The Life Cycle of Upper-Level Troughs and Ridges: A Novel Detection Method, Climatologies and Lagrangian Characteristics

• Main Authors: Schemm, Sebastian, Sprenger, Michael
• Format: Text
• Language: English
• Published: 2020
• Subjects: Greenland; Pacific; North Atlantic
• Online Access: https://doi.org/10.5194/wcd-2020-9; https://wcd.copernicus.org/preprints/wcd-2020-9/

A novel method is introduced to identify and track the life cycle of upper-level troughs and ridges. The aim is to close the existing gap between methods that detect the initiation phase of upper-level Rossby wave development and methods that detect Rossby wave breaking and decaying waves. The presented method quantifies the horizontal trough and ridge orientation and identifies the corresponding trough and ridge axes. The trough and ridge axes allow us to study the dynamics of pre- and post-trough or ridge regions separately. The tracking allows us to study the temporal evolution of the trough or ridge orientation. The method is based on the curvature of the geopotential height at a given isobaric surface and is computationally efficient. First, the algorithm is introduced in detail, and several illustrative applications, such as a downstream development from the North Atlantic into the Mediterranean, and seasonal climatologies are discussed. For example, the climatological trough and ridge orientations reveal strong zonal and meridional asymmetry. Over land, most troughs and ridges are anticyclonically oriented, while they are cyclonically oriented over the main oceanic storm tracks. The cyclonic orientation increases towards the poles, while the anticyclonic orientation increases towards the equator. Trough detection frequencies are climatologically high downstream of the Rocky Mountains and over East Asia and Eastern Europe, but are remarkably low downstream of Greenland. Furthermore, the detection frequencies of troughs are high at the end of the Pacific storm track, but no comparable signal is seen over the North Atlantic. During El Niño-affected winters, troughs and ridges tilt anomalously strong cyclonically over North America and the North Atlantic, in agreement with previous findings based on traditional variance-based diagnostics such as E vectors. During La Niña the situation is essentially reversed. Finally, the identified troughs and ridges are used as starting points for 24-hour backward parcel trajectories, and a discussion of the distribution of pressure, potential temperature and potential vorticity changes along the flow path is provided to give insight into the three-dimensional nature of troughs and ridges.