Preferred locations of weak surface field in numerical dynamos with heterogeneous core-mantle boundary heat flux: consequences for the South Atlantic Anomaly

The present-day geomagnetic field is characterized by a region of weak surface intensity, the so-called South Atlantic Anomaly (SAA). We identify the locations of surface intensity minima in modern, historical and archeomagnetic field models. We then investigate whether lowermantle thermal heterogeneity may explain the location of the SAA. We run numerical dynamos with heterogeneous core-mantle boundary (CMB) heat flux inferred from a lowermost mantle tomography model, varying dynamo internal control parameters as well as the amplitude of the CMB heat flux heterogeneity. Histograms of the longitude and latitude of surface intensity minima show the persistence of different locations. We find two preferred longitudes of surface intensity minima, one close to the present SAA minimum longitude. In contrast, in the dynamo models and in the archeomagnetic field models the surface intensity minima are often close to the equator, whereas the present-day SAA is at mid-latitudes. We demonstrate that the determining ingredients in dynamo models to reproduce the SAA latitude are related to north-south asymmetries of reversed and normal geomagnetic flux on the CMB. The imposed heterogeneous heat flux leads to more convective and magnetic activities in the Northern Hemisphere. Large time-average upwelling structure below the South Atlantic in the dynamo models correlates well with the present-day SAA region. Scaling laws analysis indicates that the persistence of surface minima longitudes is favored by slow rotation, strong convection and large heat flux heterogeneity. Furthermore, increasing mantle control yields two preferred longitudes and southern surface minima, the latter indicating that the present-day southern location of the SAA is mantle controlled. However, the rareness of mid-latitude minima in dynamo models and archeomagnetic field models leads us to speculate that the SAA mid-latitude value at present is possibly unusual.