Summer Sea Ice Research Articles

Interannual Variation in Top-of-Atmosphere Upward Shortwave Flux over the Arctic Related to Sea Ice, Snow Cover, and Land Cloud Cover in Spring and Summer

Abstract We investigated the interannual variations in the annual mean and seasonal cycle of upward shortwave radiation at the top of the atmosphere (TOA SW↑) over the Arctic using the Clouds and the Earth’s Radiant Energy System (CERES) observation data during 2001–20. The annual mean TOA SW↑ over the Arctic showed a decreasing trend from 2001 to 2012 (−2.5 W m−2 decade−1) and had a large interannual variability after 2012. The standard deviation of detrended TOA SW↑ increased from 0.4 W m−2 in 2001–12 to 1.1 W m−2 in 2012–20. Over land, TOA SW↑ variation was related to snow cover in May; snow cover, cloud fraction, and cloud optical depth (COD) in June; and cloud fraction and COD in July. Over ocean, TOA SW↑ variation in June and July was linked to sea ice cover. TOA SW↑ variation over ocean in June and July after 2012 was highly related to the North Atlantic Oscillation (NAO). This study suggests that changes in the large annual mean TOA SW↑ variability after 2012 are explained by the timing of land snow and sea ice melt in spring and summer and cloud variability over land in summer.

Loss of sea ice and intermittent winds alter distributions and diet resources of young forage fish in the Chukchi sea

Loss of sea ice alters habitat for organisms that function as energetic links in Arctic food webs. To address the impacts of such changes on lower trophic levels we focus on forage fish during the early life stages (polar cod: Boreogadus saida and saffron cod: Eleginus gracilis) and copepods (Calanus spp. and Pseudocalanus spp.) in the Chukchi Sea with differing reliance on sea ice environments. We assess distributions throughout years of varying ice extent (2010–2013, 2015, 2017–2018) and diet resources during unprecedented warm years (2017, 2018) using stable isotope analyses (bulk δ13C and δ15N and δ13C compound-specific isotope analysis of amino acids). Calanus spp. and polar cod were found at relatively higher latitudes in closer proximity to recent sea ice, whereas saffron cod were rare and Pseudocalanus spp. were ubiquitous. Polar cod juveniles, but not larvae, were more common in open water, suggesting that low ice and northward ocean currents interact to influence juvenile distributions. Low summer sea ice also coincided with northward expansion of warm Pacific-origin water across the shelf. Stable isotope δ13C values reflected this latitudinal variation in water masses as well as inshore-offshore gradients due to convergence of ocean currents. When summer sea ice was absent during 2017, juvenile polar cod were dispersed across the shelf, abundances were low for both copepod taxa, isotopic niche was reduced for polar cod, and carbon sources for some larval polar cod reflected boreal-associated phytoplankton. Distributions and isotopic patterns suggest that under low ice conditions, along-shelf wind reversals and weak currents can lead to dominance of Pacific-origin water, mixing of organisms from various source locations, and reductions in sea ice-influenced environments. Northward expansion of subarctic water masses and organisms across the Chukchi shelf alters basal resources for lower trophic levels and modifies pelagic habitats to more uniformly resemble lower latitude marine ecosystems.

Arctic amplification has already peaked

It has been demonstrated that the Arctic has warmed at almost four times the global average rate since 1979, a phenomenon known as Arctic amplification. However, this rapid Arctic warming is tightly linked to the retreat and thinning of summer sea ice, and so may be expected to weaken as the Arctic transitions to seasonal ice cover. Here we show evidence from gridded observations and climate reanalysis that Arctic amplification peaked sometime in the early 2000s. This occurred concurrently with a maximum in the rate of loss of sea ice area, thickness, and volume. From CMIP6 projections and the CESM2 large ensemble we see that Arctic amplification is unlikely to be so high again at any future point in the 21st century except in the lowest emissions scenarios in which global temperatures stabilize while the Arctic continues to warm.

