Influence of a basal thermal anomaly on mantle convection

We perform laboratory experiments to study the effect of the variable basal thermal anomaly on convection. For Rayleigh number Ra < 107, which is the regime of cellular convection, the response of the convection pattern changes as the horizontal temperature variation in the bottom boundary increases. When the temperature variation is less than critical, there is no effect to the convection pattern. Above critical, an upwelling is fixed at the site of the anomaly, which for a larger temperature variation, develops into a wide upwelling with a horizontally elongated cell. For this case, the time-averaged temperature in the isothermal core above the thermal anomaly becomes higher than that in the other regions. For Rayleigh number Ra ≥ 107, which is the regime of plume dominant convection, the horizontal temperature variation which exceeds critical, similarly affects the location of a dominant hot plume. However, the plume generated by the thermal anomaly straddle around the site of the thermal anomaly. For a larger temperature variation, hot plumes cluster together which also straddle around the anomaly. The straddling nature of the hot plumes generated by the thermal anomaly causes the time-averaged temperature above the thermal anomaly to remain unchanged despite the anomalous heating. Different from the cellular convection cases, the temperature variation less than critical is also capable of generating intermittent hot plumes but they do not dominate the convection pattern. The critical horizontal temperature variation to affect the convection pattern is scaled by the maximum standard deviation of the time variation of the temperature σmax* around the lower thermal boundary layer. We estimate the possible temperature variation which can be generated by a partially molten region at the CMB, and find that a region less viscous than the surrounding region by an order of magnitude can affect the location of a hotspot.