Identification and global climatology of upper-level troughs at low latitudes

Abstract

A new algorithm has been developed to identify upper-level troughs propagating from the extratropics into low latitudes. In contrast to previous studies the identification algorithm analyzes vertical averages of potential vorticity (VAPV) in the 400-100 hPa layer. This way the detection is less sensitive to seasonal variations and only responds to intense troughs with a coherent signal throughout the upper troposphere. Regions with VAPV values ≥ |2PVU| between 17° North and South are defined as intrusion systems. The detected systems are tracked and attributes such as lifetime, size, vertical depth and horizontal tilt are assessed. This approach is applied to ERA-40 reanalysis data for the 22-year period from 1980 to 2001. The resulting climatology indicates frequency maxima over the Pacific and Atlantic Oceans in both hemispheres during boreal winter. In boreal summer the intrusion activity is generally reduced with exception of the central North Pacific, where rather long-lived, shallow and strongly tilted systems frequently occur. The over-all geographical distribution and seasonal variations reflect the previously documented relation to mean westerly winds in the low-latitude upper troposphere. The mean lifetime of an intrusion system is nearly two days with a maximum in summer in both hemispheres. The vertical depth is on average larger during winter and in the Southern Hemisphere. The majority of intrusion systems are positively tilted. The detection is sensitive to the chosen latitudinal and VAPV thresholds with biggest effects on frequency and seasonality, while the geographical distribution is rather robust. The present study provides the basis for further investigations on tropical-extratropical interactions related to upper-troughs at low latitudes.