Geochemistry
Geophysics
Geosystems
3
G
Research Letter
Volume 9, Number 5
29 May 2008
Q05V04, doi:10.1029/2007GC001774
AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES
Published by AGU and the Geochemical Society
ISSN: 1525-2027
Click
Here
for
Full
Article
Ocean temperature forcing by aerosols across the Atlantic
tropical cyclone development region
Amato T. Evan
Cooperative Institute for Meteorological Satellite Studies, University of Wisconsin at Madison, 1225 W. Dayton Street,
Madison, Wisconsin 53706, USA (amatoe@ssec.wisc.edu)
Also at Department of Atmospheric and Oceanic Sciences, University of Wisconsin at Madison, Madison, Wisconsin
53706, USA
Andrew K. Heidinger
Cooperative Institute for Meteorological Satellite Studies, University of Wisconsin at Madison, 1225 W. Dayton Street,
Madison, Wisconsin 53706, USA
Ralf Bennartz and Val Bennington
Department of Atmospheric and Oceanic Sciences, University of Wisconsin at Madison, Madison, Wisconsin 53706, USA
Natalie M. Mahowald
Climate and Global Dynamics Division, National Center for Atmospheric Research, Boulder, Colorado 80307, USA
Hector Corrada-Bravo
Department of Computer Sciences, University of Wisconsin at Madison, Madison, Wisconsin 53706, USA
Christopher S. Velden
Cooperative Institute for Meteorological Satellite Studies, University of Wisconsin at Madison, Madison, Wisconsin
53705, USA
Gunnar Myhre
Department of Geosciences, University of Oslo, N-0315 Oslo, Norway
Now at Center for International Climate and Environmental Research, N-0318 Oslo, Norway
James P. Kossin
Cooperative Institute for Meteorological Satellite Studies, University of Wisconsin at Madison, Madison, Wisconsin
53705, USA
[1] Recent work has shown a statistical climatological link between African dust outbreaks and North
Atlantic tropical cyclone frequency and intensity. However, a definite causal link between year-to-year
changes in African dust and Atlantic tropical cyclones has yet to be proven. Here we show that variability
in Atlantic dust cover is linked to changes in tropical cyclone activity through the aerosols’ surface
radiative forcing, which has a net cooling effect on tropical Atlantic Ocean temperatures. In this manuscript
we describe a new methodology for incorporating more than 25 years of satellite observations of aerosols
into a simple model that estimates the aerosol direct effect and its impact on tropical eastern Atlantic Ocean
temperatures. The output from our model suggests that African dust outbreaks play a nonnegligible role in
the evolution of eastern Atlantic Ocean temperatures. Using the strong relationship between temperatures
Copyright 2008 by the American Geophysical Union
1 of 11
Geochemistry
Geophysics
Geosystems
3
G
evan et al.: dust forcing of tropical atlantic sst 10.1029/2007GC001774
in the so-called main development region and the seasonal power dissipation index (PDI), we estimate that
about one third of the increase in PDI over the last 25 years can be attributed to decreases in dust loadings
over the same period. Our results imply that efforts aimed at attributing causality to past variability of, or at
predicting future changes in, North Atlantic tropical cyclone activity must consider the important radiative
influence of African dust.
Components: 7117 words, 4 figures.
Keywords: tropical cyclones; mineral aerosols; climate; satellite.
Index Terms: 1616 Global Change: Climate variability (1635, 3305, 3309, 4215, 4513); 0305 Atmospheric Composition
and Structure: Aerosols and particles (0345, 4801, 4906); 3374 Atmospheric Processes: Tropical meteorology.
Received 31 July 2007; Revised 29 January 2008; Accepted 12 February 2008; Published 29 May 2008.
Evan, A. T., A. K. Heidinger, R. Bennartz, V. Bennington, N. M. Mahowald, H. Corrada-Bravo, C. S. Velden, G. Myhre, and
J. P. Kossin (2008), Ocean temperature forcing by aerosols across the Atlantic tropical cyclone development region,
Geochem. Geophys. Geosyst., 9, Q05V04, doi:10.1029/2007GC001774.
————————————
Theme: Interactions Between Climate and Tropical Cyclones on All Timescales
1. Introduction
via reductions in African dust cover, which gives way
to increased tropical cyclone activity.
[2] It is thought that anomalously warm equatorial
North Atlantic sea surface temperatures weaken the
West African monsoon, decreasing precipitation
across the Sahel during the boreal summer [Giannini
et al., 2003; Fontaine and Janicot, 1996]. Reductions in precipitation would lead to decreases in
vegetation across the Sahel [Nicholson et al.,
1998], increases in source regions for mineral
aerosols [Mahowald et al., 2002], and therefore
an increase in dust cover over the tropical North
Atlantic during the following boreal summer
[Moulin and Chiapello, 2004; Prospero and Lamb,
2003]. An increase in summertime dust activity
may have a direct dynamical effect on tropical
cyclogenesis through enhancement of the mid level
temperature inversion, increases in vertical wind
shear, and reductions in relative humidity [Dunion
and Velden, 2004; Wong and Dessler, 2005]. However, here we show that these dust plumes also
have an indirect effect on tropical cyclone activity
through cooling of sea surface temperatures. We
propose that increases in dust cover also lead to
cooler tropical North Atlantic Ocean temperatures
(in the region of 10–20°N and 15–65°W) and
reduced tropical cyclone activity [Emanuel, 1987;
Gray, 1968; Goldenberg et al., 2001; Saunders and
Harris, 1997; Shapiro and Goldenberg, 1998]. We
suggest that the opposite argument can also be made,
where anomalously cool equatorial Atlantic sea surface temperatures lead to a warming of the more
northerly tropical Atlantic during the following year,
[3] In this paper we attempt to quantify this relationship between dust and ocean temperature (and
ultimately tropical cyclone activity) over the last
quarter century using observations and output from
a radiative transfer and general circulation model. In
next section of this paper we present empirical
evidence demonstrating a statistical relationship
between African dust storm activity and North
Atlantic tropical cyclones via the oceanic cooling
associated with dust cover. The subsequent section
describes a simple radiative transfer model that
assimilates a 25-year record of satellite aerosol observations to estimate the surface aerosol forcing, and
the oceanic response to that forcing. We then utilize
the output from our satellite data driven model to
discuss the contribution dust has made to eastern
Atlantic Ocean temperatures, and estimate what
tropical cyclone activity over the last 25 years may
have looked like in the absence of dust. The paper
concludes with final remarks that put our results into
a larger context and suggests future efforts.
