The winter of 2013/14 in the northern hemisphere

Large parts of Japan were hit by catastrophic snowfall in February 2014, while the US suffered record low temperatures in December 2013. Meteorologically, these two winter events are probably connected.


Mark Bove and Eberhard Faust

The Arctic is dominated by a large, quasi-stable area of high pressure caused by sinking cold air. Surrounding this area of high pressure is the polar front, a region where cold, dry Arctic air interacts with warmer, moister air being advected poleward. The temperature and moisture gradients along the polar front cause extratropical storms to form along the boundary, and also give rise to the polar night jet stream that circles the North Pole, moving from west to east.

The strength of the polar night jet, as well as storms along the polar front, depend on the magnitude of the temperature and moisture gradients in the region. These gradients are typically at their strongest in the autumn, causing the polar night jet to intensify and impart additional vorticity, or spin, into the upper troposphere and lower stratosphere. This helps to form and contain a uniform mass of cold air over the North Pole, known as the polar vortex. The vortex acts to keep cold air in place at the pole.

The stronger the vortex, the more likely Arctic air will remain there. But as winter begins, the gradients along the polar front weaken, and the polar vortex cools and stops growing. As winter progresses towards spring, sunlight returns to the Arctic. Some of the light is absorbed by ozone in the stratosphere, warming up the upper atmosphere and weakening the polar vortex. The resulting destabilisation of the polar vortex allows for pieces of the Arctic air to move southward, resulting in cold outbreaks in the mid-latitudes.

Other outside influences can also cause weakening of the polar vortex during the winter season.One such phenomenon is known as Sudden Stratospheric Warming (SSW). An SSW event occasionally occurs when a stationary area of high pressure develops, forcing cyclonic storms to move around it. These blocking patterns create persistent atmospheric flows that can produce large amplitude planetary-scale waves in the troposphere, particularly when moving over mountainous terrain. The energy and momentum of these waves propagates into the polar stratosphere, and act to destabilise the polar jet via a warming of the stratosphere. The stratospheric warming disrupts or destroys the polar vortex, which allows for pieces of the polar air to be pushed southwards.

In late 2013, a blocking pattern developed over the northeastern Pacific Ocean and persisted throughout the entire 2014 winter season. High pressure over this region generated high amplitude waves that destabilized the polar front jet, allowing for pieces of the polar vortex to stream southward across eastern North America. The same ridge caused Arctic air to flow into eastern Asia, resulting in severe winter storms in Japan, and produced anomalously warm and dry weather in western North America, leading to worsening drought conditions in California. At the same time, Europe experienced an unusually mild winter season.

The winter in Japan Snow is not an unknown commodity in many parts of Japan, but the amount of snow seen in February in the heavily insured regions in and around Kanto was exceptional. The heavy snowfalls occurred between 6 and 9 February and 13 and 16 February and came from a similar weather pattern. Initially, a trough-like loop of the high-altitude air flow moved across eastern China and, in the days after 6 February, to the northeast across Japan. This trough in the high-altitude air flow was connected to a low pressure area in the lower atmosphere, which accompanied the displacement of the trough off Japan’s eastern coast to the northeast. At the southern to eastern side of this low pressure area, warm, moist air from the Pacific was drawn towards the north at the same time that Japan lay under the Arctic air on the rear side of the low. Where the warm air met the cold air, heavy precipitation developed that fell as snow over Japan.

On 8 February, Tokyo was already buried under 27 cm of snow – a height not measured since 12 March 1969 – when a second heavy snowfall from 13 to 16 February followed. The second weather pattern developed in much the same way as the previous snow event. The cold front’s extensive snowfall reached southern Honshu on 13 February and the Tokyo region the next day.

Snowfall was especially heavy on the east coast and in the Honshu mountains. On 16 February, there was 250 cm of snow in Tsunan and 43 cm in Fukushima. In Kofu (Yamanashi) the snow level reached 114 cm – the most snow ever recorded since recordkeeping began in 1894. The large volumes of snow brought traffic to a standstill over large areas of the country and cut off towns and villages. At least 16 people lost their lives; several hundred were injured, many of them in traffic accidents. Many parts of the area affected by the snowfall were without electricity. On 15 February, a major airline cancelled 350 flights; leading car production companies temporarily closed their factories.

Record losses for Japanese insurers Private individuals were particularly affected by the collapse of their carports under the snow’s weight. Commercially available carports in the particularly affected prefectures are normally designed to withstand a maximum snow load of 25 cm. Not only were parked cars and other items stored in the ports damaged, but snow falling from roofs and traffic accidents contributed to car insurance losses of about 22 billion yen (US$ 215m). Altogether, almost 66,000 losses were reported.

The snowfall also resulted in significant damage to roofs and gutters of residential property. Some 212,000 claims for insured damage were filed, with losses totalling 232 billion yen (US$ 2.26bn). If waterproof membranes under roof tiles had been damaged, the entire roof structure often had to be rebuilt. On top of this, the hefty demand for skilled tradesman significantly added to repair costs. Additionally, some factory buildings, warehouses, schools and gyms could not support the massive snowfall and collapsed either partly or entirely. In some areas, the snow’s weight on the buildings was twice as heavy as the heaviest weight recorded from previous years. Production materials, stored goods and inventory were also destroyed when roofs collapsed.

