Modulation of Saharan dust export by the North African dipole

S Rodríguez,S Alonso-Pérez,J M Prospero,X Querol,M I García,E Cuevas,J López-Solano,A Alastuey

doi:10.5194/acp-15-7471-2015

Abstract

Abstract. We have studied the relationship between the long-term interannual variability in large-scale meteorology in western North Africa – the largest and most active dust source worldwide – and Saharan dust export in summer, when enhanced dust mobilization in the hyper-arid Sahara results in maximum dust impacts throughout the North Atlantic. We address this issue by analyzing 28 years (1987–2014) of summer averaged dust concentrations at the high-altitude Izaña observatory (~ 2400 m a.s.l.) on Tenerife, and satellite and meteorological reanalysis data. The summer meteorological scenario in North Africa (aloft 850 hPa) is characterized by a high over the the subtropical Sahara and a low over the tropics linked to the monsoon. We measured the variability of this high–low dipole-like pattern in terms of the North African dipole intensity (NAFDI): the difference of geopotential height anomalies averaged over the subtropics (30–32° N, Morocco) and the tropics (10–13° N, Bamako region) close to the Atlantic coast (at 5–8° W). We focused on the 700 hPa standard level due to dust export off the coast of North Africa tending to occur between 1 and 5 km a.s.l. Variability in the NAFDI is associated with displacements of the North African anticyclone over the Sahara and this has implications for wind and dust export. The correlations we found between the 1987–2014 summer mean of NAFDI with dust at Izaña, satellite dust observations and meteorological re-analysis data indicate that increases in the NAFDI (i) result in higher wind speeds at the north of the Inter-Tropical Convergence Zone that are associated with enhanced dust export over the subtropical North Atlantic, (ii) influence the long-term variability of the size distribution of exported dust particles (increasing the load of coarse dust) and (iii) are associated with enhanced rains in the tropical and northern shifts of the tropical rain band that may affect the southern Sahel. Interannual variability in NAFDI is also connected to spatial distribution of dust over the North Atlantic; high NAFDI summers are associated with major dust export (linked to winds) in the subtropics and minor dust loads in the tropics (linked to higher rainfall), and vice versa. The evolution of the summer NAFDI values since 1950 to the present day shows connections to climatic variability (through the Sahelian drought, ENSO (El Niño–Southern Oscillation) and winds) that have implications for dust export paths. Efforts to anticipate how dust export may evolve in future decades will require a better understanding of how the large-scale meteorological systems represented by the NAFD will evolve.

Highlights

Desert dust aerosols influence global climate by scattering and absorbing radiation (Forster et al, 2007), influencing rainfall (Creamean et al, 2013), and by modulating ocean–atmosphere CO2 exchange through the deposition of dust that supplies iron, a micronutrient for marine biota (Jickells et al, 2005)
Our research focuses on one key question: “What is the relationship between long-term interannual variability in Saharan dust export in summer and large-scale meteorology in North Africa?” To address this issue, we used (i) the UV Aerosol Index determined by the Total Ozone Mapping Spectrometer and Ozone Monitor Instrument satellite-borne spectrometers (Herman et al, 1997) for studying long-term and interannual spatial distribution of dust and (ii) gridded meteorological National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) re-analysis data (Kalnay et al, 1996) for studying the variability of large-scale meteorological processes
Ife) and satellite and meteorological reanalysis data shows that summer Saharan dust export is highly dependent on the variability of the large-scale meteorology in North Africa, which is characterized by a high over the subtropical Sahara and a low over the tropics linked to the monsoon

Summary

Introduction

Desert dust aerosols influence global climate by scattering and absorbing radiation (Forster et al, 2007), influencing rainfall (Creamean et al, 2013), and by modulating ocean–atmosphere CO2 exchange through the deposition of dust that supplies iron, a micronutrient for marine biota (Jickells et al, 2005). There is considerable interest in climate variability, the global distribution of dust (Adams et al, 2012; Ginoux et al, 2012) and dust microphysical properties including particle size, which modulates dust impacts (Mahowald et al, 2014), e.g., the interaction with radiation (Otto et al, 2007), iron solubility and supply to the ocean (Baker and Jickells, 2006), its role as cloud and ice nuclei (Welti et al, 2009), and health effects due to dust exposure (Pérez et al, 2008, 2014; Mallone et al, 2011; Díaz et al, 2012). Dust size variability is observed over timescales of individual dust events (∼ days) (Ryder et al, 2013) and in ice cores, over thousands of years, linked to changes in wind speeds, transport pathways and dust sources attributed to climate variability (Delmonte et al, 2004)

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