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- Author or Editor: Igor V. Polyakov x
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Abstract
This analysis evaluates the thermal state of the intermediate (depth range of 150–900 m) Atlantic Water (AW) of the Arctic Ocean, beginning in the 1950s and with particular focus on the transition from the 1990s to the 2000s and on changes during the 2000s. Using an extensive array of observations, the authors document AW warming trends across various time scales and demonstrate that the 2000s were exceptionally warm, with no analogy since the 1950s or probably in the history of instrumental observations in the Arctic Ocean. Warming in the recent decade was dominated by a warm AW pulse in addition to the underlying trend. Since 1997, the Canadian Basin experienced a faster warming rate compared with the Eurasian Basin. The relative role of the AW warmth in setting the net energy flux and mass balance of the Arctic sea ice is still under debate. Additional carefully orchestrated field experiments are required in order to address this question of ongoing Arctic climate change.
Abstract
This analysis evaluates the thermal state of the intermediate (depth range of 150–900 m) Atlantic Water (AW) of the Arctic Ocean, beginning in the 1950s and with particular focus on the transition from the 1990s to the 2000s and on changes during the 2000s. Using an extensive array of observations, the authors document AW warming trends across various time scales and demonstrate that the 2000s were exceptionally warm, with no analogy since the 1950s or probably in the history of instrumental observations in the Arctic Ocean. Warming in the recent decade was dominated by a warm AW pulse in addition to the underlying trend. Since 1997, the Canadian Basin experienced a faster warming rate compared with the Eurasian Basin. The relative role of the AW warmth in setting the net energy flux and mass balance of the Arctic sea ice is still under debate. Additional carefully orchestrated field experiments are required in order to address this question of ongoing Arctic climate change.
Abstract
This study uses an extensive dataset of monthly surface air temperature (SAT) records (including previously unutilized) from high-latitude (>60°N) meteorological land stations. Most records have been updated by very recent observations (up to December 2008). Using these data, a high-latitude warming rate of 1.36°C century−1 is documented for 1875–2008—the trend is almost 2 times stronger than the Northern Hemisphere trend (0.79°C century−1), with an accelerated warming rate in the most recent decade (1.35°C decade−1). Stronger warming in high-latitude regions is a manifestation of polar amplification (PA). Changes in SAT suggest two spatial scales of PA—hemispheric and local. A new stable statistical measure of PA linking high-latitude and hemispheric temperature anomalies via a regression relationship is proposed. For 1875–2008, this measure yields PA of ∼1.62. Local PA related to the ice–albedo feedback mechanisms is autumnal and coastal, extending several hundred kilometers inland. Heat budget estimates suggest that a recent reduction of arctic ice and anomalously high SATs cannot be explained by ice–albedo feedback mechanisms alone, and the role of large-scale mechanisms of PA of global warming should not be overlooked.
Abstract
This study uses an extensive dataset of monthly surface air temperature (SAT) records (including previously unutilized) from high-latitude (>60°N) meteorological land stations. Most records have been updated by very recent observations (up to December 2008). Using these data, a high-latitude warming rate of 1.36°C century−1 is documented for 1875–2008—the trend is almost 2 times stronger than the Northern Hemisphere trend (0.79°C century−1), with an accelerated warming rate in the most recent decade (1.35°C decade−1). Stronger warming in high-latitude regions is a manifestation of polar amplification (PA). Changes in SAT suggest two spatial scales of PA—hemispheric and local. A new stable statistical measure of PA linking high-latitude and hemispheric temperature anomalies via a regression relationship is proposed. For 1875–2008, this measure yields PA of ∼1.62. Local PA related to the ice–albedo feedback mechanisms is autumnal and coastal, extending several hundred kilometers inland. Heat budget estimates suggest that a recent reduction of arctic ice and anomalously high SATs cannot be explained by ice–albedo feedback mechanisms alone, and the role of large-scale mechanisms of PA of global warming should not be overlooked.
