Seasonal and Regional Signatures of ENSO in Upper Tropospheric Jet Characteristics from Reanalyses

Abstract

The relationship of upper tropospheric jet variability to El Niño–Southern Oscillation (ENSO) in reanalysis datasets is analyzed for 1979–2018, revealing robust regional and seasonal variability. Tropical jets associated with monsoons and the Walker circulation are weaker and the zonal mean subtropical jet shifts equatorward in both hemispheres during El Niño, consistent with previous findings. Regional and seasonal variations are analyzed separately for subtropical and polar jets. The subtropical jet shifts poleward during El Niño over the Northern Hemisphere (NH) eastern Pacific Ocean in December–February (DJF) and in some Southern Hemisphere (SH) regions in March–May and September–November (SON). Subtropical jet altitudes increase during El Niño, with significant changes in the zonal mean in the NH and during summer/autumn in the SH. Although zonal mean polar jet correlations with ENSO are rarely significant, robust regional/seasonal changes occur: The SH polar jet shifts equatorward during El Niño over Asia and the western Pacific in DJF and significantly poleward over the eastern Pacific in June–August and SON. During El Niño, polar jets are weaker in the Western Hemisphere and stronger in the Eastern Hemisphere, especially in the SH; conversely, subtropical jets are stronger in the Western Hemisphere and weaker in the Eastern Hemisphere during El Niño in winter and spring. These opposing changes, along with an anticorrelation between subtropical and polar jet wind speeds, reinforce subtropical–polar jet strength differences during El Niño and suggest ENSO-related covariability of the jets. ENSO-related jet latitude, altitude, and wind speed changes can reach 4°, 0.6 km, and 6 m s-1, respectively, for the subtropical jets and 3°, 0.3 km, and 3 m s-1, respectively, for the polar jets.