Future projections of cyclone activity in the Arctic for the 21st century from regional climate models (Arctic-CORDEX)

Highlights

• Changes in the characteristics of cyclone activity in the Arctic are analyzed based on simulations with RCMs (Arctic-CORDEX)

• Most of RCMs show an increase of cyclone frequency in winter and a decrease in summer to the end of the 21st century

• Different lateral BCs from the GCMs have larger effects on the simulated RCM’s than setup and/or physics

Abstract

Changes in the characteristics of cyclone activity (frequency, depth and size) in the Arctic are analyzed based on simulations with state-of-the-art regional climate models (RCMs) from the Arctic-CORDEX initiative and global climate models (GCMs) from CMIP5 under the Representative Concentration Pathway (RCP) 8.5 scenario. Most of RCMs show an increase of cyclone frequency in winter (DJF) and a decrease in summer (JJA) to the end of the 21st century. However, in one half of the RCMs, cyclones become weaker and substantially smaller in winter and deeper and larger in summer. RCMs as well as GCMs show an increase of cyclone frequency over the Baffin Bay, Barents Sea, north of Greenland, Canadian Archipelago, and a decrease over the Nordic Seas, Kara and Beaufort Seas and over the sub-arctic continental regions in winter. In summer, the models simulate an increase of cyclone frequency over the Central Arctic and Greenland Sea and a decrease over the Norwegian and Kara Seas by the end of the 21st century. The decrease is also found over the high-latitude continental areas, in particular, over east Siberia and Alaska. The sensitivity of the RCMs' projections to the boundary conditions and model physics is estimated. In general, different lateral boundary conditions from the GCMs have larger effects on the simulated RCM projections than the differences in RCMs' setup and/or physics.

Introduction

The Arctic warming in recent decades has been proceeding at least two times faster than the global temperature increase and is accompanied by an unprecedented reduction of sea ice extent, and these changes affect large scale atmospheric circulation and weather patterns in high- and mid-latitudes (e.g. Vihma, 2014; Semenov and Latif, 2015). The Arctic Ocean has become more accessible for marine shipping along the Northern Sea Route (Khon et al., 2017), extraction of oil and natural gas resources and other activities such as tourism or fishing. However, all these activities are affected by weather conditions, in particular, cyclone activity.

Cyclones play an important role in the coupled dynamics of the Arctic climate system, in particular, they are contributing to the meridional transport of atmospheric heat and moisture from mid-latitudes, thereby changing wind, temperature, precipitation and sea ice distribution in the Arctic (e.g. Alexeev et al., 2017). The influence of a changing climate on cyclone activity characteristics is complicated as the response is dependent on many dynamical and thermodynamical processes (e.g. Mokhov et al., 1992; Inoue et al., 2012; Akperov and Mokhov, 2013). Therefore, understanding changes in storminess in the Arctic region is important to properly manage the risks associated with these events in a changing climate system.

One of the powerful tools to assess the impacts of climate change on cyclone activity are global climate models (GCMs), which are widely used to analyze midlatitude cyclones (e.g. Ulbrich et al., 2013). However, the results for the response of the Arctic cyclones to climate change from GCM studies show some disagreement. One of the reasons may be related to inter-model variability of cyclone activity characteristics across the GCMs in midlatitudes (Zappa et al., 2013).

Using an ensemble of CMIP3 models under SRES-A1B scenario, Lang and Waugh (2011) found a significant decrease in the number of cyclones in the Norwegian Sea and an increase over the Barents Sea, Baffin Bay, Davis Strait, and near the southern tip of Greenland in winter, and no significant changes in summer by 2100. They also noted a large decrease in the number of intense winter cyclones over the Arctic Ocean. Changes of cyclone activity in the 21st century from simulations with the ECHAM5/MPI–OM GCM under SRES-A1B scenario were analyzed by Ulbrich et al. (2013) using different methods of cyclone identification. They also found a decrease of cyclone numbers in the Barents and Greenland Seas for the winter in the second half of the 21st century. Orsolini and Sorteberg (2009) found an increase in the number of cyclones entering the Arctic in the summer as well as for the mean intensity by the end of the 21st century using BCM v2.0 GCM under SRES-A1B and SRES-A2 scenarios. They also noted that the cyclone increase is associated with an increase in zonal wind and meridional temperature gradient at high latitudes in summer, due to the slower Arctic Ocean warming compared to the surrounding land. Using an ensemble of CMIP3 as well as CMIP5 simulations, Nishii et al. (2015) found an increase in Arctic summer storminess across these ensembles. They found, in agreement with Orsolini and Sorteberg (2009), that the magnitude of the response of cyclone activity was strongly correlated with the magnitude of change in the zonal mean wind and the surface air temperature gradient along the Eurasian coastline.