Abstract
The fast decline of Arctic sea ice necessitates a stronger focus on understanding the Arctic sea ice predictability and developing advanced forecast methods for all seasons and for pan-Arctic and regional scales. In this study, the operational forecasting system combining an advanced eddy-permitting ocean–sea ice ensemble reanalysis ORAS5 and state-of-the-art seasonal model-based forecasting system SEAS5 is used to investigate effects of sea ice dynamics and thermodynamics on seasonal (growth-to-melt) Arctic sea ice predictability in 1993–2020. We demonstrate that thermodynamics (growth/melt) dominates the seasonal evolution of mean sea ice thickness at pan-Arctic and regional scales. The thermodynamics also dominates the seasonal predictability of sea ice thickness at pan-Arctic scale; however, at regional scales, the predictability is dominated by dynamics (advection), although the contribution from ice growth/melt remains perceptible. We show competing influences of sea ice dynamics and thermodynamics on the temporal change of ice thickness predictability from 1993–2006 to 2007–20. Over these decades, there was increasing predictability due to growth/melt, attributed to increased winter ocean heat flux in both Eurasian and Amerasian basins, and decreasing predictability due to advection. Our results demonstrate an increasing impact of advection on seasonal sea ice predictability as the region of interest becomes smaller, implying that correct modeling of sea ice drift is crucial for developing reliable regional sea ice predictions. This study delivers important information about sea ice predictability in the “new Arctic” conditions. It increases awareness regarding sea ice state and implementation of sea ice forecasts for various scientific and practical needs that depend on accurate seasonal sea ice forecasts.
Abstract
The fast decline of Arctic sea ice necessitates a stronger focus on understanding the Arctic sea ice predictability and developing advanced forecast methods for all seasons and for pan-Arctic and regional scales. In this study, the operational forecasting system combining an advanced eddy-permitting ocean–sea ice ensemble reanalysis ORAS5 and state-of-the-art seasonal model-based forecasting system SEAS5 is used to investigate effects of sea ice dynamics and thermodynamics on seasonal (growth-to-melt) Arctic sea ice predictability in 1993–2020. We demonstrate that thermodynamics (growth/melt) dominates the seasonal evolution of mean sea ice thickness at pan-Arctic and regional scales. The thermodynamics also dominates the seasonal predictability of sea ice thickness at pan-Arctic scale; however, at regional scales, the predictability is dominated by dynamics (advection), although the contribution from ice growth/melt remains perceptible. We show competing influences of sea ice dynamics and thermodynamics on the temporal change of ice thickness predictability from 1993–2006 to 2007–20. Over these decades, there was increasing predictability due to growth/melt, attributed to increased winter ocean heat flux in both Eurasian and Amerasian basins, and decreasing predictability due to advection. Our results demonstrate an increasing impact of advection on seasonal sea ice predictability as the region of interest becomes smaller, implying that correct modeling of sea ice drift is crucial for developing reliable regional sea ice predictions. This study delivers important information about sea ice predictability in the “new Arctic” conditions. It increases awareness regarding sea ice state and implementation of sea ice forecasts for various scientific and practical needs that depend on accurate seasonal sea ice forecasts.
Abstract
The Arctic Ocean is strongly stratified by salinity in the uppermost layers. This stratification is a key attribute of the region as it acts as an effective barrier for the vertical exchanges of Atlantic Water heat, nutrients, and CO2 between intermediate depths and the surface of the Eurasian and Amerasian basins (EB and AB, respectively). Observations show that from 1970 to 2017, the stratification in the AB has strengthened, whereas, in parts of the EB, the stratification has weakened. The strengthening in the AB is linked to freshening and deepening of the halocline. In the EB, the weakened stratification is associated with salinification and shoaling of the halocline (Atlantification). Simulations from a suite of CMIP6 models project that, under a strong greenhouse gas forcing scenario (ssp585), the overall surface freshening and warming continue in both basins, but there is a divergence in hydrographic trends in certain regions. Within the AB, there is agreement among the models that the upper layers will become more stratified. However, within the EB, models diverge regarding future stratification. This is due to different balances between trends at the surface and trends at depth, related to Fram Strait fluxes. The divergence affects projections of the future state of Arctic sea ice, as models with the strongest Atlantification project the strongest decline in sea ice volume in the EB. From these simulations, one could conclude that Atlantification will not spread eastward into the AB; however, models must be improved to simulate changes in a more intricately stratified EB correctly.