2. Empirical Observations
[4] To explore the oceanic cooling exhibited by
dust we created a time series of dust cover averaged over months that may be of importance to
tropical cyclone activity (May through October),
for the so-called main (or hurricane) development
region, defined here as 10–20°N and 15–65°W
2 of 11
Geochemistry
Geophysics
Geosystems
3
G
evan et al.: dust forcing of tropical atlantic sst 10.1029/2007GC001774
Figure 1. Time series of dust and tropical cyclone days. Valued on the left-hand axis is a time series of mean May –
September dust cover averaged over the hurricane development region for the period 1982 –2006. The shaded regions
represent the range of monthly values for each year, and the black line shows the mean. The dashed line is a time
series of seasonal tropical cyclone days over the hurricane development region, valued on the right-hand axis. The
dotted line is the cyclone days time series after the application of a 1-4-7-4-1 filter.
[Goldenberg and Shapiro, 1996] (Figure 1, solid
black line). Our dust index is a 25-year satellite
record of over-water dust frequency from the
Pathfinder Atmospheres Extended data set [Evan
et al., 2006a]. The gray shaded region corresponds
to monthly maximums and minimums in dustiness
for each year’s 6-month period. Here, dust activity
during the first 7 years of the record is much higher
than that of the remainder of the time series. Some
of the highest dust concentrations occurred in
1984, 1985 and 1987, and the lowest in 1996,
2004 and 2006. A time series of tropical cyclone
days [Jarvinen et al., 1984] (the number of days
where a tropical cyclonic system with 1-min-averaged maximum sustained winds greater than or
equal to 34 knots) averaged over the hurricane
development region for each year (Figure 1, dashed
line) shows that tropical cyclone activity from 1982
through 1994 is much less pronounced than over
the last 12 years of this record [Goldenberg et al.,
2001]. The dotted line in Figure 1 is the cyclone
days time series after the application of a 1-4-7-4-1
filter, and highlights the general increase over time
of tropical cyclone activity in the basin. The
interannual variability in this dust series, including
a general decrease in aerosols from the beginning
of the record until 1996 and the subsequent leveling-off of values, is somewhat similar to the
increasing time series of tropical cyclone days.
This is not surprising since there is a statistically
significant correlation between this dust record and
tropical cyclone activity in the Atlantic basin; Evan
et al. [2006b] showed a correlation coefficient
between dust and tropical cyclone days of 0.7
when ENSO was taken into account.
[5] We note that there is some contamination of
our dust record by the volcanic eruptions of El
Chichón (for 1982 values) and Mount Pinatubo
(for 1991 and 1992 values). However, the year-toyear variability in our dust record has been validated against other dust data sets [Evan et al.,
2006a], and surface observations of summertime
dustiness from Barbados do show similar results
[Chiapello et al., 2005].
[6] Monthly lead/lag correlations between our dust
time series and another of Hadley ocean temperatures [Rayner et al., 2003], both averaged over the
hurricane development region, show that consistently the strongest correlations between dust and
temperatures occur when sea surface temperature
lags behind dustiness from 1 to 3 months (Figure 2).
The plot for September dustiness is the exception,
where the strongest correlation occurs with September ocean temperatures; this may reflect the
shoaling of the mixed layer (Figure S2 in auxiliary
material) and increased ocean temperature sensitivity to changes in surface solar insolation.1 Generally, the dust time series can explain about 25%
of the observed variance in ocean temperatures at
the strongest point in the lead/lag correlations. The
plots in Figure 2 are consistent with a causal
relationship between dust and ocean temperature
[Schollaert and Merrill, 1998], implying that dust
cover has a nonnegligible effect on ocean temperature variability during the hurricane season [Evan,
2007; Lau and Kim, 2007a, 2007b]. Since the plots
1
Auxiliary materials are available in the HTML. doi:10.1029/
2007GC001774.
3 of 11
Geochemistry
Geophysics
Geosystems
3
G
evan et al.: dust forcing of tropical atlantic sst 10.1029/2007GC001774
Figure 2. Plot of lag/lead relationship between Hadley ocean temperature (SST) and dust cover averaged over the
main development region for the months of June– September. All correlations are for individual months (e.g., June
SST and June dust). Asterisks denote correlations that are significant at the 95% level on the basis of the t score of a
two-tailed t test.
in Figure 2 imply an inverse relationship between
dust and ocean temperatures on a year-to-year
basis, we suggest that changes in dust cover may
play a role in modulating the interannual and longterm variations in ocean temperature, and therefore
tropical cyclone activity [Emanuel, 2005a, 2005b;
Goldenberg et al., 2001; Landsea, 2005; Pielke,
2005; Webster et al., 2005]. Since aerosols may
skew satellite ocean temperature retrievals, we
repeated the correlation analysis in Figure 2 with
the ICOADS [Woodruff et al., 2005] data set of in
situ Sea Surface Temperature, finding similar
results (Figure S1 in auxiliary material).
[7] While these correlations do suggest a causal
relationship between dust and ocean temperatures
that is nonnegligible, they do not explicitly demonstrate that the ocean is responding to the direct
aerosol effect. The following section describes a
method for estimating the effect of aerosol cover
on ocean temperatures using our long-term satellite
record and a simple radiative transfer model.
3. Method for Quantifying the Aerosol
Impact on Ocean Temperature
[8] Using observations of aerosol optical thickness
from our satellite data set [Ignatov and Stowe,
2002a, 2002b], we estimate the aerosol radiative
forcing over the hurricane development region
from the mean difference between distributions of
surface solar insolation for an atmosphere with a
moderate background aerosol optical thickness of
0.1, and for one containing an aerosol layer whose
optical thickness is prescribed by the satellite
retrievals. Our background value of 0.1 represents
average levels of sulfates and sea salt (including
carbonaceous aerosols) in this region, in agreement
with simulations from a global aerosol-chemistry
transport model [Myhre et al., 2007]. Summertime
aerosol optical thickness is generally well above
the value of 0.1. With the exception of the two
volcanic events mentioned in the previous section,
summertime aerosol optical thickness values above
the background can be attributed to mineral species. However, this should not be surprising since
the hurricane development region is not downwind
of any major industrial regions, but is downwind of
the planet’s largest dust source.
