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WEST AFRICAN STORM TRACKS AND THEIR RELATIONSHIP WITH
ATLANTIC TROPICAL CYCLONES
Susanna HopschDepartment of Earth and Atmospheric SciencesUniversity 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
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
METHOD
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
0
1
2
3
4
5
6
May Jun Jul Aug Sep Oct Nov Dec
nu
mb
ers
0
5
10
15
20
25
30
me
an
sh
ea
r
MDR TC's NHC TC's South tracks 200-850hPa Shear
Jul - Oct tracks (10-20W)
0
5
10
15
20
25
1958
1962
1966
1970
1974
1978
1982
1986
1990
1994
1998
2002
year
nu
mb
er
ST 850 hPa, 9-18N 11 yr RM ST to MDR
Jul - Oct tracks (10-20W)
0
5
10
15
20
25
1958
1962
1966
1970
1974
1978
1982
1986
1990
1994
1998
2002
year
nu
mb
er
NT 850 hPa, 19-28N 11 yr RM NT to MDR
So
uth
tra
ck
N
ort
h tr
ack
Number of vortices along the southern and northern track:
• year-to-year variability for both northern and southern tracks and their efficiency to reach the MDR is large
• noticeable low-frequency variability found for south tracks, which is associated with moist convection and precipitation
• northern track has negligible low-frequency variability
Composite of major hurricane index and all storm tracks for active/inactive years (Jul – Oct)
Active Inactive
Genesis density for active years ( 1 )
Strong contributions from southern track and oceanic genesis maximum
Can be explained by low-freq. variability
Interannual variability
Jul - Oct tracks (10-20W)
-10
-5
0
5
10
1958
1962
1966
1970
1974
1978
1982
1986
1990
1994
1998
2002
year
no
. tra
cks
-11
yr r
un
. me
an
ST, 9-18N NT, 19-28N
Why are ST and tropical storms poorly correlated on interannual scales?
1988
1989
MDR best track data
MDR best track data
Vorticity tracks and SST-anom
Vorticity tracks and SST-anom
Does the synoptic scale offer clues? Example: using the 2-6 day filtered v-variance
Interannual correlation of tropical storms in the MDR (from best track data) and 2-6 day filtered meridional wind variance from the 850 hPa-level for July through October for 1963 to 1996 from ERA40. Contouring starts at 95% significance level.
MDR tropical storms MDR hurricanes MDR major hurricanes
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?