Characteristics of the Northern Hemisphere warm season tropopause-level jet and associated trends

Abstract

One of the most familiar structural features of Earth’s atmosphere are the tropopause-level wind speed maxima known as jet streams or jets. These jets can be separated into two main types based on their differing means of formation. The polar jet arises as a result of eddy momentum flux convergence associated with developing waves within the region of midlatitude baroclinicity whereas the subtropical jet emerges in response to angular momentum transport from the Hadley Cell circulation. These jets are seasonally variable in both intensity and location. In the Northern Hemisphere, these two jets are assumed to exist throughout the full calendar year. This study aims to identify Northern Hemisphere warm season (April - November) tropopause-level jet streams based on their physical characteristics and to evaluate whether the two jet structure characteristic of winter persists into the warm season. In this study, a calendar-year analysis of the isentropic housing of Northern Hemisphere tropopause-level jets is undertaken using the JRA-55, ERA5, and NCEP reanalysis datasets from 1979 - present. The analysis recreates a result of Christenson et al. (2017) which clearly shows a bimodal distribution of jet frequency of occurrence in the Northern Hemisphere cold season (November – April) with the two peaks corresponding to the polar and subtropical jets, respectively. The investigation uniquely reveals that the vast majority of the warm season (May – October) is characterized by a unimodal distribution suggestive of a single tropopause-level jet during these months. Furthermore, the waviness of this warm season unimodal tropopause-level jet is considered from the perspective of the average latitudinal displacement (ALD) diagnostic of Martin (2021). Trends in waviness, speed, and latitudinal location of the unimodal jet over 60-plus year and 41-year time periods are also calculated. While results vary across data sets, the unimodal jet does seem to be getting wavier over the full 61-year period while exhibiting no clear trend in speed and no significant trend in latitudinal location. Over the 41-year time span, only trends in jet speed lack consensus as only two of the three data sets suggest that the jet has slowed with time.