[ Updated 25 November 2017 with deletion of Lajes and Ponta Delgada ]
Published data were used to analyse the long-term annual temperature trends at six locations located in the Atlantic Ocean.
The data used were all from the website of KNMI (the Royal Netherlands Meteorological Institute). Only raw data were used (ie as originally recorded, without any adjustments). Data were only used from stations located in rural areas or in small towns so as to avoid the possible influence of urbanisation on the temperatures.
The periods of the data ranged from 46 years to 139 years of complete annual data. The earliest record, at Funchal, commenced in 1865.
The plots shown below are of the annual mean temperatures at each of the six locations. On each plot a polynomial “best fit” trendline has been added. It should be noted that the trendlines near each extremity (ie near the start and end of the records) are quite sensitive to individual data points, whereas trendlines within the main body of the record are much more stable and reliable.
COMPARISON OF TRENDLINES
Annual Mean Temperature Trends
[ The basis of these trendlines can be seen from the analyses for the six individual stations – see next section. ]
Annual Mean Temperature Residual Trends
[ The individual trend-lines have been adjusted by constant amounts to be approximately zero at the trend-lines in 1960.
ANNUAL MEAN TEMPERATURE TRENDS FOR INDIVIDUAL STATIONS
Horta (Azores, Portugal) 1902-2007
Funchal (Madeira, Portugal) 1865-2014
Izana (Canary Islands, Spain) 1916-2015
Wide Awake (Ascension, UK) 1924-2014
St Helena (St Helena, UK) 1893-2015
Gough Island (Gough Island, UK) 1956-2014
DISCUSSION AND COMMENTS
- The data analysed was limited to that which were available on the KNMI website.
- The annual mean temperature trend-lines for the six stations located on Atlantic Ocean islands generally showed weak cyclical patterns.
- Most of the stations had a peak trend around 2000.
- The increase in temperature from ~1970 to ~2010 was generally 0.5-1.0 deg C.
- The trend-lines for the six stations were all weakly cyclical.
- St Helena was an exception with an increase in temperature of 2.7 degC between 1940 and 2000. However, there has been a decrease of about 1.0 degc since 2000.
- The temperature rise between around 1970 and around 2010 was generally 0.5-1.0 deg C.
This article was written by Brian Gunter of Narooma, NSW. In his previous life Brian was an engineering hydrologist involved over many years in the analysis of rainfall and river flow data for the planning of water resources projects in Australia, Asia and Africa. In recent years he has been one of the Marine Rescue NSW (previously Royal Volunteer Coastal Patrol) volunteer weather observers who operate the Narooma station for the Bureau of Meteorology.