Winter Subtropical Highs, the Hadley Circulation and Baroclinic Instability.

Subtropical highs have a profound influence on the weather and climate of adjacent continents. In this study, we use reanalysis data to investigate the interannual variability and trends in winter subtropical highs from 1979 to 2021. We find dynamical relationships between subtropical high intensity, the Hadley and Ferrel Circulation intensity, and the Eady Growth Rate (EGR). A poleward shift of the maximum in EGR is associated with a strengthening of the descending branches of the Ferrel and Hadley Cells, with subtropical troposphere adiabatic warming and an increased intensity and poleward movement of the subtropical highs. Shifts in the poleward EGR are dominated by changes in vertical wind shear which, in turn, are in thermal wind balance with variations and trends in temperature. The mechanism for the intensification of the subtropical highs involves feedbacks from high‐frequency transient eddies. Strong North Pacific and South Pacific Subtropical highs are associated with La‐Niña conditions. We also show that the mechanisms for interannual variations are similar to those for trends in the highs. Plain Language Summary: Wintertime subtropical highs have large climatic impacts but the dynamical explanation for their interannual variability and trends is incomplete. Here we find that wintertime subtropical high intensity is related to overturning cell intensities and the Eady Growth Rate (EGR), which is a measurement of baroclinic instability, in both interannual variability and long‐term trends. The northward (southward) shift of the northern (southern) hemisphere EGR is associated with an increasing intensity of the Ferrel Cell and Hadley Cell. The enhancement of their descending branches strengthens the intensity of subtropical highs. The EGR change is dominated by the vertical wind shear change. The North Pacific Subtropical High and South Pacific Subtropical high are intensified by cold anomalies of sea surface temperature in the Central and Eastern Pacific but are also trending stronger and moving polewards. Key Points: Winter subtropical highs get stronger and shift polewards when the Eady Growth Rate (EGR) shifts polewards and the Hadley Cell strengthensThe EGR shift is dominated by the vertical wind shear, in thermal wind balance with the variations and trends of temperatureSimilar mechanisms are at play on interannual and longer time scales and the Pacific subtropical highs are strengthening over time [ABSTRACT FROM AUTHOR]