[9] We calculate the surface aerosol forcing by
modeling the atmospheric transmittance accounting for extinction by stratospheric ozone, water
vapor, carbon dioxide, and an aerosol layer, assuming that gas absorption and scattering can be
treated independently using multiple scattering via
the Meador and Weaver two stream approximation
[Meador and Weaver, 1980]. We obtained monthly
climatological mean and standard deviation values
of stratospheric ozone and water vapor from the
4 of 11
Geochemistry
Geophysics
Geosystems
3
G
evan et al.: dust forcing of tropical atlantic sst 10.1029/2007GC001774
NCEP reanalysis [Kalnay et al., 1996]. We assumed a constant aerosol optical thickness across
the visible spectrum to be the mean of the satellite
0.68 and 0.86mm channels. In the radiative transfer
calculations we use a single scatter albedo (0.98 ±
0.01) and an asymmetry parameter (0.78 ± 0.01)
consistent with the AVHRR aerosol optical thickness retrievals set [Ignatov and Stowe, 2002a,
2002b], and recent observational [Haywood et al.,
2003] and modeling studies [Myhre et al., 2003].
[10] Cloud cover also alters the flux of shortwave
radiation incident on the ocean. To account for this
we scaled values of our solar insolation by climatological mean monthly values of cloudiness from
the International Satellite Cloud Climatology Project [Rossow and Schiffer, 1991]. It is likely that
mean monthly cloud amounts, especially those
when dust activity peaks, are artificially high since
optically thick dust plumes are probably classified
as clouds because of their thermal contrast with the
clear-sky oceanic background [Evan et al., 2006a],
therefore our aerosol forcing estimates may be
somewhat conservative.
[11] Since we used monthly climatological mean
and standard deviation values for stratospheric
ozone, water vapor, carbon dioxide, and cloudiness, and monthly mean and standard deviation
values for the satellite retrievals of aerosol optical
thickness, we employed a Monte Carlo technique,
in order to sample distributions of these variables
for creating the distributions of surface solar insolation for the dust and dust-free cases.
[12] We used climatological monthly temperature
and salinity profiles from the World Ocean Atlas
[Conkright et al., 2002] to calculate distributions of
density profiles also using a Monte Carlo technique,
and we defined the mixed layer depth to be the
point at which the potential density is 0.125 kg/m3
greater than that at the surface [Levitus, 1982]. Our
climatological mixed layer depth averaged over the
hurricane development region has a maximum in
January of 40 m, and a minimum in September of
20 m (Figure S2 in auxiliary material).
[13] We assumed all sunlight incident on the ocean
surface is distributed throughout the mixed layer
[Schollaert and Merrill, 1998] and calculated the
local change in mixed layer temperature with
respect to time in a manner similar to Schollaert
and Merrill [1998, equation 1] (equation (1)).
@T=@t ¼ ð4QÞ= rcp MLD
ð1Þ
[14] Here, DQ is the change in solar insolation
between an atmosphere with aerosols and one that
is clear (already accounting for the fraction of the
sky that is cloudy), MLD is the depth of the mixed
layer, and rcp is the density of the ocean water
times its specific heat, which we assumed to be
constant at 4.1 106Jm3K1 [Schollaert and
Merrill, 1998]. We estimated @T/@t at a 5° resolution across the hurricane development region.
[15] This methodology for estimating the aerosol
surface forcing and resultant change in ocean
temperature is a first-order approximation. Here
we are ignoring direct and indirect impacts on the
surface longwave radiation budget, oceanic heat
transport, variability in the optical properties of
dust across the solar spectrum, and interannual
variability in the mixed layer depth and cloud
cover. Although some of these assumptions may
be justified by slow ocean currents in our region of
interest [Schollaert and Merrill, 1998], they also
are reasonable since we are looking at ocean
forcing on monthly timescales, while the resultant
anomalies are dissipated on timescales of 4 –
8 months via upward radiative and turbulent energy fluxes [Deser and Timlin, 1997].
[16] We should note there is a positive correlation
between cloudiness and dust across the tropical
Atlantic. However, this is likely an artifact in the
cloud detection (as previously discussed). While
there is a long-term downward trend in Atlantic
dustiness [Evan et al., 2007] and global aerosol
optical thickness [Mishchenko et al., 2007], there is
no trend in cloudiness over the tropical Atlantic
[Heidinger et al., 2007]. Furthermore, we are
ignoring aerosol indirect effects on cloud condensation nuclei number concentration [Forster et al.,
2007, among others], which would also act to cool
the Atlantic.
4. Model Results
[17] From Figure 3a, mean monthly aerosol forcing
values, the average monthly value of DQ strongly
reflects the seasonality of solar insolation (maximum in June, minimum in December), cloudiness
(maximum in October, minimum in April), and dust
activity (maximums in March and June, minimum
in November). The average monthly aerosol forcing
is strongest in June, and weakest in December, with
the range of values being about 7 W/m2. Our
climatological mean of 8 W/m2 compares well
to recent model-based estimates [Yoshioka et al.,
2007]. Furthermore, the variability in the dust
5 of 11
Geochemistry
Geophysics
Geosystems
3
G
evan et al.: dust forcing of tropical atlantic sst 10.1029/2007GC001774
Figure 3. Climatology the surface forcing and subsequent oceanic cooling exhibited by dust. (a) The climatological
monthly aerosol surface forcing (solid line) and the annual mean value (dotted line). (b) The time series of seasonal
June through October aerosol surface forcing (black line), the range of monthly values (gray shaded region), and the
mean (dotted line). (c) The climatological monthly surface temperature forcing by aerosols from this study (black
line) and from model output for the month following that shown on the x axis (gray line). (d) The time series of
seasonal June through October surface temperature forcing by aerosols (black line), the range of monthly values (gray
shaded region), and the mean (dotted line). All values are averaged over the hurricane development region.
forcing is of a larger magnitude than other surface
aerosol forcings [Kvalevåg and Myhre, 2007].
