dynamics Joint Experiment (AIDJEX) data. The oceanic heat flux is about 18 W m−2 during summer and about 1 W m−2, during winter. It is found that the mean ice thickness decreases with increasing ice export in close agreement with other investigations. However, the dependence of the mean ice thickness on ridging activity is found to be very weak. The change of the mean ice thickness is only 12 cm when increasing the ridging activity in the model from 0 to 0.4 yr−1; where the larger value represents an estimate of the actual ridging activity in the Arctic Ocean. In a sensitivity study, different types of ridging and albedo parameterizations are tested, and this weak dependence is found to be very robust. Other properties of the model ice cover are sensitive to ridging though: The equilibrium thickness of undeformed ice decreases with both export and ridging activity. The annual maximum open water fraction and the amplitude of the seasonal thickness variation increases with export and ridging.

Solar heating of the Arctic mixed layer

Data from the 1975 Arctic Ice Dynamics Joint Experiment (AIDJEX) are used to examine energy exchange between the Arctic mixed layer and the ice pack. Conductivity‐temperature‐depth profiles from four drifting stations reveal significant heat storage in the upper 50 m of the water column during summer, with mixed layer temperature elevation above freezing δT reaching as high as 0.4°C. Combining δT with turbulent friction velocity obtained from local ice motion provides an estimate of heat flux from the ocean to the ice Fw which was found to be strongly seasonal, with maximum values reaching 40–60 W m−2 in August. The annual average value of Fw was 5.1 W m−2, about half again as large as oceanic heat flux inferred from bottom ablation measurements in undeformed ice at the central station. Solar heat input to the upper ocean through open leads and thin ice, estimated using an ice thickness distribution model, totaled about 150 MJ m−2, in general agreement with integrated values of Fw. Results indicate that oceanic heat flux to the ice in the central Arctic is derived mainly from shortwave radiation entering the ocean through the ice pack, rather than from diffusion of warm water from below. Indeed, during the AIDJEX project the mixed layer appears to have contributed 15–20 MJ m−2 of heat to the upper pycnocline. During the summer, Fw was found to vary by as much as 10–30 W m−2 over separations of 100 to 200 km and thus represents an important term in the surface heat budget not controlled by purely local deformation and thermodynamics.

A year of Bowen ratios over the frozen Beaufort Sea

The data set obtained during the Arctic Ice Dynamics Joint Experiment (AIDJEX) contains almost a year of 6‐hourly measurements of sensible (Hs) and latent (HL) heat fluxes over multiyear ice in the Beaufort Sea. Here I compute the Bowen ratio, Bo = Hs/HL, for each set of observations and plot histograms of Bo values for the four seasons, spring, summer, autumn, and winter. In every season except summer the Bowen ratio is as likely to be negative as positive; the histograms are, thus, generally bimodal. The Bowen ratio shows a seasonal progression that is related to the seasonally averaged air temperature. In summer, when moisture exchange is most rapid, the peaks in the histogram are near Bowen ratios of 1 and −1. In winter, when the rate of moisture exchange is smallest, the peaks are at Bo>20 and Bo < −20. Spring and autumn are transition seasons with a broader range of values.

The refractive index structure parameter, C2n, for a year over the frozen Beaufort Sea

The Arctic Ice Dynamics Joint Experiment (AIDJEX) produced a year of continuous, 6‐hourly observations of the surface fluxes of momentum and sensible and latent heat over the drifting ice of the Beaufort Sea. From this data set I compute the refractive index structure parameter C2n at four electromagnetic wavelengths—one each in the visible (0.55 μm), the mid‐infrared (10.6 μm), the near‐millimeter (0.337 mm or 890 GHz), and the radio regions. I present the resulting C2n values as histograms sorted according to the four seasons, spring, summer, autumn, and winter. Because each histogram derives from at least 300 C2n values, this is one of the largest studies of C2n climatology ever reported. For all four wavelengths the winter C2n histograms are the narrowest and have the highest peaks; the summer histograms are the broadest and most uniform. For the shorter wavelengths (0.55 and 10.6 μ) the mode value of C2n shows a seasonal progression; the maximum is in winter, the minimum in summer. The longer wavelengths (0.337 mm and radio) do not show a clear seasonal trend in the mode C2n value; although the C2n distributions change shape with season, the mode value is fairly constant. The beta distribution is a useful probabilistic model for the C2n histograms.

