Winter '19 / '20 - 500 mb Geopotential Heights and 2m T Anomaly Composites: La Nada Years

La Nada or neutral ENSO comes in two flavors -
Cool (-0.5 < Region 3.4 SSTa < 0)
Warm (0 < Region 3.4 SSTa < 0.5)

Current consensus expects 'warm' La Nada for Winter '19 / '20.

Recent La Nada years by decade:
1967
1979
1981 ... 1982 ... 1986 ... 1990
1991 ... 1993 ... 1994 ... 1997
2002 ... 2004
2013 ... 2014 ... 2017

5H GPHa composite - all La Nada years (n = 15)

Prevailing feature common to cool and warm La Nadas ...
- Positive 5H GPHa INVOF Aleutian Is. & negative 5H GPHa INVOF Hawaii = EPO < 0 ==> Arctic outbreaks into central CONUS



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5H GPHa composite - cool La Nada years (n = 10)

Cool La Nada/s unique features ...
- Ridge-W / Trof-E planetary flow regime over CONUS
- Positive 5H GPHa over Greenland ==> NAO < 0
- Negative 5H GPHa over North Pole & positive 5H GPHa INVOF Aleutian Is. ==> AO > 0
- Positive 5H GPHa over SE CONUS ==> primary storm track west of Appalachian mountains
- Negative 5H GPHa over south-central Canada & north-central CONUS ==> storm track above 40°N.
- Near normal 2m T across most of CONUS

2m T anomaly composite - cool La Nada years (n = 10)

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5H GPHa composite - warm La Nada years (n = 5)

Warm La Nada/s unique features ...
- Trof-W / Ridge-E planetary flow regime over CONUS
- Negative 5H GPHa over Greenland ==> NAO > 0
- Positive 5H GPHa over eastern CONUS ==> primary storm track across Central and Upper Plains
- Below normal 2m T across western ... north-central CONUS ... and New England
- Above normal 2m T over south and SE CONUS.

2m T anomaly composite - warm La Nada years (n = 5)

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5H ≡ 500 mb
GPHa ≡ geopotential height anomaly
2m ≡ 2 meters above ground level
T ≡ dry-bulb air temperature
INVOF ≡ in the vicinity of
CONUS ≡ continental United States