[18] A time series of mean June–September aerosol forcing over the hurricane development region
shows the strongest mean forcing of 15 W/m2
occurring at the beginning of the record in 1982
(Figure 3b), this is followed by a weakening of
the magnitude of aerosol forcing until 1991, and
then another weakening until the 1994 value of
7 W/m2. As previously mentioned, stratospheric
aerosols associated with the eruption of El Chichón
and Mount Pinatubo bias our estimated mean
summertime aerosol forcing for 1982, 1991, and
1992 (and possibly to a lesser degree in 1983).
However, a reduction in surface solar insolation
associated with these events is well known [Minnis
et al., 1993; Santer et al., 2006], and our estimates
of surface cooling still appear to be reasonable
(although in this case the cooling is not just due to
mineral aerosols). These volcanic eruptions add to
the perception of a division between aerosol activity pre- and post-1995, although a summertime
reduction of dust over the Atlantic since the mid
1980s is well known [e.g., Chiapello et al., 2005].
The mean forcing values for the period of 1995–
2006 shows values ranging from 7 to 9 W/m2,
and the years 2005 and 2006, years of hyperactive
tropical cyclone activity, exhibited the weakest
mean dust forcing over our entire record. The gray
shaded region in Figure 3b represents the range
of values for the months of June–September for
each year. These ranges are generally between 5 to
10 W/m2 and demonstrate that throughout the
entire record, although the highest mean seasonal
forcings occurred in the beginning of the record,
some monthly values in the 1990s and 2000s were
as high as those from the 80s (gray shaded region,
Figure 3b).
[19] The monthly climatology of aerosol temperature forcing (equation (1) and Figure 3c) shows
that ocean temperatures are most sensitive to aerosols during the boreal summer, when the mixed
layer is most shallow, and is the strongest in
September (0.55 K/month), during the height of
the hurricane season. There is a secondary peak in
aerosol forcing in March, which reflects the increased levels of dust and biomass burning that
occur in this region during the boreal winter and
spring [Husar et al., 1997].
[20] The gray line in Figure 3c is the monthly mean
surface temperature forcing over the hurricane
6 of 11
Geochemistry
Geophysics
Geosystems
3
G
evan et al.: dust forcing of tropical atlantic sst 10.1029/2007GC001774
development region from a general circulation
model that includes dust direct radiative forcing
[Yoshioka et al., 2007; Mahowald et al., 2006]. In
the model, ocean temperature changes due to dust
forcings are determined by subtracting the mean
ocean temperature for runs with dust from those
where the dust is absent. The temperature values
observed in the model output also include longwave surface warming from dust, but this contribution is much smaller than that from the
shortwave over the hurricane development region
[Myhre et al., 2003; Yoshioka et al., 2007]. Additionally, the general circulation model includes
possible feedbacks on ocean temperatures associated with top of the atmosphere aerosol forcing
[e.g., Miller and Tegen, 1998], although more
recent studies suggest that the difference between
top of the atmosphere and surface forcing for dust
is small over the Atlantic [Kaufman et al., 2002].
The magnitudes of the monthly temperature forcing from the model and from our satellite-based
calculations are very similar (Figure 3c), suggesting a validation of our methods. The slightly larger
magnitude of forcing we calculate during the
months of September and October result from the
stronger summertime shoaling of our mixed layer
depths (see Figure S2 in auxiliary material).
[21] The exact effect aerosol forcing has on the
measured sea surface temperature is difficult to
estimate since temperatures for a given month are
dependant on, among other factors, solar radiation
received during previous months, and relative
changes in ocean heat transport and radiative cooling. However, the excellent agreement between our
satellite based estimates and those from the transport model speaks to the validity of the assumptions we have made, although other models that
estimate surface forcing from dust disagree both
with our study and the work of Mahowald et al.
[2006] and Yoshioka et al. [2007] [e.g., Miller and
Tegen, 1998]. Also, the data from our model are
estimations of the temperature change due to dust
for the top 20 to 40 m of the ocean, not just the
ocean surface. Since there is a temperature gradient
at the ocean’s surface, it is likely that observations
of sea surface temperature made with satellites,
especially after the passage of a particular dust
event, will show a stronger temperature response to
the aerosol forcing.
[22] Our time series of average July, August and
September aerosol ocean temperature forcing averaged over the hurricane development region
(Figure 3d) is similar to the series for dustiness
(Figure 1). Therefore, correlations between observations of ocean temperature and our aerosol
forced temperatures are also similar to those for
dust and ocean temperatures (Figure 2), and correlate well with tropical cyclone activity [Evan et
al., 2006b]. The time series in Figure 3d shows a
reduction in mean temperature forcing from about
0.9 to 0.4 K/month. Embedded in that decline
is a spike of about 0.4 K/month in temperature
forcings in 1991 that is enhanced by stratospheric
aerosols. From 1995 onward the mean temperature
changes due to aerosols are consistently around
0.4 to 0.5 K/month, and the monthly values
ranging from 0.6 to 0.3 K/month. We also note
a marginally weaker mean aerosol temperature
forcing for 2006 relative to 2005, in disagreement
with the findings of Lau and Kim [2007a, 2007b],
and more in agreement with Evan [2007].
[23] Holland and Webster [2007] found that since
the mid 1970s eastern tropical North Atlantic sea
surface temperatures exhibited a warm anomaly
that was not observed in the western tropical North
Atlantic or Caribbean basins. They speculated that
this anomaly of roughly 0.2°C was not related to
global process and was therefore local in nature.
When we account for dust’s effect on ocean temperatures by subtracting our monthly estimations of
oceanic cooling by dust from mean Hadley ocean
temperatures, we see about a 30% reduction in the
trend of mean July, August, and September ocean
temperatures over the hurricane development region for the last 25 years (or a reduction in the
trend of Hadley ocean temperatures of about 0.1°C
per decade, after removing years with substantial
stratospheric aerosol loadings: 1982, 1991, 1992).
Since the magnitudes of the ocean temperature
anomalies given by Holland and Webster [2007]
and our estimations of oceanic cooling by dust are
very similar, and given the increases in dust across
the eastern tropical North Atlantic region that
existed from the late 1970s through the 1980s
[Evan et al., 2007; Prospero and Lamb, 2003],
we believe future analysis similar to Holland and
Webster [2007] that account for tropospheric aerosols are warranted.
