Manifestations of climate change are often shown as gradual changes in physical or biogeochemical properties
. Components of the climate system, however, can show stepwise shifts from one regime to ...another, as a nonlinear response of the system to a changing forcing
. Here we show that the Arctic sea ice regime shifted in 2007 from thicker and deformed to thinner and more uniform ice cover. Continuous sea ice monitoring in the Fram Strait over the last three decades revealed the shift. After the shift, the fraction of thick and deformed ice dropped by half and has not recovered to date. The timing of the shift was preceded by a two-step reduction in residence time of sea ice in the Arctic Basin, initiated first in 2005 and followed by 2007. We demonstrate that a simple model describing the stochastic process of dynamic sea ice thickening explains the observed ice thickness changes as a result of the reduced residence time. Our study highlights the long-lasting impact of climate change on the Arctic sea ice through reduced residence time and its connection to the coupled ocean-sea ice processes in the adjacent marginal seas and shelves of the Arctic Ocean.
This study evaluates the performance of six atmospheric reanalyses (ERA-Interim, ERA5, JRA-55, CFSv2, MERRA-2, and ASRv2) over Arctic sea ice from winter to early summer. The reanalyses are evaluated ...using observations from the Norwegian Young Sea Ice campaign (N-ICE2015), a 5-month ice drift in pack ice north of Svalbard. N-ICE2015 observations include surface meteorology, vertical profiles from radiosondes, as well as radiative and turbulent heat fluxes. The reanalyses simulate surface analysis variables well throughout the campaign, but have difficulties with most forecast variables. Wintertime (January–March) correlation coefficients between the reanalyses and observations are above 0.90 for the surface pressure, 2-m temperature, total column water vapor, and downward longwave flux. However, all reanalyses have a positive wintertime 2-m temperature bias, ranging from 1° to 4°C, and negative (i.e., upward) net longwave bias of 3–19 W m−2. These biases are associated with poorly represented surface inversions and are largest during cold-stable periods. Notably, the recent ERA5 and ASRv2 datasets have some of the largest temperature and net longwave biases, respectively. During spring (April–May), reanalyses fail to simulate observed persistent cloud layers. Therefore they overestimate the net shortwave flux (5–79 W m−2) and underestimate the net longwave flux (8–38 W m−2). Promisingly, ERA5 provides the best estimates of downward radiative fluxes in spring and summer, suggesting improved forecasting of Arctic cloud cover. All reanalyses exhibit large negative (upward) residual heat flux biases during winter, and positive (downward) biases during summer. Turbulent heat fluxes over sea ice are simulated poorly in all seasons.
Near‐surface air temperatures close to 0°C were observed in situ over sea ice in the central Arctic during the last three winter seasons. Here we use in situ winter (December–March) temperature ...observations, such as those from Soviet North Pole drifting stations and ocean buoys, to determine how common Arctic winter warming events are. Observations of winter warming events exist over most of the Arctic Basin. Temperatures exceeding −5°C were observed during >30% of winters from 1954 to 2010 by North Pole drifting stations or ocean buoys. Using the ERA‐Interim record (1979–2016), we show that the North Pole (NP) region typically experiences 10 warming events (T2m > −10°C) per winter, compared with only five in the Pacific Central Arctic (PCA). There is a positive trend in the overall duration of winter warming events for both the NP region (4.25 days/decade) and PCA (1.16 days/decade), due to an increased number of events of longer duration.
Plain Language Summary
During the last three winter seasons, extreme warming events were observed over sea ice in the central Arctic Ocean. Each of these warming events were associated with temperatures close to or above 0°C, which lasted for between 1 and 3 days. Typically temperatures in the Arctic at this time of year are below −30°C. Here we study past temperature observations in the Arctic to investigate how common winter warming events are. We use time temperature observations from expeditions such as Fram (1893–1896) and manned Soviet North Pole drifting ice stations from 1937 to 1991. These historic temperature records show that winter warming events have been observed over most of the Arctic Ocean. Despite a thin network of observation sites, winter time temperatures above −5°C were directly observed approximately once every 3 years in the central Arctic Ocean between 1954 and 2010. Winter warming events are associated with storm systems originating in either the Atlantic or Pacific Oceans. Twice as many warming events originate from the Atlantic Ocean compared with the Pacific. These storms often penetrate across the North Pole. While observations of winter warming events date back to 1896, we find an increasing number of winter warming events in recent years.
