A Global Climatology of Baroclinically Influenced Tropical Cyclogenesis Authors: Ron McTaggart-Cowan, et. Al. Monthly Weather Review June 2013

Purpose The authors had previously produced a dynamically based genesis pathway climatology for the North Atlantic in hopes of expanding upon the understanding, and thusly the prediction of TC genesis. (McTaggart-Cowan, 2008) This paper expands upon the climatology to include all of the world’s active tropical basins.

Data and Methodology Period of “combined reanalysis” – NCEP-NCAR reanalysis I – ERA-40 – ERA-Interim – JRA-25 (Japan Meteorological Agency) T₀ defined as first report in IBTrACS

Data and Methodology 2 metrics were used for characterization Q – average convergence of the hPa Q vector. – Represents synoptic scale forcing for ascent. TH – max difference between 1000–700-hPa thickness values in two hemispheres within 10⁰ of point of interest. – lower level thickness asymmetry. Note: Background state rather than vortex itself.

Data and Methodology From figure 1 of Ron McTaggart-Cowan, et. Al Do these seem like good metrics to use?

Data and Methodology Linear discriminant analysis is employed. 5 development pathways are identified. Note: different classification technique was used here from the Atlantic paper. – As a result 1 less category is used (transient trough interaction pathway is omitted)

The 5 development pathways The reclassification of the North Atlantic TC pathways provides a look at the classification metric space.

Global climatology frequency Non baroclinic –within 15⁰ of equator Low level baroclinic – areas with land- sea contrasts or SST gradients Trough induced – equatorward and west of Rossby wave breaking centers Weak TT – near TUTT axis Strong TT – higher latitudes due to Rossby wave breaking.

Classification Accuracy Assessment Do you think this is a good technique to accomplish the objective? Do you foresee any issues that might influence the accuracy of the classification? A thorough comparison with previous studies and a subjectively classificatied sample was done. [ Gray (1968), Hess et al.(1995), Elsner et al.(1996) ] Their classification compared very favorably!

Relative frequency of TC development pathway by basin Non-baroclinic pathway dominates in all basins. But, secondary pathway varies. Strong TT is generally least prolific What can we learn from this figure?

Location of developments via each pathway.

North Atlantic Figure 8

East Pacific Figure 9

West Pacific Figure 10

Bay of Bengal – North Indian Figure 11

Arabian Sea – North Indian Figure 12

South Indian Figure 13

South Pacific Figure 14

Development Efficiency Figure 15 What can we learn form this figure? Equivalent to dividing figure 7 by figure 4

Climatology in Metric Space Figure 16 EnvironmentalGenesis events How do we interperate these figures? Left: are all of the formation events plotted in metric space along with formation density. Note maximum in non-baroclinic space Right: is the percentage of time that the metrics lie close to a given position in metric space.

TC efficiency in Metric Space Figure 16 Figure 17 Yield value of 1 is average The presence of an upper level disturbance is more favorable than purely non-baroclinic (Gray1988) Fig. B divided by fig. A Trough Ridge

Discussion Non-baroclinic TC developments dominant globally, but a “rich spectrum” of formation events happen under baroclinic conditions. – 70% non-baroclinic – 8% Low level baroclinic – 6% Trough induced – 11% weak Tropical transition – 5% Strong TT Could be combined with traditional genesis potential evaluation techniques to develop a pathway conditional index. – It is hoped that this could be an improvement on current indices.

Discussion So, what do you think? – Were they successful? – Was this a worthwhile exercise? – Could it help improve genesis prediction? – Do you feel there are any other important takeaways from the climatology?