Unprecedented Warming of the Arctic

FIGURE 1: PALEO TEMPERATURE DATA FOR THE ARCTIC FROM 20 SOURCES

FIGURE 2: SUMMARY TABLE

FIGURE 3: GIF IMAGE CYCLES THROUGH ALL DATA STATIONS

FIGURE 4: ARCTIC OCEAN GEOTHERMAL ACTIVITY [JAMES KAMIS]

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RELATED: Does Global Warming Drive Changes in Arctic Sea Ice?

In late 2017 and early 2018 a number of high profile news media sources cited a US Federal Government report to claim that “warming in the Arctic is unprecedented in 1,500 years”. Here are some examples:[DIGITAL JOURNAL, Scientists alarmed by unprecedented warming in Arctic: By Karen Graham, Feb 28, 2018 in Environment], [USA TODAY, The Arctic is warming faster than it has in 1,500 years: Doyle Rice, Dec. 12, 2017], [MASHABLE ASIA, Recent Arctic warming ‘unprecedented’ in human history, Andrew Freedman, DEC 13, 2017], [DISCOVER MAGAZINE, A major federal report about the Arctic released yesterday finds that the current rate of Arctic warming is unprecedented in at least the past 2,000 years. And the pace of Arctic sea ice loss experienced in the past few decades has not been seen in at least the past 1,450 years. Tom Yulsman, December 13, 2017], [WASHINGTON POST, Warming of the Arctic is ‘unprecedented over the last 1,500 years, Chris Mooney December 12, 2017], [THE GUARDIAN, Record warmth in the Arctic this month could yet prove to be a freak occurrence, but experts warn the warming event is unprecedented. Jonathan Watts, environment editor, Tue 27 Feb 2018]. Their source was the 2017 NOAA Arctic Report Card a summary of which appears in the next paragraph. NOAA: “The Arctic shows no sign of returning to reliably frozen region of recent past decades. Despite relatively cool summer temperatures, observations in 2017 continue to indicate that the Arctic environmental system has reached a ‘new normal’, characterized by long-term losses in the extent and thickness of the sea ice cover, the extent and duration of the winter snow cover and the mass of ice in the Greenland Ice Sheet and Arctic glaciers, and warming sea surface and permafrost temperatures. The average surface air temperature for the year ending September 2017 is the 2nd warmest since 1900; however, cooler spring and summer temperatures contributed to a rebound in snow cover in the Eurasian Arctic, slower summer sea ice loss, and below-average melt extent for the Greenland ice sheet. The sea ice cover continues to be relatively young and thin with older, thicker ice comprising only 21% of the ice cover in 2017 compared to 45% in 1985. In August 2017, sea surface temperatures in the Barents and Chukchi seas were up to 4° C warmer than average, contributing to a delay in the autumn freeze-up in these regions. Pronounced increases in ocean primary productivity, at the base of the marine food web, were observed in the Barents and Eurasian Arctic seas from 2003 to 2017. Arctic tundra is experiencing increased greenness and record permafrost warming. Pervasive changes in the environment are influencing resource management protocols, including those established for fisheries and wildfires. The unprecedented rate and global reach of Arctic change disproportionately affect the people of northern communities, further pressing the need to prepare for and adapt to the new Arctic.“ The claim that “warming in the Arctic is unprecedented” can be interpreted as a reference to temperature or to the rate of warming, or perhaps to both. The importance of the rate of warming in climate science is underscored by this statement from the NASA website “If Earth has warmed and cooled throughout history, what makes scientists think that humans are causing global warming now?The first piece of evidence that the warming over the past few decades isn’t part of a natural cycle is how fast the change is happening. The biggest temperature swings our planet has experienced in the past million years are the ice ages. Based on a combination of paleoclimate data and models, scientists estimate that when ice ages have ended in the past, it has taken about 5,000 years for the planet to warm between 4 and 7 degrees Celsius. The warming of the past century of 0.7C is roughly eight times faster than the ice-age-recovery warming on average.” [SOURCE]. We therefore interpret the Arctic warming claim broadly to include both temperature and the rate of warming. This work is a test of the claim of