When will Arctic sea ice disappear? Projections of area, extent, thickness, and volume

Rapidly diminishing Arctic summer sea ice is a strong signal of the pace of global climate change. We provide point, interval, and density forecasts for four measures of Arctic sea ice: area, extent, thickness, and volume. Importantly, we enforce the joint constraint that these measures must simultaneously arrive at an ice-free Arctic. We apply this constrained joint forecast procedure to models relating sea ice to atmospheric carbon dioxide concentration and models relating sea ice directly to time. The resulting “carbon-trend” and “time-trend” projections are mutually consistent and predict a nearly ice-free summer Arctic Ocean by the mid-2030s with an 80% probability. Moreover, the carbon-trend projections show that global adoption of a lower carbon path would likely delay the arrival of a seasonally ice-free Arctic by only a few years.

Arctic Ocean sediments as important current and future sinks for marine microplastics missing in the global microplastic budget

To better understand unexpectedly low plastic loads at the ocean’s surface compared with inputs, unidentified sinks must be located. Here, we present the microplastic (MP) budget for multi-compartments in the western Arctic Ocean (WAO) and demonstrate that Arctic sediments serve as important current and future sinks for MPs missing from the global budget. We identified an increase of 3% year −1 in MP deposition from sediment core observations. Relatively elevated MP abundances were found in seawater and surface sediments around the summer sea ice retreat region, implying enhanced MP accumulation and deposition facilitated by the ice barrier. We estimate 15.7 ± 2.30 × 10 16 N and 0.21 ± 0.14 MT as total MP loads in the WAO with 90% (by mass) buried in the post-1930 sediments, which exceeds the global average of the current marine MP load. The slower increase in plastic burial versus production implies a lag in plastic delivery to the Arctic, indicating more pollution in the future.

Concentrations and controls of dissolved inorganic carbon in Arctic summer sea ice and adjacent surface seawaters

The carbonate chemistry of sea ice plays a critical role in global ocean carbon cycles, particularly in polar regions which are subject to significant climate change-induced sea ice variation. However, less is known about the interaction of carbonate system between sea ice and its adjacent seawaters due to sparse sampling and disparities in reported results. Here we provide an insight into this issue by collecting and measuring dissolved inorganic carbon (DIC) and associated environmental parameters in Arctic sea ice during a cruise in the summer of 2014. Our observations show that DIC in Arctic summer sea ice has a mean concentration of 463.3 ± 213.0 μmol/kg and appears to be controlled mainly by the fraction of brine water in the ice. The low Chl a and nutrients content in sea ice indicate minor contribution of biological uptake to sea-ice DIC in the western Arctic Ocean. The DIC concentration in surface water (<100 m depth) decreased from a mean of 2108.3 ± 45.4 μmol/kg in 1994 to a mean of 2052.4 ± 98.6 μmol/kg in 2014, due to the enhanced sea ice melting that dilutes the DIC concentrations of surrounding seawaters.

Changing sources and burial of organic carbon in the Chukchi Sea sediments with retreating sea ice over recent centuries

Abstract. Decreasing sea ice extent caused by climate change is affecting the carbon cycle of the Arctic Ocean. In this study, surface sediments across the western Arctic Ocean are investigated to characterize sources of sedimentary organic carbon (OC). Bulk organic parameters (total organic carbon, total nitrogen, δ13Corg, and δ15N) and molecular organic biomarkers (e.g., sterols and highly branched isoprenoids – HBIs) are combined to distinguish between sympagic, pelagic, and terrestrial OC sources. Their downcore profiles generated at the Chukchi Sea R1 core site (74∘ N) are then used to evaluate changes in the relative contribution of these components of sedimentary OC over the last 200 years with decreasing sea ice. Our data evidence that, from the 1820s to the 1930s, prevailing high sea ice cover inhibited in situ primary production, resulting in prominent land-derived material in sediments. Then, from the 1930s to the 1980s, primary production started increasing with the gradual decline of summer sea ice. The ratio of sympagic and pelagic OC began to rise to account for the larger portion of sedimentary OC. Since the 1980s, accelerated sea ice loss led to enhanced primary production, stabilizing over the last decades due to freshwater-induced surface ocean stratification in summer.

