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Climate bulletin - October 2017

Summary of the world's climate in October 2017

Global land and sea-surface temperature

The global average temperature for October 2017 as estimated from the HadCRUT. data set was 0.57±0.16°C above the 1961-1990 average. Globally, October 2017 was most likely the sixth warmest October on record. Accounting for uncertainty it was one of the nineteen warmest Octobers on record. Global temperature data sets maintained by NASA GISS, NOAA NCEI, Berkeley Earth and C3S also show that October was a warm month globally. October was nominally between 2nd and 6th warmest in these data sets. Sea-surface temperatures east of the dateline in the tropical Pacific were mostly below average. ENSO conditions remain neutral but there has been cooling over the past few months at the surface and below indicating a developing La Niña.

The global average land temperature for October 2017 was 0.99 ± 0.26°C above the 1961-1990 average. For global land areas, October 2017 was nominally the 5th warmest October on record and very likely one of the top seventeen.  Unusual warmth – defined as temperatures exceeding the 90th percentile for the month – was recorded across parts of Alaska, parts of Mexico, Brazil, parts of western Europe (Portugal had its warmest October   and Spain its second warmest), Iceland and eastern Greenland, western parts of northern and tropical Africa, a region around Madagascar, southern Asia and the Middle East (Bahrain had its second warmest October on record) and parts of Australia (minimum temperatures were particularly high in southeast Queensland and northeast New South Wales). Vostok station on Antarctica had a record warm October (with a monthly average of -51.2°C). Eastern Canada and the northeast US were unusually warm with some states having their warmest October on record. Severe wildfires in California spread rapidly in warm, windy conditions. Few areas were unusually cold – defined as regions experiencing temperatures below the 10th percentile. These were: an island in the southern Pacific, and limited areas of China.

The global average sea-surface temperature (SST) for October 2017 was 0.49±0.09°C above the 1961-1990 average, nominally the 4th warmest on record and very likely between the 2nd and 13th warmest. Sea surface temperatures in the equatorial Pacific east of the dateline were mostly below average with SSTs in parts of the eastern Tropical Pacific falling below the 10th percentile of the 1961-1990 distribution. There has been cooling over the past few months in the tropical Pacific at the surface and below. Ocean and atmospheric indicators were close to La Niña thresholds in October.

Areas of unusually warm SST included: the western Pacific from the east coast of Australia to Japan; much of the North Atlantic, the Gulf of Mexico, the Caribbean Sea up to the Norwegian Sea; the north American Great Lakes; northern and western parts of the Indian Ocean. Two warm bands around 30°N and 30°S in the Pacific which have been a persistent pattern, were still evident in October. Areas of unusually low SST included parts of the Tropical eastern Pacific, limited areas of the South Atlantic, and an area in the eastern Indian Ocean. SSTs in the Atlantic hurricane Main Development Region (MDR) remained much warmer than average, with SSTs in some areas exceeding the 98th percentile. Another area of below average SST – where historical coverage is too low to assess accurately the significance of current anomalies – was an extended area in the Atlantic sector of the Southern Ocean that has persisted for several months and has been spreading slowly east and weakening.

An area of cooler-than-average surface waters in the North Pacific at around 45°N,  which was evident from May through September was absent in October. That pattern, of a relatively cool area surrounded by warmer waters, was characteristic of the positive phase of the Pacific Decadal Oscillation (PDO); the PDO index was close to zero in October. 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 and La Niña. ENSO conditions have recently been neutral or close to La Niña thresholds while the PDO remains positive. This suggests that there has been 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.


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 the eastern US and Canada; an area of Europe extending from southern Norway down to the Black Sea (heavy rain was noted in Norway, Poland, Romania, Serbia and Turkey ); various locations in southeast Asia; Japan (where western areas received record October rainfall); an area covering Southern Brazil, Paraguay and the north of Argentina; parts of Central America and Colombia; and eastern Australia. Drier than average areas included: a broad swath from northwest Africa and southwest Europe (much of continental Portugal is now in severe or extreme drought unusually late in the year  and France was exceptionally dry) to northern India; parts of Brazil; northern China; and western parts of the US and northern Mexico. The interior of the South Island of New Zealand was unusually dry.

Three tropical storms formed in the North Atlantic in October. Hurricane Nate and Tropical Storm Philippe made landfall over the US in October with heavy rains and high winds. The remnants of TS Philippe later brought high winds and heavy rain to the northeast of the US. Nate also brought heavy rain and flooding to Costa Rica. Ex-Hurricane Ophelia reached Ireland on 16th October bringing high winds with a range of impacts. The high winds also brought dust-laden air from wildfires in Spain and Portugal over the UK.

Sea Ice

Based on data from the HadISST. data set and from the National Snow and Ice Data Center [link], the Northern Hemisphere (Arctic) sea ice extent in October 2017 was likely to have been between the 4th and 5th least extensive in the satellite record. There is some uncertainty in the ranking as a number of years have similar extents.

Southern Hemisphere (Antarctic) sea ice extent was nominally between the 2nd and 5th least extensive October on record. Sea-ice extent in the Antarctic has been low since late last year. For more details and analysis of the ice extents including updates throughout the summer months, see the sea-ice monitoring brief.

Note that NSIDC have recently changed the way that they calculate their monthly extents and now use an average of the daily extents. Previously, they calculated monthly extents from the monthly-average fields of sea-ice concentrations. Extents from HadISST. are still calculated from the monthly-average fields for compatibility with the sea-ice charts that are used to extend the record back before the satellite era.


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