unprecedented warming in the Arctic. Here we use paleo temperature data provided by the NOAA Pages2k database [LINK] (caution: the link automatically initiates the download of a large Excel file). Twenty sources for Arctic air temperature were found in this database and the data from all twenty sources are shown in the charts in Figure 1. Eighteen of these sources provide d180 data. The term d180 is a reference to delta-O-18, a reference to the ratio of oxygen isotopes O(18) to O(16) compared against a standardized ratio. There is a linear relationship between this ratio and temperature and because of that d180 is often used as a proxy for temperature. The magnitudes are different but in a comparative study such as this, the d180 values can be used directly instead of temperature. The other two are lake bed sediment data and are included for completeness but the greater weight will be given to the cleaner d180 air temperature proxies. Each data set is presented graphically in Figure 1 as two side by side frames. The left frame traces the maximum d180 value (in black) in a moving 100-year window as a test of whether the maximum temperature occurs at the end of the series. The right frame traces the d180 OLS trend (in red) in a moving 100-year window as a test of whether the greatest rate of warming occurs at the end of the series. If the highest value in the curve is found at the end of the series, the data are taken to be consistent with the “unprecedented warming” hypothesis when stated in terms of the variable being tested. Otherwise, the data are taken to be inconsistent with the “unprecedented warming” hypothesis (when stated in terms of the variable being tested). The chart titles are color coded. Green indicates that the data are consistent with the hypothesis and red indicates that they are not. These interpretations of the data displayed in Figure 1 are summarized in the table in Figure 2. The lake sediment are marked in red as they may not be directly comparable with the surface temperature proxies in the other 18 datasets. The summary statistics at the bottom of the table in Figure 2 are in two parts – the part in black does not include the lake sediment data and the part in red does. Here we find as follows: (a) If we remove the lake sediment data, support for unprecedented high temperature in the Arctic at the end of the datasets is an impressive 50% but that for highest rate of warming is rather low at 22% if the end of the dataset is strictly defined as the trend in the last of the 100-year moving windows. If this definition is broadened to the “post industrial era” as any date after 1900, then support for the “unprecedented” hypothesis grows to 61%. The last column of Figure 2 shows that support for “unprecedented warming” defined as both highest temperature and highest rate of warming is a low 39% even when including the broad definition of the current era. An issue raised by James Kamis in this regard [LINK] is that extensive natural geothermal sources of heat in the Arctic Ocean are not included in the analysis. These volcanic and mantle plume sources are shown in Figure 4. We conclude from this analysis that the paleo data presented in Figure 1 (not including the Lake data) do not provide convincing evidence of “unprecedented” warming in the Arctic in the current era either in terms of temperature or in terms of the rate of warming. However, the evidence for unprecedented warming trend is strengthened significantly if the current era is defined broadly to include the period from 1900 to the present. A further conclusion from these data relate to the claim by NASA and by climate science in general (paragraph#3) that the usual argument by skeptics that “the climate has always changed” ignores the speed issue. The response by climate science is that yes, the climate has always changed but what makes the current warming different, and therefore human caused, is the high rate of warming never before seen in natural climate change. The paleo data presented above shows that in 12 out of 20 temperature datasets (including lake sediment data) and in 10 out of 18 temperature datasets with only d180 data, we find evidence that climate has changed faster than it is changing now. These changes occurred within the last 2,000 years in the pre-industrial era.

MAP SHOWING SOME OF THE DATA STATIONS

RELATED POST: Does Global Warming Drive Changes in Arctic Sea Ice?