Using CMIP5 models under various Radiative Concentration Pathway (RCP) scenarios, Colle et al. (2013) found a reduction of cyclone number over the western Atlantic and an increase near Nova Scotia in southeast Canada in cold season. Zappa et al. (2013) noted that the number and the wind intensity of cyclones decreases in the Norwegian Sea in the cold season and increases near the southern tip of Greenland in the warm season. Harvey et al. (2015) also found reductions in Arctic winter storminess at the end of the 21st century. Day and Hodges (2018) investigated the response of Arctic cyclones to climate change in a large initial value ensemble of future climate projections with the CESM1-CAM5 (CESM-LE). They found a significant reduction in cyclone frequency in winter and insignificant changes in summer. It has been also noted a reduction of cyclone intensity across the Arctic basin in winter, but with contrasting increase in summer intensity within the Arctic Ocean cyclone maximum. The study also showed a significant reduction in winter cyclogenesis events within the Greenland–Iceland–Norwegian Sea region. They emphasized that the seasonal response of cyclone intensity and cyclogenesis appears to be closely linked to changes in surface temperature gradients in the high latitudes, with Arctic poleward temperature gradients increasing in summer, but decreasing in winter. Crawford and Serreze (2017) investigated the relationship between the Arctic frontal zone and summer Arctic cyclone activity for the RCP8.5 scenario using the same CESM-LE ensemble. They showed a decrease of cyclones in the Barents, Kara, and Laptev Seas and an increase along the eastern side of Greenland and Chukchi and Beaufort Seas in summer. Detailed information about the changes of cyclone frequency as a function of region, seasons, climate models, scenarios is presented in Supplementary Table 1.

In addition to using different models and scenarios, each of these studies uses different periods of analysis, and storm intensity/activity measures, all of which may influence the results. Also, the relative coarse resolution of the GCMs and deficits in the representation of physical processes in the Arctic cause uncertainties to the projections of future changes of cyclone activity. The interest in better representing the climate variability and change at regional scales has driven the development of regional climate models (RCMs). RCMs run on limited area domains thereby allowing increased spatial resolution, and thus enabling a better representation of mesoscale atmospheric processes, which are important for cyclone activity. The international CORDEX (Coordinated Regional Climate Downscaling Experiment) (Giorgi et al., 2009) has provided multi-model RCM simulations at high spatial resolution over different regions in the world. As a part of the CORDEX framework, the Arctic-CORDEX initiative (http://www.climate-cryosphere.org/activities/targeted/polar-cordex/arctic) provides RCM projections for the Arctic at ca. 50 km (ARC-44) resolution.

Several studies demonstrated the usefulness of RCMs for studying extratropical cyclones (Côté et al., 2015) and Arctic cyclones (Shkolnik and Efimov, 2013; Akperov et al., 2015; Akperov et al., 2018). Recently, Akperov et al. (2018) showed that the state-of-the-art RCMs from Arctic-CORDEX are able to simulate realistically the present-day cyclone activity characteristics in the Arctic compared to reanalysis data.

The general aim of this paper is to analyze possible future changes of cyclone characteristics (frequency, depth, and size) over the Arctic region using a multi-model ensemble of RCM simulations (Arctic-CORDEX) for the 21st century. We further address how different GCM as lateral boundary conditions affect the RCMs results.