Exhibition Reviews

In the almost four years since The Arctic: While the Ice Is Melting opened (October 2019), we have experienced a global pandemic and now find ourselves teetering on the edge of catastrophic ice melt. With exhibits, elaborate installations, ceiling projections, and interactive stations, this award-winning exhibition frames Indigenous communities and the work of non-Indigenous researchers through the omnipresence of ice and ice melt around the Arctic, encompassing Alaska, Inuit Nunangat (Canada), Kalaallit Nunaat (Greenland), Iceland, Svalbard and Norway, Sweden, Finland, and Russia. It is expansive in its thematic outlook and asks us to think about an Arctic without ice and the implications of this for Indigenous societies and cultures. With the exhibition extended throughout 2023, and given the latest news on predicted summer sea ice levels, the urgency of its message is perhaps now even more palpable.

Estimation of summer pan-Arctic ice draft from satellite passive microwave observations

Pan-Arctic observations of summer sea ice thickness have been a challenging problem despite their global significance in weather/climate analysis and prediction. To solve this problem, this study developed a method for estimating the pan-Arctic ice draft (depth of sea ice below sea surface level) using spaceborne passive microwave-measured brightness temperatures (TBs). This study found that the time series of TBs is highly correlated with the corresponding time series of ice draft (D)s measured by three upward-looking sonar (ULS) sites over the Beaufort Sea during the ice melting season. Based on this finding, a D estimation equation was derived by relating satellite-measured TBs at microwave frequencies to Ds. Pan-Arctic D distributions from June to August obtained from this study describe well the general thinning process of Arctic sea ice during the summer of 2021. The validation of estimated Ds with other ULS measurements, which were not used for training data, showed good agreement between them suggesting the robustness of the developed estimation method over the Arctic basin. In addition, validation was made against ice mass balance buoy-measured sea ice thickness, and result shows a good agreement between buoy-measured ice thickness and estimated ice thickness converted from the D. Meanwhile, relatively high uncertainty in D estimation above 2 m shown in the result and validation suggests the potential for future improvement of D estimation in the early melting period. As the proposed method can provide summer pan-Arctic D distributions on a daily basis, the D from this study can be applied for generating the initial field for the sea ice and global climate models through data assimilation.

Effects of sea ice form drag on the polar oceans in the NEMO-LIM3 global ocean–sea ice model

The surface roughness of sea ice is highly variable because of the diversity of discrete obstacles to the flow present on the sea ice surface. These obstacles result in form drag, an effect poorly accounted for in the calculation of surface fluxes over sea ice in climate models. In this study, we implement in the ocean–sea ice model NEMO-LIM3 a variable formulation of the atmospheric and oceanic neutral drag coefficients over sea ice that includes the form drag effect. We examine the impact of this formulation on the ice cover and ocean surface properties over the past decades in the Arctic and Antarctic. Including form drag in the simulations leads to 10–50cm thinner sea ice in the peripheral Arctic seas and ∼10cm thicker sea ice in the Antarctic ice pack on annual means. Form drag significantly impacts summer sea surface temperatures in the marginal ice zone (+0.5 to +1.5°C in the Arctic and -0.5°C in the Antarctic) and sea surface salinities under the ice cover (+0.52 to +0.89g/kg in both polar regions). The seasonality of the ocean surface stress under sea ice is reversed, with a maximum (minimum) ocean surface stress under sea ice in summer (winter) when including form drag. However, this seasonal reversal is likely too intense in our simulations due to the underestimation of the prescribed mean floe lengths in summer. In the Arctic, the long-term declines in sea ice volume and multi-year ice are associated with decreasing neutral drag coefficients and negative trends in ocean surface stress. In the Antarctic, the trends in the neutral drag coefficients control the increase (decrease) in ocean surface stress in the Weddell and Ross (Bellingshausen and Amundsen) Seas. Form drag results in a 22–38% deeper summer mixed layer under polar sea ice and a year-long 10–25m deeper mixed layer in the Central Arctic. All these impacts on the ocean surface properties demonstrate the relevance of form drag for polar ocean modelling.