ARCTIC WARMING BIBLIOGRAPHY

2002: Polyakov, Igor V., et al. “Observationally based assessment of polar amplification of global warming.” Geophysical research letters 29.18 (2002): 25-1. Arctic variability is dominated by multi‐decadal fluctuations. Incomplete sampling of these fluctuations results in highly variable arctic surface‐air temperature (SAT) trends. Modulated by multi‐decadal variability, SAT trends are often amplified relative to northern‐hemispheric trends, but over the 125‐year record we identify periods when arctic SAT trends were smaller or of opposite sign than northern‐hemispheric trends. Arctic and northern‐hemispheric air‐temperature trends during the 20th century (when multi‐decadal variablity had little net effect on computed trends) are similar, and do not support the predicted polar amplification of global warming. The possible moderating role of sea ice cannot be conclusively identified with existing data. If long‐term trends are accepted as a valid measure of climate change, then the SAT and ice data do not support the proposed polar amplification of global warming. Intrinsic arctic variability obscures long‐term changes, limiting our ability to identify complex feedbacks in the arctic climate system. 2002: Rigor, Ignatius G., John M. Wallace, and Roger L. Colony. “Response of sea ice to the Arctic Oscillation.” Journal of Climate 15.18 (2002): 2648-2663.Data collected by the International Arctic Buoy Programme from 1979 to 1998 are analyzed to obtain statistics of sea level pressure (SLP) and sea ice motion (SIM). The annual and seasonal mean fields agree with those obtained in previous studies of Arctic climatology. The data show a 3-hPa decrease in decadal mean SLP over the central Arctic Ocean between 1979–88 and 1989–98. This decrease in SLP drives a cyclonic trend in SIM, which resembles the structure of the Arctic Oscillation (AO). Regression maps of SIM during the wintertime (January–March) AO index show 1) an increase in ice advection away from the coast of the East Siberian and Laptev Seas, which should have the effect of producing more new thin ice in the coastal flaw leads; 2) a decrease in ice advection from the western Arctic into the eastern Arctic; and 3) a slight increase in ice advection out of the Arctic through Fram Strait. Taken together, these changes suggest that at least part of the thinning of sea ice recently observed over the Arctic Ocean can be attributed to the trend in the AO toward the high-index polarity. Rigor et al. showed that year-to-year variations in the wintertime AO imprint a distinctive signature on surface air temperature (SAT) anomalies over the Arctic, which is reflected in the spatial pattern of temperature change from the 1980s to the 1990s. Here it is shown that the memory of the wintertime AO persists through most of the subsequent year: spring and autumn SAT and summertime sea ice concentration are all strongly correlated with the AO index for the previous winter. It is hypothesized that these delayed responses reflect the dynamical influence of the AO on the thickness of the wintertime sea ice, whose persistent “footprint” is reflected in the heat fluxes during the subsequent spring, in the extent of open water during the subsequent summer, and the heat liberated in the freezing of the open water during the subsequent autumn. 2003: Semenov, Vladimir A., and Lennart Bengtsson. “Modes of the wintertime Arctic temperature variability.” Geophysical Research Letters 30.15 (2003). It is shown that the Arctic averaged wintertime temperature variability during the 20th century can be essentially described by two orthogonal modes. These modes were identified by an Empirical Orthogonal Function (EOF) decomposition of the 1892–1999 surface wintertime air temperature anomalies (40°N–80°N) using a gridded dataset covering high Arctic. The first mode (1st leading EOF) is related to the NAO and has a major contribution to Arctic warming during the last 30 years. The second one (3rd leading EOF) dominates the SAT variability prior to 1970. A correlation between the corresponding principal component PC3 and the Arctic SAT anomalies is 0.79. This mode has the largest amplitudes in the Kara‐Barents Seas and Baffin Bay and exhibits no direct link to the large‐scale atmospheric circulation variability, in contrast to the other leading EOFs. We suggest that the existence of this mode is caused by long‐term sea ice variations presumably due to Atlantic inflow variability. 2003: Polyakov, Igor V., et al. “Variability and trends of air temperature and pressure in the maritime Arctic, 1875–2000.” Journal of Climate 16.12 (2003): 2067-2077. 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. 2003: Johannessen, Ola M., et al. “Arctic climate change—will the ice disappear this century?.” Elsevier Oceanography Series. Vol. 69. Elsevier, 2003. 490-496. A new set of multi-decadal and century-scale sea-ice data is compared with coupled atmosphere-ocean model simulations in order to understand Arctic sea ice and climate variability. It is evident that the two pronounced 20th-century warming events—both amplified in the Arctic—were linked to sea-ice variability. The area of sea ice is observed to have decreased by 8× 105km2 (7.4%) since 1978, with record-low summer ice coverage in 2002. Model predictions are used to quantify changes in the ice cover through the 21st century. A predominantly ice-free Arctic in summer is predicted for the end of this century. 