ALBERT

Description: By performing two sets of high-resolution atmospheric general circulation model (AGCM) experiments, we find that the atmospheric response to a sea surface temperature (SST) anomaly in the extratropical North Pacific is sensitive to decadal variations of the background SST on which the SST anomaly is superimposed. The response in the first set of experiments, in which the SST anomaly is superimposed on the observed daily SST of 1981-1990, strongly differs from the response in the second experiment, in which the same SST anomaly is superimposed on the observed daily SST of 1991-2000. The atmospheric response over the North Pacific during 1981-1990 is eddy-mediated, equivalent barotropic and concentrated in the east. In contrast, the atmospheric response during 1991-2000 is weaker and strongest in the west. The results are discussed in terms of Rossby wave dynamics, with the proposed primary wave source switching from baroclinic eddy vorticity forcing over the eastern North Pacific in 1981-1990 to mean flow divergence over the western North Pacific in 1991-2000. The wave source changes are linked to the decadal reduction of daily SST variability over the eastern North Pacific and strengthening of the Oyashio Extension front over the western North Pacific. Thus, both daily and frontal aspects of the background SST variability in determining the atmospheric response to extratropical North Pacific SST anomalies are emphasized by our AGCM experiments.

Description: The atmospheric response to large-scale extratropical North Pacific sea surface temperature (SST) anomalies associated with the Pacific Decadal Oscillation (PDO) is assessed using observational data, coupled ocean-atmosphere general circulation model (CGCM) simulations, and forced high-resolution atmospheric general circulation model (AGCM) experiments. The characteristics of the atmospheric response as well as its sensitivity to daily to decadal variability in the background SST, on which the SST anomalies are superimposed, are inspected. Here, only the boreal winter (DJF) season is considered. The analysis of the observational data reveals that the PDO exhibits significant variability on interannual, decadal and multidecadal timescales, and is highly correlated with the El Niño/Southern Oscillation (ENSO). In the atmosphere, significant spectral peaks at similar frequencies as those identified in the PDO are found in the observed sea level pressure over the North Pacific, suggesting potential links between the ocean and the atmosphere. This link is further supported by a number of regression and correlation analyses. It is proposed that SST anomalies associated with the PDO can persist for several winters through the “reemergence” mechanism, repeatedly exerting anomalous SST forcing on the atmosphere and causing the latter to respond. The atmospheric response is of the “cold SST-low pressure”-type, also known as the “cold-trough” (or “warm-ridge”) response. Exactly the same analyses as those described above are repeated on the output of a coarse-resolution CGCM, the Kiel Climate Model (KCM). Results show that the KCM depicts the reemergence mechanism but does not simulate an atmospheric response consistent with that suggested by the observations. It is conjectured that too coarse horizontal resolution in the atmospheric component of the KCM inhibits a realistic representation of the atmospheric response to extratropical North Pacific SST anomalies. To further investigate the mechanism of the atmospheric response and to examine the reason for the failure of the KCM to simulate such a response, a high-(horizontal) resolution version of the ECHAM5 AGCM is integrated in stand-alone mode forced by prescribed SSTs. The SST forcing consists of the observed daily SSTs of 1981–1990, which serve as the background SSTs, on which a fixed PDO-like SST anomaly, which as been derived from observations, is superimposed. Results show that the winter-mean atmospheric response to the SST anomaly (averaged over all 10 winters) is equivalent barotropic, with a significant low pressure anomaly over the eastern North Pacific and a northeast-southwest shift in the storm track. It is further pointed out that the response is established and maintained by anomalous eddy vorticity and momentum flux convergence. Noticeable changes in zonal and vertical winds are also found. In terms of surface pressure, the model results compare reasonably well with the observations. In order to further investigate the role of the background SSTs in the atmospheric response, a set of sensitivity experiments with the ECHAM5 AGCM is designed employing identical SST anomalies but different background SSTs. It is found that daily background SST variability plays a key role in the atmospheric response, whereas interannual variability of the background SST is only of minor importance. It is thus proposed that sufficient daily extratropical SST variability must be simulated by the ocean components and resolved by the atmospheric components of CGCMs to enable realistic simulation of decadal climate variability in the North Pacific sector. Identical AGCM integrations were conducted for the subsequent 10 winters of 1991–2000. Together, the two sets of integrations reveal a remarkable decadal transition with sign reversal in terms of the area-averaged 500 hPa height anomalies over the North Pacific. Although the response is characterized by an equivalent barotropic, circumglobal Rossby wave train in both periods, the primary wave source switches from baroclinic eddy vorticity forcing over the eastern North Pacific in 1981–1990 to mean-flow divergence over the western North Pacific in 1991–2000. By examining the decadal changes of the background SSTs, it is then proposed that the wave source transition can be linked to the decadal reduction of daily SST variability in the eastern North Pacific and the decadal strengthening of the Oyashio Extension front over the western North Pacific. In this view, the decadal variations of small-scale features of the background SSTs, temporal and spatial, are emphasized.