5. Relevance to Tropical Cyclone
Activity
[24] One way to quantify the effect dust has tropical
cyclone activity is to utilize the strong linear relationship between September ocean temperatures in
the hurricane development region and the seasonal
tropical cyclone power dissipation index (PDI) for
7 of 11
Geochemistry
Geophysics
Geosystems
3
G
evan et al.: dust forcing of tropical atlantic sst 10.1029/2007GC001774
Figure 4. Seasonal Atlantic and Caribbean tropical
cyclone power dissipation index (PDI). The dark gray
region represents the measured seasonal PDI, and the
light gray region represents our estimation of PDI for a
‘‘dust-free’’ atmosphere.
the entire tropical North Atlantic and Caribbean
basins, as was done by Emanuel [2005a]. The PDI
is a proxy for the power a tropical cyclone dissipates at the surface, and is directly proportional to
the cyclone’s wind speed cubed integrated over the
lifetime of the storm [Emanuel, 2005a]. Since
tropical Atlantic Ocean temperature serves as a
proxy for basinwide circulation changes that affect
tropical cyclone genesis location, track, duration,
and intensity [Kossin and Vimont, 2007], this relationship between PDI and ocean temperature is
probably indirect. To estimate a seasonal PDI for
a dust-free atmosphere we first determine the slope
of the least-squared linear best fit line between
mean September ocean temperatures in the hurricane development region to seasonal PDI for the
years 1982 through 2006. We then use this slope in
conjunction with the time series of mean September
oceanic cooling by dust, which we have already
calculated, to determine the increase in seasonal
PDI that would occur in a dust-free atmosphere.
[25] Figure 4 shows the measured seasonal PDI
(dark gray shaded region), and the increase in PDI
that we estimate for the dust-free scenario (light
gray shaded region). The most striking feature of
this plot is the large increase in PDI across the
entire time series that result from warmer ocean
temperatures in the dust-free scenario. On average,
PDI for the dust-free case is about 2.2 times greater
than the measured PDI. This value of 2.2 excludes
years where volcanic aerosols probably contaminate our dust estimations (1982, 1991 and 1992).
[26] It is well known that hurricane activity in the
Atlantic entered a more active phase in 1995
[Goldenberg et al., 2001]. This active phase is
reflected in the difference between mean values
of the measured PDI (dark gray region in Figure 4);
from 1995 onward the mean seasonal PDI is about
2.5 times greater than the mean seasonal PDI for
1983 through 1994 (again, ignoring 1982, 1991,
and 1992 for this comparison). PDI for our dustfree scenario post-1994 is only about 1.5 times
greater than it is up to this year. This reduction in
the difference between mean PDI values for the
dust-free case implies that the reduction in dust that
has occurred after the 1980s contributed to the
recent increase in (1) ocean temperatures across the
hurricane development region and (2) seasonal
Atlantic and PDI. We estimate that about 35% of
the increase in PDI over the last quarter century can
be attributed to reductions in dust from West
Africa, and the subsequent loss of the cooling
effect these aerosols have on ocean temperatures
in the eastern tropical North Atlantic basin.
[27] Including the years of 1982, 1991, and 1992,
for which the surface forcing is due to a combination of sulfates from volcanic event as well as dust,
only slightly reduces the difference in PDI for postand pre-1995 for the ‘‘dust-free’’ case. Here, we
estimate that almost 45% of the recent increase in
PDI can be explained by the oceanic cooling
associated with dust and stratospheric aerosols.
6. Concluding Remarks
[28] Our results demonstrate that during the boreal
summer dust plays a nonnegligible role in shaping
the temperature of the eastern tropical Atlantic,
suggesting that the exclusion of mineral aerosols
in tropical Atlantic Ocean temperature attribution
studies [e.g., Santer et al., 2006] may not be justified. Our results also imply that the tropical Atlantic
is unique in terms of the processes that force the
surface temperatures. Holland and Webster [2007]
demonstrated that ocean temperatures in the eastern
tropical Atlantic have been increasing more sharply
than have been those in the Caribbean and Western
Atlantic, after global processes have been accounted
for. It is possible that this additional warming can
be attributed to the decreasing dustiness that we
observe over the same time period. This would
imply that recent decreases in dustiness have actually helped to force the tropical cyclone activity to
into a new, more destructive regime [Holland and
Webster, 2007]. Therefore new analysis is warranted
to determine how our results fit into what is know
8 of 11
Geochemistry
Geophysics
Geosystems
3
G
evan et al.: dust forcing of tropical atlantic sst 10.1029/2007GC001774
about changing ocean temperature across the
tropics. Recent work linking Atlantic tropical
cyclone activity to the Atlantic Meridional Mode
[Kossin and Vimont, 2007] has shown that a marginal increase (decrease) in tropical Atlantic surface
temperatures could ‘‘push’’ the basin into a state
that is more favorable (unfavorable) for cyclogenesis (less than 0.5 K). Therefore, we also suggest
that investigations into a potential relationship
between dust and the Atlantic Meridional Mode
are warranted.
[29] Understanding the exact causes of the year-toyear changes in Atlantic dust cover is not completely straightforward. For example, some variability in Sahelian precipitation (and therefore dust
cover) may result from feedbacks associated with
the dust loadings themselves [Yoshioka et al.,
2007], and from nonlinear feedbacks associated
with vegetation changes across the Sahel [Foley
et al., 2003]. Changes in year-to-year dustiness
may be modified by land use change [Moulin and
Chiapello, 2006; Tegen et al., 2004a], but to what
extent is still disputed [Mahowald et al., 2004;
Tegen et al., 2004b]. It is also possible that anthropogenic climate change has played a role in recent
variability of Sahelian precipitation [Held et al.,
2005]. Additionally, while it is known that Atlantic
Ocean temperatures play a strong role in modulating the strength of the West African monsoon, the
oceanic regions where temperature variability has
been linked to Sahelian precipitation [Giannini et
al., 2003; Fontaine and Janicot, 1996] tend to lie
outside the latitudes where dust is observed in the
summer months [Husar et al., 1997], but the
meridional gradients of Atlantic surface temperature may be more important than regional behavior
[Hoerling et al., 2006]. However, it is likely that
exists a feedback between oceanic forcing by
mineral aerosols and the strength of the West
African monsoon.
