By Xiangdong Zhang, International Arctic Research Center and Department of Atmospheric Sciences, University of Alaska Fairbanks

Amplification of Arctic surface air temperature (SAT) increase, at more than twice the rate of average global temperatures since the mid-twentieth century, has been well-recognized (e.g., Serreze et al. 2011; Cohen et al. 2014). Recently, a large number of studies and the Intergovernmental Panel on Climate Change Fifth Assessment Report (IPCC AR5) suggested a hiatus or slow-down of global warming from 1998–2012. However, accurate estimates of Arctic warming have been limited due to sparse observational networks in this remote, harsh, and climatically sensitive area and representation of the Arctic observations in global SAT data sets is problematic. U.S. and Chinese government agencies, including the National Science Foundation, supported recent work by our group of researchers at University of Alaska Fairbanks and Tsinghua University in Beijing, China, that revealed gaps in the spatial and temporal observation datasets analyzed in the earlier global warming studies. Even though various extrapolation approaches were employed in constructing those datasets, the gaps may have caused a biased estimate of contributions of the Arctic warming amplification to the global warming.

Our collaborative groups reconstructed a new global dataset with improved spatial and temporal representation of the Arctic SATs (Huang et al. 2017). In the reconstruction, we employed Arctic-specific observations (i.e., data from the International Arctic Buoy Programme/Polar Exchange at the Sea Surface for the period of 1979–2004; IABP/POLES; Rigor et al. 2000). We also employed the Data Interpolating Empirical Orthogonal Functions (DINEOF) method for reconstruction of the dataset, which is constrained by the available Arctic observations both spatially and temporally.

By analyzing this newly reconstructed global SAT dataset, we found that the Arctic warming amplification has significantly contributed to the global warming trend, causing a continued and even accelerated increase in global mean SAT, rather than a hiatus or slowdown of the trend as earlier studies suggested. The newly estimated rate of global SAT for 1998–2012 is around 0.112℃/decade, instead of 0.05℃/decade from IPCC AR5 (see Figure 1). In extending the analysis time period to the beginning of the twentieth century, we found a global warming rate at 0.091℃/decade from 1900–2014. A comparison with the rate of 0.075℃/decade and 0.089℃/decade from 1900–1998 and 1900–2012, respectively, suggests that global warming has even accelerated. Further analysis indicates that the Arctic SAT has increased at a rate of 0.755℃/decade during 1998–2012, which is more than six times the global average for the same time period. This indicates that the Arctic amplification has also been further enhanced.

Figure 1. Annual mean SAT anomalies (solid lines) relative to 1979–2004 climatology and their linear trends (dashed lines) over 1998–2012 for (a) the Arctic region (60–90ºN) and (b) the globe. In (a), the black lines are the results using the conventional Kriging interpolation, the red lines are the mean of two reconstructed datasets using the new method of DINEOF, and the blue shading represents the range of the two reconstructed datasets. In (b), the black lines show the results using global SATs from Kar
Figure 1. Annual mean SAT anomalies (solid lines) relative to 1979–2004 climatology and their linear trends (dashed lines) over 1998–2012 for (a) the Arctic region (60–90ºN) and (b) the globe. In (a), the black lines are the results using the conventional Kriging interpolation, the red lines are the mean of two reconstructed datasets using the new method of DINEOF, and the blue shading represents the range of the two reconstructed datasets. In (b), the black lines show the results using global SATs from Karl et al. (2015), and the red lines and blue shading are the same as in (a) but for the globe. The numbers in the panels are the trends corresponding to the two dashed lines in the same colors. Figure courtesy of Huang et al. (2017).

Along with the enhanced Arctic warming amplification, the frequency of occurrence of extreme climate and weather events has ostensibly increased across the Northern Hemisphere mid-latitudes, including adverse cold spells, severe heat waves, destructive floods, and persistent droughts (see Figure 2). The coincidence of the changes between the Arctic and mid-latitudes has stimulated broad interest in their possible linkages in the climate community, the general public, media agencies, and decision-makers. There is also a particular interest in changes of occurrence frequency of future extreme events considering the model-projected continuation and acceleration of Arctic warming and associated sea ice loss into the next several decades (e.g., Zhang and Walsh 2006; Wang and Overland 2012; Stroeve et al. 2012). So, it is hypothesized that Arctic warming amplification can influence the mid-latitudes through a number of pathways, including change in the atmospheric circulation, wavier jet stream, and troposphere-stratosphere coupling1, (e.g., a U.S. Climate Variability and Predictability Program (CLIVAR) white paper by Cohen et al. 2018; as well as review papers by Vihma 2014; Cohen et al. 2014; Overland et al. 2016; and Zhang et al. 2018). However, a strong debate emerged and continues in the studies on the topic due to controversial results, especially regarding inconsistencies of atmospheric circulation response to the changed Arctic sea ice.

