Linkage between cross-equatorial potential vorticity flux and surface air temperature over the mid–high latitudes of Eurasia during boreal spring

Abstract

The source of potential vorticity (PV) for the global domain is located at the Earth’s surface. PV in one hemisphere can exchange with the other through cross-equatorial PV flux (CEPVF). This study investigates the features of the climatic mean CEPVF, the connection in interannual CEPVF with the surface thermal characteristics, and the associated mechanism. Results indicate that the process of positive (negative) PV carried by a northerly (southerly) wind leads to the climatologically overwhelming negative CEPVF over almost the entire equatorial cross-section, while the change of the zonal circulation over the equator is predominately responsible for CEPVF variation. By introducing the concept of “PV circulation” (PVC), it is demonstrated that the interannual CEPVF over the equator is closely linked to the notable uniform anomalies of spring cold surface air temperature (SAT) over the mid–high latitudes of Eurasia by virtue of the PVC, the PV-θ mechanism, and the surface positive feedback. Further analysis reveals that equatorial sea surface temperature (SST) forcing, such as the El Niño–Southern Oscillation and tropical South Atlantic uniform SST, can directly drive anomalous CEPVF by changing the zonal circulation over the equator, thereby influencing SAT in the Northern Hemisphere. All results indicate that the equilibrium linkage between CEPVF and extratropical SAT is mainly a manifestation of the response of extratropical SAT to tropical forcing by virtue of PVC, and that the perspective of PVC can provide a reasonably direct and simple connection of the circulation and climate between the tropics and the mid–high latitudes.