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March 2014

Climate Change 2013: Data, facts, background

Research Climate Change

For weather and climate researchers, 2013 was marked by prolonged wet weather conditions and temperature extremes. The Antarctic sea ice extent set a new record for the second year in succession.

According to preliminary data issued by the World Meteorological Organization (WMO), 2013, like the preceding year, was among the ten warmest years since 1850. The mean global temperature was about 0.5°C above the 30-year reference period (1961–1990) defined by the WMO and therefore close to the mean value of the ten warmest years. While it was clearly excessively warm in large parts of northern and eastern Europe, central and east Asia, Australia and in parts of Brazil, it was only cooler than the reference period in a few areas such as Canada and northern Russia. However, when viewed on a month-by-month basis, a very differentiated picture emerges: pronounced regional summer heatwaves in many Asian countries and in Australia contrasted with massive outbreaks of cold polar air during the northern winter and spring across large parts of Europe and eastern North America.

Global precipitation (restricted to land-based data) was generally lower than in the reference period (1961–1990) defined by the US Weather Service, NOAA. This was particularly true for parts of Australia, the western US and Brazil. More relevant – also because of their impact on the overall claims costs of the insurance industry – were the sustained periods of rain in some regions, which brought severe flooding. Europe, western Canada and the border area between Russia and China were particularly affected.

The El Niño/Southern Oscillation Index (ENSO index) remained neutral throughout 2013. This underlying weather phenomenon, which depends on fluctuations in the surface temperature of the equatorial Pacific Ocean, therefore had no relevant influence on weather patterns and weather extremes.

The global sea level reached a new record in 2013. The rise of approximately 3 mm per year observed between 2001 and 2010 was almost twice the secular trend of 1.6 mm per year in the 20th century.

Cold spells in the north

Mild temperatures predominated in large parts of Europe at the beginning of the year, breaking records in the northeast of Iceland. But in the course of January, weather conditions changed completely. The persistent flow of cold polar air, which lasted into March, resulted in one of the most intense cold spells in central Europe and Russia for many decades. Temperatures were up to 10°C below the long-term average. In many parts of Russia, March was colder than February. In North America cold air from the Arctic held the eastern part of the continent firmly in its grip right into April.

By contrast, Australia began the year with one of the most intense heatwaves in recent memory. In January, the highest ever nationwide average daytime temperature of 40.3°C was recorded; Sydney and Hobart reached new records of 41.8 and 45.8°C respectively. The plateau in the northeast of Brazil suffered the worst drought for 50 years. Crop failures and shortfalls in hydroelectric power supplies produced economic losses costing billions.

Is the loss of sea ice promoting cold advection?

The cold in late winter and early spring in the midlatitudes of the northern hemisphere was caused by a negative phase in what is called the Arctic Oscillation. During these phases, there are comparatively small differences in temperature and air pressure between the Arctic and the more southerly latitudes, resulting in weak westerly winds. For Europe this means, for example, that there are only slight differences in temperature and air pressure between the subtropical highpressure area in the south of the eastern North Atlantic and the low-pressure area around Iceland. Mild air masses which have been warmed by the Atlantic do not reach the continent in these circumstances and cold conditions can develop there. In North America, the area of this type of polar outbreak stretches over the central and eastern parts of the continent, as in the late winter of 2013. Some research groups suspect that the polar outbreaks are linked to the dwindling sea ice in the Arctic Ocean. They have discovered that the probability of weather patterns of this nature with inland cold spells during the continental winter increases as the ice cover decreases in the Arctic. This results in an intensification of the Siberian high-pressure area during the autumn, which in turn influences the regime of the atmospheric circulation into the winter months (see Cohen et al., 2012). However, the physical mechanisms behind this interrelation are not yet adequately understood.

Time Series global annual Temperature Anomalies - Munich Re

Temperature anomalies, Jan–Dec 2013 (with respect to a 1981–2010 base period)

In 2013, northern and eastern Europe, central Asia and Australia in particular were warmer than the reference period, while the annual mean temperature in the eastern US and Canada was below the long-term mean. In global terms, 2013 is among the ten warmest years since 1850.

Time Series global annual Temperature Anomalies - Munich Re

Precipitation anomalies, Jan–Dec 2013 (with respect to a 1961–1990 base period)

Regional anomalies of total annual precipitation in 2013 in comparison with the reference period 1961–1990. Note the above-average wet conditions in Europe and the eastern US.

