• Characteristics of Arctic storms and their influence on surface climate

      Yang, Yang; Zhang, Xiangdong; Danielson, Seth; Fochesatto, Javier; Hock, Regine (2020-05)
      Impacts of intense synoptic storms on Chukchi Sea and Beaufort Sea surface environmental conditions are examined, focusing on storms moving into the regions with northward and eastward pathways. Both storms alter the prevailing northeasterly wind to southerly and southwesterly wind. The storms moving from the East Siberian Sea that follow a west to east route are most active in summer and have the longest duration. Increasing southwesterly wind plays a key role in the decline of thin sea ice within the warm season. Storms traveling from the relatively warm Pacific Ocean into the Arctic over the Bering Strait are more common in winter, and are typically more intense than the summer storms that propagate west to east. Downward longwave radiation increases considerably with the passage of intense winter storms over the ice-covered Chukchi Sea; the sea ice concentration decreases accordingly. The impact of different sea ice conditions on Arctic synoptic storm systems in autumn are investigated in the North Pacific and Atlantic sectors, based on the ten ensembles of hindcast simulations from coupled regional climate model HIRHAM-NAOSIM. In both the Pacific and Atlantic sectors, greater transfers of heat and moisture fluxes from the open ocean to the atmosphere occur in low sea ice years than in high sea ice years. The largest increase of upward heat fluxes and baroclinicity occurs over the Laptev, southern Chukchi and Beaufort Seas in the Pacific sector, and over the southern Greenland and Barents Seas in the Atlantic sector. Enhanced baroclinity plays a dominant role in the development of intense storm systems. Therefore, storms in reduced sea ice years are more intense than in enhanced sea ice years in both Atlantic and Pacific sectors. The storm count also increases over locations exhibiting high baroclinicity. Sea ice volume anomalies are significantly correlated with synoptic storm counts based on maximum covariance analysis (MCA) leading modes of covariance between sea ice volume and storm count over Pacific and Atlantic sectors are identified respectively. The results are consistent with our findings in the composite analysis. In the Pacific sector, the first pattern of the MCA demonstrates that increasing storm counts over the Laptev Sea corresponds to decreasing sea ice volume over that region. In the Atlantic sector, the decrease of sea ice volume is highly correlated with decreasing storm counts over the northern Greenland Sea. Connection of storm activity over the North Pacific Ocean with the tropical stratosphere quasi-biennial oscillation (QBO) is investigated following a composite analysis of intense storm vertical cross sections. An observed stronger potential vorticity anomaly of intense storms is associated with the QBO west phase and results in enhanced warm air advection near the surface. A warm core structure forms over the east or northeast direction relative to the surface low center, which bows the isentropes downward. Upward motion following the isentropes reduces the surface low pressure, which in turn, facilitate storms to keep propagating in east and northeast directions. Under the QBO east phase, a weak surface warm core forms to the southeast of the storm center, resulting in a slow development of the storms, and these storms tend to move southeastward.
    • Emergent impacts of rapidly changing climate extremes in Alaska

      Lader, Rick T.; Walsh, John E.; Bhatt, Uma S.; Rupp, T. S.; Zhang, Xiangdong (2018-08)
      The frequency and intensity of certain extreme weather events in Alaska are increasing, largely due to climate warming from greenhouse gas emissions. Future projections indicate that these trends will continue, potentially leading to billions of dollars in climate-related damages this century. Expected damages arise from increases in extreme precipitation, severe wildfire, altered ocean chemistry, land subsidence from permafrost thaw, and coastal erosion. This dissertation applies new downscaled reanalysis and climate model simulations from the fifth phase of the Coupled Model Intercomparison Project to enhance current understanding of climate extremes in Alaska. Model output is analyzed for a historical period (1981-2010) and three projected periods (2011-2040, 2041-2070, 2071-2100) using representative concentration pathway 8.5. Unprecedented heat and precipitation are expected to occur when compared to the historical period. Maximum 1-day and consecutive 5-day precipitation amounts are expected to increase by 53% and 50%, respectively, and the number of summer days per year (Tmax > 25°C) increases from a statewide average of 1.5 from 1981-2010 to 29.7 for 2071-2100. Major alterations to the landscape of Alaska are anticipated due to a decreasing frequency of freezing temperatures. Growing season length extends by 48-87 days by 2071-2100 with the largest changes in northern Alaska. In contrast, projections indicate a reduced snow season length statewide and many locations in southwest Alaska no longer have continuous winter snow cover. Changes to these metrics indicate that a climate-warming signal emerges from the historical inter-annual variability, meaning that future distributions are entirely outside of those previously observed. The largest changes to extremes may be avoided by following a lower emissions trajectory, which would reduce the impacts and associated costs to maintain infrastructure and human health.
    • Impacts of storm on sea ice: from case study to climate scale analysis