[30] Since summertime Atlantic dust cover is well
correlated with Sahelian precipitation from the
previous year [Prospero and Lamb, 2003; Moulin
and Chiapello, 2004], reconstructions of dust cover
over the last 50 years can be made with time series
of Sahelian precipitation [Prospero and Lamb,
2003]. Therefore, it is possible that the noted
long-term correlations between Sahel precipitation
and tropical cyclones [Landsea and Gray, 1992]
could be partially attributed to the cooling effect
mineral aerosols have on tropical ocean temperatures. The modeling study of Mahowald et al.
[2006] showed increases in vegetation across the
Sahel and resultant reductions in Atlantic dust
cover associated with a doubling of CO2. Therefore, not only may there be a less dusty future for
the Atlantic Ocean, but coming multidecadal periods of Sahelian drought commonly associated with
reductions in Atlantic tropical cyclone frequency
and intensity [Landsea and Gray, 1992], may not
lead to the same upswings in dust activity, cooling
of ocean temperatures, and dampening of tropical
cyclone activity, as they probably did in the past.
However, Held et al. [2005] showed a strong
drying of the Sahel with further increases in
greenhouse gases, implying possible increases in
dust storm activity, and therefore future cooling of
the tropical North Atlantic.
Acknowledgments
[31] We would like to thank Chris Landsea, Greg Holland,
Michael Mann, and Ron Miller for their detailed review of this
manuscript. Funding for this research was provided by the
NOAA/NESDIS Polar Program and the NOAA/NESDIS/ORA
AVHRR Reprocessing Program. The views, opinions, and
findings contained in this report are those of the authors and
should not be construed as an official NOAA or U.S. Government position, policy, or decision. International Satellite
Cloud Climatology Project cloud data are available from
http://isccp.giss.nasa.gov/. Data from the World Ocean Atlas
are available from http://www.nodc.noaa.gov/, ICOADS data
are available from http://dss.ucar.edu/, and Hadley HadISST1
data are available from http://www.hadobs.org. Pathfinder
Atmospheres Extended satellite products are available through
http://cimss.ssec.wisc.edu/clavr/; more detailed information
about the project and products can also be found there.
Tropical cyclone data were obtained from the National Hurricane Center Hurxricane Best Track Files.
References
Chiapello, I., C. Moulin, and J. M. Prospero (2005), Understanding the long-term variability of African dust transport
across the Atlantic as recorded in both Barbados surface
concentrations and large-scale Total Ozone Mapping Spectrometer (TOMS) optical thickness, J. Geophys. Res., 110,
D18S10, doi:10.1029/2004JD005132.
Conkright, M. E., R. A. Locarnini, H. E. Garcia, T. D. O’Brien,
T. P. Boyer, C. Stephen, and J. I. Antonov (2002), World
Ocean Atlas 2001: Objective Analyses, Data Statistics, and
Figures, CD-ROM Documentation, 17 pp., Natl. Oceanogr.
Data Cent., Silver Spring, Md.
Deser, C., and M. S. Timlin (1997), Atmosphere-ocean interaction on weekly timescales in the North Atlantic and Pacific, J. Clim., 10, 393 – 408, doi:10.1175/1520-0442
(1997)010<0393:AOIOWT>2.0.CO;2.
Dunion, J. P., and C. S. Velden (2004), The impact of the Saharan air layer on Atlantic tropical cyclone activity, Bull. Am.
Meteorol. Soc., 85, 353 – 365, doi:10.1175/BAMS-85-3-353.
Emanuel, K. A. (1987), The dependence of hurricane intensity
on climate, Nature, 326, 483 – 485, doi:10.1038/326483a0.
9 of 11
Geochemistry
Geophysics
Geosystems
3
G
evan et al.: dust forcing of tropical atlantic sst 10.1029/2007GC001774
Emanuel, K. (2005a), Increasing destructiveness of tropical
cyclones over the past 30 years, Nature, 436(7051), 686 –
688, doi:10.1038/nature03906.
Emanuel, K. (2005b), Meteorology—Emanuel replies, Nature,
438(7071), E13, doi:10.1038/nature04427.
Evan, A. T. (2007), Comment on ‘‘How nature foiled the 2006
hurricane forecasts,’’, Eos Trans. AGU, 88(26), 271,
doi:10.1029/2007EO260009.
Evan, A. T., A. K. Heidinger, and M. J. Pavolonis (2006a),
Development of a new over-water Advanced Very High Resolution Radiometer dust detection algorithm, Int. J. Remote Sens.,
27(18), 3903 – 3924, doi:10.1080/01431160600646359.
Evan, A. T., J. Dunion, J. A. Foley, A. K. Heidinger, and C. S.
Velden (2006b), New evidence for a relationship between
Atlantic tropical cyclone activity and African dust outbreaks,
Geophys. Res. Lett., 33, L19813, doi:10.1029/
2006GL026408.
Evan, A. T., N. Mahowald, and L. Remer (2007), Aerosols, in
State of the Climate in 2006, edited by A. Horvitz, Bull. Am.
Meteorol. Soc., 88(6), S23 – S25.
Foley, J. A., M. T. Coe, M. Scheffer, and G. L. Wang (2003),
Regime shifts in the Sahara and Sahel: Interactions between
ecological and climatic systems in northern Africa, Ecosystems, 6, 524 – 539, doi:10.1007/s10021-002-0227-0.
Fontaine, B., and S. Janicot (1996), Sea surface temperature
fields associated with West African rainfall anomaly types,
J. Clim., 9, 2935 – 2940, doi:10.1175/1520-0442 (1996)009<
2935:SSTFAW>2.0.CO;2.
Forster, P., et al. (2007), Changes in Atmospheric Constituents
and Radiative Forcing, Cambridge Univ. Press, New York.
Giannini, A., R. Saravanan, and P. Chang (2003), Oceanic
forcing of Sahel rainfall on interannual to interdecadal time
scales, Science, 302, 1027 – 1030, doi:10.1126/
science.1089357.
Goldenberg, S. B., and L. J. Shapiro (1996), Physical mechanisms for the association of El Niño and West African rainfall
with Atlantic major hurricane activity, J. Clim., 9, 1169 – 1187,
doi:10.1175/1520-0442(1996)009<1169:PMFTAO>2.0.
CO;2.
