Predictability of Extreme Dust Events in South Florida

Dust loading at Miami, located at 25.76N, 80.19W, is dependent on the characteristics of the lower free-tropospheric winds due to the vast distance the dusty air must travel and Miami’s northern latitude relative to the North African coast where most dust is initially emitted (~10N). On average, dust arrives to Miami 10 days after emission from the African coast and experiences mesoscale and synoptic scale influences. The Miami, Florida 43-year daily dust record is useful for assessing Saharan dust variability on daily to decadal scales, despite being a point source. Daily dust measurements from July and August, the largest contributors to Miami dust season, are used to characterize synoptic conditions that are favorable for dust transport.

While dust emission is episodic and variable on its own, this study focuses on the efficiency of transporting dust to Miami from the North African coast: the maximum potential for dust to arrive after emission takes place with limited loss to deposition or mixing. Two key-regions have been linked with the highest dust concentrations: (i) zonal wind over the Tropical West Atlantic [15-25N, 45-80W] and (ii) meridional wind over the Florida Peninsula [20-30N,75-80W]. These regions are synoptically enhanced when the North Atlantic subtropical high (NASH) is displaced south and zonally elongated, with the western edge located over Florida. This encourages a favorable wind flow that efficiently transports Saharan dust to the Florida Peninsula with minimal dispersion and time for deposition.

After identifying the key wind regions, a dust-transport-efficiency (DTE) index is defined to identify extreme dust cases on the sub-seasonal scale. Monthly DTE index calculations show good agreement with July and August dust extremes over the 43-year span. The DTE calculation for July and August combined provides useful information for dust loading at the seasonal scale. While seasonal dust emissions have been decreasing, the transport efficiency has been increasing, possibly due to trends or low frequency changes in the NASH, and may continue to do so in the future.