Figure 2. Linear trend of winter (December–January–February) seasonal mean surface temperatures over the Northern Hemisphere mid- and high-latitudes from 1997–2017. Arctic warming is amplified but cooling occurs in the North American and Eurasian mid-latitudes. The data used for this analysis is from the Goddard Institute for Space Studies (GISS) Surface Temperature Analysis (GISTEMP; GISTEMP Team, 2018). Figure courtesy of Xiangdong Zhang and Liran Peng.
Figure 2. Linear trend of winter (December–January–February) seasonal mean surface temperatures over the Northern Hemisphere mid- and high-latitudes from 1997–2017. Arctic warming is amplified but cooling occurs in the North American and Eurasian mid-latitudes. The data used for this analysis is from the Goddard Institute for Space Studies (GISS) Surface Temperature Analysis (GISTEMP; GISTEMP Team, 2018). Figure courtesy of Xiangdong Zhang and Liran Peng.

To understand where consensus has been reached and discrepancies continuously exist, we recently participated in a collaborative effort with international groups to synthesize the results from the latest modeling experiments, especially those from the recent fully coupled2 climate modeling experiments (Screen et al. 2018). The fully coupled climate models unanimously show an Arctic warming amplification and Arctic-mid-latitude linkage in response to Arctic sea ice loss, though there are large diversities in atmosphere-only models. The changes in Northern Hemisphere and Arctic sea level pressure or the surface atmospheric circulation/wind pattern resemble the negative phase of the Arctic Rapid change Pattern (ARP; Zhang et al. 2008), which is characterized by an intensified Siberian High and Aleutian Low and plays a driving role in amplification of Arctic warming and acceleration in sea ice decrease.

Figure 3. (a) Atmospheric moisture transport (color arrows) associated with a shifted atmospheric circulation pattern (and (b) corresponding surface air temperature anomalies (red and blue color shadings) and sea level pressures (red and green contours). Changes in the atmospheric circulation enhance moisture transport into the Arctic Ocean, contributing to an increase in surface air temperature (adapted from Zhang et al. 2013). Image courtesy of Xiangdong Zhang.
Figure 3. (a) Atmospheric moisture transport (color arrows) associated with a shifted atmospheric circulation pattern (and (b) corresponding surface air temperature anomalies (red and blue color shadings) and sea level pressures (red and green contours). Changes in the atmospheric circulation enhance moisture transport into the Arctic Ocean, contributing to an increase in surface air temperature (adapted from Zhang et al. 2013). Image courtesy of Xiangdong Zhang.

The improvement in Arctic SAT representation highlighted above (Huang et al. 2017) further indicates the importance to continue and enhance current efforts to improve the spatial and temporal coverage of Arctic observations and reduce observational and data processing biases. Improved observations are not only crucial for monitoring, assessing, and predicting/projecting Arctic regional climate change, but also global climate change, for example the Arctic-mid-latitude linkages.

Although the recent synthesis, based on the existing fully coupled modeling experiments, exhibits consistency in atmospheric circulation response to sea ice loss (Screen et al. 2018), discrepancies still exist, including signs, strength, and patterns of the simulated temperature anomalies across the models. In particular, physical mechanisms causing the spatially shifted atmospheric circulation structure still remains unknown, though this shift has been identified previously (Zhang et al. 2008) and simulated by the latest fully coupled models. The U.S. CLIVAR Arctic–mid-latitude working group has proposed coordinated modeling experiments, the Polar Amplification Model Intercomparison Project (PAMIP; a U.S. CLIVAR White Paper by Cohen et al. 2018; Smith et al. 2018), which would be an excellent venue to address the need for further study.

For more information about this work, please see “Recently amplified arctic warming has contributed to a continual global warming trend” and “Consistency and discrepancy in the atmospheric response to Arctic sea-ice loss across climate models”.

Author Bio

Xiangdong Zhang Xiangdong Zhang is a professor at the International Arctic Research Center and the Department of Atmospheric Sciences, University of Alaska Fairbanks. His research interests include weather and climate extremes in the present and projected future climate, Arctic-lower latitude interactions, Arctic freshwater and sea ice and their roles in global climate, and development of climate model and improvement of physical treatments.

Footnotes

1 Editor's Note: Coupling of the troposphere-stratosphere refers to interaction between the troposphere from the earth's surface to about 3.7–6.2 miles (6–10 km) and the stratosphere just above the troposphere.

2 Editor's Note: Coupling of refers to communication and interchange of information between models.

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