A Global Process-Based Study of Marine CCN Trends and Variability

Abstract:

One proposed climate feedback mechanism is that, in a warming climate, wind speeds might change, altering emission rates of natural marine aerosols. This would affect the albedo of marine clouds, which have a strong cooling effect on a global scale.

To investigate the sign and extent of this proposed feedback mechanism, we performed a fifteen-year simulation using the global aerosol microphysics model GLOMAP-mode. We then analysed wind speed values and CCN concentrations from GLOMAP using a dynamic linear model to find non-linear trends in the data.

We compared trends in wind speed and CCN concentrations in three marine regions: the Northern Equatorial Pacific, the North Atlantic, and the Southern Ocean. No correlation was evident between wind speed and CCN concentrations in the NEP and the NA, indicating that a wind-based feedback is unlikely to have much effect in these regions. In the Southern Ocean, there is a stronger link between wind speed and CCN, suggesting a greater importance of natural, wind-driven aerosols. 

To determine which processes overwhelmed the influence of emissions to control CCN concentrations, we ran a set of six two-month simulations with high temporal resolution output: one control simulation, and five simulations with one aerosol microphysical process switched off (new particle formation, primary sea spray emissions, dimethyl sulphide emissions, nucleation scavenging, and impaction scavenging). We found that in-cloud scavenging determines the base level of CCN concentration, with a large contribution from new particle formation and marine emissions in two of the regions. However, we also found that day-to-day fluctuations in CCN concentrations were more closely linked to transport of continental emissions in the Northern Equatorial Pacific and the North Atlantic.

Given the low sensitivity of CCN to changes in emissions, and the importance of transported continental aerosol, we expect that the proposed feedback mechanism is unlikely to have a strong effect except in pristine marine environments. However, based on our finding that nucleation scavenging determines the overall level of CCN concentrations, we propose that a precipitation-based feedback mechanism is more likely to have an effect throughout the oceans.