1. Investigating the Correlation Between Equatorial African Rainfall and Tropical Cyclone Activity in the
North Atlantic Basin
Michael Willette
University of Northern Colorado, MET 452, Spring 2016
Introduction
The North Atlantic’s tropical cyclone season has the
potential to affect millions of people living along the eastern
seaboard of the United States, Mexico, and the Caribbean.
Predicting the variability of tropical cyclone activity each
year is vital for human safety and the protection of economic
interests. Although interdecadel oscillations, such as the
NAO, may be responsible for some of this variability, the
formation and movement of African waves that act as
“seeds” for tropical cyclones is likely more important. This
study acts to investigate this possible correlation between
the North Atlantic tropical cyclone activity and rainfall
anomalies in Equatorial Africa.
Fig. 1. A figure from Jackson, Nicholson, and Klotter (2009) that shows the five-year
mean number of lightning flashes for the year and for the SON season. These areas of
lightning maximas are indicative of the genesis of MCCs and African waves that will move
out to the Atlantic ocean,
Figure 2: A storm track density analysis from Hopsch et al. ((2007) relating the origination of storm tracks in
equatorial Africa to the positioning of tropical cylcone formation in the Atlantic ocean basin. Notice that the
origination comes from two hotspots in northern and central Africa.
Movement into the Atlantic Basin
In order to get a comprehensive analysis of the link between
tropical cyclone genesis in the North Atlantic basin and
rainfall in equatorial (and specifically the Sahel region of
Africa), the location of where these equatorial waves move
off the western African coast into the North Atlantic is vital.
Hopsch et. al. (2006) state that there is year-to-year
variability in the positioning of the storm tacks of equatorial
waves as they move off the coast of western Africa.
Although SSTs and west African Rainfall can explain some of
the variability, variation of the meridional wind shows the
strongest positive correlation with tropical cyclone activity in
year-to-year time scales. (Hopsch et. al., 2006) There ends
up being two sources for storm tracks over the Atlantic;
north and south of 15˚N in North Africa (Hopsch et. al.,
2006), and that the southern storm track provides the
highest amount of tropical cyclone activity.
The Origination of African Waves
The western portion of equatorial Africa undergoes some of
the most unstable processes leading to intense
thunderstorms and arguably some of the least understood.
The climate is warm, wet, and volatile meteorologically. The
area of maximum radiation from the sun and its associated
ITCZ Studies show that wind patterns largely drive the
location and intensity of these convective events. With the
core at 600mb, the positioning of the African Easterly Jet
corresponds well with convective and lightning maximums in
equatorial Africa. (Jackson et. al., 2009) Specifically, the
location of the right front quadrant of jet streaks associated
with the African Easterly Jet appears to focus the most
intense convective development. (Jackson et. al., 2009)
Figure 3: A comparison of Burundi Highlands Precipitation Anolamlies from data produced by Bonnefille and
Chalie (2000) on the left and a statistical model of tropical cyclone activity from Mann et al. (2009). Both sets of
data were then compiled in Excel in this study. There are some correlations between the two plots, mostly being
correlations when precipitation and tropical cyclone activity are both low.
Discussion of Methods and Results
To investigate this correlation, a comparison of data
sets between two studies is used. Paleo-pollen proxy
data from Bonnefille and Chalie (2000) is compared
with a statistical model produced by Mann et al.
(2009). The results are shown in figure 3 and some
interesting correlations (or lack thereof) are seen.
• Positive correlations are hard to find. A good
correlation can be seen at 600 AD.
• There seems to be some sort of lag in correlations,
but is hard to determine.
• Negative anomalies seem to correlate better than
positive anomalies
• The decrease in tropical cyclone activity starting at
1000AD is not reflected in the precipitation
anomalies
• The lack of data points in the precipitation
anomalies makes it hard for correlation with the
statistical model
Summary
Overall, there is evidence for correlation between
equatorial rainfall in Africa and tropical cyclone activity
in the North Atlantic. Theoretically, there should be a
nice correlation between the two regions with time
periods of high equatorial African rainfall correlating
with areas of high tropical cyclone activity and vice
versa. There are areas of local minima in the TA activity
that correlate with minima’s in rainfall in Africa,
specifically at 1380AD, 1150 AD, and 580 AD. Since low
rainfall in Equatorial Africa would produce fewer
waves that propagate out to the eastern Atlantic and
provide the “seeds” for tropical cyclone development,
less tropical cyclones would form. What is interesting is
that this correlation does not apply to maximas.
Possibly, there are other forces that are responsible for
letting these equatorial waves become tropical
cyclones, such as wind shear, sea surface
temperatures, and multi-decadal weather/ocean
patterns not investigated in this study.
References
Hopsch, S. B., Thorncroft, C. D., Hodges, K. and Aiyyer, A., 2007: West African storm
tracks and their relationship to Atlantictropical
cyclones. J. Climate, 20, 2468-2483, doi: 10.1175/JCLI4139.1
Bonnefille, R., Chalie, F., 2000: Pollen-inferredtime-series from equatorial
mountains, Africa, the last 40 kyr BP. Global and Planetary
Climate Change, 26, 25-50.
Mann, M. E , Kozar, M. E.,. Emanuel, K. A., and Evans, J. L., 2009: Long-term
variations of North Atlantic tropicalcyclone activity
downscaled from a coupled model simulation of the last
millennium.Journal of Geophysical Research: Atmospheres, 118,
13383-13392.