In this study, the effects of volcanic forcing on North Pacific climate variability, on interannual to decadal time scales, are examined using climate model simulations covering the last 600 years. The model used is the Bergen Climate Model, a fully coupled atmosphere–ocean general circulation model. It is found that natural external forcings, such as tropical strong volcanic eruptions (SVEs) and variations in total solar irradiance, play an important role in regulating North Pacific Decadal Variability (NPDV). In response to tropical SVEs the lower stratospheric pole–to–equator temperature gradient is enhanced. The North polar vortex is strengthened, which forces a significant positive Arctic Oscillation (AO). At the same time, dipole zonal wind anomalies associated with strong polar vortex propagate downward from the lower stratosphere. Through positive feedbacks in the troposphere, the surface westerly winds across the central North Pacific are significantly weakened, and positive sea level pressure anomalies are formed in the North Pacific. This anomalous surface circulation results in changes in the net heat fluxes and the oceanic advection across the North Pacific. As a result of this, warm water converges in the subtropical western North Pacific, where the surface waters in addition are heated by significantly reduced latent and sensible heat fluxes from the ocean. In the eastern and high–latitude North Pacific the ocean loses more heat, and large–scale decreases in sea surface temperatures are found. The overall response of this chain of events is that the North Pacific enters a negative phase of the Pacific decadal oscillation (PDO), and this negative phase of the PDO is maintained for several years. It is thus concluded that the volcanic forcing plays a key role in the phasing of the PDO. The model results furthermore highlight the important role of troposphere–stratosphere coupling, tropical–extratropical teleconnections and extratropical ocean–atmosphere interactions for describing NPDV.

Wang, T., O.H. Otter？, Y.Q. Gao, and H.J. Wang, 2012: The response of the North Pacific Decadal Variability to strong tropical volcanic eruptions, Climate Dynamics, doi:10.1007/s00382-012-1373-5

(http://www.springerlink.com/content/06314h0126u76123/?MUD=MP)

Figure (a) Volcanic and TSI forcings based on Crowley et al., 2003. The dashed lines represent tropical strong volcanic eruptions. (b) Reconstructed (Mann et al., 2009, green) and simulated (CTL600 black, EXT600 blue and ALL150 red; 9–year running mean) annual SST anomalies over the PDO region. The mean SST in ALL150 is used as a reference value for these three simulations. The PDO region is defined as the region from 22.5o N to 57.5o N, and from 152.5o E to 132.5o W (Mantua et al., 1997).