Strong rivers of westerly winds, known as jet streams, are driven primarily by temperature differences between low and high latitudes as well as the rotation of the Earth. The jet streams create and impact weather systems and steer them in the midlatitudes of both hemispheres. Often, these jet streams do not flow directly from west to east, but rather meander north and south in a wave pattern of alternating high- and low-pressure regions. These meanders are Rossby waves, which influence the jet streams via baroclinic instability caused by temperature gradients. Depending on their wavelength, latitude, and the background wind speed, these waves can move to the east or to the west and under certain conditions also be (quasi)stationary. Jet streams can locally increase the gradient of vorticity (atmospheric spin), so that atmospheric wave guides may be formed. These waveguides affect the propagation pathways of Rossby waves, often leading to more zonal propagation, and potentially amplification of waves. Rossby waves, jets, and waveguides affect atmospheric eddies, such as anticyclonic blocks, and can create prolonged weather conditions that lead to extreme weather impacts. Anthropogenic climate change is weakening the north–south temperature gradient. The rapid warming of the Arctic, known as Arctic amplification, is one factor affecting the poleward temperature gradient. It has been hypothesized that a weakened jet stream tends to be “wavier,” which may increase the probability of atmospheric blocking. On the other hand, anthropogenic climate change also cools the polar lower stratosphere and warms the tropical upper troposphere, which should act to strengthen the jet. Jet streams may therefore be experiencing a “tug-of-war” between these effects with opposing signs. Together, these changes tend to increase the vertical wind shear in midlatitudes, leading to increases in turbulence at the cruising altitude of aircraft. It is not yet clear which of these two effects of amplified warming will be dominant, nor how this will depend on season, region, and background state, but changes in the jet would have potential implications for the frequency of extreme weather in a future, warmer climate.

Keywords: Blocking, Climate change, Jet streams, Rossby waves, Stationary waves, Wave guide

Abstract The influence of adiabatic and diabatic processes on the midlatitude circulation is a formidable research question, especially considering their projected changes under global warming. This study presents the prospects, merits, and caveats of a potential vorticity (PV) gradient perspective as a means to disentangle the contributions of adiabatic and diabatic processes affecting the midlatitude circulation. Theoretical considerations reassess the link between the PV gradient and the jet stream. They reveal that the maximum isentropic PV gradient is consistently located on the stratospheric side of the jet, whereas the gradient of $$ ln(PV) $$ is shifted to the tropospheric side but, in general, is better aligned with the jet axis. The stratospheric shift of the PV gradient results from variations in stability across the tropopause, whereas the tropospheric shift of the $$ ln(PV) $$ gradient results from variations in vorticity. Regions of high PV gradient may serve as a proxy for the curvature of the wind field in the case of sufficiently small variations in stability. Otherwise, they depict variations in both wind and thermal stratification along tropopause-intersecting isentropic surfaces. Lagrangian “PV gradient thinking” is demonstrated in two case studies of jet streak evolution in a simulation with 1.1 km grid spacing performed with the graphics-processing-unit-enabled numerical weather prediction model Consortium for Small-Scale Modelling featuring on-line air parcel trajectories. Dry deformation drives the Lagrangian evolution of the PV gradient in the first case, whereas there is a pronounced influence of diabatic modification in the second case. The Lagrangian PV gradient perspective presented offers fresh insight into adiabatic and diabatic processes underlying the midlatitude circulation variability and change.

Keywords: cloud–circulation interactions, jet streak, jet stream, km-scale simulation, midlatitude dynamics, potential vorticity