Fifteen–Year Trend in African Dust Outbreaks across the US Caribbean

Although several field campaigns have been conducted to have a better understanding of optical, chemical and physical properties of the African Mineral dust across the Eastern Caribbean; there are still fundamental questions unanswered regarding frequency and magnitude of dust outbreaks particularly in the vicinity of Puerto Rico and the US Virgin Islands (US Caribbean). The goal of this study was to identify the trends in frequency and magnitude of African Dust Days (ADD) across the US Caribbean since most the analyses have been performed well south in the vicinity of Barbados. Even more important, and has not been done before, the North Africa Dipole Intensity (NAFDI) index was correlated with the US Caribbean dustiness as a potential predictor of enhanced activity.

To identify the trends in frequency and magnitude of ADD; the daily area-averaged Aerosol Optical Depth (AOD) retrieved from the MODerate Resolution Imaging Spectroradiometer (MODIS-Aqua) between latitudes 13° and 23° North and longitudes 61° and 71° West was validated using AOD data retrieved from the AErosol RObotic NETwork (AERONET) at Cape San Juan (CSJ), Fajardo, Puerto Rico. Once the validation was completed, ADD was identified across the US Caribbean using the 90^th (D90) percentile within the log-normal distribution of AOD MODIS-retrieved combined with the Extinction Ångström Exponent (EAE) AERONET-retrieved at CSJ or identifying the Saharan Air Layer (SAL) on meteorological soundings launched at the National Weather Service Forecast Office, San Juan, Puerto Rico (WFO SJU). Having the number of ADD per season, the trends in frequency and magnitude were calculated using the Rate of Change (ROC) with time.

A moderate level of agreement was found between MODIS and AERONET retrievals during the period of record 2003-2018 with a correlation coefficient, R, of 0.57. Nevertheless, the agreement between both sensors increases during ADD with R of 0.61. Although there were no changes observed in the frequency or magnitude of ADD across the US Caribbean between 2003 and 2018, the years with highest frequency of ADD were years with a positive NAFDI index which is an indicator of dust source intensification over central-south Argelia and transport toward the Subtropical Atlantic.