Abstract
The Arctic Ocean is strongly stratified by salinity in the uppermost layers. This stratification is a key attribute of the region as it acts as an effective barrier for the vertical exchanges of Atlantic Water heat, nutrients, and CO2 between intermediate depths and the surface of the Eurasian and Amerasian basins (EB and AB, respectively). Observations show that from 1970 to 2017, the stratification in the AB has strengthened, whereas, in parts of the EB, the stratification has weakened. The strengthening in the AB is linked to freshening and deepening of the halocline. In the EB, the weakened stratification is associated with salinification and shoaling of the halocline (Atlantification). Simulations from a suite of CMIP6 models project that, under a strong greenhouse gas forcing scenario (ssp585), the overall surface freshening and warming continue in both basins, but there is a divergence in hydrographic trends in certain regions. Within the AB, there is agreement among the models that the upper layers will become more stratified. However, within the EB, models diverge regarding future stratification. This is due to different balances between trends at the surface and trends at depth, related to Fram Strait fluxes. The divergence affects projections of the future state of Arctic sea ice, as models with the strongest Atlantification project the strongest decline in sea ice volume in the EB. From these simulations, one could conclude that Atlantification will not spread eastward into the AB; however, models must be improved to simulate changes in a more intricately stratified EB correctly.
Abstract
Arctic atmospheric variability during the industrial era (1875–2000) is assessed using spatially averaged surface air temperature (SAT) and sea level pressure (SLP) records. Air temperature and pressure display strong multidecadal variability on timescales of 50–80 yr [termed low-frequency oscillation (LFO)]. Associated with this variability, the Arctic SAT record shows two maxima: in the 1930s–40s and in recent decades, with two colder periods in between. In contrast to the global and hemispheric temperature, the maritime Arctic temperature was higher in the late 1930s through the early 1940s than in the 1990s. Incomplete sampling of large-amplitude multidecadal fluctuations results in oscillatory Arctic SAT trends. For example, the Arctic SAT trend since 1875 is 0.09 ± 0.03°C decade−1, with stronger spring- and wintertime warming; during the twentieth century (when positive and negative phases of the LFO nearly offset each other) the Arctic temperature increase is 0.05 ± 0.04°C decade−1, similar to the Northern Hemispheric trend (0.06°C decade−1). Thus, the large-amplitude multidecadal climate variability impacting the maritime Arctic may confound the detection of the true underlying climate trend over the past century. LFO-modulated trends for short records are not indicative of the long-term behavior of the Arctic climate system. The accelerated warming and a shift of the atmospheric pressure pattern from anticyclonic to cyclonic in recent decades can be attributed to a positive LFO phase. It is speculated that this LFO-driven shift was crucial to the recent reduction in Arctic ice cover. Joint examination of air temperature and pressure records suggests that peaks in temperature associated with the LFO follow pressure minima after 5–15 yr. Elucidating the mechanisms behind this relationship will be critical to understanding the complex nature of low-frequency variability.
Abstract
Arctic atmospheric variability during the industrial era (1875–2000) is assessed using spatially averaged surface air temperature (SAT) and sea level pressure (SLP) records. Air temperature and pressure display strong multidecadal variability on timescales of 50–80 yr [termed low-frequency oscillation (LFO)]. Associated with this variability, the Arctic SAT record shows two maxima: in the 1930s–40s and in recent decades, with two colder periods in between. In contrast to the global and hemispheric temperature, the maritime Arctic temperature was higher in the late 1930s through the early 1940s than in the 1990s. Incomplete sampling of large-amplitude multidecadal fluctuations results in oscillatory Arctic SAT trends. For example, the Arctic SAT trend since 1875 is 0.09 ± 0.03°C decade−1, with stronger spring- and wintertime warming; during the twentieth century (when positive and negative phases of the LFO nearly offset each other) the Arctic temperature increase is 0.05 ± 0.04°C decade−1, similar to the Northern Hemispheric trend (0.06°C decade−1). Thus, the large-amplitude multidecadal climate variability impacting the maritime Arctic may confound the detection of the true underlying climate trend over the past century. LFO-modulated trends for short records are not indicative of the long-term behavior of the Arctic climate system. The accelerated warming and a shift of the atmospheric pressure pattern from anticyclonic to cyclonic in recent decades can be attributed to a positive LFO phase. It is speculated that this LFO-driven shift was crucial to the recent reduction in Arctic ice cover. Joint examination of air temperature and pressure records suggests that peaks in temperature associated with the LFO follow pressure minima after 5–15 yr. Elucidating the mechanisms behind this relationship will be critical to understanding the complex nature of low-frequency variability.
