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The Role of the West African Monsoon on Atlantic Tropical Cyclone Activity

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Title: The Role of the West African Monsoon on Atlantic Tropical Cyclone Activity


1
The Role of the West African Monsoon on Atlantic
Tropical Cyclone Activity
  •  THIS TALK
  • Introduction into the WAM
  • Atlantic Tropical Cyclone Variability
  • 3. A Climate Perspective
  • 4. A Weather Perspective
  • 5. Final Comments

Acknowledgements Anantha Aiyyer (North Carolina
State University) Gareth Berry (SUNY at
Albany) Susanna Hopsch (SUNY at Albany) George
Kiladis (ESRL, Boulder) Nick Hall (Universite
Paul Sabatier, Toulouse) NOAA and NSF
2
1. Introduction to the West African Monsoon
NDVI image for 21-31 August 2000, from Pathfinder
AVHRR, highlighting the marked meridional
gradients in surface conditions over tropical
North Africa and zonal symmetry.
3
1. Introduction to the West African Monsoon
Key features of the West African Monsoon Climate
System during Boreal summer
Heat Low
SAL
AEJ
ITCZ
Cold Tongue
4
1. Introduction to the West African Monsoon
Key features of the West African Monsoon Climate
System during Boreal summer
Heat Low
SAL
AEJ
Sahel
ITCZ
Cold Tongue
5
1. Introduction to the West African Monsoon
Motivation Societal need for improved climate
prediction over West Africa
Time series (1941-2001) of average normalized
April-October rainfall departure for 20 stations
in the West African Soudano-Sahel zone (11-18N
and West of 10E) following methodology of Lamb
and Peppler, 1992).
6
1. Introduction to the West African Monsoon
Motivation Societal need for improved climate
prediction over West Africa
What will happen next?
Time series (1941-2001) of average normalized
April-October rainfall departure for 20 stations
in the West African Soudano-Sahel zone (11-18N
and West of 10E) following methodology of Lamb
and Peppler, 1992).
7
1. Introduction to the West African Monsoon

