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 statistics maps for eastern MDR storm tracks

Jul Aug

Tra

ck d

ensi

ty

Sep Oct

Seasonal cycle continued…

AugJul

Gen

esis

den

sity

OctSep

Seasonal cycle

0

1

2

3

4

5

6

May Jun Jul Aug Sep Oct Nov Dec

nu

mb

ers

0

5

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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

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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

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1982

1986

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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?