Midlatitude-tropical intrusions and extrusions: territory invasion

The transitioning nature of the atmosphere, especially in terms of midlatitude-tropical interactions, engenders considerable forecast difficulty and case-study material. The introduction of a midlatitude feature such as a trough fracture (wave breaking) or potential vorticity (PV) tail into a tropical environment may induce tropical transition (TT) or seed a powerful cut-off low pressure system. Conversely, the recurvature and/or extratropical transition (ET) of a tropical cyclone may advect warmer, moister air poleward (heat pool) to affect the longwave pattern for many days both upstream and downstream (i.e. blocking onset).

This presentation is a detailed extension of Hart and Grumm (2001), which introduced an objective ranking system for synoptic-scale events as a metric of atmospheric rarity. Here, the ERA40 (plus EC operational) reanalysis is used to construct an overarching global database of normalized departures from climatology in terms of the standard deviation of a variable (i.e. surface and aloft) from a long-term mean. This method has the distinct advantage of quantitatively assessing weather extremes built upon an evolving (day by day, point by point) climatology.

Midlatitude/tropical interactions/transitions are easily detected as examples of highly anomalous events (sigmas away from the mean) both in geopotential height and temperature throughout the troposphere. Along with a cursory climatology, specific (extra)tropical transition, heat pool, and trough fracture cases will be shown to highlight the utility of detecting, tracking, and assessing the predictability of these high-impact events using a sigma-schema technique. A rich tapestry of synoptic-dynamical structure on a variety of spatiotemporal scales results from this analysis germane to predictability studies