Key Points
Arctic winter warming events are a normal part of the Arctic winter climate. Observations of these events date back to the Fram expedition
North Pole region typically experiences 10 distinct warming events per winter, compared with 5 in the Pacific Central Arctic
Positive trends in the number and duration of Arctic winter warming events (1980–2016), with strongest trends for North Pole domain
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.
Atmospheric measurements were made over Arctic sea ice north of Svalbard from winter to early summer (January–June) 2015 during the Norwegian Young Sea Ice (N‐ICE2015) expedition. These measurements, ...which are available publicly, represent a comprehensive meteorological data set covering the seasonal transition in the Arctic Basin over the new, thinner sea ice regime. Winter was characterized by a succession of storms that produced short‐lived (less than 48 h) temperature increases of 20 to 30 K at the surface. These storms were driven by the hemispheric scale circulation pattern with a large meridional component of the polar jet stream steering North Atlantic storms into the high Arctic. Nonstorm periods during winter were characterized by strong surface temperature inversions due to strong radiative cooling (“radiatively clear state”). The strength and depth of these inversions were similar to those during the Surface Heat Budget of the Arctic Ocean (SHEBA) campaign. In contrast, atmospheric profiles during the “opaquely cloudy state” were different to those from SHEBA due to differences in the synoptic conditions and location within the ice pack. Storm events observed during spring/summer were the result of synoptic systems located in the Barents Sea and the Arctic Basin rather than passing directly over N‐ICE2015. These synoptic systems were driven by a large‐scale circulation pattern typical of recent years, with an Arctic Dipole pattern developing during June. Surface temperatures became near‐constant 0°C on 1 June marking the beginning of summer. Atmospheric profiles during the spring and early summer show persistent lifted temperature and moisture inversions that are indicative of clouds and cloud processes.
Key Points
Analysis of a new comprehensive meteorological data set over Arctic sea ice from winter to summer
Measurements of Arctic storms during winter show large but short‐lived impact on atmospheric temperature
Spring/summer atmosphere is characterized by persistent temperature and humidity inversions indicative of clouds
During two consecutive cruises to the Eastern Central Arctic in late summer 2012, we observed floating algal aggregates in the melt-water layer below and between melting ice floes of first-year pack ...ice. The macroscopic (1-15 cm in diameter) aggregates had a mucous consistency and were dominated by typical ice-associated pennate diatoms embedded within the mucous matrix. Aggregates maintained buoyancy and accumulated just above a strong pycnocline that separated meltwater and seawater layers. We were able, for the first time, to obtain quantitative abundance and biomass estimates of these aggregates. Although their biomass and production on a square metre basis was small compared to ice-algal blooms, the floating ice-algal aggregates supported high levels of biological activity on the scale of the individual aggregate. In addition they constituted a food source for the ice-associated fauna as revealed by pigments indicative of zooplankton grazing, high abundance of naked ciliates, and ice amphipods associated with them. During the Arctic melt season, these floating aggregates likely play an important ecological role in an otherwise impoverished near-surface sea ice environment. Our findings provide important observations and measurements of a unique aggregate-based habitat during the 2012 record sea ice minimum year.
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 people 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. Significant progress has been made in recent years in understanding of Arctic sea ice and its role in climate, the ecosystem, and human activities. However, significant challenges remain in furthering the knowledge of the processes, impacts, and future evolution of the system.
