The areal extent, concentration and thickness of sea ice in the Arctic Ocean and adjacent seas have strongly decreased during the recent decades, but cold, snow-rich winters have been common over mid-latitude land areas since 2005. A review is presented on studies addressing the local and remote effects of the sea ice decline on weather and climate. It is evident that the reduction in sea ice cover has increased the heat flux from the ocean to atmosphere in autumn and early winter. This has locally increased air tempera- ture, moisture, and cloud cover and reduced the static stability in the lower troposphere. Several studies based on observations, atmospheric reanalyses, and model experiments suggest that the sea ice decline, together with increased snow cover in Eurasia, favours circulation patterns resembling the negative phase of the North Atlantic Oscillation and Arctic Oscillation. The suggested large-scale pressure patterns include a high over Eurasia, which favours cold winters in Europe and northeastern Eurasia. A high over the western and a low over the eastern North America have also been suggested, favouring advection of Arctic air masses to North America. Mid-latitude winter weather is, however, affected by several other factors, which generate a large inter-annual variability and often mask the effects of sea ice decline. In addition, the small sample of years with a large sea ice loss makes it difficult to distinguish the effects directly attributable to sea ice conditions. Several studies suggest that, with advancing global warming, cold winters in mid-latitude continents will no longer be common during the second half of the twenty-first century. Recent studies have also suggested causal links between the sea ice decline and summer precipitation in Europe, the Mediterranean, and East Asia.

/pdf/effects-of-arctic-sea-ice-decline-on-weather-and-climate-a-44t894nz9i.pdf

Effects of Arctic Sea Ice Decline on Weather and Climate: A Review

Sea ice in the Arctic is one of the most rapidly changing components of the global climate system. Over the past few decades, summer areal extent has declined over 30, and all months show statistically significant declining trends. New satellite missions and techniques have greatly expanded information on sea ice thickness, but many uncertainties remain in the satellite data and long-term records are sparse. However, thickness observations and other satellite-derived data indicate a 40 decline in thickness, due in large part to the loss of thicker, older ice cover. The changes in sea ice are happening faster than models have projected. With continued increasing temperatures, summer ice-free conditions are likely sometime in the coming decades, though there are substantial uncertainties in the exact timing and high interannual variability will remain as sea ice decreases. The changes in Arctic sea ice are already having an impact on flora and fauna in the Arctic. Some species will face increasing challenges in the future, while new habitat will open up for other species. The changes are also affecting peoples living and working in the Arctic. Native communities are facing challenges to their traditional ways of life, while new opportunities open for shipping, fishing, and natural resource extraction.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2013RG000431

Arctic sea ice in transformation: A review of recent observed changes and impacts on biology and human activity

Atmospheric reanalyses depend on a mix of observations and model forecasts. In data-sparse regions such as the Arctic, the reanalysis solution is more dependent on the model structure, assumptions, and data assimilation methods than in data-rich regions. Applications such as the forcing of ice–ocean models are sensitive to the errors in reanalyses. Seven reanalysis datasets for the Arctic region are compared over the 30-yr period 1981–2010: National Centers for Environmental Prediction (NCEP)–National Center for Atmospheric Research Reanalysis 1 (NCEP-R1) and NCEP–U.S. Department of Energy Reanalysis 2 (NCEP-R2), Climate Forecast System Reanalysis (CFSR), Twentieth-Century Reanalysis (20CR), Modern-Era Retrospective Analysis for Research and Applications (MERRA), ECMWF Interim Re-Analysis (ERA-Interim), and Japanese 25-year Reanalysis Project (JRA-25). Emphasis is placed on variables not observed directly including surface fluxes and precipitation and their trends. The monthly averaged surface tem...

/pdf/evaluation-of-seven-different-atmospheric-reanalysis-vzeafi1hma.pdf

Evaluation of Seven Different Atmospheric Reanalysis Products in the Arctic

The Arctic Ocean’s freshwater budget comprises contributions from river runoff, precipitation, evaporation, sea-ice and exchanges with the North Pacific and Atlantic. More than 70,000km3 of freshwater are stored in the upper layer of the Arctic Ocean, leading to low salinities in upper-layer Arctic sea water, separated by a strong halocline from warm, saline water beneath. Spatially and temporally limited observations show that the Arctic Ocean’s freshwater content has increased over the past few decades, predominantly in the west. Models suggest that wind-driven convergence drives freshwater accumulation. Here we use continuous satellite measurements between 1995 and 2010 to show that the dome in sea surface height associated with the western Arctic Beaufort Gyre has been steepening, indicating spin-up of the gyre. We find that the trend in wind field curl—a measure of spatial gradients in the wind that lead to water convergence or divergence—exhibits a corresponding spatial pattern, suggesting that wind-driven convergence controls freshwater variability. We estimate an increase in freshwater storage of 8,000±2,000km3 in the western Arctic Ocean, in line with hydrographic observations, and conclude that a reversal in the wind field could lead to a spin-down of the Beaufort Gyre, and release of this freshwater to the Arctic Ocean.

