Historical Contribution of the Saharan Air Layer to Atlantic Mixed Layer Temperatures in the Hurricane Development Region

Dust storms from Africa are a persistent feature in the skies over the northern tropical Atlantic, and strong variability in Atlantic dust cover has been observed on seasonal to decadal time scales. It is well known that over water surfaces the net radiative effect from mineral aerosols at the surface is negative, and recent work has shown that this reduction in downwelling radiation translates into localized cooling of the mixed layer. Recent studies have also demonstrated that over the last quarter century roughly 25% of the observed upward trend in sea surface temperatures can be attributed to declines in Atlantic dust cover over the same time period. 

While there is compelling evidence suggesting that African dust storms contribute to Atlantic surface temperatures on decadal time-scales, to-date studies investigating dust-forcing of temperatures have generally neglected other important environmental factors that are associated with Atlantic dust outbreaks; namely the dry air, mid-level warm anomaly, and increased surface wind speed. The Saharan Air Layer (SAL) is the term given to this dry air mass that is associated with the dust, and the net effect of the reduction in water vapor, warm anomaly, and increase in surface winds is to further cool the mixed layer via negative longwave radiative forcing at the surface, and wind-driven latent and sensible heat fluxes, and vertical turbulent mixing. Therefore, it is likely that the SAL, considered in its entirety, has a stronger role in shaping Atlantic temperatures on monthly to decadal time scales than does dust alone. 

At the same time, satellite, in-situ, and proxy dust records all show that Atlantic dust cover has strong decadal variability, and recent work has shown that a simple statistical model can reproduce month-to-month variability in Atlantic dust cover by considering reanalysis winds, climate indices, and observational records. Therefore, the opportunity exists to reconstruct spatial and temporal Atlantic dust storm variability from the mid-20th century to the present. 

Here we propose to capitalize on studies that demonstrate techniques for modeling the radiative effects of the SAL, and that provide a basis for developing a statistical model to reconstruct historical dust cover, in order to estimate the impact of the Saharan Air Layer on Atlantic Ocean 4 surface temperatures over the last 60 years. By exploring the effect of the SAL on temperatures using an ocean general circulation model (GCM), we will quantify the effect of the SAL in shaping decadal scale surface temperature variability. We will also analyze any effect of the SAL on deep ocean temperatures and determine if there is a SAL contribution to the Atlantic meridional overturning circulation (AMOC). 

In order to evaluate how aerosol cover and atmospheric winds have impacted Atlantic tropical cyclone activity on decadal time scales, we propose the following course of action: 

1) Use several data sets to create a climatology of the SAL that extends back to the mid-20th century, estimating SAL vertical profiles of dust optical depth, water vapor, temperature, and winds. 

2) Employ established techniques to model SAL-forced changes in surface radiative fluxes and surface turbulent heat fluxes, along with observation-based calculations of horizontal oceanic heat advection and vertical turbulent mixing, to determine the dominant processes associated with the mixed layer temperature response to the SAL. 

3) Force an ocean GCM with SAL-forced surface radiative fluxes and surface wind anomalies in order to quantify the ocean temperature response to SAL variability, and determine to what degree the SAL contributes to meridional heat transport, from 1950 through the present.

The main objective of this proposal is to understand the role of the SAL in shaping Atlantic surface and subsurface temperature anomalies over the last 60 years, focusing on the SAL’s contribution to decadal scale upper ocean temperature variability, and the AMOC.