2003: Polyakov, Igor V., et al. “Long-term ice variability in Arctic marginal seas.” Journal of Climate 16.12 (2003): 2078-2085. 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. 2004: Overland, James E., et al. “Seasonal and regional variation of pan-Arctic surface air temperature over the instrumental record.” Journal of Climate 17.17 (2004): 3263-3282. Instrumental surface air temperature (SAT) records beginning in the late 1800s from 59 Arctic stations north of 64°N show monthly mean anomalies of several degrees and large spatial teleconnectivity, yet there are systematic seasonal and regional differences. Analyses are based on time–longitude plots of SAT anomalies and principal component analysis (PCA). Using monthly station data rather than gridded fields for this analysis highlights the importance of considering record length in calculating reliable Arctic change estimates; for example, the contrast of PCA performed on 11 stations beginning in 1886, 20 stations beginning in 1912, and 45 stations beginning in 1936 is illustrated. While often there is a well-known interdecadal negative covariability in winter between northern Europe and Baffin Bay, long-term changes in the remainder of the Arctic are most evident in spring, with cool temperature anomalies before 1920 and Arctic-wide warm temperatures in the 1990s. Summer anomalies are generally weaker than spring or winter but tend to mirror spring conditions before 1920 and in recent decades. Temperature advection in the trough–ridge structure in the positive phase of the Arctic Oscillation (AO) in the North Atlantic establishes wintertime temperature anomalies in adjacent regions, while the zonal/annular nature of the AO in the remainder of the Arctic must break down in spring to promote meridional temperature advection. There were regional/decadal warm events during winter and spring in the 1930s to 1950s, but meteorological analysis suggests that these SAT anomalies are the result of intrinsic variability in regional flow patterns. These midcentury events contrast with the recent Arctic-wide AO influence in the 1990s. The preponderance of evidence supports the conclusion that warm SAT anomalies in spring for the recent decade are unique in the instrumental record, both in having the greatest longitudinal extent and in their associated patterns of warm air advection. 2004: Johannessen, Ola M., et al. “Arctic climate change: observed and modelled temperature and sea-ice variability.” Tellus A: Dynamic Meteorology and Oceanography 56.4 (2004): 328-341. Changes apparent in the arctic climate system in recent years require evaluation in a century-scale perspective in order to assess the Arctic’s response to increasing anthropogenic greenhouse-gas forcing. Here, a new set of centuryand multidecadal-scale observational data of surface air temperature (SAT) and sea ice is used in combination with ECHAM4 and HadCM3 coupled atmosphere’ice’ocean global model simulations in order to better determine and understand arctic climate variability. We show that two pronounced twentieth-century warming events, both amplified in the Arctic, were linked to sea-ice variability. SAT observations and model simulations indicate that the nature of the arctic warming in the last two decades is distinct from the early twentieth-century warm period. It is suggested strongly that the earlier warming was natural internal climate-system variability, whereas the recent SAT changes are a response to anthropogenic forcing. The area of arctic sea ice is furthermore observed to have decreased~8 · 105 km2 (7.4%) in the past quarter century, with record-low summer ice coverage in September 2002. A set of model predictions is used to quantify changes in the ice cover through the twenty-first century, with greater reductions expected in summer than winter. In summer, a predominantly sea-ice-free Arctic is predicted for the end of this century. 2004: Bengtsson, Lennart, Vladimir A. Semenov, and Ola M. Johannessen. “The early twentieth-century warming in the Arctic—A possible mechanism.” Journal of Climate 17.20 (2004): 4045-4057. The huge warming of the Arctic that started in the early 1920s and lasted for almost two decades is one of the most spectacular climate events of the twentieth century. During the peak period 1930–40, the annually averaged temperature anomaly for the area 60°–90°N amounted to some 1.7°C. Whether this event is an example of an internal climate mode or is externally forced, such as by enhanced solar effects, is presently under debate. This study suggests that natural variability is a likely cause, with reduced sea ice cover being crucial for the warming. A robust sea ice–air temperature relationship was demonstrated by a set of four simulations with the atmospheric ECHAM model forced with observed SST and sea ice concentrations. An analysis of the spatial characteristics of the observed early twentieth-century surface air temperature anomaly