Medium-range to seasonal precursor conditions to higher latitude landfalls of extratropically transitioning hurricanes

Statistical composites of synoptic and climatic patterns leading to observed tracks of North Atlantic tropical cyclones (TCs) during and after extratropical transition (ET) are presented for transitioning tropical cyclones over a 34 year (1970-2003) time period. Historical values of major climate indices are correlated to the observed number and percentage of transitioning tropical cyclones as determined from the National Hurricane Center Best Track (Jarvinen 1984; Neumann et al. 1993) database on a monthly basis for a given month and the three months prior to it during the Atlantic hurricane season. A climatology of the synoptic patterns in the 60 days leading to transition is developed and analyzed for statistically significant differences from a full climatology to determine large- and small-scale patterns favorable for a particular transition evolution. Distinctions are drawn between cases of all transitioning TCs and those that impact a particular region (New England, Canadian Maritimes, Europe, or some combination thereof) with respect to the findings.

It is shown that with respect to the major climate indices, a few select statistically significant (to 90% or higher) correlations are demonstrated for the composite cases. To a large degree, it is believed that the broad-scale patterns contributing to extratropical transition across the basin or across a given subset of the basin become diffuse in the monthly values of the climate indices. Thus, those indices that most greatly reflect consistent longer-term synoptic patterns that may impact ET (e.g. blocking regimes across the North Atlantic) are those which have the greatest correlations to the frequency of ET. A weak correlaton is shown between the indices that are well-correlated with overall tropical cyclone activity, such as the Southern Oscillation Index (SOI), and the total number of transitioning cyclones across the basin..

Statistically significant (to 99%) results are noted, particularly at the surface, through the 60 day climatology prior to the approach of a TC to a given region (or, in the full ET composite, prior to transition). Upper-level features are progressively less significant with height from 850hPa up to 200hPa; however, those features that are significant at upper levels are often found in conjunction with stronger surface anomalies (e.g. deviations from climatology), suggesting that the deepest synoptic features may have the greatest impacts upon tracks of storms during and prior to transition. Rossby wave patterns are evident within each composite through the entire lag climatology, becoming progressively less significant and well-defined back into the past. Positive phases of the Madden-Julian Oscillation (MJO) are noted within all of the composites across the tropical basins within the month leading to transition into the midlatitudes, suggesting a connection between an enhanced potential for TC formation and ultimately undergoing ET. Other significant impacts are noted with polar-latitude surface pressure anomalies, predominantly in the 20-40 days prior to transition, suggesting a transitional correlation between midlatitude TC impacts and the Arctic Oscillation (AO). Finally, there may be relationships between Pacific Typhoons and future Atlantic transitions; this relationship is being investigated.

The results presented here will ultimately lead to a statistical formulation toward predicting where a tropical cyclone, once formed, may ultimately impact in the western and northern Atlantic.