8
1. Introduction to the West African Monsoon

?
9
1. Introduction to the West African Monsoon
Key weather systems in the West African and
Tropical Atlantic regions An ideal region to
study scale interactions
SAL
AEWs
TC
MCSs
10
1. Introduction to the West African Monsoon
11
2. Atlantic Tropical Cyclone Variability
12
2. Atlantic Tropical Cyclone Variability
There exists marked Atlantic Tropical Cyclone
Variability (ATCV).
What are the causes of this variability?
13
2. Atlantic Tropical Cyclone Variability
Known factors Atlantic Sea Surface
Temperatures ENSO West African rainfall Phase
of QBO!!!
14
2. Atlantic Tropical Cyclone Variability
Known factors Atlantic Sea Surface
Temperatures ENSO West African rainfall Phase
of QBO!!!
15
2. Atlantic Tropical Cyclone Variability
Goldenberg and Shapiro (1996) Linear correlation
coefficients ENSO ATCV -0.41 Sahel
Rainfall ATCV 0.70 This is usually
interpreted in terms of the impact ENSO and Sahel
rainfall have on the large-scale environment
where tropical cyclones form e.g. vertical shear.
16
3. A Climate Perspective
Aiyyer and Thorncroft (2008)
17
3. A Climate Perspective
Understanding the processes that influence the
MDR shear and its variability is very
important West Africa and East Pacific both
provide important anomalous heat sources that can
impact the MDR shear through tropical
teleconnections
Thorncroft and Pytharoulis (2001)
18
3. A Climate Perspective Approach
Data NCEP-NCAR Reanalyses , 1948-2004 Here we
take a shear-centered view focusing on
May-October vertical wind shear between 850mb and
200mb We separate the wind data into Low
Frequency (LF, gt8yrs) and High Frequency (HF,
1.5-8 yrs) components Dominant modes of
vertical shear are extracted using EOF analysis
of LF and HF shear.
Aiyyer and Thorncroft (2008)
19
3. A Climate Perspective
Shading shows percentage variance in shear
explained
20
3. A Climate Perspective
21
3. A Climate Perspective
22
3. A Climate Perspective
23
3. A Climate Perspective
24
3. A Climate Perspective
25
3. A Climate Perspective
26
3. A Climate Perspective
27
3. A Climate Perspective
28
3. A Climate Perspective
Some Conclusions Variability in the large-scale
environment in the tropical Atlantic strongly
influences ATCV. ATCV is influenced by ENSO
and West African rainfall associated with
changes in shear patterns in the MDR (consistent
with Bell and Chelliah, 2006) ENSO is more
important on interannual timescales West African
rainfall is more important on multidecadal
timescales
29
4. A Weather Perspective
Most Atlantic tropical cyclones form from African
weather systems but which one?
30
Hurricane Bonnie
A reminder of the importance of West African
weather systems on hurricanes
Hurricane Charlie
Hurricane Frances
Hurricane Ivan
Provided by A. Aiyyer
31
4. A Weather Perspective
OLR and 850 hPa Flow Regressed against
TD-filtered OLR (scaled -20 W m2) at 10?N, 10?W
for June-September 1979-1993
Day 0
Streamfunction (contours 1 X 105 m2 s-1) Wind
(vectors, largest around 2 m s-1) OLR (shading
starts at /- 6 W s-2), negative blue
Kiladis, Thorncroft, Hall (2006)
32
OLR and 850 hPa Flow Regressed against
TD-filtered OLR (scaled -20 W m2) at 10?N, 10?W
for June-September 1979-1993
Day-4
Streamfunction (contours 1 X 105 m2 s-1) Wind
(vectors, largest around 2 m s-1) OLR (shading
starts at /- 6 W s-2), negative blue
33
OLR and 850 hPa Flow Regressed against
TD-filtered OLR (scaled -20 W m2) at 10?N, 10?W
for June-September 1979-1993
Day-3
Streamfunction (contours 1 X 105 m2 s-1) Wind
(vectors, largest around 2 m s-1) OLR (shading
starts at /- 6 W s-2), negative blue
34
OLR and 850 hPa Flow Regressed against
TD-filtered OLR (scaled -20 W m2) at 10?N, 10?W
for June-September 1979-1993
Day-2
Streamfunction (contours 1 X 105 m2 s-1) Wind
(vectors, largest around 2 m s-1) OLR (shading
starts at /- 6 W s-2), negative blue
35
OLR and 850 hPa Flow Regressed against
TD-filtered OLR (scaled -20 W m2) at 10?N, 10?W
for June-September 1979-1993
Day-1
Streamfunction (contours 1 X 105 m2 s-1) Wind
(vectors, largest around 2 m s-1) OLR (shading
starts at /- 6 W s-2), negative blue
36
OLR and 850 hPa Flow Regressed against
TD-filtered OLR (scaled -20 W m2) at 10?N, 10?W
for June-September 1979-1993
Day 0
Streamfunction (contours 1 X 105 m2 s-1) Wind
(vectors, largest around 2 m s-1) OLR (shading
starts at /- 6 W s-2), negative blue
37
OLR and 850 hPa Flow Regressed against
TD-filtered OLR (scaled -20 W m2) at 10?N, 10?W
for June-September 1979-1993
Day1
Streamfunction (contours 1 X 105 m2 s-1) Wind
(vectors, largest around 2 m s-1) OLR (shading
starts at /- 6 W s-2), negative blue
38
4. A Weather Perspective Multi-scale
Diagnostics
315K Potential Vorticity (Coloured contours every
0.1PVU greater than 0.1 PVU) with 700hPa trough
lines and easterly jet axes from the GFS analysis
(1 degree resolution), overlaid on METEOSAT-7 IR
imagery.
See Berry, Thorncroft and Hewson (2007)
39
4. A Weather Perspective Multi-scale
Diagnostics
315K Potential Vorticity (Coloured contours every
0.1PVU greater than 0.1 PVU) with 700hPa trough
lines and easterly jet axes from the GFS analysis
(1 degree resolution), overlaid on METEOSAT-7 IR
imagery.
40
4. A Weather Perspective Multi-scale
Diagnostics
315K Potential Vorticity (Coloured contours every
0.1PVU greater than 0.1 PVU) with 700hPa trough
lines and easterly jet axes from the GFS analysis
(1 degree resolution), overlaid on METEOSAT-7 IR
imagery.
41
4. A Weather Perspective Multi-scale
Diagnostics
315K Potential Vorticity (Coloured contours every
0.1PVU greater than 0.1 PVU) with 700hPa trough
lines and easterly jet axes from the GFS analysis
(1 degree resolution), overlaid on METEOSAT-7 IR
imagery.
42
4. A Weather Perspective Multi-scale
Diagnostics
315K Potential Vorticity (Coloured contours every
0.1PVU greater than 0.1 PVU) with 700hPa trough
lines and easterly jet axes from the GFS analysis
(1 degree resolution), overlaid on METEOSAT-7 IR
imagery.
43
4. A Weather Perspective Multi-scale
Diagnostics
315K Potential Vorticity (Coloured contours every
0.1PVU greater than 0.1 PVU) with 700hPa trough
lines and easterly jet axes from the GFS analysis
(1 degree resolution), overlaid on METEOSAT-7 IR
imagery.
44
4. A Weather Perspective Multi-scale
Diagnostics
315K Potential Vorticity (Coloured contours every
0.1PVU greater than 0.1 PVU) with 700hPa trough
lines and easterly jet axes from the GFS analysis
(1 degree resolution), overlaid on METEOSAT-7 IR
imagery.
45
4. A Weather Perspective Multi-scale
Diagnostics
315K Potential Vorticity (Coloured contours every
0.1PVU greater than 0.1 PVU) with 700hPa trough
lines and easterly jet axes from the GFS analysis
(1 degree resolution), overlaid on METEOSAT-7 IR
imagery.
46
4. A Weather Perspective Multi-scale
Diagnostics
315K Potential Vorticity (Coloured contours every
0.1PVU greater than 0.1 PVU) with 700hPa trough
lines and easterly jet axes from the GFS analysis
(1 degree resolution), overlaid on METEOSAT-7 IR
imagery.
47
4. A Weather Perspective Multi-scale
Diagnostics
315K Potential Vorticity (Coloured contours every
0.1PVU greater than 0.1 PVU) with 700hPa trough
lines and easterly jet axes from the GFS analysis
(1 degree resolution), overlaid on METEOSAT-7 IR
imagery.
48
4. A Weather Perspective Multi-scale
Diagnostics
315K Potential Vorticity (Coloured contours every
0.1PVU greater than 0.1 PVU) with 700hPa trough
lines and easterly jet axes from the GFS analysis
(1 degree resolution), overlaid on METEOSAT-7 IR
imagery.
49
4. A Weather Perspective Multi-scale
Diagnostics
315K Potential Vorticity (Coloured contours every
0.1PVU greater than 0.1 PVU) with 700hPa trough
lines and easterly jet axes from the GFS analysis
(1 degree resolution), overlaid on METEOSAT-7 IR
imagery.
50
4. A Weather Perspective Multi-scale
Diagnostics
315K Potential Vorticity (Coloured contours every
0.1PVU greater than 0.1 PVU) with 700hPa trough
lines and easterly jet axes from the GFS analysis
(1 degree resolution), overlaid on METEOSAT-7 IR
imagery.
51
4. A Weather Perspective Multi-scale
Diagnostics
315K Potential Vorticity (Coloured contours every
0.1PVU greater than 0.1 PVU) with 700hPa trough
lines and easterly jet axes from the GFS analysis
(1 degree resolution), overlaid on METEOSAT-7 IR
imagery.
52
4. A Weather Perspective Multi-scale
Diagnostics
315K Potential Vorticity (Coloured contours every
0.1PVU greater than 0.1 PVU) with 700hPa trough
lines and easterly jet axes from the GFS analysis
(1 degree resolution), overlaid on METEOSAT-7 IR
imagery.
53
4. A Weather Perspective Multi-scale
Diagnostics
315K Potential Vorticity (Coloured contours every
0.1PVU greater than 0.1 PVU) with 700hPa trough
lines and easterly jet axes from the GFS analysis
(1 degree resolution), overlaid on METEOSAT-7 IR
imagery.
54
4. A Weather Perspective Importance of the
Guinea Highlands
AEWs often get a boost before they leave
Africa associated with mergers of PV from
upstream and in situ generation. The Guinea
Highlands region is one of the wettest regions of
tropical North Africa.