revealed that it was associated with similar sea ice variations. Further investigation of the variability of Arctic surface temperature and sea ice cover was performed by analyzing data from a coupled ocean–atmosphere model. By analyzing climate anomalies in the model that are similar to those that occurred in the early twentieth century, it was found that the simulated temperature increase in the Arctic was related to enhanced wind-driven oceanic inflow into the Barents Sea with an associated sea ice retreat. The magnitude of the inflow is linked to the strength of westerlies into the Barents Sea. This study proposes a mechanism sustaining the enhanced westerly winds by a cyclonic atmospheric circulation in the Barents Sea region created by a strong surface heat flux over the ice-free areas. Observational data suggest a similar series of events during the early twentieth-century Arctic warming, including increasing westerly winds between Spitsbergen and Norway, reduced sea ice, and enhanced cyclonic circulation over the Barents Sea. At the same time, the North Atlantic Oscillation was weakening. 2005: Stroeve, J. C., et al. “Tracking the Arctic’s shrinking ice cover: Another extreme September minimum in 2004.” Geophysical Research Letters 32.4 (2005). Satellite passive microwave observations document an overall downward trend in Arctic sea ice extent and area since 1978. While the record minimum observed in September 2002 strongly reinforced this downward trend, extreme ice minima were again observed in 2003 and 2004. Although having three extreme minimum years in a row is unprecedented in the satellite record, attributing these recent trends and extremes to greenhouse gas loading must be tempered by recognition that the sea ice cover is variable from year to year in response to wind, temperature and oceanic forcings. 2014: Woollings, Tim, Ben Harvey, and Giacomo Masato. “Arctic warming, atmospheric blocking and cold European winters in CMIP5 models.” Environmental Research Letters 9.1 (2014): 014002. Amplified Arctic warming is expected to have a significant long-term influence on the midlatitude atmospheric circulation by the latter half of the 21st century. Potential influences of recent and near future Arctic changes on shorter timescales are much less clear, despite having received much recent attention in the literature. In this letter, climate models from the recent CMIP5 experiment are analysed for evidence of an influence of Arctic temperatures on midlatitude blocking and cold European winters in particular. The focus is on the variability of these features in detrended data and, in contrast to other studies, limited evidence of an influence is found. The occurrence of cold European winters is found to be largely independent of the temperature variability in the key Barents–Kara Sea region. Positive correlations of the Barents–Kara temperatures with Eurasian blocking are found in some models, but significant correlations are limited. 2015: Francis, Jennifer, and Natasa Skific. “Evidence linking rapid Arctic warming to mid-latitude weather patterns.” Phil. Trans. R. Soc. A 373.2045 (2015): 20140170. The effects of rapid Arctic warming and ice loss on weather patterns in the Northern Hemisphere is a topic of active research, lively scientific debate and high societal impact. The emergence of Arctic amplification—the enhanced sensitivity of high-latitude temperature to global warming—in only the last 10–20 years presents a challenge to identifying statistically robust atmospheric responses using observations. Several recent studies have proposed and demonstrated new mechanisms by which the changing Arctic may be affecting weather patterns in mid-latitudes, and these linkages differ fundamentally from tropics/jet-stream interactions through the transfer of wave energy. In this study, new metrics and evidence are presented that suggest disproportionate Arctic warming—and resulting weakening of the poleward temperature gradient—is causing the Northern Hemisphere circulation to assume a more meridional character (i.e. wavier), although not uniformly in space or by season, and that highly amplified jet-stream patterns are occurring more frequently. Further analysis based on self-organizing maps supports this finding. These changes in circulation are expected to lead to persistent weather patterns that are known to cause extreme weather events. As emissions of greenhouse gases continue unabated, therefore, the continued amplification of Arctic warming should favour an increased occurrence of extreme events caused by prolonged weather conditions. 