Tropical Cyclogenesis in the Subtropics Andy Hulme AOS 575 May 9

Background Formations: Tropical-only or baroclinically-enhanced Baroclinic storm counts may have no relationship to basin- wide conditions Hindcasts improved with baroclinic systems thrown out (Hess et al. 1995) Forecasts use parameters from deep tropical locations only (e.g. SOI, African precip, Caribbean SLP) DeMaria et al. (2001) Hypothesis: Deep Tropics TCs result from basinwide environmental conditions, whereas Baroclinic TCs are a random result of intrusions of mid-latitude weather systems

Active Subtropical

Active Tropical

R(DC,SC) = Storm Counts

ACE all year vs. ASO R(taa,taf) = 0.97 R(saa,saf) = 0.93 R(daa,daf) = 0.99 deep trop vs. subtrop R(daa,saa) = R(daf,saf) = R(daf3,saf3) = R(daf15,saf15) = -0.65

Compositing 26 storms that formed in box 1000 mb height CI = 90%

Compositing 200 mb height CI = 90%

Correlation/ Regression Time series: box averaged 1000 mb height anomaly during ASO ( ) Regressed onto 200 mb height

EOF Analysis Time series: box averaged 1000 mb height anomaly during ASO ( )

Conclusions Subtropical activity more consistent during the season and in yearly storm counts Subtropical activity negatively correlated to deep-tropical activity Upper-level troughs precede most subtropical developments Subtropical SLP is related to mid-latitude weather systems

Future Ideas Classify subtropic dominated years and conditions that define them Throw out subtropical TC numbers and rerun TC activity hindcasts Find conditions leading to subtropical developments and forecast them seperately Look at potential instability instead of SST