GPCP rainfall (mm/day) for Aug-Sep, 1997-2007)
55
4. A Weather Perspective Importance of the
Guinea Highlands


Does the nature of the AEWs and embedded MCSs
impact the probability of tropical cyclogenesis
downstream?

Hopsch and Thorncroft, 2008
56
4. A Weather Perspective
What about variability in the weather systems?
57
4. A Weather Perspective
There is a hint that the number of strong
vortices leaving the West African coast impacts
ATCV but this is far from being a sure
case. Recent analysis in the ERA40 datset (Hopsch
et al, 2006) suggests this relationship to be
weak on interannual timescales - but not on
interdecadal timescales!
58
5. Final Comments
Multi-decadal timescales West Africa is currently
in a wet phase linked to naturally varying AMO
or an anthropogenic AMO type SST pattern
(Mann and Emanuel, 2006)? or the Atlantic
Meridional Mode (Vimont and Kossin, 2007)? The
associated low-shear is associated with enhanced
ATCV. There are more favorable seedlings
leaving West Africa in the wet phase. Vecchi and
Soden (2007) suggest that shear in the Atlantic
will increase during the next century.
59
5. Final Comments
Interannual timescales ENSO is the most important
influence on Atlantic shear and related ATCV Its
influence has increased in recent decades (not
shown) Intraseasonal? Needs more work - See
Maloney (2008) paper on MJO Daily timescales The
nature of the AEWs leaving West Africa can
influence the probability of tropical
cyclogenesis (especially in the East
Atlantic) Convection and associated PV generation
over the Guinea Highlands appears to be
particularly important and yet there are no
observations there!
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