Climate Variability of the Arctic from an Isentropic Potential Vorticity Perspective

The background state of the Arctic atmosphere under prolonged warming is analyzed by evaluating blended CFSR-CFSv2 reanalysis data from an isentropic lens, where potential temperature is used as the vertical coordinate and the principle dynamical framework is potential vorticity (PV). Since the quantity is materially conserved along adiabatic, friction-less flow, PV can be used to describe the background vorticity and static stability as well as to give insight on how diabatic processes might alter this balance. Of particular interest is how the background state of the lower troposphere has evolved in recent years to become preferential to inducing anticyclonic wind stress at the Arctic ice-ocean surface. We compare the trends in Arctic PV to the Arctic Ocean Oscillation (AOO), which entered a phase of a dominantly anticyclonic circulation of the Beaufort Gyre (BG) in the mid-to-late 1990's and has persisted since. We find that PV has a relationship with sea surface height (SSH), which in turn can be used to quantify the index of the AOO, and that the strongest relationship occurs during the ice melt season. The reduced meridional thermal gradient during the ice melt season, here designated as the summer (JJA) and autumn (SON) seasons, allows for increased meridional transport of mid-latitude air masses. This is quantified by correlating melt season PV flux and the surface heat budget as well as wintertime geopotential height and zonal wind to connect the background environment preceding the peak Arctic Oscillation (AO) signal, which can enhance or suppress the typical sea level pressure (SLP) maximum that exists over the Beaufort Sea during this time. We find that positive meridional fluxes of PV during summer correlate with increased net shortwave fluxes at the surface and increased outgoing longwave fluxes in autumn. The relationship between the influx of low-latitude PV and the surface heat budget is consistent with previous works that demonstrate the role Arctic cyclones have on sea ice transport, which has major implications on the absorption and release of heat to and from the Arctic Ocean. The transport of lower-PV air associated with latent heat release via saturation of air parcels and higher specific humidity modulates the environmental PV, stabilizing the boundary layer and reducing the cyclonic tendency over the Beaufort Sea region. This may have further impacts in reducing the baroclinicity of the Arctic and suppress cyclonic development during fall, which in return may promote conditions favorable to downwelling earlier that promote an anticyclonic ocean current to develop and persist for much of the year.