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Climate bulletin - August 2017

Summary of the world's climate in August 2017

Global land and sea surface temperature

The global average temperature for August 2017 as estimated from the HadCRUT.4.6.0.0 data set was 0.72±0.22°C above the 1961-1990 average. Globally, August 2017 was most likely the third warmest on record, but accounting for uncertainty was one of the eighteen warmest Augusts on record. Global temperature data sets maintained by NASA GISS, NOAA NCEI, Berkeley Earth and C3S also show that August was a very warm month globally. August was nominally 2nd or 3rd warmest in these data sets. Sea-surface temperatures east of the dateline in the tropical Pacific were near or below average indicating neutral ENSO conditions.

The global average land temperature for August 2017 was 1.06 ± 0.27°C above the 1961-1990 average. For global land areas August 2017 was nominally the 2nd warmest August on record and very likely one of the top fifteen.  Unusual warmth – defined as temperatures exceeding the 90th percentile for the month – was recorded across an area that extended from Spain and Morocco, through the Mediterranean, southern Europe, and the Middle East. More details are available from the WMO Region VI monthly bulletin. Bahrain had its warmest August on record and the east coast of Africa and Madagascar were also affected. Other areas experiencing unusual warmth included parts of southern and eastern Asia, western areas of North America, areas across Central America and the Caribbean, eastern areas of South America and parts of Australia. The states of California, Oregon and Washington had their warmest Augusts on recordNew Zealand had its 3rd warmest August on record. Few areas were unusually cold – defined as regions experiencing temperatures below the 10th percentile. These were central parts of North America, an area on the coast of Brazil and a limited area in central Asia.

The global average sea-surface temperature (SST) for August 2017 was 0.61 ± 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 east of the dateline were near or below average, indicating neutral ENSO conditions. Sub-surface temperatures in the tropical Pacific were cooler than average in the eastern Pacific in August.

Areas of unusually warm SST included: the western Pacific, two bands around 30°N and 30°S in the Pacific (a persistent pattern for some months now), the Mediterranean and large areas of the Indian Ocean, except for an area just west of Australia. The tropical Atlantic, Gulf of Mexico and western north Atlantic were much warmer than average in August. SSTs in the Atlantic hurricane Main Development Region (MDR) were warmer than average, with SSTs in some areas exceeding the 98th percentile. Areas of unusually low SST included limited regions of the north Pacific, and a larger area in the eastern Indian Ocean. Another area of below average SST – where historical coverage is too low to accurately assess the significance of current anomalies – was an extended area in the Atlantic sector of the Southern Ocean which has persisted for several months and has been spreading slowly east.

As in May through July, there was a band of cooler-than-average waters in the North Pacific at around 45°N surrounded by areas of warmer-than-average waters. The pattern is less clear in August. 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 suggests a more persistent shift to the positive phase of the PDO. 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.

Precipitation

Higher than average precipitation totals (based on the monthly first-guess analysis by the Global Precipitation Climatology Centre, GPCC) were recorded in Alaska; parts of South America including western Brazil and eastern parts of Argentina; Spain and Morocco; Western Turkey; northern Europe and Scandinavia; western China, southern and western India and southern Australia. Above average rainfall was recorded in many areas along the eastern edge of Asia. Typhoon Hato and Tropical Storm Pakhar, made landfall over southern China in the space of a week, bringing flooding from heavy rain and an associated storm surge.

Hurricane Harvey made landfall as a major hurricane during August. Rainfall from the slow-moving storm exceeded 1200 mm in some locations in Texas. It was a record wet month for the state as a whole and Louisiana had its second wettest August on record.

Areas of northeast India, Pakistan, Bangladesh and Nepal saw above average rainfall for the month, which led to severe flooding. Mumbai and parts of Pakistan also saw heavy rain and associated impacts.

Drier than average areas included: the western US and large areas of Canada; central and eastern Australia; parts of northern South America; and parts of South Africa, Zimbabwe, Namibia and Botswana. An area of drier than average conditions affected coastal areas in the central Mediterranean, extending into Eastern Europe and beyond, into central Asia. The area of drier than average conditions also extended south from Russia and Kazakhstan down to Iran and Afghanistan.

Sea Ice

Based on data from the HadISST.2.2.0.0 data set and from the National Snow and Ice Data Center, the Northern Hemisphere (Arctic) sea ice extent in August 2017 was likely to have been between the 3rd and 4th least extensive in the satellite record. There is some uncertainty in the ranking as a number of years have very similar extents. Southern Hemisphere (Antarctic) sea ice extent was nominally between the 2nd and 4th least extensive August on record in both data sets. Sea-ice extent in the Antarctic has been unusually low since late last year perhaps due to high remnant sea-surface temperatures following the El Niño and unusual atmospheric circulation. For more details and analysis of the ice extents including updates throughout the summer, see the sea-ice monitoring brief.

 

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