Volcanic aerosols from huge volcanoes directly reduce global mean sea surface temperature by approximately 0.2-0.3 °C, milder than global total surface temperature drop, which is ~0.3 to 0.5 °C, according to both global temperature records and model simulations. It usually takes several years to be back to normal.
The volcanic cooling signals in ocean heat content can persist for much longer time, far beyond the duration of volcanic forcing. Several studies have revealed that Krakatau’s effect in the heat content can be as long as one-century. Relaxation time of the effects recent volcanoes is generally shorter than those before the 1950s. For example, the recovery time of ocean heat content of Pinatubo, which caused comparable radiative forcing to Krakatau, seems to be much shorter. The reason is that Pinatubo happened under a warm and non-stationary background with increasing greenhouse gas forcing. However, its signal still could penetrate down to ~1000 m deep.
As the thermal expansion is the key factor to the sea level variability, decreased heat content should result in a reduction in global mean sea level on a decadal time scale. However, Grinsted argued that a significant sea level rise is the first direct response to the volcanic eruption, and after that sea level becomes to drop. One possible explanation for this phenomenon is the imbalance of ocean mass fluxes. After the volcano eruption, evaporation over ocean becomes lower, because it is largely determined by the ocean skin temperature change. The quick response of evaporation to the surface cooling and the delayed response of river runoff to the associated lower precipitation lead to an increased sea level. About 1~2 years later, river discharge becomes less due to the reduced precipitation and less sea ice melting, which cause sea level to drop.
Enhancing AMOC
Results from a number of modeling studies suggest that the Atlanticmeridional overturning circulation is enhanced by the volcano activities. The deepwater formation at the northern end of the Atlantic Ocean allows SST anomalies to be subducted into the deep ocean efficiently because the rate of overturning is altered by changes in salinity. The decreasing summer-time ice melting and precipitation due to the volcano cooling enhance the salinity near the Greenland Sea, and further reduces static stability, which means more surface water sinks into the deep ocean. The studies of Stenchikov et al. and Iwi suggest that both Krakatau and Pinatubo may have strengthened the overturning circulation. And the increase in AMOC seems to be strongest at about one decade after the volcano eruption, with a magnitude of about one sverdrup for Krakatau and Pinatubo.
