African dust days across the northeast Caribbean

Abstract

Although field campaigns have been conducted to improve the understanding of African dust across the Caribbean, there are still fundamental unanswered questions regarding frequency and magnitude over the Puerto Rico area, which is located at the north edge of the dust corridor. This study aims to identify the African Dust Days (ADD) across Puerto Rico between 2004 and 2018; to assess trends in the frequency and magnitude during June, July, and August (JJA) when the strong seasonal cycle of dust transport into the Caribbean is observed; to show how the intra-seasonal variations of dust transport into the northeast Caribbean is modulated by the North African Dipole Intensity (NAFDI) index, and to identify impacts on rainfall patterns. A day was classified as ADD if the Aerosol Optical Depth (AOD) was equal to or greater than 0.20 combined with an Extinction Ångström Exponent equal to or lower than 0.60. These optical properties were retrieved from the AErosol RObotic NETwork at Cape San Juan and La Parguera, Puerto Rico. The frequency of ADD was the number of ADD divided by the total number of observations. The magnitude was the mean AOD during ADD. The AOD monthly anomaly, which was calculated as well, was then correlated with the NAFDI index to determine if the NAFDI index modulates the AOD anomaly. A total of 674 days was identified as ADD between 2004 and 2018, with 505 of them observed during JJA (75%). Although no quantifiable trend in the frequency or magnitude of ADD is observed, data suggest that the NAFDI index modulates the intra-seasonal variation in dust transport into the northeast Caribbean and associated AOD anomaly. Meteorological data retrieved at the Weather Service Forecast Office in San Juan, Puerto Rico, during the days classified as ADD also suggest that rainfall activity occurs if the static energy values are equal to or greater than 334 kJ kg^-1combined with mixing ratio values equal to or greater than 12 g kg^−1 at 850 mb.