October 31, 2008

Atlantic SSTs and Saharan Dust (and Hurricanes)

In our last World Climate Report article, we described new findings that verified older findings that the patterns of sea surface temperature (SST) variations in the Atlantic Ocean (including in the tropical Atlantic region which is the birthplace of Atlantic tropical storms and hurricanes), are largely a reflection of natural variability, with some anthropogenic warming thrown in for good measure.

This time, we report on new research that finds that rather than a large dose of anthropogenic warming, a decline in the amount of dust coming off of the Saharan desert may have collaborated with multidecadal natural oscillations to produce the observed warming trend in Atlantic tropical SST over recent decades. An implication of this finding is to further lessen any impact than human emissions of greenhouse gases may have had on the observed behavior of Atlantic hurricanes, including the recent upturn in activity.

Researchers Gregory Foltz and Michael McPhaden investigated the relationship between tropical Atlantic SSTs, Sahel precipitation and Saharan dust over the period 1980-2006. This period is characterized by rising SSTs, increasing rainfall over Africa’s Sahel region, and a resultant decrease in dust production from the region. Additionally, there was a phase change in the Atlantic Multidecadal Oscillation (AMO)—the predominant pattern of SST variations in the Atlantic Ocean (and the subject of our last WCR article).

Foltz and McPhaden introduce their work as follows:

During the past century tropical North Atlantic sea surface temperatures (SST) have fluctuated strongly with a period of ~70 years (Goldenberg et al., 2001). The oscillations are part of a basin-scale Atlantic multidecadal oscillation (AMO) thought to be driven by changes in the strength of the Atlantic thermohaline circulation (e.g., Delworth and Mann, 2000; Knight et al., 2005). The AMO exerts a significant influence on weather and climate in the tropical Atlantic sector, with positive phases of the AMO contributing to enhanced rainfall in the Sahel region of Africa (Zhang and Delworth, 2006) and above-normal hurricane activity in the Atlantic basin (Goldenberg et al., 2001). Superimposed on the AMO is a strong warming trend that has been attributed to anthropogenic greenhouse gas forcing (Mann and Emanuel, 2006).

These references should sound familiar to anyone who read our last WCR which reaffirmed the natural occurrence of the AMO (rather than being a simple manifestation of anthropogenic global warming).

Foltz and McFaden wanted to see what role Sarahan dust may play in this picture, as there have been previous studies which have linked atmospheric dust concentrations over the tropical North Atlantic ocean with precipitation in the Sahel—the more rain that falls over the Sahel, the less dust is produced the following summer.

By combining data collected from several satellites that measure the “aerosol optical depth”—basically a measure of dustiness in the atmosphere—Foltz and McFaden piece together a dust record for the tropical North Atlantic beginning in 1980. Figure 1 shows the timeseries of SST, rainfall and dust. Tropical North Atlantic SST increased from 1980-2006, Sahel rainfall increased, and the dust load in the atmosphere over the tropical North Atlantic decreased.


Figure 1. Summer means and linear trends in (top) tropical North Atlantic SST, (middle) Sahel precipitation, and (bottom) tropical North Atlantic aerosol optical depth (AOD). Circles in Figure 1c denote the years of significant volcanic eruptions (1982 and 1991). Dashed line is the linear trend calculated after removing these data points (source: Foltz and McPhaden, 2008).

A decline in dustiness implies that the atmosphere is growing clearer and thus more sunshine can reach the surface and heat things up. So, Foltz and McPhaden performed a calculation as to just how much the decline in dustiness could warm up the tropical SST.

What they found was astounding.

They calculated that the decline in dustiness from 1980 to 2006 could have warmed the underlying SST by 3ºC—and amount that is 5 times the observed increase during that period!

The authors explain away this excess heat by involving other physical process which move heat in, around, and through the atmosphere/ocean system. But, the point is, is that a positive feedback loop is set up over the tropical North Atlantic is which rising SSTs there lead to increasing Sahel precipitation which lead to decreased dust production which leads to risings SSTs. And the magnitude of the SST rise produced by the atmospheric dust declines is quite substantial. Thus, natural variations in tropical Atlantic SSTs, modulated by the AMO, can couple with declines in dust loading to more than account for the entirety of the observed temperature increase—no anthropogenic influence need be involved!

At the very least, the influence of Saharan dust needs to be considered in discussions of the assessment of the causes of the observed temperature patterns of the Atlantic Ocean (including the warming trend in recent decades)—something that is not widely done currently. And these same discussions eventually get around to hurricanes.

Foltz and McPhaden conclude with the following:

The observed relationships between SST, dust, and rainfall during 1980–2006 may have important implications for numerical models simulating North Atlantic climate on multidecadal time scales. Recent studies suggest that coupled models tend to underestimate the amplitude of tropical North Atlantic SST variations associated with the AMO (e.g., Delworth and Mann, 2000; Knight et al., 2005). The inclusion of dust in the atmospheric component of these models may therefore lead to improved modeling and prediction of the AMO. Changes in dust concentration in the tropical North Atlantic may also influence the timing and intensity of the Atlantic meridional SST gradient mode, which affects rainfall in Brazil and the Sahel and tropical cyclone activity in the Atlantic basin (e.g., Kossin and Vimont, 2007).

Add this paper to the ever-expanding list of findings in the scientific literature that do not provide overwhelming support for the idea that anthropogenic activities are significantly effecting the pattern of hurricane frequency and intensity in the Atlantic basin, and instead suggest that natural processes, past, present, and future, play a leading role in the patterns of Atlantic tropical cyclone activity.

Reference:

Foltz, G. R., and M. J. McPhaden, 2008. Trends in Saharan dust and tropical Atlantic climate during 1980-2006. Geophysical Research Letters, 35, L20706, doi:10.1029/2008GL035042.




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