Surface Cyclolysis in the North Pacific Ocean. Part III: Composite Local Energetics of Tropospheric-Deep Cyclone Decay Associated with Rapid Surface Cyclolysis

Abstract

Two regional local energetics composites of tropospheric-deep cyclone decay were constructed based upon 49 cyclones in the Gulf of Alaska region and 18 cyclones in the Bering Sea region whose decay was marked by rapid surface cyclolysis. Both composites indicate that surface drag is only a secondary sink of eddy kinetic energy (EKE) during the decay. This result holds even when a generous accounting is made for uncertainty in the surface drag calculation. The subordinate role of surface drag in the Gulf of Alaska region composite is particularly interesting, given that the cyclones in this composite decay in close proximity to rugged and extensive high-elevation terrain. Both composites also display two of the fundamental characteristics of the downstream development model of cyclone decay: the role of radiative dispersion as the chief sink of EKE during decay, and the occurrence of prominent downstream EKE dispersion. Furthermore, the two composites illustrate that an unusually pronounced decline in baroclinic conversion occurs simultaneously with the intense radiative dispersion. Taken together, these results suggest that the energetic decay of cyclones marked by rapid surface cyclolysis is driven from the upper troposphere, not from the surface. Some notable differences also emerge from the two composites. Considerable downstream development occurs in the immediate vicinity of the decaying cyclone in the Bering Sea region composite, but not in the Gulf of Alaska region composite. Meanwhile, the areal extent of the downstream dispersion is greater in the Gulf of Alaska region composite. The latter circumstance suggests that decay events in the Gulf of Alaska region, while not producing significant downstream development in their near vicinity, may have important energetic implications for subsequent development farther downstream over North America. The composites also indicate that the decline of EKE in the vicinity of the decaying cyclone is more pronounced in the Gulf of Alaska region. In the Bering Sea region composite, this EKE is maintained via a persistent convergence of ageostrophic geopotential flux (AGF) that emanates from regions well south of the primary cyclone. Similar evidence for the influence of upstream disturbances on the cyclone decay does not appear in the Gulf of Alaska region composite.