A seasonal and quasibiennial climatology of Rossby wave breaking in the 320-2000 K layer

A seasonal climatology of potential vorticity gradient (P_y) and Rossby wave breaking (RWB) statistics is presented for the 320-850 K layer using NCEP reanalysis data during 1979-2005, and for the 320-2000 K layer using UKMO data during 1991-2003. A primary goal is to depict the spatial extent and seasonality of RWB maxima. A striking PV gradient maximum exists at the equator throughout the layer 360-2000 K, flanked by subtropical RWB maxima, integral components of the Lagrangian cross-equatorial flow. Strong RWB occurs in the polar night vortex where beta is small. Over the summer pole strong poleward RWB associated with synoptic waves decays into small amplitude motions in the upper stratosphere, where heating gradients cause P_y < 0. Seven distinct spatial regimes are linked to three different dynamical causes of reversals: wave breaking associated with westerly jets, a combined barotropic/inertial instability in cross-equatorial flow, and on the periphery of monsoon anticyclones.

When there is westerly shear associated with the quasibiennial oscillation (QBO), P_y is enhanced at the equator, and is accompanied by increased RWB in the subtropics. During boreal winter when QBO westerly shear is in the lower stratosphere, the north polar night jet is more symmetric, with an elongated trough over eastern Canada and a diminished Aleutian High. When QBO westerly shear approaches the tropopause, subtropical westerly jets extend farther across the Pacific, with increased RWB on their poleward sides. This causes a poleward shift of the East Pacific troughs toward Northern California and Southern Chile, with reduced westerly flow at the equatorial tropopause, hence a reduced Walker circulation.