We have been doing observational studies of cyclogenesis in the eastern Pacific. The previous study, presented at the last tropical conference, focused on large-scale aspects of the problem. The current study looks at specific aspects of the genesis of Hurricane Hernan in the eastern Pacific in September/October 1996. The pre-genesis state is described using ECMWF gridded analyses. A key aspect of the analysis is the use of time filtering to separate the easterly wave scale from the slowly varying background state.
The roles of and interactions among the pre-existing easterly wave, the Central American mountains, a surge in the wind field, and an unstable background state will be described. Among the results are the following:
Hernan developed in association with a wave in the easterlies that could be tracked back to Africa in longitude-time plots of the filtered v component of the wind (2-6 day period) at 700 mb. The wave crossed Central America near Lake Nicaragua with only slight distortion from the mountains. The wave intensified as it moved through a barotropically unstable background state (defined by a low-pass filter with a 20-day cutoff) in the western Caribbean and eastern Pacific. As it approached the Isthmus of Tehuantepec, the 700 mb wave organized strong northerly flow through the Isthmus in the lower troposphere. Only at that time did a surface circulation develop.
A westerly surge in the surface wind field occurred prior to depression formation. The time scale of this surge was intermediate between the wave scale and the background state variation. It appeared to represent a northward extension of the monsoon trough, the cause of which remains uncertain.
On the basis of this and previous work, it is argued that the steady supply of waves in the easterlies from upstream and the wave-induced flow through the Isthmus of Tehuantepec play a major role in the climatology of eastern Pacific tropical cyclogenesis