Phytoplankton responses to increasing Arctic river discharge under the present and future climate simulations

In recent decades, the unprecedented rate of Arctic warming has accelerated the high-latitude landmass hydrological cycle, leading to increased river discharge into the Arctic Ocean. This study elucidates the role of Arctic river discharge, which was the large model uncertainty in the Coupled Model Intercomparison Project 6, for the phytoplankton responses in present-day and future climate simulations by adding fresh water into the model. In the present-day climate simulation, additional river discharge decreases the spring phytoplankton biomass. Freshening of Arctic seawater facilitates freezing, increasing sea ice concentration in spring and eventually decreasing phytoplankton due to less availability of light. On the other hand, in the summer, phytoplankton increases due to the surplus of surface nitrate and the increase in the vertical mixing induced by the reduced summer sea ice melting water. In the future climate, the plankton response to the additional freshwater input is similar to the present-day climate. Nevertheless, the major phytoplankton responses are shifted from the Eurasian Basin to the Canada Basin and the East-Siberian Sea, mainly due to the marginal sea ice zone shift from the Barents-Kara Sea to the East Siberian-Chukchi Sea in the future.

Interdecadal change in the linkage of early summer sea ice in the Barents Sea to the variability of West China Autumn Rain

Interdecadal change in the linkage of early summer sea ice in the Barents Sea to the variability of West China Autumn Rain

Reducing the Spring Barrier in Predicting Summer Arctic Sea Ice Concentration

AbstractThe predictive skill of summer sea ice concentration (SIC) in the Arctic presents a steep decline when initialized before June, which is the so‐called spring predictability barrier for Arctic sea ice. This study explores the potential influence of surface heat flux, cloud and water vapor anomalies on monthly to seasonal predictions of Arctic SIC anomalies. The results show an enhancement in skill predicting Arctic September SIC in the models that use surface fluxes, clouds, or water vapor in combination with SIC and surface sea temperature as predictors when initialized in boreal spring. This result shows the potential to reduce the spring barrier for Arctic SIC predictions by including the surface heat budget. The enhanced predictive skill can be very likely linked to the improved representation of the thermodynamics associated with water vapor and cloudiness anomalies in spring.

Optical Insight Into Riverine Influences on Dissolved and Particulate Organic Carbon in a Coastal Arctic Lagoon System

AbstractArctic coastal margins receive organic material input from rivers, melted sea ice, and coastal erosion, phenomena that are all undergoing changes related to global climate. The optical properties of coastal Arctic waters contain information on this organic material, and we examined three optical properties of seawater (absorption, backscatter, and fluorescence) for their relationships to variability in dissolved and particulate organic carbon (DOC and POC) in Stefansson Sound, Alaska, a coastal Arctic embayment. During open water periods in 2018 and 2019, DOC was inversely correlated with salinity (r2 = 0.97) and positively correlated with dissolved organic matter fluorescence (fDOM; r2 = 0.67). DOC showed strong correlation with the nonparticulate absorption coefficient at 440 nm (ag(440)) only in 2018 (r2 = 0.95). The vertical structure of fDOM in Stefansson Sound aligned with density profiles more strongly in 2018 than in 2019, and higher levels of fDOM, ag(440), and backscatter seen near the bottom in 2019 suggest wind‐driven mixing and/or bottom resuspension events. In both years, DOC correlated strongly with the spectral slope of the absorption coefficient between 412 and 550 nm (r2 = 0.70), and POC was well correlated with spectral backscattering at 470, 532, and 660 nm (r2 = 0.90, 0.71, and 0.59). These interannual differences in the spatial and vertical distributions of DOC and POC, and their respective correlations with optical proxies, likely reflect regional climatological factors such as precipitation over the adjacent watersheds, wind patterns, and residual sea ice in late summer.

