North Pacific Cooling from an abrupt increase in Black Carbon in a coupled-climate model

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Wednesday, 7 January 2015
Anna A. Scott, Johns Hopkins University, Baltimore, MD

An equilibrium coupled-climate model experiment examining the effects of black carbon was found to produce warming over most land areas but a strong cooling centered over the North Pacific Ocean. We relate this signal to the semi-direct or non-Twomey-Albrecht indirect forcing (NTAIF), referring to changes in clouds forced by black carbon's redistribution of atmospheric heating. The use of a coupled climate model allows for ocean-atmosphere interaction allowing for feedbacks between sea surface temperatures and clouds. Three regions are analyzed- the source region with high black carbon aerosol concentration but little temperature response, land over central Asia experiencing warming, and the North Pacific Ocean, which experiences a cooling of 1 degree as compared to pre-industrial BC levels. Changes in the energy budget show that in the source region, changes in radiation are partially balanced by hot air being advected away, stabilizing the boundary layer away from the source region. Over land, this decreases cloudiness, helping to drive the heating seen over Central Asia. Over the ocean, the influx of warm air aloft stabilizes the boundary layer and reduces deep convection. This allows moisture to build up and enhances cloudiness, causing a North Pacific cold anomaly of one degree. The response exceeds the natural variability of the model and is manifest both in atmospheric and oceanic variables.