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Thursday, May 16, 2013

Guest Post: How cold has it really been in the Northern Hemisphere?

While parts of Australia have been experiencing a warmer than average autumn, Europe and parts of North America have experienced extreme cold weather caused by a strong Arctic Oscillation, similar to the weather in 2010.

According to Peter Hannam in the Brisbane Times, the temperatures in Sydney has reached 20 degrees C for 26 consecutive days, a new record for this late in May in more than 150 years of record keeping. It is also likely to be one of the driest 30 days in autumn on record for Sydney. Offshore along most of the NSW coast, sea surface temperatures are between 1.5 and 3 degrees above normal.

Globally, March 2013 was tied with 2006 for 9th warmest March on record, since records started in 1880. The northern Hemisphere averaged 0.78 degrees C. above normal, placing it tied for 11th warmest on record. It was also the 10th warmest on record for the southern Hemisphere, averaging 0.52 degrees C. above the 1951-1980 average. The global temperature for March 2013 averaged 0.59 degrees C. above the 1951-1980 average.

In the article below republished from the Conversation under Creative Commons licence, Blair Trewin, Australian Bureau of Meteorology and Karl Braganza, Australian Bureau of Meteorology explain what is happening with cold extremes and why average temperatures over the hemisphere and the global average are still increasing.

How cold has it really been in the Northern Hemisphere?

If you’ve been following the news in Australia in recent months, you would probably be under the impression that it has been exceptionally cold in the Northern Hemisphere. We’ve seen heavy snow on multiple occasions in places such as the United Kingdom and the United States.

In fact, some places experienced cold conditions, while other places experienced warmth that was anomalous for the time of year. These events were driven by unusual atmospheric circulation, which redistributed typical weather patterns across the Northern Hemisphere.

Warm, with pockets of cold

While some places were cold, the Northern Hemisphere was warmer than average in March, and indeed across the winter, consistent with long-term warming trends. The US National Climatic Data Centre (NCDC) has recently described such conditions as “Pockets of cold in a warming world”.

The last time the Northern Hemisphere recorded a month — any month — that was cooler than the 1961-1990 long-term average was in February 1994. The last time a whole Northern Hemisphere winter was colder than average was 1984.

Cold weather still happens — but it is less frequent, and most often less cold, than it was during most of the 20th century.

The month of March has been locally reported as being intensely cold in the Northern Hemisphere. But when averaged over the globe or the hemispheres, March 2013 was actually notable for its warmth. Globally, temperatures were 0.46°C above the 1961-1990 average, almost exactly matching average values of the last decade, and it was the tenth-warmest March on record for both the Northern and Southern Hemispheres.

In the Northern Hemisphere, it was a spring of extremes in particular places. But overall the warm regions more than cancelled out the cold regions, as the hemispheric average shows.

Why do we hear about snow in Europe?

In reality, the impression of a cold Northern Hemisphere winter is simply the result of hearing more about the cold extremes, partly because they have happened in places the Australian media pays a lot of attention to. These are places where normal conditions are close enough to freezing that unusually cold weather makes the difference between having lots of snow and lots of grey dampness.

Big dumps of snow are newsworthy since they can have high impacts and provide good visuals for the media. Of course, they are also highly disruptive to modern life, often shutting down airports, freeways, trains and many of the other transport options we take for granted.

While heatwaves and bushfires can tick the same media boxes in providing news value, they are also distinguished by the fact that they are indeed happening more frequently, and this increase is linked to global warming.

The heatwaves that have been reported from around the world since the turn of the century have all set new records for high temperatures, and have been associated with very-much-warmer conditions, or record warmth, on hemispheric and global scales — consistent with a climate system that is heating up.

For Australia, recent experience has been that record hot weather is occurring three to five times more frequently than record cold weather, and this shift reflects the background warming trend.

Around the world, exceptional cold weather still occurs, but it is happening much less often than in the past.

What happened in March 2013?

The major influence on the extreme conditions — both warm and cold — in March in the Northern Hemisphere was a highly abnormal air pressure pattern.

Under normal circumstances, winter sees high pressure systems sitting over subtropical latitudes and low pressure at higher latitudes (the “Azores high” and “Icelandic low”). The westerly winds between those two systems — equivalent to the “Roaring Forties” south of Australia — direct relatively mild maritime air over Europe, making its winters much warmer than those of most other places at the same latitude.

For example, the average winter temperature in Paris is more than 10°C warmer than Montreal, and 25°C warmer than Harbin in northeastern China, despite Paris being further north than these cities.

In March 2013 this situation was reversed, with persistent high pressure over the Arctic and sub-Arctic, and low pressure in the central Atlantic. Monthly averaged air pressure was well above normal near and north of Iceland, and well below normal west of Portugal.

