Articles | Volume 14, issue 24
Research article
19 Dec 2014
Research article |  | 19 Dec 2014

A global process-based study of marine CCN trends and variability

E. M. Dunne, S. Mikkonen, H. Kokkola, and H. Korhonen

Abstract. Low-level clouds have a strong climate-cooling effect in oceanic regions due to the much lower albedo of the underlying sea surface. Marine clouds typically have low droplet concentrations, making their radiative properties susceptible to changes in cloud condensation nucleus (CCN) concentrations. Here, we use the global aerosol model GLOMAP to investigate the processes that determine variations in marine CCN concentrations, and focus especially on the effects of previously identified wind speed trends in recent decades. Although earlier studies have found a link between linear wind speed trends and CCN concentration, we find that the effects of wind speed trends identified using a dynamic linear model in the Northern Equatorial Pacific (0.56 m s−1 per decade in the period 1990–2004) and the North Atlantic (−0.21 m s−1 per decade) are largely dampened by other processes controlling the CCN concentration, namely nucleation scavenging and transport of continental pollution. A CCN signal from wind speed change is seen only in the most pristine of the studied regions, i.e. over the Southern Ocean, where we simulate 3.4 cm−3 and 0.17 m s−1 increases over the 15-year period in the statistical mean levels of CCN and wind speed, respectively. Our results suggest that future changes in wind-speed-driven aerosol emissions from the oceans can probably have a climate feedback via clouds only in the most pristine regions. On the other hand, a feedback mechanism via changing precipitation patterns and intensities could take place over most oceanic regions, as we have shown that nucleation scavenging has by far the largest absolute effect on CCN concentrations.

Short summary
Marine clouds have a strong effect on the Earth's radiative balance. One proposed climate feedback is that, in a warming climate, marine aerosol emissions will change due to changing wind speeds. We have examined the processes that affect aerosol emissions and removal over 15 years, and high-temporal-resolution output over 2 months. We conclude that wind trends are unlikely to cause a strong feedback in marine regions, but changes in removal processes or transport from continental regions may.
Final-revised paper