Temporal evolution of under-ice meltwater layers and false bottoms and their impact on summer Arctic sea ice mass balance

Low-salinity meltwater from Arctic sea ice and its snow cover accumulates and creates under-ice meltwater layers below sea ice. These meltwater layers can result in the formation of new ice layers, or false bottoms, at the interface of this low-salinity meltwater and colder seawater. As part of the Multidisciplinary drifting Observatory for the Study of the Arctic Climate (MOSAiC), we used a combination of sea ice coring, temperature profiles from thermistor strings and underwater multibeam sonar surveys with a remotely operated vehicle (ROV) to study the areal coverage and temporal evolution of under-ice meltwater layers and false bottoms during the summer melt season from mid-June until late July. ROV surveys indicated that the areal coverage of false bottoms for a part of the MOSAiC Central Observatory (350 by 200 m2) was 21%. Presence of false bottoms reduced bottom ice melt by 7–8% due to the local decrease in the ocean heat flux, which can be described by a thermodynamic model. Under-ice meltwater layer thickness was larger below first-year ice and thinner below thicker second-year ice. We also found that thick ice and ridge keels confined the areas in which under-ice meltwater accumulated, preventing its mixing with underlying seawater. While a thermodynamic model could reproduce false bottom growth and melt, it could not describe the observed bottom melt rates of the ice above false bottoms. We also show that the evolution of under-ice meltwater-layer salinity below first-year ice is linked to brine flushing from the above sea ice and accumulating in the meltwater layer above the false bottom. The results of this study aid in estimating the contribution of under-ice meltwater layers and false bottoms to the mass balance and salt budget for Arctic summer sea ice.

Causes of the record-low Antarctic sea-ice in austral summer 2022

The Antarctic sea-ice area reached a record minimum during austral summer 2022 (December 2021 to February 2022), with particularly considerable loss in the West Antarctic. Based on lead–lag correlation analysis, the authors found that the record minimum was closely linked to the preceding near-strongest positive-phase Southern Annular Mode during August–October 2021 and stronger positive SST anomaly in the Maritime Continent during July–September 2021. The former remained at a historically near-strongest or strongest level because of the stratospheric cooling effect of the unprecedented stratospheric ozone depletion, which induced a deepened and southwestward-shifted Amundsen Sea Low (ASL), causing sea-ice retreat via horizontal wind anomalies. The latter persisted into summer and favored the development of La Niña, which generated deep convection over the southwestern subtropical Pacific and triggered a southeastward-propagated Rossby wave train, deepening the ASL remotely. The ozone reduction also induced surface warming in the West Antarctic by increasing the net downward shortwave radiation, thereby intensifying the sea-ice loss. Additionally, positive feedback between the SST, net shortwave radiation and cloudiness, along with the Ekman heat transport, amplified the surface warming.摘要2022年夏季 (2021年12月至2022年2月) 南极海冰面积达到历史新低, 西南极减少最显著. 2021年8∼10月南半球环状模接近历史最强和7∼9月海洋性大陆附近海温显著增暖是两个关键因素. 由于平流层臭氧破纪录减少使得前者维持历史最强或接近最强, 导致阿蒙森低压 (ASL) 加深并向西南移动, 有利于海冰减少. 后者持续到夏季, 有利于拉尼娜的发展, 通过激发罗斯贝波列加深ASL. 在热力上, 臭氧减少导致向下净短波辐射增加, 引起西南极增暖. 此外, 净短波辐射-海温-云形成正反馈, 与埃克曼输送一起, 放大表面增暖, 从而促进海冰融化.

