Diabatic processes on synoptic timescales drive variability in midlatitude storm tracks

The storm tracks along the two main western boundary currents, the Kuroshio-Oyashio and Gulf Stream, are an integral feature of the Northern Hemisphere climate. Even though diabatic processes play a fundamental role in the evolution of storm tracks, especially related to the enhanced water cycle along sea surface temperature fronts, our theoretical understanding of the impact of moist dynamic processes is still incomplete. To shed light on the relative importance of diabatic effects on storm tracks, we quantify diabatic and adiabatic contributions to variations in baroclinicity using a framework based on isentropic slope tendencies.

We reveal a dichotomy in the maintenance of baroclinicity between the near-surface and free troposphere. Specifically, changes in baroclinicity due to adiabatic and diabatic processes have opposite phases with adiabatic depletion preceding diabatic generation of baroclinicity in the near-surface, while diabatic generation precedes adiabatic depletion in the free troposphere.

In the near-surface troposphere, cold air outbreaks (CAOs) are the primary contributors to variability in baroclinicity, while outside of CAOs variability is significantly weaker and largely incoherent with the overall near-surface variability. In the free troposphere, on the other hand, most of the variability in baroclinicity is attributable to extra-tropical cyclones and fronts. Despite their limited areal extent, they explain more than half the total variance in baroclinicity. The contribution to total variability from atmospheric rivers is small, indicating that the presence of moisture alone does not necessarily translate into diabatic production of baroclinicity in the absence of a mechanism for ascent.