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Climate bulletin - April 2017

Summary of the world's climate system in April 2017

The global average temperature for April 2017 as estimated from the HadCRUT4 data set was 0.74±0.15°C above the 1961-1990 average. Globally April 2017 was one of the ten warmest Aprils on record and most likely second warmest. Global temperature data sets maintained by NASA GISS and NOAA NCEI also show that April was a very warm month globally. Sea-surface temperatures in much of the tropical Pacific were near or above average indicating neutral ENSO conditions. However, SSTs in the eastern Tropical Pacific, close to the coast of South America, were warmer than average.

The global average land temperature for April 2017 was 1.08 ± 0.26°C above the 1961-1990 average. For global land areas April 2017 was nominally the third warmest April on record and very likely in the top twelve, though very unlikely to have been the warmest. Unusual warmth – temperatures exceeding the 90th percentile for the month – was recorded in: southern and eastern areas of the US (8 states reported record April warmth), western Alaska; an area stretching from Spain and Morocco through the Mediterranean, Middle East, Pakistan and India and into east Asia; areas of Brazil; a number of islands in the Western Pacific; as well as a number of areas of Africa for which we receive information from long-term stations.  Few areas were unusually cold – temperatures below the 10th percentile: a small area on the Hudson Bay, part of western Australia, western Greenland, and along the coast of Antarctica.

The global average sea-surface temperature for April 2017 was 0.60 ± 0.09°C above the 1961-1990 average, nominally the second warmest on record and very likely one of the six warmest. Sea surface temperatures in the tropical Pacific were near average with some areas warmer than the long-term mean, indicating neutral ENSO conditions. However, in the east, near the coast of South America SSTs continued to be higher than average. Areas of unusually warm SST included: large areas of the north Atlantic and the tropical Atlantic; areas of the Indian Ocean; the Western Tropical Pacific and two bands around 20°N and 20°S across the Pacific. Unusually low SSTs were recorded in limited areas of the North Atlantic, south of Greenland, parts of the Indian Ocean, and parts of the Tropical Atlantic. Other areas of below average SST – where historical coverage is too low accurately assess the significance of current anomalies - include an extended area in the Atlantic sector of the Southern Ocean which has persisted for several month and parts of the South Pacific.

In the North Pacific there was a band of cooler-than-average waters around 45°N surrounded by areas of warmer-than-average waters. This pattern is characteristic of the positive phase of the Pacific Decadal Oscillation (PDO). Some measures of the PDO have been indicating a shift to its positive phase since the start of 2014. However, on short time scales, SST patterns associated with the PDO look very similar to those associated with El Niño. As ENSO conditions are currently neutral, this does suggest a more persistent shift to the positive phase of the PDO. However, other indicators (TPI) have returned to near-neutral, so it is not yet clear. The negative phase of the PDO has been associated with a reduction in the rate of global temperature increase since the start of the millennium.

Higher than average precipitation totals (based on the monthly first-guess analysis by GPCC) were recorded across: all but the more southerly parts of New Zealand (associated with remnants of Cyclone’s Debbie and Cook in the first half of the month); Argentina; much of the USA (it was the second wettest April overall for the country); northern India, Nepal, Bangladesh, Myanmar and parts of southern and western China; Norway; areas of Central and Eastern Europe and western Russia; and parts of southern Africa including Botswana and Namibia. Heavy rain in Haiti caused flooding and landslides in some areas. Drier than average areas included: parts of east Africa including Ethiopia, Somalia, Kenya and Tanzania; Mexico; the west coast of Canada; Western Europe and the Mediterranean; Alaska had its second driest April on record. Australia had two large areas of below average rainfall in the north east and south west of the country, separated by a band of above average rainfall.

Based on data from the HadISST. data set, both Northern Hemisphere (Arctic) and Southern Hemisphere (Antarctic) sea-ice extent were nominally the second least extensive April sea ice on record for the respective hemispheres. This analysis is consistent with data from the National Snow and Ice Data Center in the US. In the Arctic, sea-ice was marginally – but not significantly – more extensive than in April 2016. In the Southern Hemisphere, ice-extent for April was the lowest since April 1980. Sea-ice extent has been unusually low in both hemispheres since late 2016. For more details and analysis of the winter ice extents and updates throughout the summer, see the sea-ice monitoring brief.


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