Goldenberg, S. B., C. W. Landsea, A. M. Mestas-Nunez, and
W. M. Gray (2001), The recent increase in Atlantic hurricane
activity: Causes and implications, Science, 293(5529), 474 –
479, doi:10.1126/science.1060040.
Gray, W. M. (1968), Global view of the origin of tropical
disturbances and storms, Mon. Weather Rev., 96, 669 – 700,
doi:10.1175/1520-0493(1968)096<0669:GVOTOO>2.0.CO;2.
Haywood, J., P. Francis, S. Osborne, M. Glew, N. Loeb,
E. Highwood, D. Tanré, G. Myhre, P. Formenti, and E. Hirst
(2003), Radiative properties and direct radiative effect of
Saharan dust measured by the C-130 aircraft during SHADE:
1. Solar spectrum, J. Geophys. Res., 108(D18), 8577,
doi:10.1029/2002JD002687.
Heidinger, A. K., A. T. Evan, and B. Baum (2007), Cloudiness, in State of the Climate in 2006, edited by A. Horvitz,
Bull. Am. Meteorol. Soc., 88(6), S17 – S18.
Held, I. M., T. L. Delworth, J. Lu, K. L. Findell, and T. R.
Knutson (2005), Simulation of Sahel drought in the 20th and
21st centuries, Proc. Natl. Acad. Sci. U.S.A., 102, 17,891 –
17,896, doi:10.1073/pnas.0509057102.
Hoerling, M., J. Hurrell, J. Eischeid, and A. Phillips (2006),
Detection and attribution of twentieth-century northern and
southern African rainfall change, J. Clim., 19, 3989 – 4008,
doi:10.1175/JCLI3842.1.
Holland, G. J., and P. J. Webster (2007), Heightened tropical
cyclone activity in the North Atlantic: Natural variability or
climate trend?, Philos. Trans. R. Soc. London, Ser. A, 365,
2695 – 2716, doi:10.1098/rsta.2007.2083.
Husar, R. B., J. M. Prospero, and L. L. Stowe (1997), Characterization of tropospheric aerosols over the oceans with the
NOAA advanced very high resolution radiometer optical
thickness operational product, J. Geophys. Res., 102(D14),
16,889 – 16,910, doi:10.1029/96JD04009.
Ignatov, A., and L. Stowe (2002a), Aerosol retrievals from individual AVHRR channels. Part I: Retrieval algorithm and
transition from Dave to 6S radiative transfer model, J. Atmos.
Sci., 59, 313 – 334, doi:10.1175/1520-0469(2002)059<0313:
ARFIAC>2.0.CO;2.
Ignatov, A., and L. Stowe (2002b), Aerosol retrievals from
individual AVHRR channels. Part II: Quality control, probability distribution functions, information content, and consistency checks of retrievals, J. Atmos. Sci., 59, 335 – 362,
doi:10.1175/1520-0469 (2002)059<0335:ARFIAC>2.0.
CO;2.
Jarvinen, B. R., C. J. Neumann, and M. A. Davis (1984),
A tropical cyclone data tape for the North Atlantic Basin,
1886 – 1983: Contents, limitations, and uses,NOAA Tech.
Memo., NWS NHC 22, 21 pp., NOAA, Coral Gables, Fla.
Kalnay, E., et al. (1996), The NCEP/NCAR 40-year reanalysis
project, Bull. Am. Meteorol. Soc., 77, 437 – 471, doi:10.1175/
1520-0477(1996)077<0437:TNYRP>2.0.CO;2.
Kaufman, Y. J., D. Tanre, and O. Boucher (2002), A satellite
view of aerosols in the climate system, Nature, 419, 215 –
223, doi:10.1038/nature01091.
Kossin, J. P., and D. J. Vimont (2007), A more general framework for understanding Atlantic hurricane variability and
trends, Bull. Am. Meteorol. Soc., 88, 1767 – 1781,
doi:10.1175/BAMS-88-11-1767.
Kvalevåg, M. M., and G. Myhre (2007), Human impact
on direct and diffuse solar radiation during the industrial
era, J. Clim., 20, 4874 – 4883, doi:10.1175/JCLI4277.1.
Landsea, C. W. (2005), Meteorology—Hurricanes and global
warming, Nature, 438(7071), E11 – E13, doi:10.1038/nature04477.
Landsea, C. W., and W. M. Gray (1992), The strong association between western Sahelian monsoon rainfall and intense
Atlantic hurricanes, J. Clim., 5, 435 – 453, doi:10.1175/15200442 (1992)005<0435:TSABWS>2.0.CO;2.
Lau, K. M., and J. M. Kim (2007a), How nature foiled the
2006 hurricane forecasts, Eos Trans. AGU, 88(9), 105 – 107,
doi:10.1029/2007EO090002.
Lau, W. K. M., and K.-M. Kim (2007b), Reply to comment on
‘‘How nature foiled the 2006 hurricane forecasts’’, Eos
Trans. AGU, 88(26), 271, doi:10.1029/2007EO260010.
Levitus, S. (1982), Climatological Atlas of the World Ocean,
NOAA Prof. Pap., 13, 120 pp., NOAA, Rockville, Md.
Mahowald, N. M., C. S. Zender, C. Luo, D. Savoie, O. Torres,
and J. del Corral (2002), Understanding the 30 year Barbados desert dust record, J. Geophys. Res., 107(D21), 4561,
doi:10.1029/2002JD002097.
Mahowald, N. M., G. D. R. Rivera, and C. Luo (2004), Comment on ‘‘Relative importance of climate and land use in
determining present and future global soil dust emission’’
by I. Tegen et al., Geophys. Res. Lett., 31, L24105,
doi:10.1029/2004GL021272.
Mahowald, N. M., D. R. Muhs, S. Levis, P. J. Rasch,
M. Yoshioka, C. S. Zender, and C. Luo (2006), Change in
atmospheric mineral aerosols in response to climate: Last
glacial period, preindustrial, modern, and doubled carbon
dioxide climates, J. Geophys. Res., 111, D10202,
doi:10.1029/2005JD006653.