      Peng, Liran; Zhang, Xiangdong; Collins, Richard; Fochesatto, Javier; Polyakov, Igor (2019-12)
      Recent studies have shown that intense and long-lasting storms potentially facilitate sea ice melting. Under the background of extratropical storm tracks poleward shift, significant reductions of Arctic sea ice coverage, and thinning of sea ice thickness over the last several decades, a better understanding on how storms impact sea ice mass balance is obviously of great importance to better predict future sea ice and the Arctic climate changes. This thesis presents a multi-scale study on how storms impact sea ice, consisting of three different parts of the effort. In the first part, we examined the impacts of the 2016 summer intense storm on sea ice changes over the Chukchi Sea using ship-borne observations. The results show that the intense storm can accelerate ice melt through enhanced upper-ocean mixing and upward heat transport. The satellite-observed long-term sea ice variations potentially can be impacted by many factors. In the second part, we first explore key physical processes controlling sea ice changes under no-storm condition. We examined and compared results from 25 sensitivity experiments using the NCAR's Community Earth System Model (CESM). We found that sea ice volume, velocity, and thickness are highly sensitive to perturbed air-ice momentum flux and sea ice strength. Increased sea ice strength or decreased air-ice momentum flux causes counter-clockwise rotation of the transpolar drift, resulting in an increase in sea ice export through Fram Strait and therefore reduction of the pan-Arctic sea ice thickness. Following four tracers released over the Arctic, we found the sea ice thickness distributions following those tracers are broader over the western Arctic and becomes narrower over the eastern Arctic. Additionally, thermodynamic processes are more dominant controlling sea ice thickness variations, especially over periphery seas. Over the eastern Arctic, dynamic processes play a more important role in controlling sea ice thickness variation. Previous studies show that thin ice responds to external perturbations much faster than the thick ice. Therefore, the impacts of storms on sea ice are expected to be different compared with the western/eastern Arctic and the entral/periphery seas. In the third part, we conduct a new composite analysis to investigate the storm impact on sea ice over seven regions for all storms spanning from 1979 to 2018. We focused on sea ice and storm changes over seven regions and found storms tend to have different short-term (two days before and after storm passage), mid-term (one-two weeks after storm passage), and long-term (from 1979 to 2018) impact on sea ice area over those regions. Over periphery seas (Chukchi, East Siberian, Laptev, Kara, and Barents Seas), storms lead to a short-term sea ice area decrease below the climatology, and a mid-term sea ice increase above the climatology. This behavior causes sea ice area to have a small correlation with the storm counts from 1979 to 2018, which suggest that storms have a limited long-term impact on sea ice area over periphery seas. Both the short term and mid-term storm impacts on sea ice area are confined within a 400 km radius circle with maximum impacts shown within a 200 km radius circle. Storms over the western Arctic (Chukchi, East Siberian, and Laptev Seas) have a stronger short-term and mid-term impact on sea ice area compared with the Eastern Arctic (Barents and Kara Seas). Storms over both Atlantic and Pacific entrance regions have a small impact on sea ice area, and storms over the Norwegian, Iceland, and Greenland Seas have the smallest impact on the sea ice area. Compared to the periphery seas, storms tend to have a stronger long-term impact on sea ice area over the central Arctic. The correlation coefficients between the storm count and sea ice area exceed 0.75.