An analysis of the equatorward penetration of Rossby Wave breaks in the Polar Jet stream in the Northern Hemisphere

The penetration of upper-level troughs into low latitudes and their amplification and thinning accompanying the equatorward breaking of Rossby Waves (RWB) has been observed to trigger heavy precipitation events and massive dust storms over subtropical areas like North Africa and the Middle East. Conversely, the poleward extension of a subtropical ridge structure as part of the amplified wave is associated with heat waves and drought events in mid-latitudes.

We present a long-term (2000- 2022) analysis of RWB in the Polar Jet Stream (PJ) by analyzing the large-scale and irreversible overturning of high-PV contours on the 330K isentropic surface. The identification of RWBs and their anticyclonic or cyclonic character is followed by the further geometrical analysis of the contours: the length of the overturning; the centroid and area of the 2 PVU tongues extending equatorward and the subtropical ridges built poleward as part of the amplified wave train; the location and timing of the maximum penetration equatorward and poleward of 2 PVU contours on the isentropic surface over the time; the equivalent latitude of the 2 PVU contours; and the ratio short-circle length at the equivalent latitude to the great-circle distance along the PVU contour.

There is a clear seasonal dependence of the preferred southernmost locations where RWBs penetrate, both in latitude and longitude. Locations and seasonality coincide with those of dust outbreaks and/or precipitation. The equivalent latitude has a strong seasonal component with a superimposed small but significant upward trend, which is more prominent in summer, suggesting a poleward retreat of the polar incursions.