Weather news

Atlantic hurricane season ends

Early December marked the end of the 2012 Atlantic hurricane season and the US National Oceanic and Atmospheric Administration (NOAA) recently issued its annual summary. They describe the latest hurricane season as busy, continuing the current era of high-activity in the north Atlantic which began in the mid-1990s.

The 2012 season saw 19 named storms, of which 10 reached hurricane strength and one was classified as a major hurricane. This puts the number of named storms well above the average of 12, with the number of hurricanes also above the long-term average of 6.

Overall, then, 2012 was an above-normal year, but not exceptionally so: 10 seasons were more active in the last three decades. The season also saw an early start, with two named storms developing before it officially began in June.

Notably, this year was the second-in-a-row in which parts of the Caribbean and eastern United States suffered devastation from a storm. As we described last month, Sandy caused dozens of fatalities and significant destruction from heavy rain, strong winds and a coastal storm surge in some highly-populated areas.

The only major hurricane of the season was Michael, which reached category 3 on the Saffir-Simpson scale, but this particular storm remained over the open ocean. NOAA describes how a persistent jet-stream pattern tended to steer many of this season's storms towards the eastern portion of the United States. They add that the number of named storms was higher than early predictions indicated, owing largely to the lack of El Niño developments which were signalled by some climate models at the time: El Niño tends to suppress storm activity.

Sea-level rise from polar-ice melt

There is a history of disagreement between polar research groups on the effect on sea-level rise from polar-ice melt. A new study, published in the journal Science, brings together teams of researchers in an attempt to provide a definitive assessment.

The paper describes how different observing methods have led to varying estimates of the mass of ice across Greenland, west and east Antarctica. These differences stem from such difficulties as distinguishing snow from ice, the depth of ice and a phenomenon known as post-glacial rebound, a process by which the level of the land rises as ice melts, thus reducing the massive weight pushing down on the land surface. New technologies like GPS are now allowing these processes to be better understood.

The group's new estimate, obtained by comparing several dozen studies made over the last two decades, is that polar-ice sheets have contributed around 11mm to sea-level rise, with error bars of approximately 4mm. They have confirmed their belief that Antarctica is losing ice and that losses from Greenland are accelerating as the ocean and climate warms.

These issues are becoming increasingly important given the threat of rising oceans on coastal communities, and the group hopes that these new estimates can be used by modellers to more accurately predict future changes.

State of the global climate

In related news, the WMO recently released its provisional annual statement on the state of the global climate. They describe the years 2001–2011 as all amongst the warmest on record, with the first ten months of 2012 likely to be the ninth warmest such period since records began in 1850, thus continuing the pattern. They also highlight the unprecedented melt of Arctic sea ice.

Using information to October 2012 from three separate global data sets, the WMO says that the 2012 global average temperature was just over 14°C, around 0.5 degC above the long-term average. The higher-than-average temperatures experienced in 2012 came despite the year beginning during a La Niña phase of the El Niño Southern Oscillation. Such periods tend to have an overall lowering effect on global temperatures. Indeed, 2012 saw such an effect, but global land and ocean temperatures rose quickly above the long-term average once La Niña ended in April.

The full statement is available via the WMO website:

Weather extremes

Parts of southeast Australia experienced record-breaking temperatures during late November as the Australian Bureau of Meteorology warned of a widespread and severe heatwave. The highest temperatures affected inland parts of South Australia, New South Wales (NSW) and Victoria as hot air was drawn southeast. Temperatures peaked at 46°C at Ouyen in Victoria and Pooncarie in NSW, with eighteen stations recording their hottest November days on record. At the same time as the very hot days, night-time minima were also at record levels with the temperature not falling below 25°C in places. The heat led to the slow-running of trains, for fear of buckled rails, and had impacts on agriculture and human health.

Around the same time, algal blooms resulted in what locals described as blood-red water off some Sydney beaches; several, including the world-famous Bondi, were closed to swimmers over fears for their health.

At the other temperature extreme, heavy snow and ice in a region around Moscow led to an astonishing 125-mile traffic jam with some vehicles stranded for two days. Emergency responders were forced to set up food stations as thousands of lorries and cars became stranded on the M10 motorway.

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