| Summary: | A quantification of local energy dispersion is employed to distinguish cases of downstream baroclinic development, as described by Orlanski and Sheldon, from among 41 cold-season cyclones that intensified strongly over the eastern North Pacific Ocean. Complete summaries of the eddy energy budget are calculated for each event, and about half are found to be in general accord with the proposed evolution. Almost all of this subset appear to have been influenced by a dispersion of energy from separate cyclones developing over the western North Pacific a day or two earlier. The primary source for eddy energy dispersion downstream and subsequent generation near the eastern cases is a baroclinic conversion associated with ascent in the warm sector of the upstream cyclones. The importance of downstream baroclinic development is confirmed for one eastern North Pacific cyclone in two complementary ways. First, the original eddy energy diagnosis is compared to one based on wave activity. In terms of local group velocity, only minor differences are found during much of the initial evolution. It is only once the tropopause undulations lose their wave-like appearance that the group velocity calculated using eddy energy becomes faster than that depicted by wave activity. Second, by employing numerical simulations, the importance of downstream baroclinic development to the intensification of this cyclone is quantified. Various initial conditions are produced using potential vorticity inversion. Simulations in which an upstream trough/ridge couplet are removed from the initial conditions result in both the absence of a downstream baroclinic development and a weakening of the downstream surface cyclone. The remainder of this study investigates the relationship between cold-season cyclones and sea surface temperature anomalies for small groups of strong cyclones occurring in the western North Pacific region. Previous studies have emphasized the importance of the western ocean boundary currents and their strong sea surface temperature gradients to rapid cyclone development. Physical mechanisms governing this relationship have been studied extensively elsewhere. Here, proxy evidence of systematic changes in the role of surface heat and moisture fluxes during the cold season is presented. Cyclones of similar intensification rates are grouped according to their occurrence either during midwinter or during the early and late cold season. Systematic differences in sea surface temperature anomalies beneath these two groups are interpreted as a proxy for corresponding differences in preconditioning by the upperoceanic mixed layer. Submonthly sea level pressure variations for the same North Pacific cyclones appear to support an interpretation in terms of an upward oceanic influence. It is suggested that the role of preconditioning heat fluxes in cyclones varies because of large-scale seasonal changes in baroclinicity and in the availabilitv of water vapour already in the atmosphere. Similar differences are obtained using a group of strong western North Atlantic cyclones.
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