2015: Kug, Jong-Seong, et al. “Two distinct influences of Arctic warming on cold winters over North America and East Asia.” Nature Geoscience 8.10 (2015): 759. Arctic warming has sparked a growing interest because of its possible impacts on mid-latitude climate1,2,3,4,5. A number of unusually harsh cold winters have occurred in many parts of East Asia and North America in the past few years2,6,7, and observational and modelling studies have suggested that atmospheric variability linked to Arctic warming might have played a central role1,3,4,8,9,10,11. Here we identify two distinct influences of Arctic warming which may lead to cold winters over East Asia or North America, based on observational analyses and extensive climate model results. We find that severe winters across East Asia are associated with anomalous warmth in the Barents–Kara Sea region, whereas severe winters over North America are related to anomalous warmth in the East Siberian–Chukchi Sea region. Each regional warming over the Arctic Ocean is accompanied by the local development of an anomalous anticyclone and the downstream development of a mid-latitude trough. The resulting northerly flow of cold air provides favourable conditions for severe winters in East Asia or North America. These links between Arctic and mid-latitude weather are also robustly found in idealized climate model experiments and CMIP5 multi-model simulations. We suggest that our results may help improve seasonal prediction of winter weather and extreme events in these regions. 2015: Park, Jong-Yeon, et al. “Amplified Arctic warming by phytoplankton under greenhouse warming.” Proceedings of the National Academy of Sciences (2015): 201416884. One of the important impacts of marine phytoplankton on climate systems is the geophysical feedback by which chlorophyll and the related pigments in phytoplankton absorb solar radiation and then change sea surface temperature. Yet such biogeophysical impact is still not considered in many climate projections by state-of-the-art climate models, nor is its impact on the future climate quantified. This study shows that, by conducting global warming simulations with and without an active marine ecosystem model, the biogeophysical effect of future phytoplankton changes amplifies Arctic warming by 20%. Given the close linkage between the Arctic and global climate, the biologically enhanced Arctic warming can significantly modify future estimates of global climate change, and therefore it needs to be considered as a possible future scenario. 2015: Francis, Jennifer A., and Stephen J. Vavrus. “Evidence for a wavier jet stream in response to rapid Arctic warming.” Environmental Research Letters 10.1 (2015): 014005. New metrics and evidence are presented that support a linkage between rapid Arctic warming, relative to Northern hemisphere mid-latitudes, and more frequent high-amplitude (wavy) jet-stream configurations that favor persistent weather patterns. We find robust relationships among seasonal and regional patterns of weaker poleward thickness gradients, weaker zonal upper-level winds, and a more meridional flow direction. These results suggest that as the Arctic continues to warm faster than elsewhere in response to rising greenhouse-gas concentrations, the frequency of extreme weather events caused by persistent jet-stream patterns will increase. 2016: Goss, Michael, Steven B. Feldstein, and Sukyoung Lee. “Stationary wave interference and its relation to tropical convection and Arctic warming.” Journal of Climate 29.4 (2016): 1369-1389.The interference between transient eddies and climatological stationary eddies in the Northern Hemisphere is investigated. The amplitude and sign of the interference is represented by the stationary wave index (SWI), which is calculated by projecting the daily 300-hPa streamfunction anomaly field onto the 300-hPa climatological stationary wave. ERA-Interim data for the years 1979 to 2013 are used. The amplitude of the interference peaks during boreal winter. The evolution of outgoing longwave radiation, Arctic temperature, 300-hPa streamfunction, 10-hPa zonal wind, Arctic sea ice concentration, and the Arctic Oscillation (AO) index are examined for days of large SWI values during the winter. Constructive interference during winter tends to occur about one week after enhanced warm pool convection and is followed by an increase in Arctic surface air temperature along with a reduction of sea ice in the Barents and Kara Seas. The warming of the Arctic does occur without prior warm pool convection, but it is enhanced and prolonged when constructive interference occurs in concert with enhanced warm pool convection. This is followed two weeks later by a weakening of the stratospheric polar vortex and a decline of the AO. All of these associations are reversed in the case of destructive interference. Potential climate change implications are briefly discussed. 2016: Baggett, Cory, Sukyoung Lee, and Steven Feldstein. “An investigation of the presence of atmospheric rivers over the North Pacific during planetary-scale wave life cycles and their role in Arctic warming.” Journal of the Atmospheric Sciences73.11 (2016): 4329-4347. Heretofore, the tropically excited Arctic warming (TEAM) mechanism put forward that localized tropical convection amplifies planetary-scale waves, which transport sensible and latent heat into the Arctic, leading to an enhancement of downward infrared radiation and Arctic surface warming. In this study, an investigation is made into the previously unexplored contribution of the synoptic-scale waves and their attendant atmospheric rivers to the TEAM mechanism. Reanalysis data are used to conduct a suite of observational analyses, trajectory calculations, and idealized model simulations. It is shown that localized tropical convection