10 of 11
Geochemistry
Geophysics
Geosystems
3
G
evan et al.: dust forcing of tropical atlantic sst 10.1029/2007GC001774
Meador, W. E., and W. R. Weaver (1980), Two-stream approximations to radiative transfer in planetary atmospheres:
A unified description of existing methods and a new
improvement, J. Atmos. Sci., 37, 630 – 643, doi:10.1175/
1520-0469(1980)037<0630:TSATRT>2.0.CO;2.
Miller, R. L., and I. Tegen (1998), Climate response to soil
dust aerosols, J. Clim., 11, 3247 – 3267, doi:10.1175/15200442(1998)011<3247:CRTSDA>2.0.CO;2.
Minnis, P., E. F. Harrison, L. L. Stowe, G. G. Gibson, F. M.
Denn, D. R. Doelling, and W. L. Smith (1993), Radiative
climate forcing by the Mount Pinatubo eruption, Science,
259, 1411 – 1415, doi:10.1126/science.259.5100.1411.
Mishchenko, M. I., I. V. Geogdzhayev, W. B. Rossow,
B. Cairns, B. E. Carlson, A. A. Lacis, L. Liu, and L. D. Travis
(2007), Long-term satellite record reveals likely recent aerosol trend, Science, 315, 1543, doi:10.1126/science.1136709.
Moulin, C., and I. Chiapello (2004), Evidence of the control of
summer atmospheric transport of African dust over the
Atlantic by Sahel sources from TOMS satellites (1979 –
2000), Geophys. Res. Lett., 31, L02107, doi:10.1029/
2003GL018931.
Moulin, C., and I. Chiapello (2006), Impact of human-induced
desertification on the intensification of Sahel dust emission
and export over the last decades, Geophys. Res. Lett., 33,
L18808, doi:10.1029/2006GL025923.
Myhre, G., A. Grini, J. M. Haywood, F. Stordal, B. Chatenet,
D. Tanré, J. K. Sundet, and I. S. A. Isaksen (2003), Modeling
the radiative impact of mineral dust during the Saharan Dust
Experiment (SHADE) campaign, J. Geophys. Res.,
108(D18), 8579, doi:10.1029/2002JD002566.
Myhre, G., et al. (2007), Comparison of the radiative properties
and direct radiative effect of aerosols from a global aerosol
model and remote sensing data over ocean, Tellus, Ser. B, 59,
115 – 129, doi:10.1111/j.1600-0889.2006.00226.x.
Nicholson, S. E., C. J. Tucker, and M. B. Ba (1998), Desertification, drought and surface vegetation, an example from
the West African Sahel, Bull. Am. Meteorol. Soc., 79, 815 –
829, doi:10.1175/1520-0477(1998)079<0815:DDASVA>2.0.CO;2.
Pielke, R. A.Jr. (2005), Meteorology—Are there trends in hurricane destruction?, Nature, 438(7071), E11, doi:10.1038/
nature04426.
Prospero, J. M., and P. J. Lamb (2003), African droughts and
dust transport to the Caribbean: Climate change implications,
Science, 302, 1024 – 1027, doi:10.1126/science.1089915.
Rayner, N. A., D. E. Parker, E. B. Horton, C. K. Folland, L. V.
Alexander, D. P. Rowell, E. C. Kent, and A. Kaplan (2003),
Globally complete analyses of sea surface temperature, sea
ice and night marine air temperature, 1871 – 2000, J. Geophys. Res., 108(D14), 4407, doi:10.1029/2002JD002670.
Rossow, W. B., and R. A. Schiffer (1991), ISCCP cloud data
products, Bull. Am. Meteorol. Soc., 72, 2 – 20, doi:10.1175/
1520-0477(1991)072<0002:ICDP>2.0.CO;2.
Santer, B. D., et al. (2006), Forced and unforced ocean temperature changes in Atlantic and Pacific tropical cyclogenesis regions, Proc. Natl. Acad. Sci. U.S.A., 103, 13,905 –
13,910, doi:10.1073/pnas.0602861103.
Saunders, M. A., and A. R. Harris (1997), Statistical evidence
links exceptional 1995 Atlantic hurricane season to record
sea warming, Geophys. Res. Lett., 24(10), 1255 – 1258,
doi:10.1029/97GL01164.
Schollaert, S. E., and J. T. Merrill (1998), Cooler sea surface
west of the Sahara Desert correlated to dust events, Geophys.
Res. Lett., 25(18), 3529 – 3532, doi:10.1029/98GL52591.
Shapiro, L. J., and S. B. Goldenberg (1998), Atlantic sea surface temperatures and tropical cyclone formation, J. Clim.,
11, 578 – 590, doi:10.1175/1520-0442(1998)011<0578:ASSTAT>2.0.CO;2.
Tegen, I., M. Werner, S. Harrison, and K. Kohfeld (2004a),
Relative importance of climate and land use in determining
present and future global soil dust emission, Geophys. Res.
Lett., 31, L05105, doi:10.1029/2003GL019216.
Tegen, I., M. Werner, S. P. Harrison, and K. E. Kohfeld
(2004b), Reply to comment by N. M. Mahowald et al. on
‘‘Relative importance of climate and land use in determining
present and future global soil dust emission,’’ Geophys. Res.
Lett., 31, L24106, doi:10.1029/2004GL021560.
Webster, P. J., G. J. Holland, J. A. Curry, and H. R. Chang
(2005), Changes in tropical cyclone number, duration, and
intensity in a warming environment, Science, 309(5742),
1844 – 1846, doi:10.1126/science.1116448.
Wong, S., and A. E. Dessler (2005), Suppression of deep
convection over the tropical North Atlantic by the Saharan
air layer, Geophys. Res. Lett., 32, L09808, doi:10.1029/
2004GL022295.
Woodruff, S. D., H. F. Diaz, S. J. Worley, R. W. Reynolds, and
S. J. Lubker (2005), Early ship observational data and
ICOADS, Clim. Change, 73, 169 – 194, doi:10.1007/
s10584-005-3456-3.
Yoshioka, M., N. M. Mahowald, A. J. Conley, W. D. Collins,
D. W. Fillmore, C. S. Zender, and D. B. Coleman (2007),
Impact of desert dust radiative forcing on Sahel precipitation:
Relative importance of dust compared to sea surface temperature variations, vegetation changes, and greenhouse gas
warming, J. Clim., 20, 1445 – 1467, doi:10.1175/
JCLI4056.1.
11 of 11