In recent years more months in the UK have been above the Central England Temperature (CET) average than below it. With a few notable exceptions, for example 2009-10, many winters have been milder than the long term average.
Do these changes coincide with a trend towards less deep cold air masses being present in the Northern Hemisphere? There has been much discussion elsewhere about this, but TWO decided to compare 850hPa temperatures between 1961 to 1990 and 1991 to 2020.
The 850hPa level is approximately 1500m above sea level and is high enough not to be subject to diurnal (day to night) variations. Therefore, it provides useful information on the air mass.
Example northern hemisphere 850hPa temperature plot
The Reanalysis data used is provided by the NOAA/OAR/ESRL PSL, Boulder, Colorado, USA, from their Web site at https://psl.noaa.gov/ available from NCEP. The latitude and longitude resolution of the data is 1 degree and 4 time steps are provided for each day.
In this experiment the latitude and longitude grid was thinned to 2 degrees. The time series was also thinned so that one reading is used for each day rather than the four which are available. These changes were made to reduce the amount of time it takes to run the necessary script.
The latitude range was 40 degree north to 90 degree north.
The longitude range was 0 degree to 360 degree.
The 850hPa temperature value of each point was checked and converted from Kelvin to Celsius. It was then added to the appropriate grouping where applicable.
The groupings used:
1) -10C or lower
2) From -5C to -9C
3) From 0C to -4C
Readings of greater than 0C were not counted.
The extra day in leap years was discounted.
The 850hPa temperature counts for each year from 1961 to 1990 are shown below. The final row of the table contains the average annual values for the entire 1961 to 1990 period.
The 850hPa temperature counts for each year from 1991 to 2020 are shown below. The final row of the table contains the average annual values for the entire 1991 to 2020 period.
The results show the following:
1) A 6.54% decrease in the number of data points showing -10C or lower at the 850hPa level.
2) A 0.44% increase in the number of data points showing -9C to -5C at the 850hPa level.
3) A 2.39% increase in the number of data points showing -4C to 0C at the 850hPa level.
The changes suggest the extent of deep cold pooling (considered to be -10C or lower at the 850hPa level) has decreased significantly in the Northern Hemisphere during recent decades.
It suggests that if all other factors remain unchanged, the chance of a severe cold spell in the UK during a given winter is lower now than it was several decades ago.
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