Breakdown of potential vorticity–based equivalent latitude as a vortex‐centered coordinate in the polar winter mesosphere

Abstract

Potential vorticity (PV) and stream function (ψ) are useful vortex‐centered coordinates for studying the winter stratosphere, since both exhibit monotonic relationships with tracer constituents. At each isentropic level in the stratosphere and mesosphere, ψ varies monotonically normal to the jet axis and to tracer contours, and can thus be considered vortex‐centered. In the stratosphere, PV is anticorrelated with ψ and is also vortex‐centered. We show that, contrary to this, above ∼3000 K (∼60 km) the PV‐ψ anticorrelation breaks down up to 80% of the time. Over five years of Goddard Earth Observing System analyses are used to depict the seasonal and spatial variation of the relationship between PV and ψ. The deviation of PV from a vortex‐centered coordinate is estimated by calculating differences between the mean latitude of the highest PV band and the most polar mean latitude for any PV band in the hemisphere. If PV is anticorrelated with ψ, and thus vortex centered, these differences should be small. In both winter hemispheres, latitude differences are less than 5° below 55 km but exceed 15° above 60 km (∼0.5 hPa, ∼3000 K). In the mesosphere, regions of small PV are usually found in the vortex core and the largest PV values are located along the subtropical jet. The relationship between PV and ψ in the polar lower mesosphere is interpreted in the context of the gravity wave driven warm anomaly, the associated wave breaking regime, and local static stability anomalies which affect PV much more than ψ.