over the Maritime Continent precedes the peak of the planetary-scale wave life cycle by ~10–14 days. The Rossby wave source induced by the tropical convection excites a Rossby wave train over the North Pacific that amplifies the climatological December–March stationary waves. These amplified planetary-scale waves are baroclinic and transport sensible and latent heat poleward. During the planetary-scale wave life cycle, synoptic-scale waves are diverted northward over the central North Pacific. The warm conveyor belts associated with the synoptic-scale waves channel moisture from the subtropics into atmospheric rivers that ascend as they move poleward and penetrate into the Arctic near the Bering Strait. At this time, the synoptic-scale waves undergo cyclonic Rossby wave breaking, which further amplifies the planetary-scale waves. The planetary-scale wave life cycle ceases as ridging over Alaska retrogrades westward. The ridging blocks additional moisture transport into the Arctic. However, sensible and latent heat amounts remain elevated over the Arctic, which enhances downward infrared radiation and maintains warm surface temperatures. 2016: Woods, Cian, and Rodrigo Caballero. “The role of moist intrusions in winter Arctic warming and sea ice decline.” Journal of Climate 29.12 (2016): 4473-4485. This paper examines the trajectories followed by intense intrusions of moist air into the Arctic polar region during autumn and winter and their impact on local temperature and sea ice concentration. It is found that the vertical structure of the warming associated with moist intrusions is bottom amplified, corresponding to a transition of local conditions from a “cold clear” state with a strong inversion to a “warm opaque” state with a weaker inversion. In the marginal sea ice zone of the Barents Sea, the passage of an intrusion also causes a retreat of the ice margin, which persists for many days after the intrusion has passed. The authors find that there is a positive trend in the number of intrusion events crossing 70°N during December and January that can explain roughly 45% of the surface air temperature and 30% of the sea ice concentration trends observed in the Barents Sea during the past two decades. 2017: Tokinaga, Hiroki, Shang-Ping Xie, and Hitoshi Mukougawa. “Early 20th-century Arctic warming intensified by Pacific and Atlantic multidecadal variability.” Proceedings of the National Academy of Sciences 114.24 (2017): 6227-6232. Arctic amplification is a robust feature of climate response to global warming, with large impacts on ecosystems and societies. A long-standing mystery is that a pronounced Arctic warming occurred during the early 20th century when the rate of interdecadal change in radiative forcing was much weaker than at present. Here, using observations and model experiments, we show that the combined effect of internally generated Pacific and Atlantic interdecadal variabilities intensified the Arctic land warming in the early 20th century. The synchronized Pacific–Atlantic warming drastically alters planetary-scale atmospheric circulations over the Northern Hemisphere that transport warm air into the Arctic. Our results highlight the importance of regional sea surface temperature changes for Arctic climate and constrain model projections in this important region. 2017: Breider, Thomas J., et al. “Multidecadal trends in aerosol radiative forcing over the Arctic: Contribution of changes in anthropogenic aerosol to Arctic warming since 1980.” Journal of Geophysical Research: Atmospheres 122.6 (2017): 3573-3594. Arctic observations show large decreases in the concentrations of sulfate and black carbon (BC) aerosols since the early 1980s. These near‐term climate‐forcing pollutants perturb the radiative balance of the atmosphere and may have played an important role in recent Arctic warming. We use the GEOS‐Chem global chemical transport model to construct a 3‐D representation of Arctic aerosols that is generally consistent with observations and their trends from 1980 to 2010. Observations at Arctic surface sites show significant decreases in sulfate and BC mass concentrations of 2–3% per year. We find that anthropogenic aerosols yield a negative forcing over the Arctic, with an average 2005–2010 Arctic shortwave radiative forcing (RF) of −0.19 ± 0.05 W m−2 at the top of atmosphere (TOA). Anthropogenic sulfate in our study yields more strongly negative forcings over the Arctic troposphere in spring (−1.17 ± 0.10 W m−2) than previously reported. From 1980 to 2010, TOA negative RF by Arctic aerosol declined, from −0.67 ± 0.06 W m−2 to −0.19 ± 0.05 W m−2, yielding a net TOA RF of +0.48 ± 0.06 W m−2. The net positive RF is due almost entirely to decreases in anthropogenic sulfate loading over the Arctic. We estimate that 1980–2010 trends in aerosol‐radiation interactions over the Arctic and Northern Hemisphere midlatitudes have contributed a net warming at the Arctic surface of +0.27 ± 0.04 K, roughly one quarter of the observed warming. Our study does not consider BC emissions from gas flaring nor the regional climate response to aerosol‐cloud interactions or BC deposition on snow.

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