Calendar Year Climatology of Northern Hemisphere Tropopause-level Jets

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. Only grid columns in which the vertically integrated average wind speed exceeds 30 m s^-1 in the 100-400 hPa layer are considered. In such columns, the analysis identifies the wind speed maximum and the potential temperature (θ) at that level to create distributions of the maximum wind speeds in 5K θ intervals. The analysis recreates a result of Christenson et al. (2017) which shows a clearly bimodal distribution of jets in the cold season (November – March) with the two peaks corresponding to the polar and subtropical jets, respectively. Conversely, the warm season (May – October) is characterized by a unimodal distribution suggestive of a single tropopause-level jet during this time of year. The months of April and November feature transitional periods during which a switch to unimodal from bimodal (April), or vice-versa, is evident. A first look at trends in the waviness of the warm season NH jet, determined using the average latitudinal displacement (ALD) diagnostic of Martin (2021), is offered.