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Air mass temperatures since 1961

Northern hemisphere deep cold declining?


06/01/2021

NH air mass analysis

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.

GFS 850hPa temperature example chart
Example northern hemisphere 850hPa temperature plot

Data used and experiment constraints

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.   

1961 to 1990

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.

Year -10C or lower -5C to -9C 0C to -4C
1961 664225 272958 307736
1962 676505 279379 301392
1963 660637 291005 321637
1964 687602 270094 306067
1965 666371 282379 322399
1966 681930 269515 300945
1967 650699 298710 307809
1968 674827 271156 312216
1969 674821 290442 308200
1970 676711 286715 309382
1971 681230 265067 304688
1972 681581 305724 311166
1973 675437 290221 301391
1974 669782 280928 318286
1975 670631 290340 306379
1976 674914 307091 308435
1977 657753 281895 301296
1978 664147 308044 308951
1979 665081 288705 306534
1980 676487 293470 302479
1981 646729 290346 318847
1982 680592 294937 307026
1983 664174 278305 319225
1984 660049 284406 310302
1985 662436 290629 308670
1986 667508 312753 313240
1987 670034 280059 308357
1988 656031 289186 301903
1989 655120 299830 294563
1990 629270 285498 322759
1961-1990 667444 287660 309076

 

1991 to 2020

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.

Year -10C or lower -5C to -9C 0C to -4C
1991 640995 282320 313280
1992 684549 303471 298480
1993 658166 297895 312334
1994 670524 288933 297991
1995 625165 301770 322528
1996 664317 311320 307257
1997 646880 280886 315328
1998 631563 283859 310856
1999 646361 275018 311760
2000 651230 280008 316318
2001 653655 270879 300472
2002 631868 289292 314691
2003 627411 283541 316272
2004 637186 278199 317218
2005 605216 276934 331372
2006 617729 299759 310218
2007 615850 285234 317184
2008 623276 278917 317892
2009 618308 291859 320164
2010 594220 297619 335125
2011 609134 273033 323785
2012 613341 281291 320904
2013 637307 289512 297737
2014 625208 275271 314600
2015 606023 292371 322196
2016 571791 322423 317277
2017 603931 302124 327290
2018 596445 299314 332176
2019 573608 294614 334404
2020 573633 280296 319217
1991-2020 625163 288932 316544

 

Summary

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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