Yi, D. L., B. Gan, L. Wu and A. J. Miller, 2018:
The North Pacific Gyre Oscillation and mechanisms
of its decadal variability in CMIP5 models.
Journal of Climate, in press.
Based on the Simple Ocean Data Assimilation (SODA) product and 37 models
from the Coupled Model Intercomparison Project Phase 5 (CMIP5) database, we
evaluate the North Pacific Gyre Oscillation (NPGO) and its decadal generation
mechanisms, by studying the second leading modes of North Pacific sea surface
height (SSH) and sea level pressure (SLP) as well as their dynamical connections. It is
found that 17 out of 37 models can well simulate the spatial pattern and decadal time
scales (10-30 years) of the NPGO mode, which resembles to the observation-based
SODA. Dynamical connections between the oceanic mode – NPGO and the
atmospheric mode – North Pacific Oscillation (NPO) are strongly evident in both
SODA and the 17 models. Particularly, about 30%-40% of the variance of the NPGO
variability, which generally exhibits a preferred time scale, can be explained by the
NPO variability, which has no preferred time scale in most models.
Two mechanisms of the decadal NPGO variability which have been proposed by
previous studies are evaluated in SODA and the 17 models: (1) stochastic atmospheric
forcing – oceanic spatial resonance, (2) low-frequency atmospheric teleconnections
excited by the equatorial Pacific. Evaluation reveals that such two mechanisms are
valid in SODA and 2 models (CNRM-CM5 and CNRM-CM5-2), while 2 models
(CMCC-CM and CMCC-CMS) prefer the first mechanism and 2 models (CMCC-CESM
and IPSL-CM5B-LR) prefer the second mechanism. The other 11 models have
no evident relations with the proposed two mechanisms, suggesting the need for a
fundamental understanding of the decadal NPGO variability in future.