The challenges of forecasting small, but mighty, polar lows
Sailors in Scandinavian countries have told tales about dangerous encounters with small, intense storms since time immemorial. These maritime storms, known as polar lows, are believed to have claimed many small boats in North Atlantic waters [Rasmussen and Turner, 2003]. In a recent case in October 2001, strong winds associated with a polar low that developed near the Norwegian island of Vannøya capsized a boat, causing the death of one of its two crew members.
Polar lows are not only found in the North Atlantic but also are common in the North Pacific and in the Southern Ocean. In Japan, for example, tragedy struck in December 1986, when strong winds from a polar low caused a train crossing the Amarube Bridge to derail and fall from the tracks onto a factory below, killing six people [Yanase et al., 2016].
Forecasting these systems remains challenging because of their relatively small size, rapid formation, and short duration (most last less than 2 days). However, as global warming and receding sea ice make the Arctic more accessible and increase the vulnerability of coastal populations and ecosystems, it will become increasingly important to forecast these dangerous storms accurately. Studying the effects of a warming climate on where these storms form, as well as on their frequency, lifetime, and intensity, is also vital because this work will help determine which regions will be the most affected by polar lows in the future.
Dangerous High-Latitude Storms
Polar lows are a little-known part of the wider family of polar cyclones, which include polar mesoscale cyclones less than 1,000 kilometers in diameter as well as larger, synoptic-scale cyclones. With diameters between 200 and 1,000 kilometers—and most often 300–400 kilometers—polar lows are a subset of mesoscale cyclones.
The relatively small storms differ from other polar mesoscale cyclones in that they develop over the ocean and are especially intense. Polar lows are often associated with severe weather like heavy snow showers and strong winds that can reach hurricane force. Thus, they sometimes lead to poor visibility, large waves, and snow avalanches in mountainous coastal regions. Changes in meteorological conditions can be abrupt, with winds increasing from breeze to gale force in less than 10 minutes, for example. Such severe weather can force affected countries to close roads and airports.
Polar lows can even cause the formation of rare, extreme storm waves known as rogue waves. One such wave, named the Draupner wave, was observed in the North Sea in 1995 and reached a height of 25.6 meters [Cavaleri et al., 2016].
With their high winds and waves, polar lows threaten many communities and ecosystems with extreme weather as well as potential coastal erosion and effects on ocean primary productivity. They also pose significant risks to marine-based industries, such as fishing and onshore and offshore resource extraction. Roughly 25% of the natural gas and 10% of the oil produced worldwide are produced in the Arctic, and despite the strong influence of fossil fuel use on continuing climate change, interest in further extraction of offshore resources in this region is growing.
In addition, as summer sea ice extent decreases because of climate change, shipping seasons will become longer, and new shipping routes will open up, making the Arctic more accessible and potentially increasing the likelihood of storm-related accidents. The possibility of shipping accidents or other disasters causing oil spills in the Arctic is particularly concerning because the lack of infrastructure in this remote region means that it could take a long time to respond to spills. With so many concerns and at-risk communities, there is a pressing need to improve forecasting of polar lows and other extreme Arctic weather to reduce risk.
Where Do Polar Lows Form?
Polar lows are predominantly a cold season phenomenon, developing near the sea ice edge and the coasts of snow-covered continents during cold air outbreaks, when very cold air over the ice or landmass flows out over the relatively warm ocean.
Southern Hemisphere polar lows, which have received less attention from researchers, develop mainly near the Antarctic sea ice edge, far from human settlements, and they tend to be less intense than their northern counterparts. Northern Hemisphere polar lows develop above about 40°N, thus affecting several Arctic countries. They are more frequent in the North Atlantic than in the North Pacific [Stoll et al., 2018], mainly forming in the Nordic Seas, the Denmark Strait, the Labrador Sea, and Hudson Bay. Every year, some of the polar lows that develop in the Nordic Seas make landfall on the coast of Norway, affecting its coastal population.
In the North Pacific, polar lows primarily form over the Sea of Okhotsk, the Sea of Japan, the Bering Sea, and the Gulf of Alaska. Densely populated areas of Japan are especially vulnerable when marine cold air outbreaks in the Sea of Japan lead to polar lows.
An Elusive Phenomenon
The origins and characteristics of polar lows largely remained a mystery until the beginning of the satellite era in the 1960s. With resolution in atmospheric models being too coarse to capture the storms until relatively recently, satellite infrared images have been key to identifying polar lows. These images have shown that some polar lows are shaped like commas, similar to midlatitude synoptic-scale (i.e., extratropical) cyclones, whereas others are spiraliform like hurricanes (i.e., tropical cyclones; Figure 1).
How polar lows develop was long debated among researchers. Some argued that polar lows resembled small versions of synoptic-scale cyclones, which develop because of baroclinic instabilities arising from strong horizontal temperature gradients in the atmosphere. Others claimed they were akin to hurricanes, which intensify as a result of convection and are typically about 500 kilometers in diameter. Today, the research community agrees that development mechanisms of polar lows are complex and include some processes involved in the formation of synoptic-scale cyclones and some involved in hurricane formation. Among these processes are transfers of sensible heat from the ocean surface to the atmosphere through the effects of turbulent air motion, which play roles in the formation and intensification of polar lows.
In general, weather forecasting in polar regions remains challenging because atmospheric models still struggle to correctly represent certain key processes, such as air-sea interactions, in these regions. Because of their small size and short lifetimes, polar lows are particularly hard to forecast compared with larger polar cyclones. Compounding the challenge is the fact that these systems develop over the ocean at high latitudes, where conventional observations (e.g., from surface weather stations, buoys, and aircraft) are scarce.