Estimates for insured damage are over 320 billion yen (US$ 3.1bn), making these storms one of the most expensive for catastrophe damage in the Japanese insurance industry’s history. This is typical “mass damage” resulting from an extensive accumulation of small and medium-sized losses averaging US$ 3,000 to 5,000 – similar to what one sees from a winter windstorm in Europe. In comparison, there were fewer large-scale losses. Industrial policies were hardly affected.

The winter in North America

Some parts of North America also battled bitterly cold temperatures, breaking records – some more than a century old – in a dozen states. A series of frigid air masses surging south from the Arctic brought record cold temperatures to the eastern United States and Canada. The cold air also helped trigger the formation of several severe winter precipitation events, some as far south as the Florida Panhandle. Although cold air outbreaks are not uncommon during the North American winter, the unusually long duration and intensity of 2014’s cold weather has spurred questions regarding the impact of anthropogenic climate change on winters in the region.

Last winter produced two major Arctic air outbreaks over North America. The first occurred in early December 2013, causing unseasonably cold temperatures across the region, but quickly dissipated. The second outbreak, caused by an episode of SSW, started on 2 January and ushered in the coldest air of the season. Record low temperature observations, some dating back to the 19th century, were set in dozens of cities over the next week, including Chicago, Detroit, New York City, Cleveland, and Atlanta. Although temperatures moderated somewhat by mid-January, the persistent ridge over the northeast Pacific kept forcing Arctic air southward over eastern North America for the next three months, resulting in one of the coldest winter seasons in decades.

Twelve states had three-month temperature averages that ranked among the ten coldest on record. New records were set for the lowest monthly average temperatures across many midwestern cities in February, and across New England and the mid-Atlantic regions in March. The prolonged outbreak also caused one of the largest freeze-over events of the Great Lakes in decades. Large icebergs remained in Lake Superior until late May, and their slow melt prolonged unseasonably cool temperatures near the lakes through the summer of 2014. In all, the combination of extreme cold temperatures and several winter storm events caused an estimated US$ 2.3bn in insured losses, with economic losses of around US$ 4bn. The insured loss total for 2014 is the fifth highest total on record (adjusted for inflation), and almost double that of the 2009 - 2013 average, but still not extraordinary in comparison with other winter seasons over the past decade. The majority of the insured losses, US$ 1.7bn, stemmed from the January Arctic air outbreak. The majority of claims were due to frozen pipes bursting, resulting in water damage to buildings and personal property. The remainder of insured losses across the winter season were primarily associated with roof damage due to the weight of snow and ice, freezing rain events downing trees and power lines, ice damming on roofs, and automobile accidents due to slippery driving conditions.


It is currently unclear whether anthropogenic climate change played any role in the severity and duration of the 2014 winter season across eastern North America. However, new research investigating links between climate change and the northern hemisphere winter give some indications that the Arctic polar vortex is more likely to become destabilised in a warmer world, potentially leading to more mid-latitude cold outbreaks. There are several potential mechanisms being researched that could link our changing climate to more frequent cold air outbreaks.

This first is that the Arctic has warmed dramatically over the past 100 years, and is now 3.5°C warmer than in the late 19th century. Since the development of a stable polar vortex relies in part on strong temperature gradients, the warming of the Arctic means that the gradient between polar air and warmer air to its south is reduced. This gradient reduction might lead to weaker polar vortexes in the future, which would increase the potential for pieces of the vortex to break off and enter the mid-latitudes. The second potential mechanism is the dramatic drop in sea ice over the Arctic during early autumn. Recent modelling studies have shown that the dramatic loss of sea ice in the Arctic, particularly north of Scandinavia and Russia, may intensify atmospheric wave patterns that can weaken the polar vortex through SSW.

Other research gives hints that persistent areas of high pressure in the North Pacific – like 2014’s “Ridiculously Resilient Ridge”, which lasted through most of the year and is linked to both the cold weather across eastern North America and the drought in California – may form more often in the current climate regime than in one without the anthropogenic greenhouse effect, giving a third mechanism for more Arctic air outbreaks in a changed climate. It should be noted that all research noted here is preliminary, and it will take years of further research to determine what, if any, role anthropogenic climate change has on the frequency and severity of Arctic air outbreaks. But given the rapid changes being observed in the Arctic, it is likely that anthropogenic climate change will play at least some role in what to expect from winter seasons in the future.

Underwriting aspects

The snow damage in Japan clearly showed what a large effect a relatively small geographic shift in a weather pattern can have, for a country that is no stranger to weather phenonema like heavy snowfalls, when it hits an unprepared region. It remains clear that insurance purchasers need to buy coverage for precisely such events, and that it is the responsibility of the insurance industry to deal with the losses in a manner that is as efficient and uncomplicated as possible for the policyholders. Snowstorms along with wind, floods, hail, heavy rains, tidal waves (except for tsunamis) are covered in Japanese Cat XL contracts as well as in proportional fire treaties.

Despite winter perils being covered by most insurance policies in North America, insured loss potentials due to winter storms are typically not as severe as those from tropical cyclones and thunderstorm perils. For example, winter storm losses over the period 2009–2013 have averaged US$ 1.3bn per year, but losses from tropical cyclones and thunderstorms over the same time period have averaged US$ 7.7bn and US$ 15bn respectively. As a result, winter storms are often considered a secondary peril in the US reinsurance market, with Cat XL structures typically designed to protect against tropical cyclone or earthquake loss potentials. But despite this designation, winter storms still need to be considered by the underwriter assessing business risks.

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