The pulsating nature of large-scale Saharan dust transport as a result of interplays between mid-latitude Rossby waves and the North African Dipole Intensity

E Cuevas,A.J Gómez-Peláez,S Rodríguez,E Terradellas,S Basart,R.D García,O.E García,S Alonso-Pérez

doi:10.1016/j.atmosenv.2017.08.059

Abstract

It was previously shown that during August the export of Saharan dust to the Atlantic was strongly affected by the difference of the 700-hPa geopotential height anomaly between the subtropics and the tropics over North Africa, which was termed the North African Dipole Intensity (NAFDI). In this work a more comprehensive analysis of the NAFDI is performed, focusing on the entire summer dust season (June–September), and examining the interactions between the mid-latitude Rossby waves (MLRWs) and NAFDI. Widespread and notable aerosol optical depth (AOD) monthly anomalies are found for each NAFDI-phase over the dust corridors off the Sahara, indicating that NAFDI presents intra-seasonal variability and drives dust transport over both the Mediterranean basin and the North Atlantic. Those summer months with the same NAFDI-phase show similar AOD-anomaly patterns. Variations in NAFDI-phase also control the displacement of the Saharan Heat Low (SHL) westwards or eastwards through horizontal advection of temperature over Morocco-Western Sahara or eastern Algeria-Western Libya, respectively. The connection between the SHL and the NAFDI is quantified statistically by introducing two new daily indexes that account for their respective phases (NAFDI daily index -NAFDIDI-, and SHL longitudinal shift index -SHLLSI-) and explained physically using the energy equation of the atmospheric dynamics. The Pearson's correlation coefficient between the one–day-lag SHLLSI and the NAFDIDI for an extended summer season (1980–2013) is 0.78. A positive NAFDI is associated with the West-phase of the SHL, dust sources intensification on central Algeria, and positive AOD anomalies over this region and the Subtropical North Atlantic. A negative NAFDI is associated with the East-phase of the SHL, and positive AOD anomalies over central-eastern Sahara and the central-western Mediterranean Sea. The results point out that the phase changes of NAFDI at intra-seasonal time scale are conducted by those MLRWs that penetrate deeply into the low troposphere.

Highlights

Mineral dust from deserts is the second most important contributor to the global atmospheric aerosol burden after sea-salt aerosols (Huneeus et al, 2013 and references )
To properly address these questions we set the following objectives in this study: 1) investigate intra-seasonal changes in dust transport over the Sahara and outflow regions driven by changes in North African Dipole Intensity (NAFDI) during summertime (JuneeSeptember; hereinafter denoted as JJAS); 2) identify and analyse the atmospheric processes which might connect NAFDI variations with changes in the SHL position; and 3) identify and quantify the atmospheric physical mechanisms by which MLRWs might modulate at intra-seasonal scale the SHL
The two points chosen by those authors in the definition of the NAFDI were located over Mali and Morocco following the 7W meridian because they wanted to analyse and characterize the transport of Saharan dust toward the North Atlantic

Summary

Introduction

Mineral dust from deserts is the second most important contributor to the global atmospheric aerosol burden after sea-salt aerosols (Huneeus et al, 2013 and references ). We wonder about the role that MLRWs might play in the intra-seasonal variability of NAFDI, and if it is possible to explain the mechanisms that control the interaction between Rossby waves and the NAFDI and SHL phases To properly address these questions we set the following objectives in this study: 1) investigate intra-seasonal changes in dust transport over the Sahara and outflow regions driven by changes in NAFDI during summertime (JuneeSeptember; hereinafter denoted as JJAS); 2) identify and analyse the atmospheric processes which might connect NAFDI variations with changes in the SHL position; and 3) identify and quantify the atmospheric physical mechanisms by which MLRWs might modulate at intra-seasonal scale the SHL-.

MACC reanalysis

MODIS data

Role of NAFDI in dust outflows and associated meteorological patterns

Physical mechanics by which NAFDI drives the SHL longitudinal shifts

Connection between mid-latitude Rossby waves and NAFDI

Conclusions