Time Series global annual Temperature Anomalies - Munich Re

Annual variations of the global annual mean temperatures in the period 1950 to 2013 compared with the 1961 to 1990 mean

The ten warmest years in the observation period 1850 to 2013 were all after 1998. The time series commences in 1850; the period 1950 to 2013 is shown here.

Time Series global annual Temperature Anomalies - Munich Re

Change in winter temperatures based on the Arctic Oscillation Index

If the Arctic Oscillation Index decreases by a standard deviation, the mean temperature from December to February changes as shown in the graph. The cooling in large parts of Europe, northern Asia and the eastern part of North America is clearly shown.

Severe flooding in central Europe

In Russia, the unusually cold weather suddenly shifted to extraordinarily warm weather in April, with positive regional temperature deviations of up to 9°C compared with the long-term average. It was also very warm at the beginning of the Australian autumn. At the end of May/beginning of June, a trough of low pressure developed over central Europe, channelling warm, damp air round the Alps. The violent precipitation, which in places reached 400 litres per square metre in the space of a few days, led to the most costly natural disaster of the year in terms of overall economic losses. In southwestern Asia, the monsoon broke very early in June and caused the most severe flooding of the last 50 years in the border regions of India and Pakistan.

Record ice in the Antarctic

Both the northern summer and the southern winter were marked by extensive warm periods with temperature anomalies of up to 5°C compared with the NASA reference period of 1981 to 2010. Central and eastern Europe, western North America and Australia were particularly affected. In stark contrast to this was the extension of the sea ice in the Antarctic, which set a new record for the second time in a row. The maximum surface area of 19.5 million km2 measured by satellites was 2.6% over the mean level of the reference period. At the North Pole, the melting of the ice cap was less pronounced than in recent years. While its minimum level of 3.4 million km2 set a new record low in 2012, the melting process in 2013 stopped in September at 5.1 million km2. But this was still approximately 18% (1.1 million km2) less than the average of the reference period 1981 to 2010.

Sustained, intense rain between the end of July and the middle of August in the border region between China and Russia led to extensive floods, particularly in the catchment area of the Amur River, which reached new record levels.

Weather becoming more persistent

Stationary high- and low-pressure systems triggered a series of extreme weather events in 2013. Persistent troughs of low pressure with high precipitation activity were responsible for the floods in central Europe and on the Russian/Chinese border. During the time of the trough over central Europe, a persistent highpressure zone formed further to the east in Russia and Scandinavia, which caused a prolonged heatwave.  

According to the findings of the latest research, these persistent weather patterns could be linked to the warming of the high latitudes as a consequence of climate change. A meandering band of strong winds at high altitudes which encircles the Earth in a wave-like pattern normally determines the arrangement and movement (normally from west to east) of the large high- and low-pressure areas in the mid-latitudes. Analysis of past extreme summer events (US heatwave in 2011; European floods in 2002) has revealed evidence of a resonant reinforcement of a stationary occurrence of this wave structure with particularly large amplitudes. The resultant intense high- and lowpressure areas therefore increased their regional impact due to their extreme persistence. The basic conditions for a stationary wave structure capable of determining weather conditions described by research scientists occurred twice as frequently in the period 2002 to 2012 as in the periods 1991 to 2001 and 1980 to 1990. A correlation with the reduced temperature difference between the higher and lower latitudes as a result of climate change is assumed (Petoukhov et al., 2013) but has not been conclusively proved. Future research projects must explain the extent to which climate change promotes the formation of stationary wave structures.

Low-energy tropical cyclones

The figure of 86 tropical cyclones observed worldwide in 2013 is in line with the long-term average (1981–2010 average 89). In the North Atlantic, the number of 13 named tropical storms was below the average for the warm phase of the “Atlantic Multidecadal Oscillation”, which has predominated since 1995 (average since 1995: 15). What was more striking was the low energy of the storms: the “Accumulated Cyclone Energy“ (ACE), which is determined by the intensity and duration of the storms, was only about 30% of the long-term average.

On the other hand, rather more than the usual number of cyclones occurred in the northwest Pacific. One of them, Super Typhoon Haiyan, which struck the southern Philippines with wind speeds far exceeding 300 km/h in places, caused the greatest human catastrophe of the year. A detailed description can be found starting on page 6 under the heading “In Focus“.

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