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WEST AFRICAN STORM TRACKS AND THEIR RELATIONSHIP WITH ATLANTIC TROPICAL CYCLONES Susanna Hopsch Department of Earth and Atmospheric Sciences University at Albany/SUNY,NY Funded by: NSF (PTAEO:1023911-1-24796) NOAA (NA03OAR4310007) Motivation • Previous studies have shown a strong relationship between Sahelian rainfall variability and Atlantic tropical cyclone variability (e.g. Landsea and Gray, 1992) • This may be associated with: (i) Large scale/teleconnections and changes in the environment where storms form (e.g. shear) (ii) Variability in the West African weather systems • Improve knowledge about nature and variability of West African storm tracks METHOD Data set: • The ECMWF ERA40 data set 1958 –2002 is used to track relative vorticity centers • Method used for identification as of Hodges (1995) Results shown here are based on analysis of 850hPa track statistics How to choose a track Thresholds: • 0.5 x 10-5 s-1 intensity • lifetimes > 2 days • travel > 10o lon/lat 850hPa Climatology • ERA40 Track density, May – Nov, 1958-2002 • MDR region outlined in red • What are the relative roles of northern and southern tracks on TC activity in the MDR? Seasonal cycle statistics maps for eastern MDR storm tracks Jul Aug Sep Oct Seasonal cycle continued… Jul Aug Sep Oct 6 30 5 25 4 20 3 15 2 10 1 5 0 0 May Jun MDR TC's Jul Aug NHC TC's Sep South tracks Oct Nov Dec 200-850hPa Shear mean shear numbers Seasonal cycle Number of vortices along the southern and northern track: Jul - Oct tracks (10-20W) 25 number 20 15 North track 10 5 2002 1998 1994 1990 1986 1982 1978 1974 1970 1966 1962 1958 0 year NT 850 hPa, 19-28N 11 yr RM NT to MDR • noticeable low-frequency variability found for south tracks, which is associated with moist convection and precipitation Jul - Oct tracks (10-20W) 25 • northern track has negligible low-frequency variability 15 10 5 year ST 850 hPa, 9-18N 11 yr RM ST to MDR 2002 1998 1994 1990 1986 1982 1978 1974 1970 1966 1962 0 1958 number 20 South track • year-to-year variability for both northern and southern tracks and their efficiency to reach the MDR is large Composite of major hurricane index and all storm tracks for active/inactive years (Jul – Oct) Active Inactive Strong contributions from southern track and oceanic genesis maximum Genesis density for active years ( 1 ) Can be explained by low-freq. variability Interannual variability Jul - Oct tracks (10-20W) no. tracks -11yr run. mean 10 5 0 -5 year ST, 9-18N NT, 19-28N 2002 1998 1994 1990 1986 1982 1978 1974 1970 1966 1962 1958 -10 Why are ST and tropical storms poorly correlated on interannual scales? 1988 MDR best track data Vorticity tracks and SST-anom MDR best track data Vorticity tracks and SST-anom 1989 Does the synoptic scale offer clues? Example: using the 2-6 day filtered v-variance MDR tropical storms MDR hurricanes MDR major hurricanes Interannual correlation of tropical storms in the MDR (from best track data) and 2-6 day filtered meridional wind variance from the 850 hPalevel for July through October for 1963 to 1996 from ERA40. Contouring starts at 95% significance level. Summary • Seasonal variation in e.g. number density and intensity coincides with that of Atlantic tropical cyclone activity • Low-frequency variability of south tracks correlates well with West African precipitation, SSTs, TC activity etc • Interannual variability is large but uncorrelated to Atlantic tropical cyclone activity etc • The synoptic scale AEW are important to consider and correlate well with TCs …discussion • inspection of two extreme years (’88 vs. 89) shows that location of track-genesis might be important to consider for TC activity • can synoptic scale weather systems (AEWs) be used to determine whether the vorticity tracks are in a favorable position for continued existence or even intensification Future Work: • Need to consider the vertical structure of the tracked storms • Are tracks that are associated with named storms found in strong synoptic AEWs? • What are the differences between land-based storm tracks and ocean genesis storm tracks?