December 2015 was the last month of 2015. It had 31 days. December was also an unusually warm December, globally. The temperature anomaly in December was the highest recorded by some margin. In the NOAAGlobalTemp analysis the margin was 0.29degC. In HadCRUT4, it was around 0.31degC. Uncertainties in monthly values are typically larger than they are for annual values, but even so, the margin is significant in so far as the 95% uncertainty ranges for this December and the previous record holder don’t overlap.
What made it so warm? Well, there’s the ongoing El Nino, which peaked in December. Typically, the global temperature response to El Nino lags a few months behind. So, despite El Nino developing (definitively) around May/June, global temperatures didn’t strongly respond till around September. It’s hard to say exactly how much El Nino contributed to the warmth of a single month, but, according to the NOAA SST analysis, areas of the tropical oceans in all three major ocean basis reached record highs for the month. Tropical areas of the Americas and Africa also saw very high temperatures for the time of year. Another factor is the circulation pattern, which saw higher than average temperatures across large areas of north America and Eurasia.
The contiguous US was the warmest and wettest December since records began in 1895. In contrast Alaska tied with 1968 as the 4th driest since the statewide record began in 1925. The Canadian province of Ontario was record warm in December with anomalies that were record high for any month. Central and south America were unusually warm, with areas experiencing their warmest December on record. Australia had its 6th warmest December since records began. A heat wave brought record warm maximum temperatures to southern South Australia and Victoria. A number of locations in northern Victoria had their hottest night on record for any month.
Europe experienced the warmest December on record with a number of countries reporting a record warm month (UK, France, Germany and the Netherlands, in most cases by a wide margin), or close to it (Austria 4th, Spain 2nd, Norway 7th). In the UK, the long-running Central England Temperature series, which starts in 1659, had its warmest December, more than 10 degrees warmer on average than December 2010, which was the second coldest in the same time period. On 30 December, Svalbard Airport recorded a temperature of 8.7C which is a new record for December for the station (quite a bit higher than the old record of 7.2degC, recorded 4th December 1995), and the measurement is higher than the record for November.
Western parts of Russia were also very warm, with the Volga region experiencing record December temperatures. For Russia overall, it was only fifth warmest. Very warm weather was observed in Central Asia. New daily maximum temperature records were established in Turkmenistan, Uzbekistan, Kyrgyzstan and southern Kazakhstan (it was so warm that lilacs bloomed in parts of southern Kazakhstan). Monthly mean temperatures were above normal across Japan (as was precipitation) with temperatures reaching December-record levels in eastern Japan. Monthly precipitation amounts on the Pacific side of western Japan were the highest on record for December. In South Korea, December was also the warmest on record (since 1973 at least).
A few areas were colder than average. An area extending from north Africa through to the middle east experienced colder than average conditions. Western Greenland, northeast Asia, New Zealand and the southernmost tip of South America were also colder than average. An area of the Atlantic, to the south of Greenland has been persistently cold throughout the year and a large are of the high latitude north Atlantic has cooled relative to 2014. Parts of the southern ocean also remain cooler than the long-term mean.
Unusual warmth in Europe was coupled in places with unusually wet conditions and, in others, with unusually dry conditions depending on which half of the +ve NAO pressure anomaly dipole they were sat on – the split in dry and wet is particularly marked. The anomalous flow was from the southwest. To the north, under low pressure anomalies, the Norway and the UK were both warm and wet. In the UK, it was the wettest December on record and the recorded highest 24-hour rainfall total (341.4mm) was recorded at Honister Pass in Cumbria in the aptly named Lake District. There was flooding in a number of areas, with record river levels reported on some rivers. Above average precipitation totals were seen deep into Eurasia in areas that were also very warm (for the time of year). Norway had its 4th wettest December (169% of normal). Kazakhstan and northern parts of Uzbekistan received 2-3 times the average monthly rainfall.
In contrast, in Spain where higher-than-average pressure prevailed, conditions were unusually dry. Countrywide, rainfall was only around 20% of normal. Some areas had no rain and, in places, it was the driest December on record. In other places, the warmth translated into a lack of seasonal precipitation. In Austria, Seefield in the Tirol recorded no snow in December; the average for the station is 80cm, with 26 days of snow cover. The northern coast of North Africa was also dry.
An area from Central America through Colombia to Brazil was drier than average. Brazil was mostly much drier than usual, except in the south of the country. An area covering parts of Brazil, Paraguay, Uruguay and Argentina was much wetter than average. Argentina saw monthly records broken at some stations. This pattern (dry across the north and east of the continent with wetter conditions in Argentina/Uruguay) is broadly consistent with the “typical” El Nino pattern. Also consistent with the “typical” pattern were the dry conditions recorded in the south eastern countries of Africa – South Africa, Botswana, Zambia, Zimbabwe, parts of Mozambique (though the latter saw above average rain further north). In New Zealand, high pressure prevailed over and to the west of the country. To the east, lower than normal pressure was recorded. December was consequently very dry; several locations reported record low December rainfall totals and less than half of normal rainfall was observed for most of the North Island and below normal rainfall (50-79% of average) was recorded across most of the South Island. Fiji had mixed rainfall totals with some stations reporting above average precip, while Vanuabalavu recorded a total rainfall of 16mm, the lowest December total in its 31-year record.
In Australia, it was a very wet month for the Northern Territory, where it was the wettest December on record (at 197% of mean rainfall).
Ocean Heat Content estimates produced by the ex-NODC at NCEI show that OHC anomalies in each quarter of 2015 were the highest on record for each quarter.