Key Points
Arctic sea ice is rapidly changing; thinning and summer extents are decreasingChanges are faster than model forecasts; feedbacks play a key roleChanging sea ice is impacting biology and human activity in the Arctic
During the Norwegian young sea ICE (N‐ICE2015) campaign in early 2015, a deep snowpack was observed, almost double the climatology for the region north of Svalbard. There were significant amounts of ...snow‐ice in second‐year ice (SYI), while much less in first‐year ice (FYI). Here we use a 1‐D snow/ice thermodynamic model, forced with reanalyses, to show that snow‐ice contributes to thickness growth of SYI in absence of any bottom growth, due to the thick snow. Growth of FYI is tightly controlled by the timing of growth onset relative to precipitation events. A later growth onset can be favorable for FYI growth due to less snow accumulation, which limits snow‐ice formation. We surmise these findings are related to a phenomenon in the Atlantic sector of the Arctic, where frequent storm events bring heavy precipitation during autumn and winter, in a region with a thinning ice cover.
Key Points
Combination of high autumn and winter precipitation and thinning ice make snow‐ice formation prevalent in the Atlantic sector of the Arctic
Snow‐ice is significant for second‐year ice mass balance, and the heavy snow load in autumn results in little thermodynamic bottom growth
First‐year ice growth and snow contribution are tightly controlled by the timing of growth onset relative to precipitation events
Winter time atmospheric observations from the 2015 Norwegian young sea‐ICE campaign (N‐ICE2015) are compared with data from the 1997–1998 Surface Heat Budget of the Arctic (SHEBA) campaign. Both data ...sets have a bimodal distribution of the net longwave radiative flux for January–February, with modal values of −40 W m−2 and 0 W m−2. These values correspond to the radiatively clear and opaquely cloudy states, respectively, and are likely to be representative of the wider Arctic. The new N‐ICE2015 observations demonstrate that the two winter states operate in the Atlantic sector of the Arctic and regions of thin sea ice. We compare the N‐ICE2015 and SHEBA data with ERA‐Interim and output from the coupled Arctic regional climate model HIRHAM‐NAOSIM. ERA‐Interim simulates two Arctic winter states well and captures the timing of transitions from one state to the other, despite underestimating the cloud liquid water path. HIRHAM‐NAOSIM has more cloud liquid water compared with ERA‐Interim but simulates the two states poorly. Our results demonstrate that models must simulate realistic synoptic forcing and temperature profiles to accurately capture the two Arctic winter states, and not only the presence of mixed‐phase clouds. Using ERA‐Interim, we find a positive trend in the number of opaquely cloudy days in the western Atlantic sector of the Arctic, and a strong correlation with the mean winter temperature over much of the Arctic Basin. Hence, the two Arctic winter states are important for understanding interannual variability in the Arctic. The N‐ICE2015 data set will help improve our understanding of these relationships.
Key Points
Two Arctic winter states are observed during the N‐ICE2015 and SHEBA campaigns with similar net longwave fluxes
ERA‐Interim captures the transitions from radiatively clear to opaquely cloudy states during N‐ICE2015 and SHEBA
Strong correlation between number of opaquely cloudy days and mean winter temperature over large areas of the Arctic
Climate change affects the Arctic with regards to permafrost thaw, sea-ice melt, alterations to the freshwater budget and increased export of terrestrial material to the Arctic Ocean. The Fram and ...Davis Straits represent the major gateways connecting the Arctic and Atlantic. Oceanographic surveys were performed in the Fram and Davis Straits, and on the east Greenland Shelf (EGS), in late summer 2012/2013. Meteoric (f
), sea-ice melt, Atlantic and Pacific water fractions were determined and the fluorescence properties of dissolved organic matter (FDOM) were characterized. In Fram Strait and EGS, a robust correlation between visible wavelength fluorescence and f
was apparent, suggesting it as a reliable tracer of polar waters. However, a pattern was observed which linked the organic matter characteristics to the origin of polar waters. At depth in Davis Strait, visible wavelength FDOM was correlated to apparent oxygen utilization (AOU) and traced deep-water DOM turnover. In surface waters FDOM characteristics could distinguish between surface waters from eastern (Atlantic + modified polar waters) and western (Canada-basin polar waters) Arctic sectors. The findings highlight the potential of designing in situ multi-channel DOM fluorometers to trace the freshwater origins and decipher water mass mixing dynamics in the region without laborious samples analyses.
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