Pan-Arctic marine biodiversity and species co-occurrence patterns under recent climate

The Arctic region is experiencing drastic climatic changes bringing about potential ecological shifts. Here, we explored marine biodiversity and potential species associations across eight Arctic marine areas between 2000 and 2019. We compiled species occurrences for a subset of 69 marine taxa (i.e., 26 apex predators and 43 mesopredators) and environmental factors to predict taxon-specific distributions using a multi-model ensemble approach. Arctic-wide temporal trends of species richness increased in the last 20 years and highlighted potential emerging areas of species accrual due to climate-driven species redistribution. Further, regional species associations were dominated by positive co-occurrences among species pairs with high frequencies in the Pacific and Atlantic Arctic areas. Comparative analyses of species richness, community composition, and co-occurrence between high and low summer sea ice concentrations revealed contrasting impacts of and detected areas vulnerable to sea ice changes. In particular, low (high) summer sea ice generally resulted in species gains (loss) in the inflow and loss (gains) in the outflow shelves, accompanied by substantial changes in community composition and therefore potential species associations. Overall, the recent changes in biodiversity and species co-occurrences in the Arctic were driven by pervasive poleward range shifts, especially for wide-ranging apex predators. Our findings highlight the varying regional impacts of warming and sea ice loss on Arctic marine communities and provide important insights into the vulnerability of Arctic marine areas to climate change.

Sea Ice Melt Pond Fraction Derived From Sentinel‐2 Data: Along the MOSAiC Drift and Arctic‐Wide

AbstractMelt ponds forming on Arctic sea ice in summer significantly reduce the surface albedo and impact the heat and mass balance of the sea ice. Therefore, their areal coverage, which can undergo rapid change, is crucial to monitor. We present a revised method to extract melt pond fraction (MPF) from Sentinel‐2 satellite imagery, which is evaluated by MPF products from higher‐resolution satellite and helicopter‐borne imagery. The analysis of melt pond evolution during the MOSAiC campaign in summer 2020, shows a split of the Central Observatory (CO) into a level ice and a highly deformed ice part, the latter of which exhibits exceptional early melt pond formation compared to the vicinity. Average CO MPFs are 17% before and 23% after the major drainage. Arctic‐wide analysis of MPF for years 2017–2021 shows a consistent seasonal cycle in all regions and years.

Will the summer sea ice in the Arctic reach a tipping point?

The Arctic sea-ice cover has decreased in extent, area, and thickness over the last six decades. Most global climate models project that the summer sea-ice extent (SIE) will decline to less than 1 million (mill.) km2 in this century, ranging from 2030 to the end of the century, indicating large uncertainty. However, some models, using the same emission scenarios as required by the Paris Agreement to keep the global temperature below 2°C, indicate that the SIE could be about 2 mill. km2 in 2100 but with a large uncertainty of ±1.5 mill. km2. Here, the authors take another approach by exploring the direct relationship between the SIE and atmospheric CO2 concentration for the summer–fall months. The authors correlate the SIE and ln(CO2/CO2r) during the period 1979–2022, where CO2r is the reference value in 1979. Using these transient regression equations with an R2 between 0.78 and 0.87, the authors calculate the value that the CO2 concentration needs to reach for zero SIE. The results are that, for July, the CO2 concentration needs to reach 691 ± 16.5 ppm, for August 604 ± 16.5 ppm, for September 563 ± 17.5 ppm, and for October 620 ± 21 ppm. These values of CO2 for an ice-free Arctic are much higher than the targets of the Paris Agreement, which are 450 ppm in 2060 and 425 ppm in 2100, under the IPCC SSP1-2.6 scenario. If these targets can be reached or even almost reached, the “no tipping point” hypothesis for the summer SIE may be valid.

Separation of Atmospheric Circulation Patterns Governing Regional Variability of Arctic Sea Ice in Summer

Separation of Atmospheric Circulation Patterns Governing Regional Variability of Arctic Sea Ice in Summer