Seasonal and regional signatures of ENSO in upper tropospheric jet characteristics from reanalyses

Gloria L Manney,Michaela I Hegglin,Zachary D Lawrence

doi:10.1175/jcli-d-20-0947.1

Gloria L Manney, Michaela I Hegglin + Show 1 more

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https://doi.org/10.1175/jcli-d-20-0947.1

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Abstract

AbstractThe 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 NH eastern Pacific in DJF, and in some SH regions in MAMand SON. Subtropical jet altitudes increase during El Niño, with significant changes in the zonal mean in the NH and during summer/fall in the SH. Though 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 poleward over the eastern Pacific in JJA and SON. Polar jets are weaker (stronger) during El Niño in the western (eastern) hemisphere, especially in the SH; conversely, subtropical jets are stronger (weaker) in the western (eastern) hemisphere during El Niño in winter and spring; these opposing changes, along with an anticorrelation between subtropical and polar jet windspeed, reinforce subtropical/polar jet strength differences during El Niño, and suggest ENSO-related covariability of the jets. ENSO-related jet latitude, altitude, and windspeed changes can reach 4(3)°, 0.6(0.3) km, and 6(3) ms−1, respectively, for the subtropical (polar) jets.

Highlights

Jet streams in the upper troposphere (UT) are a prominent feature of the atmospheric circulation and play an essential role in variability in phenomena such as storm tracks, precipitation, and extreme weather events (Uccellini and Johnson 1979; Nakamura et al 2004; Kolstad et al 2010; Grotjahn et al 2016; Harnik et al 2016; Mann et al 2017; Winters et al 2019, and references therein)
We show the fraction of each longitude bin and season with no jet, only a subtropical jet, only a polar jet, or both subtropical and polar jets
The total fraction of each bin with a subtropical or polar jet is the sum of the ‘‘both’’ bar and the subtropical or polar jet bar, respectively. (Figures S3 and S4 in the online supplemental material show that these relationships are very similar in the other reanalyses used .) In both hemispheres, both jets are most often present at the same time and in the same longitude bin in winter and least often present at the same time and in the same longitude bin in summer, consistent with the climatological spectrum of jets showing strongest and most persistent subtropical jets in winter (e.g., Manney et al 2014)

Summary

Introduction

Jet streams in the upper troposphere (UT) are a prominent feature of the atmospheric circulation and play an essential role in variability in phenomena such as storm tracks, precipitation, and extreme weather events (Uccellini and Johnson 1979; Nakamura et al 2004; Kolstad et al 2010; Grotjahn et al 2016; Harnik et al 2016; Mann et al 2017; Winters et al 2019, and references therein). Jet time series are constructed following Manney and Hegglin (2018): Latitude, altitude, and wind speed of the subtropical and polar jets cores were calculated at each longitude of the reanalysis grids for each day using 1200 UTC fields; these are averaged for month and season [DJF, March–May (MAM), June–August (JJA), and SON], zonally and in 208 longitude bins (from 1808–1608W to 1608E–1808).

Results

Conclusion