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 (NA). 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). 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 1D mixed layer model of the ocean, and various versions of a 3D general ocean circulation model, to study the impact of Saharan dust on the circulation of the NA. A buoyancy source generated by realistic dust-induced shortwave flux anomalies is imposed in the eastern NA 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). 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 NA suggests a local cooling of about 0.5°C on sub-seasonal to interannual time-scales. Results of the 3D simulations show that an advection of the ocean properties ocurs in response to the buoyancy source in the eastern subtropical NA. The eddies and baroclinic instabilities present in the ocean advect the signal towards the west and back towards the east. Once they have reached the African coast, they trigger westward propagating Rossby waves. 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 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.
Table of Contents
1 Introduction
1.1 Aims
1.2 Structure
2 Remote Sensing: SST, AOD. KPP-1D. MITgcm
2.1 Introduction
2.2 Data and Methodology
2.2.1 MODIS AOD
2.2.2 TMI SST
2.3 Aerosol Radiative Forcing during AEROSE-I
2.4 Observed SST and AOD anomalies
2.5 Simulated SST anomalies in the ocean mixed layer
2.6 Isolating dust-induced SST anomalies
2.6.1 Model description
2.6.2 Model setup
2.6.3 Results from 3D eddy-permitting simulation
2.7 Concluding remarks
3 Impact of Saharan Dust on the North Atlantic Circulation
3.1 Introduction
3.2 Methods
3.2.1 The experimental setup
3.2.2 Construction of a realistic perturbation
3.3 Mean ocean circulation
3.4 Perturbed response
3.4.1 Large-scale response
3.4.2 Sub-basin and local response
3.4.3 Vertical structure of temperature anomalies
3.4.4 Meridional Overturning and Heat Transport in the Atlantic
3.5 Discussion and Conclusions
4 Conclusions
4.1 Summary
4.2 Outlook
Research Objectives and Key Topics
This thesis investigates the impact of Saharan mineral dust on the North Atlantic ocean circulation and sea surface temperature (SST). The primary research aim is to determine how dust-induced changes in shortwave radiative forcing affect the heat budget of the ocean mixed layer and drive subsequent alterations in ocean currents, specifically the Atlantic Meridional Overturning Circulation (AMOC) and meridional heat transport.
- Analysis of aerosol radiative forcing mechanisms related to Saharan dust outbreaks.
- Estimation of dust-induced SST cooling through satellite observations and numerical modeling.
- Assessment of ocean dynamical responses to buoyancy source perturbations in the subtropical eastern North Atlantic.
- Investigation of westward propagating Rossby waves triggered by dust-induced anomalous signals.
- Evaluation of impacts on basin-scale circulation and heat transport pathways.
Excerpt from the Book
Aerosol Radiative Forcing during AEROSE-I
To understand the impact of Saharan mineral dust on SST of the eastern Atlantic, an important quantity to know is the shortwave (SW) radiative forcing anomaly at sea level associated with a specific dust load of the atmosphere, referred to below as aerosol radiative forcing anomaly flux (ARF in Wm−2). ARF associated with dust in the SAL is usually computed from AOD fields according to: ARF = fe · AOD (2.1) where fe is the aerosol surface forcing efficiency coefficient (in units of Wm−2AOD−1, see Ramanathan et al., 2001) and AOD is the aerosol optical depth from MODIS discussed above. A few estimates of fe are available (Li et al., 2004; Yoon et al., 2005; Zhu et al., 2007).
Fig. 2.4, provides an example of an ARF field that resulted from the March 2004 dust event shown in Fig. 2.1. In order to estimate the ARF, forcing efficiencies for that month, year and region are needed. Yoon et al. (2005) provide monthly values of fe obtained from observations during the period October 1994 - December 2003. Even though the year 2004 is not included, their results are based on a 9-year climatology, so it seems quite reasonable to use their estimation for March 2004. But, the region for which these results are valid (called Cape Verde in their manuscript) includes a large portion of the African continent (see brown box on Fig. 2.5, left panel) which will lead to misleading results when applying for the oceanic region that is being investigated in this thesis.
Summary of Chapters
1 Introduction: Provides an overview of the climatic relevance of Saharan dust and defines the specific research goals of the thesis.
2 Remote Sensing: SST, AOD. KPP-1D. MITgcm: Analyzes the local cooling effect of dust on SST using observational satellite data and a 1D mixed-layer model.
3 Impact of Saharan Dust on the North Atlantic Circulation: Investigates the basin-scale dynamical response of ocean currents, AMOC, and heat transport to dust-induced buoyancy changes using a 3D ocean circulation model.
4 Conclusions: Summarizes the key findings regarding the impact of Saharan dust on ocean temperature and circulation and provides an outlook for future research.
Keywords
Saharan dust, SST, SSH, aerosol radiative forcing, North Atlantic circulation, Rossby waves, AMOC, meridional heat transport, ocean modeling, mineral dust aerosols, mixed layer depth, air-sea interaction, biogeochemical response, radiative cooling, dynamical adjustment
Frequently Asked Questions
What is the core focus of this doctoral thesis?
The research examines the physical impacts of Saharan mineral dust on the North Atlantic Ocean, specifically its influence on sea surface temperature and subsequent changes in large-scale ocean circulation patterns.
What are the primary themes addressed in the work?
The work covers aerosol radiative forcing, ocean mixed layer dynamics, North Atlantic circulation pathways, and the sensitivity of the Atlantic Meridional Overturning Circulation (AMOC) to surface perturbations.
What is the main research question of this study?
The central question is how the dust-induced cooling at the ocean surface, caused by the atmospheric attenuation of shortwave radiation, generates density gradients that force changes in the ocean circulation and meridional heat transport.
Which scientific methodologies are utilized?
The research employs a combination of satellite remote sensing (MODIS, TMI, AVHRR), in-situ radiometric data from the AEROSE-I cruise, 1D mixed-layer modeling, and a 3D eddy-permitting general ocean circulation model (MITgcm).
What does the main body of the research cover?
The main body investigates the observational evidence of dust-induced cooling, performs numerical simulations to isolate these effects from dynamical variability, and analyzes the resulting propagation of anomalies and circulation shifts.
Which keywords best describe this research?
Key terms include Saharan dust, SST, AMOC, meridional heat transport, ocean modeling, and Rossby waves.
What is the role of the Sahara Air Layer (SAL)?
The SAL is a dry, dust-laden air layer that transports significant amounts of mineral dust over the Atlantic, which the author identifies as a critical factor in modifying surface radiation and upper-ocean heat budgets.
How do Rossby waves factor into the findings?
The study finds that the anomalous signals generated by dust in the eastern Atlantic propagate westward, eventually triggering Rossby waves that significantly influence the larger basin-scale circulation.
How does the model address potential biases?
The author uses a control run and a perturbed run to isolate dust-specific impacts, while acknowledging and correcting for model biases, such as those related to coastal upwelling and satellite data retrieval limitations.
What is the significance of the 200-260 Tg annual dust deposition into the North Atlantic?
This high deposition rate makes the North Atlantic the primary global sink for Saharan dust, thus exerting a significant and measurable influence on regional climate and ocean dynamics.
- Quote paper
- Nidia Martínez Avellaneda (Author), 2010, The Impact of Saharan Dust on the North Atlantic Circulation, Munich, GRIN Verlag, https://www.grin.com/document/146039