With the advent of high-resolution nonhydrostatic atmospheric models with grid meshes of less than about 10 kilometers (which started to be implemented for weather forecasting in the 2000s), however, polar low forecasts have improved notably. Unlike models that assume hydrostatic conditions, nonhydrostatic models do not assume balance between the vertical pressure gradient force, which results from the decrease of atmospheric pressure with altitude, and the force of gravity—a balance that does not occur in intense small-scale systems. Compared to coarser models, high-resolution models better represent processes that occur near the surface (e.g., the influence of topography on wind) as well as convection, which play important roles in polar low development. Moreover, high-resolution models can better resolve the structure of polar lows (e.g., strong wind gradients).
Nevertheless, model improvements are still needed to forecast the trajectories and intensities of polar lows accurately [Moreno-Ibáñez et al., 2021]. For instance, the parameterization of turbulence is based on approximations that are not valid at the kilometer scale. In addition, more conventional observations of atmospheric variables at high latitudes, such as winds and temperatures at different levels of the atmosphere, are required to improve the initial conditions fed into the models.
Several major scientific questions about these storms also remain unanswered: What are the best objective criteria (e.g., size, intensity, lifetime) for identifying and tracking polar lows using storm tracking algorithms? What is the main trigger for polar low development? And, most intriguing, what is the role of polar lows in the climate system?
Actors in the Climate System
Little is known about how polar lows contribute to Earth’s climate system. A few studies have analyzed the effects of polar lows on the ocean, but results so far are inconclusive. On the one hand, the large sensible heat fluxes—which can reach more than 1,000 watts per square meter—from the ocean surface to the atmosphere that favor the development of these cyclones lead to cooling of the ocean surface [e.g., Føre and Nordeng, 2012]. On the other hand, the strong winds of polar lows induce upper-ocean mixing, which can warm the ocean surface in regions where sea surface temperatures are colder than underlying waters [Wu, 2021].
The overall warming or cooling effect of polar lows on the ocean surface may influence the formation rate of deep water, a major component of Earth’s global ocean circulatory system. In one study, researchers found that polar mesoscale cyclones increase ocean convection and stretch convection to deeper depths [Condron and Renfrew, 2013]. However, this study used only a coupled ocean–sea ice model, relying on a parameterization to represent the effects (e.g., winds) of polar mesoscale cyclones in the ocean-ice model rather than explicitly resolving the cyclones. Therefore, the interactions between the ocean and the atmosphere, which are relevant for the deepwater formation, were not represented. This tantalizing, but hardly definitive, result highlights the need for further study of polar lows’ interaction with the ocean and climate.
Polar Lows in a Warmer Climate
The continuing decreases in Arctic sea ice extent and snow cover on land projected to occur with global warming, as well as increases in sea surface temperatures, will undoubtedly affect the climatology of polar lows. In the North Atlantic, polar lows have been projected to decrease in frequency, and the regions where they develop are expected to shift northward as sea ice retreats [Romero and Emanuel, 2017]. This shift means that newly opened Arctic shipping routes will not be spared from these storms.
We do not know yet what will happen in other regions because research investigating climate change impacts on the frequency, lifetime, intensity, and genesis areas of polar lows is still at an incipient stage. The few studies conducted so far have used dynamical or statistical downscaling methods to produce high-resolution information about the relatively small, localized phenomenon of polar lows from low-resolution data (e.g., from global climate models)—approaches that require far less computing resources than performing global, high-resolution climate simulations.
Unfortunately, current coarse-grained global climate models cannot resolve small-scale phenomena like polar lows. The typical resolution of the models included in the Coupled Model Intercomparison Project Phase 5 (CMIP5), endorsed by the World Climate Research Programme in 2008, was 150 kilometers for the atmosphere and 1° (i.e., 111 kilometers at the equator) for the ocean. As part of CMIP6, a High Resolution Model Intercomparison Project has been developed [Haarsma et al., 2016], including models with grid meshes ranging from 25 to 50 kilometers for the atmosphere and 10 to 25 kilometers for the ocean. These resolutions are fine enough to enable study of some mesoscale eddies in the atmosphere and the ocean [Hewitt et al., 2020], and important weather phenomena, such as tropical cyclones, can also be simulated [e.g., Roberts et al., 2020].
Nevertheless, atmospheric models at this resolution are still too coarse to resolve most polar lows. Moreover, the resolution of these ocean models is not high enough to resolve mesoscale eddies that develop poleward of about 50° latitude [Hewitt et al., 2020], so some mesoscale air-sea interactions cannot be adequately represented. Mesoscale air-sea interactions also affect sea ice formation, which influences where polar lows form. The recent Intergovernmental Panel on Climate Change report indicates that there is low confidence in projections of future regional evolution of sea ice from CMIP6 models.
Interdisciplinary Research Needed
Considering the interactions among the atmosphere, ocean, and sea ice involved in polar low development, the importance of interdisciplinary collaboration in polar low research cannot be overstated. Close cooperation among atmospheric scientists, oceanographers, and sea ice scientists is needed to enable a complete understanding of polar lows and their role in the climate system.
Improving forecasts and longer-term projections of polar lows requires coupling of high-resolution atmosphere, ocean, and sea ice models. High-resolution coupled model forecasts of polar lows are already practicable. With continuing increases in computational capabilities, it may become feasible to use coupled high-resolution regional climate models and variable-resolution global climate models to better study how polar low activity may change in a warming climate and the impact of polar lows on ocean circulation. Such interdisciplinary research will also help us better anticipate and avoid damaging effects of these small, but mighty, polar storms on people and productivity.