Trends in Some Characteristics of the Warm-Season Tropopause-Level Jet Streams in Both Hemispheres

Abstract. A calendar-year analysis of the isentropic housing of Northern Hemisphere tropopause-level jets is undertaken using three different reanalysis data sets. In contrast to the distinct polar and subtropical jets that characterize the cold season, the analysis reveals that the vast majority of the warm season (May–October) is characterized by a single tropopause-level jet. Additionally, the warm-season jet is found to occupy an isentropic space adjacent to that of the cold-season subtropical jet inviting the interpretation that the traditional polar jet erodes away during the warm-season.

Trends in the waviness, average speed, and latitudinal location of this unimodal warm-season jet are assessed using the recently developed average latitudinal displacement (ALD) methodology. While the results among the various data sets are not as uniform for the warm-season jet as prior work suggests they are for the cold-season polar and subtropical jets, the analysis concludes that the unimodal jet has experienced a slight increase in waviness. Simultaneously, it has undergone a slight decrease in average speed implying that a nearly steady circulation has stirred the NH warm season extratropics over the last 40 years. A similar analysis of the SH warm-season jet reveals that it is also unimodal, has gotten systematically wavier over the same interval and appears to have also sped up on average. Possible implications of these hemispheric differences are discussed.