The meteorological nature of variable soluble iron transport and deposition within the North Atlantic Ocean basin

Aerosol transport from the Sahara desert to the North Atlantic Ocean generates the largest annual flux of mineral dust and total Fe found in the global oceans, enriching the mixed layer with soluble iron. We use the Geophysical Fluid Dynamics Laboratory Global Chemical Transport model to examine the transport and deposition of bioavailable iron on time scales ranging from seasonal to daily. The model is compared with observed mineral dust concentrations, depositions, and soluble Fe fractions. It is shown that simulated cumulative soluble Fe deposition (SFeD) employing a variable Fe solubility parameterization compares well with observed short-term changes of dissolved iron within a thermally stratified surface mixed layer, while assuming a constant 2% solubility does not. The largest year-to-year variability of seasonal SFeD (45 to 90%) occurs throughout winter and spring in the central and northeast Atlantic Ocean. It is strongly linked to the North Atlantic Oscillation (NAO) index, producing substantially more SFeD during the positive phase than the negative phase. The ratio of wet to total SFeD increases with distance from the Saharan source region and is especially large when concentrations are small during the negative NAO. In summer, the relatively steady circulation around the Azores high results in low interannual variability of SFeD (<30%); however, regional short-term events are found to be highly episodic, and daily deposition rates can be a factor of 4 or more higher than the monthly mean flux. Three-dimensional backward trajectories are used to determine the origin and evolution of a specific SFeD event. We show that the dust mass-mean sedimentation rate should be incorporated into the air parcel dynamical vertical velocity for a more precise transport path.