Influence of aerosols in multidecadal SST variability simulations over the North Pacific

Abstract

The influence of aerosol emissions on North Pacific sea surface temperature (SST) variability during the twentieth century is investigated using a comparison between historical simulations with and without anthropogenic aerosol changes. The historical simulations using the Hadley Global Environment Model version 2 show that there is a common externally forced component in relation to the twentieth century North Pacific SST variability. This matches a number of important temporal and spatial characteristics of the observed multidecadal SST variability from the 1920s to 1990s, which is not found in experiments without aerosol changes. This paper explores both direct and indirect aerosol influences, and finds that in this model the aerosol-cloud interactions dominate the total aerosol forcing of the surface energy budget. These aerosol-cloud processes were not commonly included in most models in the previous (Coupled Model Intercomparison Project phase 3) generation, which may explain why the potential role of aerosols in Pacific variability has not been previously discussed. However, unlike recently reported aerosol drivers of Atlantic SST variability, the aerosol surface radiative forcing pattern does not map directly onto the historical spatial surface radiative and SST changes but is instead modulated by circulation changes to the Aleutian Low. These circulation changes share common features with previously reported studies of natural drivers of Pacific variability, suggesting that both forced and internally generated SST variability may be modulated via the same circulation response.

Explaining Extreme Events of 2014 from a Climate Perspective

“…Average daily mean temperature (Tmean) was also the second warmest (13.8°C) behind only 1998. Due to the unusually warm spring weather, many spring flower festivals ran earlier than scheduled, indicating the dominant influence of temperature on plant phenology (Kim et al 2015). Further, tropical nights (daily minimum temperature >25°C; e.g., Choi et al 2009) appeared in May for the first time over South Korea (Jeju and Gangneung stations) since meteorological observations began in the early 20th century.…”

“…(NCDC 2014;Environment Canada 2014). These low temperatures have been connected to warm sea surface temperatures in the North Pacific (Hartmann 2015;Lee et al 2015). A potential driver for positive sea surface temperature anomalies in the North Pacific and cold conditions in central North America further downstream is warm surface waters in the tropical west Pacific (Palmer 2014;Hartmann 2015).…”

“…Sea surface temperatures in the tropical west Pacific were exceptionally high in winter 2013/14 (Lee et al 2015) causing enhanced convective activity, indicated by negative outgoing longwave radiation (OLR) anomalies, in this region (Supplemental Fig. S7.1).…”

“…The HadGEM3-A response to SST may be exaggerated by a lack of air-sea coupling in the model (e.g., Zhu and Shukla 2013), and the SST changes themselves may have an anthropogenically driven component. In particular, drought in northern China has previously been linked to PDO and AMO variability, which are potentially driven, at least in part, by AA changes (Booth et al 2012;Allen et al 2014;Boo et al 2015).…”

The 2014 Hot, Dry Summer in Northeast Asia

“…The HadGEM3-A response to SST may be exaggerated by a lack of air-sea coupling in the model (e.g., Zhu and Shukla 2013), and the SST changes themselves may have an anthropogenically driven component. In particular, drought in northern China has previously been linked to PDO and AMO variability, which are potentially driven, at least in part, by AA changes (Booth et al 2012;Zhang et al 2013;Allen et al 2014;Boo et al 2015).…”