Showcase: Dust transport and deposition over the Atlantic ocean (text only version)

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Introduction | How do we calculate dust transport? | Measuring the dust transport | Results

Introduction

• In this article Yoram Kaufman and his team present a new way to calculate the desert dust flux over the Atlantic Ocean, which reveals for the first time the actual amount of dust which is transported each year from the Saharan Desert .
  

 

• Desert dust covers large extends of the Atlantic and Pacific Oceans. It has intriguing effects on the world`s life cycle, therefore it is important to study emissions from the deserts and transport from one continent to another.
  

 

• It is a main source of nutrients to oceanic biota and the Amazon forest. For example, Iron contained in aeolian dust was shown to be an important micro-nutrient for ocean phytoplankton, which could contribute to fluctuation of CO 2 on climatic time scales and contribute to climate variations.
  

 

• It affects the Earth radiation budget, thus participating in climate change as a feedback mechanisms. Desert dust interact with solar and thermal radiation, thus can modulate the Earth radiation balance in response to changing climate conditions . Dust particles can also interact with clouds, mainly after absorbing hygroscopic material, therefore making changes in precipitation in the Soudano-Sahel region. Dust also affects photolysis rates and heterogeneous reactions for ozone chemistry, by changing the concentration of UV radiation.

 

• The animated image originally accompanying this page was showing 3 different images showing a dust storm (sand color) emerging to the Atlantic Ocean south of the Sahara and circulating in the Atlantic ocean back to Northern Africa.

   

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How do we calculate dust transport?

• The dust flux is calculated by measuring the rate in which dust passes through a given window.

  This calculation can be understood in a few steps.

 

• First, the optical thickness is measured, for the dust aerosol particles which are checked.
  The optical thickness is a parameter from which a scientist can calculate the size of an aerosol particle and the concentration of a group of particles. The result together with the knowledge of the dust caring velocity and the wind speed determines the mass of a dust column.
  

 

• Then, measurements are made again to determine how long was the dust emitted from it`s source in the Saharan desert.

  These two parameters determine the dust rate of each dust storm that has been checked: how much dust per second was emitted.

• Now in order to find the flux, all that is left is to pass the dust rate that was found through a fixed sized window within a fixed time-frame (a day, a month, a year etc.).The results are the statistical and numerical knowledge of the amount of dust which is transported and deposited over the Atlantic.

• In this study dust transported from Africa was used at 15°W is calculated by applying the monthly average westward wind speed, W(m/s), to the monthly average dust concentration, M du (g/m 2 ), and the longitudinal length, L (m), of the segment through which the flux is being computed.

  The dust particles are transported westward across the Atlantic Ocean by the middle level easterly jet and sometimes North by the anticyclone over the Azores or Canaries Islands. The latitudinal variation of the dust belt is controlled by the movement of the West African midtropospheric jet which occupies its northernmost position (20°N) in the summer.

• The animated image in this section shows the Aerosol optical thickness (see color bar on the right) as function of time (vertical axis), longitude (top) and latitude (center).

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Measuring the dust transport

• In order to make the measurements, the scientists relied on two main data sources:

 

• First, the NCEP reanalysis data set, which is a sophisticated model of global winds, was used to calculate the surface winds, and the MODIS measurements of the aerosol optical thickness were used to calculate the dust column concentration.

 

• Second, the use of MODIS data helped to overcome two major difficulties previously unresolved:

  First, was sorting out the finer particles from the coarser ones, since desert dust aerosols has a much larger portion of coarse particles than smoke that is also emitted from Africa. This was done by using the MODIS accuracy and spectral sampling across the solar spectrum, together with several years of dust remote sensing from the ground by AERONET, which allowed accurate measurements of the aerosol optical thickness and distinction.

   

  The second challenge was sorting out the clouds from the dust. In order to screen for clouds and generate a statistically robust aerosol measurement, the analysis is performed on a grid box of 10 km's at the sub-satellite point.

 

• The average of the measured spectral radiance over cloud-free, glint-free ocean scenes, is used to derive the aerosol information by fitting it to a lookup table, that includes both fine aerosol (effective radius between 0.1, and 0.25 µm) and coarse aerosol (effective radius between 1 and 2.5 µm). In the process, the best fitting fine and coarse models are chosen and the optical thickness at 550 nm.

  Aggregation of the MODIS aerosol information from the 500 m pixels to the 10 km product, allows rigorous cloud screening, avoiding data gaps and still generates large enough statistics for a stable and accurate product.

• The animated image in this section shows Dust classified in the MODIS images from Terra and Aqua, on May 1st 2003 .

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Results

• Overall 240±80 Tg (Terra-gram, 10/12 grams) of dust are transported annually from Africa at 20°S-30°N. From that 20±10 Tg return east to Africa and Europe at 30°N-50°N, 140±40 Tg are deposited in the Atlantic Ocean, 50±15 Tg are deposited in the Amazon Basin and 50±25 Tg arrive to the Caribbean.

 

• Note that out of the flux returning East, part can be attributed to Asian dust. The net flux to the Amazon of 50±15 Tg (35 in Nov-April and 15 in May-Oct. - see table 2), may explain the paradox between the low estimate of dust deposition in the Amazon of 13 Tg and the order of magnitude larger estimate of the flux needed to sustain the forest. A good agreement is found between the dust deposition in the Atlantic Ocean as observed by MODIS and calculated in other models for dust diameter = 6 µm.

 

• The calculations made in this paper are of the dust monthly and annual transport from Africa and deposition in the Atlantic Ocean. The analysis reveals on a regional scale the large fraction of biomass burning smoke imbedded in the dust in the winter months (45% smoke & 55% dust in Dec-March) due to savanna fires in the Sahel, with purer dust (14% smoke, 86% dust) transported from Africa in the summer months (June-September).

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