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Climate bulletin - September 2017

Summary of the world's climate in September 2017

Global land and sea-surface temperature

The global average temperature for September 2017 as estimated from the HadCRUT.4.6.0.0 data set was 0.56±0.20°C above the 1961-1990 average. Globally, September 2017 was most likely the sixth warmest September on record. Accounting for uncertainty it was one of the twenty-two warmest Septembers on record. Global temperature data sets maintained by NASA GISS, NOAA NCEI, Berkeley Earth and C3S also show that September was a warm month globally. September 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. Other atmospheric indicators of La Niña, however, have not yet developed.

The global average land temperature for September 2017 was 1.02 ± 0.29°C above the 1961-1990 average. For global land areas, September 2017 was nominally the 3rd warmest September 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 parts of the Middle East (for example, Bahrain had its warmest September on record) and east Africa; scattered areas across southern Asia; parts of Argentina, Brazil and Central America; an area around the North American Great Lakes; and northern Canada. Areas of southern Africa and western Tropical Africa were also unusually warm (colder than average coastal areas on the map are associated with SSTs rather than land temperatures).

Australia as a whole experienced its warmest September day on record on 22nd September during a warm spell late in the month and a number of local September-temperature records were broken. Parts of New Zealand also experienced a heatwave on 24th and 25th September under the influence of the same weather pattern with eastern areas experiencing particularly high temperatures boosted by the Föhn effect in the north-westerly winds.

Few areas were unusually cold – defined as regions experiencing temperatures below the 10th percentile. These were: an area in northern Australia, a small area of coastal Brazil and two small areas in Russia. The wider area in northern Russia, which saw slightly cooler-than-average conditions, also saw cooler-than-average conditions in September in 2010, 2013, 2014, 2015 and 2017.

The global average sea-surface temperature (SST) for September 2017 was 0.44±0.09°C above the 1961-1990 average, nominally the sixth warmest on record and very likely between the 4th and 16th 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. ENSO conditions remain neutral but there has been cooling over the past few months at the surface and below. Other atmospheric indicators of La Niña have not yet developed.

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, though the southern band is now less pronounced), large areas of the Indian Ocean, except for an area off the west coast of Australia. The northern tropical Atlantic and central north Atlantic were much-warmer-than-average in September as were SSTs in the Atlantic sector of the Arctic. SSTs in the Atlantic hurricane Main Development Region (MDR) remained much warmer than average, with SSTs in some areas exceeding the 98th percentile. Areas of unusually low SST included limited regions of the north Pacific, parts of the Tropical eastern Pacific, and an area in the eastern Indian Ocean. 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.

As in May through August, there was an area of cooler-than-average waters in the North Pacific at around 45°N surrounded by areas of warmer-than-average waters. The pattern is more distinct than in August but does not extend so far west as it did in earlier months. 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-verging-on-La Niña, 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 and across the western half of the US; parts of South America including a coastal area of eastern Brazil and central Argentina; a band running east from the UK across Europe with particularly high totals running from Italy and the Balkans north to the Baltic states (heavy rain in Italy and the Balkans led to flooding ); western and central Australia; an area from Central to eastern Russia; North Island, New Zealand; parts of southern China; southern India, Bangladesh and Myanmar; and parts of Indonesia, Malaysia and Papua New Guinea.

In the middle of the month areas of Japan experienced heavy rain associated with Typhoon Talim. Central Argentina and Uruguay were much wetter than average. In contrast, other parts of Argentina saw unusually dry conditions that were conducive to the spread of wildfire.

Drier than average areas included: large areas of Brazil; Spain, Portugal, southeast France and northern Scandinavia (Parts of the west and north of Norway were record warm and dry); an area of central-eastern Greenland; southern Africa; eastern and some southern areas of Australia; areas bordering the Black Sea; parts of southeast Asia, northeast China and the Korean peninsula; and Northern India. An area of California was drier than average in September and parts of California also experienced extensive wildfires. An area from the Great Lakes to the Gulf Coast was drier than average, with Louisiana tied for its driest September on record in contrast to a very wet August. Portugal had its driest September in 87 years. Continental Portugal received on average only 2mm of rain, 5% of the 1971-2000 average.

Five hurricanes were active in the Atlantic in September, four of which reached major hurricane strength. Hurricane Irma (30 August-12 September) was notable in a number of ways. It made landfall over the Leeward Islands, Virgin Islands, Cuba and Florida; each time as a major hurricane. It also affected Puerto Rico. There was severe damage along its path and at least 134 deaths. Hurricane Maria (16-30 September) strengthened rapidly, reaching category 5 before making landfall over Dominica. Maria later made landfall over Puerto Rico, the strongest Hurricane to do so on the island since 1928, bringing widespread destruction to the island.

Sea Ice

September is the month during which Arctic sea ice extent reaches its annual minimum. 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 September 2017 was likely to have been between the 4th and 7th 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 3rd least extensive September on record. Sea-ice extent in the Antarctic has been unusually low since late last year. For more details and analysis of the ice extents including updates throughout the summer, 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.2.2.0.0 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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