New Study: “Pivotal role of the North African Dipole Intensity (NAFDI) on alternate Saharan dust export over the North Atlantic and the Mediterranean, and relationship with the Saharan Heat Low and mid-latitude Rossby waves”

In this study, published in Atmospheric Chemistry and Physics (Discussion phase), we start from the results obtained by Rodríguez et al. (2015) regarding the so-called North African Dipole Intensity (NAFDI). The NAFDI is the difference of geopotential height anomalies averaged over the subtropics and the tropics close to the Atlantic coast. We review the definition of the NAFDI index and assess the results obtained by Rodríguez et al. (2015) using complementary spatial fields of Aerosol Optical Depth (AOD) from MODIS (Moderate Resolution Imaging Spectroradiometer) retrievals and MACC (Monitoring Atmospheric Composition and Climate) reanalysis, and extending the analysis to every month of the summertime (June-August). Dust transport and meteorological patterns associated with the NAFDI phases (Figure 1), and the role played by the NAFDI in dust transport to the Mediterranean basin, and its impact on dust source activation over the Sahara is explored.

Figure 1. Averaged AOD anomalies from combined MODIS Dark-Target and Deep-Blue AOD at 550 nm for summer months (June, July and August) with positive and negative NAFDI phases, in the period 2003-2012. These plots were obtained by averaging the AOD anomalies for each month and each phase of the NAFDI.

We determine and analyse the physical mechanisms that link the NAFDI and the Saharan Heat Low on a daily basis (Figure 2), since the SHL modulates many of the mesoescale baroclinic atmospheric processes that mobilize dust in the Sahara. We also analyse the variations of Rossby waves (amplitude and phase) in the North-East Atlantic, identifying on a daily basis the physical mechanisms by which these waves modulate the NAFDI variations and hence, the SHL phases.

Figure 2. Daily NAFDI vs Daily SHL West-East Displacement Index 1-day lag for 20 June – 16 September 1980-2013.

Finally, a schematic conceptual model from hemispheric to meso-scale atmospheric mechanisms driving dust transport over Northern Africa is presented.

The full reference of the paper is:

Cuevas, E., Gómez-Peláez, Á. J., Rodríguez, S., Terradellas, E., Basart, S., García, R. D., García, O. E., and Alonso-Pérez, S.: Pivotal role of the North African Dipole Intensity (NAFDI) on alternate Saharan dust export over the North Atlantic and the Mediterranean, and relationship with the Saharan Heat Low and mid-latitude Rossby waves, Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2016-287, in review, 2016.
The article can be downloaded at: http://www.atmos-chem-phys-discuss.net/acp-2016-287/