Abstract
Examination of records of fast ice thickness (1936–2000) and ice extent (1900–2000) in the Kara, Laptev, East Siberian, and Chukchi Seas provide evidence that long-term ice thickness and extent trends are small and generally not statistically significant, while trends for shorter records are not indicative of the long-term tendencies due to large-amplitude low-frequency variability. The ice variability in these seas is dominated by a multidecadal, low-frequency oscillation (LFO) and (to a lesser degree) by higher-frequency decadal fluctuations. The LFO signal decays eastward from the Kara Sea where it is strongest. In the Chukchi Sea ice variability is dominated by decadal fluctuations, and there is no evidence of the LFO. This spatial pattern is consistent with the air temperature–North Atlantic Oscillation (NAO) index correlation pattern, with maximum correlation in the near-Atlantic region, which decays toward the North Pacific. Sensitivity analysis shows that dynamical forcing (wind or surface currents) dominates ice-extent variations in the Laptev, East Siberian, and Chukchi Seas. Variability of Kara Sea ice extent is governed primarily by thermodynamic factors.
Abstract
Examination of records of fast ice thickness (1936–2000) and ice extent (1900–2000) in the Kara, Laptev, East Siberian, and Chukchi Seas provide evidence that long-term ice thickness and extent trends are small and generally not statistically significant, while trends for shorter records are not indicative of the long-term tendencies due to large-amplitude low-frequency variability. The ice variability in these seas is dominated by a multidecadal, low-frequency oscillation (LFO) and (to a lesser degree) by higher-frequency decadal fluctuations. The LFO signal decays eastward from the Kara Sea where it is strongest. In the Chukchi Sea ice variability is dominated by decadal fluctuations, and there is no evidence of the LFO. This spatial pattern is consistent with the air temperature–North Atlantic Oscillation (NAO) index correlation pattern, with maximum correlation in the near-Atlantic region, which decays toward the North Pacific. Sensitivity analysis shows that dynamical forcing (wind or surface currents) dominates ice-extent variations in the Laptev, East Siberian, and Chukchi Seas. Variability of Kara Sea ice extent is governed primarily by thermodynamic factors.
Abstract
A 15-yr duration record of mooring observations from the eastern (>70°E) Eurasian Basin (EB) of the Arctic Ocean is used to show and quantify the recently increased oceanic heat flux from intermediate-depth (~150–900 m) warm Atlantic Water (AW) to the surface mixed layer and sea ice. The upward release of AW heat is regulated by the stability of the overlying halocline, which we show has weakened substantially in recent years. Shoaling of the AW has also contributed, with observations in winter 2017–18 showing AW at only 80 m depth, just below the wintertime surface mixed layer, the shallowest in our mooring records. The weakening of the halocline for several months at this time implies that AW heat was linked to winter convection associated with brine rejection during sea ice formation. This resulted in a substantial increase of upward oceanic heat flux during the winter season, from an average of 3–4 W m−2 in 2007–08 to >10 W m−2 in 2016–18. This seasonal AW heat loss in the eastern EB is equivalent to a more than a twofold reduction of winter ice growth. These changes imply a positive feedback as reduced sea ice cover permits increased mixing, augmenting the summer-dominated ice-albedo feedback.
Abstract
A 15-yr duration record of mooring observations from the eastern (>70°E) Eurasian Basin (EB) of the Arctic Ocean is used to show and quantify the recently increased oceanic heat flux from intermediate-depth (~150–900 m) warm Atlantic Water (AW) to the surface mixed layer and sea ice. The upward release of AW heat is regulated by the stability of the overlying halocline, which we show has weakened substantially in recent years. Shoaling of the AW has also contributed, with observations in winter 2017–18 showing AW at only 80 m depth, just below the wintertime surface mixed layer, the shallowest in our mooring records. The weakening of the halocline for several months at this time implies that AW heat was linked to winter convection associated with brine rejection during sea ice formation. This resulted in a substantial increase of upward oceanic heat flux during the winter season, from an average of 3–4 W m−2 in 2007–08 to >10 W m−2 in 2016–18. This seasonal AW heat loss in the eastern EB is equivalent to a more than a twofold reduction of winter ice growth. These changes imply a positive feedback as reduced sea ice cover permits increased mixing, augmenting the summer-dominated ice-albedo feedback.