/pdf/western-arctic-ocean-freshwater-storage-increased-by-wind-398jqlcvkh.pdf

Western Arctic Ocean freshwater storage increased by wind-driven spin-up of the Beaufort Gyre

. Rapid changes are occurring in the Arctic, including a
reduction in sea ice thickness and coverage and a shift towards younger and
thinner sea ice. Snow and sea ice models are often used to study these
ongoing changes in the Arctic, and are typically forced by atmospheric
reanalyses in absence of observations. ERA5 is a new global reanalysis that
will replace the widely used ERA-Interim (ERA-I). In this study, we compare
the 2 m air temperature (T2M), snowfall (SF) and total precipitation (TP)
from ERA-I and ERA5, and evaluate these products using buoy observations
from Arctic sea ice for the years 2010 to 2016. We further assess how biases in
reanalyses can influence the snow and sea ice evolution in the Arctic, when
used to force a thermodynamic sea ice model. We find that ERA5 is generally
warmer than ERA-I in winter and spring (0–1.2  ∘ C), but colder
than ERA-I in summer and autumn (0–0.6  ∘ C) over Arctic sea ice.
Both reanalyses have a warm bias over Arctic sea ice relative to buoy
observations. The warm bias is smaller in the warm season, and larger in the
cold season, especially when the T2M is below −25 ∘ C in the
Atlantic and Pacific sectors. Interestingly, the warm bias for ERA-I and new
ERA5 is on average 3.4 and 5.4  ∘ C (daily mean),
respectively, when T2M is lower than −25 ∘ C. The TP and SF along
the buoy trajectories and over Arctic sea ice are consistently higher in ERA5
than in ERA-I. Over Arctic sea ice, the TP in ERA5 is typically less than 10 mm snow water equivalent (SWE) greater than in ERA-I in any of the seasons, while the SF in ERA5 can
be 50 mm SWE higher than in ERA-I in a season. The largest increase in
annual TP (40–100 mm) and SF (100–200 mm) in ERA5 occurs in the Atlantic
sector. The SF to TP ratio is larger in ERA5 than in ERA-I, on average 0.6
for ERA-I and 0.8 for ERA5 along the buoy trajectories. Thus, the
substantial anomalous Arctic rainfall in ERA-I is reduced in ERA5,
especially in summer and autumn. Simulations with a 1-D thermodynamic sea ice
model demonstrate that the warm bias in ERA5 acts to reduce thermodynamic
ice growth. The higher precipitation and snowfall in ERA5 results in a
thicker snowpack that allows less heat loss to the atmosphere. Thus, the
larger winter warm bias and higher precipitation in ERA5, compared with
ERA-I, result in thinner ice thickness at the end of the growth
season when using ERA5; however the effect is small during the freezing
period.

/pdf/comparison-of-era5-and-era-interim-near-surface-air-6ywl4glq01.pdf

Comparison of ERA5 and ERA-Interim near-surface air temperature, snowfall and precipitation over Arctic sea ice: effects on sea ice thermodynamics and evolution

[1] The summer circulations and hydrographic fields of the Kara Sea are reconstructed for mean, positive and negative Arctic Oscillation regimes employing a variational data assimilation technique which provides the best fit of reconstructed fields to climatological data and satisfies dynamical and kinematic constraints of a quasi-stationary primitive equation ocean circulation model. The reconstructed circulations agree well with the measurements and are characterized by inflow of 0.63, 0.8, 0.51 Sv through Kara Gate and 1.18, 1.1, 1.12 Sv between Novaya Zemlya and Franz Josef Land, for mean climatologic conditions, positive and negative AO indexes, respectively. The major regions of water outflow for these regimes are the St. Anna Trough (1.17, 1.21, 1.34 Sv) and Vilkitsky/Shokalsky Straits (0.52, 0.7, 0.51 Sv). The optimized velocity pattern for the mean climatological summer reveals a strong anticyclonic circulation in the central part of the Kara Sea (Region of Fresh Water Inflow, ROFI zone) and is confirmed by ADCP surveys and laboratory modeling. This circulation is well pronounced for both high and low AO phases, but in the positive AO phase it is shifted approximately 200 km west relatively to its climatological center. During the negative AO phase the ROFI locaion is close to its climatological position. The results of the variational data assimilation approach were compared with the simulated data from the Hamburg Shelf Ocean Model (HAMSOM) and Naval Postgraduate School 18 km resolution (NPS-18) model to validate these models.

/pdf/investigation-of-the-summer-kara-sea-circulation-employing-a-ychs7xwamq.pdf

Investigation of the summer Kara Sea circulation employing a variational data assimilation technique

[1] We compare daily data from the National Center for Atmospheric Research and National Centers for Environmental Prediction “Reanalysis 1” project with observational data obtained from the North Pole drifting stations in order to validate the atmospheric forcing data used in coupled ice-ocean models. This analysis is conducted to assess the role of errors associated with model forcing before performing model verifications against observed ocean variables. Our analysis shows an excellent agreement between observed and reanalysis sea level pressures and a relatively good correlation between observed and reanalysis surface winds. The observed temperature is in good agreement with reanalysis data only in winter. Specific air humidity and cloudiness are not reproduced well by reanalysis and are not recommended for model forcing. An example sensitivity study demonstrates that the equilibrium ice thickness obtained using NP forcing is two times thicker than using reanalysis forcing.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2007GL031378

Atmospheric forcing validation for modeling the central Arctic

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Investigation of the summer Kara Sea circulation employing a variational data assimilation technique

Atmospheric forcing validation for modeling the central Arctic