Atmospheric boundary layer structure and drag coefficients over sea ice

This paper estimates the air/sea drag coefficient for first‐year ice from recent aircraft measurements and reconciles the range of observed drag coefficients (103CD = 1.2–3.7 referenced to 10 m) for all sea ice types, based on ice roughness and seasonal meteorology. For the purpose of sea ice modeling, it is necessary to define an effective drag coefficient which relates regional stress to regional wind, because sea ice is heterogeneous on scales less than 20 km. Regional stress is influenced by the distribution of surface roughness, the buoyancy flux from quasi‐periodic leads, and external atmospheric conditions, principally the inversion height. 103CD is 1.3–1.5 for smooth ice but is much greater for nonflat surfaces. For wind speeds greater than 5 m/s and air temperatures below freezing, the effective 103CD is 2.5–3.0 for nearly continuous pack ice, such as first‐year ice in seasonal ice zones and central Arctic basin. The range of values of 103CD is 3.0–3.7 for unstable boundary layers typical of off‐ice winds in the marginal ice zone (MIZ) or even greater if the ice has been broken by a recent storm. CD values at the lower end of these ranges are associated with low inversion heights. These coefficients are confirmed by 103CD of 2.9, 2.5, and 3.1 for first‐year sea ice calculated from gust probe data collected by the NOAA P‐3 aircraft, interior to the inner MIZ in the Bering Sea during the Marginal Ice Zone Experiment (MIZEX‐West) in February 1983, and 103CD of 2.6 for the Arctic calculated from Arctic Ice Dynamics Joint Experiment (AIDJEX) aircraft data for February 1976. The effective drag coefficient with the presence of even a small concentration of sea ice is greater than the oceanic value as shown by a 103CD of 2.2 calculated from NOAA P‐3 gust probe data over a 40‐km track of 0.4 ice concentration in the outer MIZ of the Greenland Sea in June 1984 during MIZEX‐84. The relation of surface wind and stress to the geostrophic wind for shallow inversion heights, typical of high latitudes, is reviewed with a turbulent closure atmospheric boundary layer model. The winter Arctic is typified by low inversions (<100 m), low geostrophic drag coefficients, and large geostrophic/surface wind turning angles, with low functional dependence on surface roughness. Seasonal ice zones are typified by a more modest influence of the inversion on boundary layer characteristics.

Mesoscale eddies of the Arctic Ocean

Subsurface baroclinic eddies in the Arctic Ocean have a diameter of 10–20 km and are confined in depth between 50 and 300m. During the Arctic Ice Dynamics Joint Experiment (AIDJEX) in the central Beaufort Sea between March 1975 and May 1976, extensive information on eddy characteristics, statistics, and behavior was collected from four manned drifting ice camps. During this 14‐month period, 146 eddies were encountered, of which 19 were found to be repeated crossings, making a total of 127 separate eddies observed during this period. On the basis of the 1975–1976 AIDJEX data set, it is concluded that eddies are (1) prevalent in the Amerasia Basin of the Arctic Ocean and, in particular, the Beaufort Sea where they may occupy up to a quarter of the available surface area, (2) located in the depth range of 50–300 m (although a few deeper eddies are also present), (3) the second largest producers (32%) of kinetic energy found within the upper 200 m of the Beaufort Sea, and (4) predominantly anticyclonic in rotational tendency. These eddies apparently originate north of Point Barrow, Alaska, as a result of instability in the eastward flowing Alaskan Coastal Current and transfer seasonally varying water properties from the Chukchi Sea and Alaskan Shelf into the Arctic Ocean. They appear to translate in response to barotropic forcing over short time scales and, over longer time periods, move with the mean geostrophic field. The season of formation for an eddy may be identified by the anomalous thermal properties still residing in its core.

