The Impact of Saharan Dust on the North Atlantic Circulation

Abstract

The erosion of Saharan soil is the World's largest annual source of mineral dust aerosols, resulting in a deposition of more than 40% of the global atmospheric dust into the North Atlantic. By changing the atmospheric opacity, mineral dust can alter the shortwave radiative forcing at the surface of the ocean, altering the ocean mixed layer heat budget and therefore affecting the sea surface temperature (SST), which plays an important role in the regional and global climate. Moreover, changes of the total amount of energy received at the ocean surface have an impact on the ocean circulation. In this thesis we combine several satellite observations, in-situ radiation measurements, a one-dimensional mixed layer model of the ocean, and various versions of a three-dimensional general ocean circulation model, to study the impact of Saharan dust on the circulation and transport of properties in the North Atlantic. A buoyancy source generated by realistic dust-induced shortwave flux anomalies is imposed in the eastern North Atlantic and the differences between this simulation and an unperturbed one are investigated in terms of the ocean dynamical adjustment and changes in the Atlantic Meridional Overturning Circulation (AMOC) and Meridional Heat Transport (HT). A joint analysis of aerosol optical depth retrievals from the MODIS sensor and SST from the TMI sensor for the period 2000-2006 shows a decrease in SST of 0.2° to 0.4°C simultaneously with, or shortly after, strong dust outbreaks, which is consistent with an independent estimate of SST decrease simulated by a local 1D mixed layer model. A comparison between observed TMI SST fields and simulated SSTs with an eddy-permitting model of the North Atlantic suggests a local cooling of about 0.5°C on sub-seasonal to interannual time-scales. A regression analysis suggests that about 9% of SST variance could be explained by dust-induced cooling in this region which is not represented in existing AVHRR sensor SST fields nor represented in surface heat fluxes from current atmospheric reanalyses, with which the model was forced. Results of the 3D eddy-permitting simulations show that an advection of the ocean properties ocurs in response to the buoyancy source in the eastern subtropical North Atlantic. These ocean anomalous signals are generally advected by eddies that arise from the instability of ocean currents. Their spatial pattern depends on the ocean local density structure, the time/space distribution of dust and the mean circulation in the Atlantic. The anomalies leave the perturbed area and take part in the equatorial circulation. The eddies and baroclinic instabilities present in the ocean advect the signal towards the west and back towards the east reaching the easternmost part of the original perturbed area in 10 months. Once they have reached the African coast at about 20°N, they trigger westward propagating Rossby waves. As a result of a realistic dust-induced perturbation in the shortwave flux in the subtropical eastern North Atlantic, the time-mean differences of AMOC between the perturbed and unperturbed simulations show an increased meridional transport at 38°N and 43°N of 0.55 and 0.45 Sv, respectively, and a decreased AMOC at 40°N and 45°N of 0.2 Sv. We observe a decrease of the total time-mean HT of 7 TW between 10°N and 35°N, which is a result of the balance between a decrease in the overturning component of 15 TW and an increase of the horizontal gyre component of 8 TW. The total HT is observed to increase everywhere else. In addition, we have shown changes in the amplitude of ocean properties and generation of westward propagating Rossby waves off the African coast, which are triggered by the anomalous signals generated on the east that are advected by the existing eddies following the main circulation path. We conclude that the effect of Saharan dust should be incorporated in ocean numerical simulations, specially under the frame of climate change studies when a changing dust load of the atmosphere in response to a changing climate could be possible.