Our previous study (Zhang et al., 2004) has indicated a poleward shift of extratropical storm tracks over the Northern Hemisphere and an intensification of storm activities in the Arctic Ocean. Along with these long-term changes, intense and long-lasting storms have more frequently occurred over the Arctic Ocean during the most recent decade.  Associated with these storms, extreme climate and weather events have been observed, such as the record lows of sea ice extent in summer 2012 and 2016 and the record highs of warm temperature in winter 2015/16 and 2017/18. However, it has remained unclear what drive occurrences of these intense and long-lasting storms and how these storms impact Arctic sea ice and contribute to the rapidly changing Arctic climate system.

The objective of this project is to identify and quantify impacts of intensified storm activity on the rapidly changing Arctic climate system and declining Arctic sea ice. To achieve this objective, we have performed the studies through an integrative approach across atmosphere, sea ice, and ocean. Major accomplishments are summarized below.

Intellectual Merit: We first have examined storm activities over each sub-regions of the Arctic Ocean in each season based on our previous study. Our results indicate that the regionally averaged storm intensity has obviously increased over the North Atlantic Arctic Ocean, in particular over the Eurasian shelf seas, including the Kara, Laptev, and East Siberian seas. The intensity mainly demonstrates inter-annual fluctuations over the North Pacific Arctic Ocean.

We then investigated mechanisms and processes responsible for occurrences of intense and long-lasting storms through conducting high-resolution modeling studies on selected cases. We found that these storms are predominantly characterized by barotropic structures from surface to the lower stratosphere over the majority of their lifetimes, which is significantly different from conventional midlatitude extratropical storms. The baroclinic instability, which is indicated by horizontal thermal contrast and a fundamental mechanism driving midlatitude extratropical storms, only plays an important role at the initial stage of the storm genesis and development. The downward intrusion of synoptic-scale, symmetric lower stratosphere vortex decisively drives intensification and persistence of the storms over an extended long time period. This improves understanding of Arctic storms and advances classic theory of extratropical storms. A schematic diagram of this type of Arctic storms is illustrated in Figure 1.

Extratropical storms are a major driver for atmospheric transient heat and moisture transport. We have therefore examined changes in large-scale and storm-induced heat and moisture transport and their roles in surface energy budgets. The results indicate that negative polarization of the Arctic Rapid change Pattern (ARP) enhanced poleward heat and moisture transport into the Arctic Ocean from the North Atlantic. The ARP is characterized by a strengthened and northwestward shifted Siberian high and a deepened Aleutian low, and has demonstrated a negative trend since the late 1990s. The ARP-driven enhancement of poleward heat and moisture transport has obviously contributed to the observed amplification of Arctic warming (Figure 2). Through close examination on single storms, we found that the storm Frank, which originated from the northeast coast of U.S. and propagated poleward to the Nordic Seas in December 2015, substantially enhanced poleward atmospheric moisture transport into the Arctic Ocean, leading to an increase in cloudiness and downward longwave radiation. As a consequence, an extreme warming event occurred with an increase in surface air temperature by about 10 degree within a few days.

Further, we investigated how storms impacts sea ice mass balance. We analyzed ensemble simulation results from an Arctic regional coupled atmosphere-sea ice-ocean model. We found that, on the North Atlantic Arctic, intense storms tend to occur over the Barents and Kara seas. Associated with these intense storms, both sea ice extent and thickness decrease. We have also participated in field observations onboard ice-breaking R/V Araon in summer 2016, which exactly captured an intense storm. Through comprehensive diagnostic analyses of in-site and satellite observational data, we found that the intense storm forces an increase in upwelling to transport more warm Pacific water to the mixed layer through Ekman pumping mechanism and enhances upper ocean mixing. As a result, there is an increase in ocean-to-sea ice bottom heat flux, accelerating sea ice melt.

Broader impacts: We have published more than thirty papers and gave presentations/lectures (including invited ones) at various workshops, conferences, summer schools, U.S. and international institutions, and webinars to disseminate our project results. Four Ph.D. students have participated in the project. One has graduated and awarded a Ph.D. degree. The other students will defend and graduate in fall 2018 and in 2019. The project PI and Co-PI have incorporated the research results into the courses they teach, including Synoptic Weather Analysis and Forecast and Advanced Synoptic Meteorology. Both PI and Co-PI have also been interviewed by various media agencies to inform the scientific community and the general public of the research results.

Last Modified: 06/11/2018
Modified by: Xiangdong Zhang