Thomas Saunders English, 1928-1985

After three years of illness, Tom English passed away in Seattle, Washington, on 5 January 1985. ... Tom was born and grew up in Washington, D.C. The undergraduate years, 1946 to 1950, were spent at Iowa State University in Ames, first as a journalism major, later as a zoology student. He continued at Ames with graduate work in zoology and statistics, receiving a master's degree in 1951 with a thesis on age, growth, and life history of carps in a local lake. ... Before receiving his doctorate at the University of Washington in 1961, Tom did graduate studies at the University of Oslo and in Bergen, Norway (1954-55); he also served in the U.S. Air Force at Fairbanks and near the North Pole (1956-58) and spent one year as instructor in fisheries biology at the University of Alaska (1958-59). His Ph.D. thesis treated the distribution and abundance of planktonic flatfish eggs in Puget Sound. Tom joined the teaching faculty of the then Department of Oceanography of the University of Washington in 1959. The next quarter of a century of his professional life was more than filled with teaching, research, participation in base-line studies for the state and federal governments and administrative work in the department. ... Tom's research interests revolved around problems of sampling design and the plankton of the Arctic Ocean. ... The common method of estimating stock size of fishes with pelagic eggs from integrating areas within contour lines of egg abundance was critically assessed and a better method was proposed. ... Tom chose to discharge his ROTC obligations of undergraduate days by entering the Air Force ... and, apparently, elected to be detailed to the Arctic Aeromedical Laboratory outside Fairbanks. ... he spent two summers and autumns on Drift Station Alpha, an ice floe near the North Pole .... The biological investigations were performed under contract with the Arctic Institute of North America. Besides being among the first to SCUBA-dive under the ice ..., Tom established in great detail what earlier polar investigators could only suggest: that the annual phytoplankton cycle under the ice is primarily driven by underwater light, which in turn depends more on snow melt than incident radiation. ... In the mid-sixties, Tom's continuing interest in the plankton of the Arctic Ocean was directed to Fletcher's Ice Island (also called T-3). Plankton collections were made in eight summer seasons between 1966 and 1973, and in all or most other months from 1968 through April 1974. The island drifted during this time from about 75° N north of Point Barrow, to beyond 85° N, to north of Ellesmere Island. In addition, summer collections were made in 1975 at the main camp of the Arctic Ice Dynamics Joint Experiment in the Beaufort Sea. Predominantly north of 80° N, Tom, his students, and his assistants collected the following (approximate numbers): 5500 samples each for temperature and salinity, 5000 for oxygen, 3300 each for phosphate, nitrate, and silicate, 6000 for chlorophyll, and 4000 measurements of photosynthesis (14C); further, 5800 phytoplankton and 11 000 zooplankton samples were gathered .... While the emphasis was on the upper layers, some sampling extended to the bottom. The enormous, truly unique body of analyses has only partly been evaluated .... Tom chose not to spend much of his energy on the dissemination of research results through publications but, like a real college teacher, to devote his time primarily to transmitting knowledge by classroom teaching, as well as guiding students doing research. ...

The seasonal variation of the mixed layer and the pycnocline under polar sea ice

A model of the seasonal variation of the mixed layer and the pycnocline under polar sea ice is compared with observations during the Arctic Ice Dynamics Joint Experiment (AIDJEX) 1975–1976. The model is of the Kraus‐Turner type, extended to include mixed layer and pycnocline. The forcing mechanisms are the mechanical mixing due to ice keel stirring and the surface buoyancy flux due to melting and freezing of sea ice. The model is designed to be used for climate studies in conjunction with atmospheric and oceanic general circulation models and with sea ice models.

Research planned for marginal ice zones

World interest in and awareness of the polar regions has developed at an accelerated pace over the past decade. This interest has stemmed from a variety of factors, including increased resource utilization, increased use of polar seas as shipping corridors, and a growing realization that sea ice must be incorporated into any viable global climate prediction scheme. In the 1970s this interest led to implementation of such programs as the Arctic Ice Dynamics Joint Experiment (AIDJEX) and the Arctic Data Buoy Programs, all of which addressed oceanographic and sea ice problems in the central Arctic Basin.The more difficult, and possibly more significant, problems remain, however, those concerning the variable extents of the sea ice fields in both hemispheres. These variations occur with the greatest intensity in the marginal ice zones, where the floating ice, oceans, and atmospheres are in continual physical contact and dynamic interaction. This realization led the U.S. National Academy of Sciences (NAS) in 1974 to recommend commencement of theoretical and field programs that would address marginal ice zone problems by the late 1970s. Similar recommendations were adopted by the Global Atmospheric Research Program (GARP) a few years following the NAS statement. Most recently, the World Meteorological Organization has reiterated the significance of the marginal ice zones in influencing climate and has recommended implementation of a study program.

Sea Ice Processes and models, Proceedings of the Arctic Ice Dynamics Joint Experiment, International Commission on Snow and Ice Symposium at Seattle, September 1977, edited by R.S. Pritchard

Sea Ice Processes and models, Proceedings of the Arctic Ice Dynamics Joint Experiment, International Commission on Snow and Ice Symposium at Seattle, September 1977, edited by R.S. Pritchard