This also reversed the usual airflow, directing easterly winds over northern and central Europe — which brought cold air, originating over Russia, across the continent.

Weather in the Northern Hemisphere

The result of the unusual pressure pattern was an extremely cold month in northern and central Europe. Monthly average temperatures were 5-7°C below normal in western Russia, much of Finland and the Baltic countries, and parts of Germany and Poland, and at least 3°C below normal over most remaining parts of the region.

March in the UK was the coldest since 1962 (and colder than any winter month between 1997 and 2009), and Germany had its fifth-coldest March since 1881. With the cold came unusual snowfalls in many regions, perhaps the most notable being falls of up to 25 cm on the islands of Jersey and Guernsey on 11-12 March — an exceptional event for any time of year. Further south, temperatures were near normal, but under the influence of low pressure it was very wet. Large parts of Spain had their wettest March on record, with rainfall totals more than four-times normal.

It was also a rather cold month in the eastern and central United States and adjacent parts of Canada, generally ranking between the 5th and 20th coldest of the last 100 years, although warm conditions in the west meant that the overall American average for the month came in at just 0.5°C below normal. Notably, this followed another warmer-than-average winter for the United States.

In the US, the month stood out for its contrast with the exceptionally warm March of 2012 — one example was in Chicago, where the maximum of −4°C on 20 March compared with a week of high 20s and low 30s at the same time last year.

Due to the vagaries of international reporting on weather and climate, the truly extraordinary US warmth in winter and spring 2012 tended not to reach an Australian audience.

In 2013, the contrast between cold air to the north and much warmer air further south has led to some wild fluctuations in temperature in some areas — one example being at Amarillo in west Texas, which had maximum temperatures of 32°C on 22 April and 2°C on the 23rd (even the cold fronts are bigger in Texas).

On the other side of the coin, it was much warmer than normal in many other parts of the Northern Hemisphere. Temperatures were well above average through most of central and eastern Asia outside of Russia, as well as in the northern half of Africa.

China had its second-warmest March on record, and monthly average temperatures were more than 7°C above normal in parts of Mongolia and eastern Kazakhstan. Large parts of the Arctic also experienced unusually high temperatures.

In the third week of March several days near 12°C, about 25°C above normal, occurred on the west coast of Greenland as far north as the Arctic Circle, and it was also much warmer (relatively speaking) than normal in much of the Canadian Arctic. Arctic Sea ice continued to be below average for March.

As previously noted, and shown in the figure above, warmer than average conditions were more widespread across the Northern Hemisphere — and indeed the entire globe — during the month.

Is there a link to climate change?

One way of assessing how unusual the atmospheric conditions were in March is to make use of an index of atmospheric circulation, known as the Arctic Oscillation (AO) index.

This is an indicator of pressure patterns in the middle and high latitudes of the Northern Hemisphere. If pressure is above normal in the Arctic and below normal in the central North Atlantic and North Pacific, the AO index will be negative. It will be positive if the Azores high and Icelandic low are both stronger than normal.

The monthly average value of the Arctic Oscillation index for March was −3.18, meaning very-much-higher pressure over the Arctic and lower pressure further south.

This easily broke the previous record low for March, −2.82 in 1962, and was the sixth-lowest value on record for any month. Three of the six lowest monthly AO values on record have now occurred in the last four years, including the lowest of all, −4.27 in February 2010.

As March 2013 has demonstrated, extreme phases of the Arctic Oscillation redistribute warmth from one region to another — and this is significant for seasonal weather.

In terms of climate change, there is no clear long-term trend in average values of the Arctic Oscillation index. However, a number of recent model-based studies have suggested a possible link between decreased Arctic Ocean sea-ice cover, driven by global warming, and extreme phases of the Arctic Oscillation.

This is still a new and active area of research, and it is too early to draw firm conclusions. The possibility will need to be monitored closely over the coming years.

This highlights a very interesting aspect of weather and climate at the poles, and particularly in the Arctic. Powerful “feedback mechanisms” operate in this part of the world — that can transiently accelerate or slow the pace of global warming. The prime example is the loss of Arctic sea-ice, which tends to accelerate the rate of warming in the Arctic Ocean.

Extreme weather in any given season in the Arctic — such as a record summer melt or particularly warm winter — can actually have lasting impacts in a climate system that is being changed by greenhouse warming.

The Bureau of Meteorology provides Australians with environmental intelligence for their safety, sustainability, well-being and prosperity. Our weather, climate and water services include observations, alerts, warnings and forecasts for extreme events. Karl Braganza does not consult to, own shares in or receive funding from any company or organisation that would benefit from this article, and has no relevant affiliations.

The Conversation

This article was originally published at The Conversation.
Read the original article. Map by NASA/GISS

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