On the mechanisms of decadal variability of the North Pacific Gyre Oscillation over the 20th century

Abstract

The decadal variability of the North Pacific gyre oscillation (NPGO) over the 20 th century is examined from a long-term integration of the Simple Ocean Data Assimilation (SODA) reanalysis. The NPGO is reflected by the second dominant pattern of sea surface height (SSH) variability in SODA, with a northsouth dipole structure over the northeast Pacific. SSH anomalies in this region exhibit distinct decadal variability with a significant spectrum peak at approximately 18 years. The upper-ocean heat budget reveals that this dipole structure associated with the NPGO is predominantly due to the anomalous Ekman pumping and Ekman advection induced by the surface wind. The NPGO mode in SODA reanalysis originates from atmosphere stochastic noise (North Pacific Oscillation) which has a meridional dipole pattern but no preferred time scale. The oceanic planetary wave, particularly the advective baroclinic mode, integration of atmospheric stochastic noise leads to a spatial resonance with preferred decadal time scale. The limitation of current study is also discussed.

Low-Frequency Variability and Possible Changes in the North Pacific Simulated by CMIP5 Models

“…Because the data length is limited, longer periods cannot be captured. The power spectrum of NPGO in the observations has two peaks, around 20 and 8 years, which is consistent with a previous study (Yi et al 2015); there is also a prominent decadal period around 15 years for the MME4 simulated NPGO. The analysis above shows that the MME4 makes some progress toward capturing the PDO and NPGO cycle, although the prominent periods of PDO and NPGO are hard to capture either in a single model or in MME.…”

Empirical Evidence Linking the Pacific Decadal Precession to Kuroshio Extension Variability

“…Importantly, these two characteristics subsequently allow the anomalies to continue to force the initial SSH anomaly (Figures 3e and 3f). Through the time integration of this forcing (Frankignoul et al, 1997;Kwon et al, 2010;Qiu et al, 2007), the resultant baroclinic Rossby wave intensifies as it propagates westward (Farneti, 2007;Yi et al, 2015), which is also a characteristic seen in our empirical analysis (Figures 4e and 4f). As the SSH anomaly passes the date line, our empirical analysis indicates that it narrows substantially as it approaches the western portion of the basin (Figure 4g), consistent with the transition from large-scale, linear Rossby wave dynamics to mesoscale, non-linear Rossby wave dynamics along a jet (Sasaki et al, 2014).…”

“…This propagation of the meridional dipole in atmospheric pressures is co-located with a similar propagating dipole in SSTs (Figures 5e, 5f, and 7) and is equivalent barotropic with higherthan-normal atmospheric pressures situated over warmer-than-normal SSTs. As noted above, previous theoretical results (Goodman & Marshall, 1999;Ferreira et al, 2001;Farneti, 2007;Fang & Yang, 2011) highlight that this equivalent barotropic structure can sustain the oceanic and atmospheric anomalies through positive feedbacks in which the wind stress curl associated with the atmospheric anomaly induces changes in SSTs via vertical and horizontal advection (Farneti, 2007;Yi et al, 2015;Kwon et al, 2010;Qiu et al, 2014;Fang & Yang, 2011, 2015. By altering the large-scale meridional temperature gradient, the SST anomalies in turn modify the baroclinicity of the overlying atmosphere and give rise to anomalous transient eddy activity that sustains the large-scale, low-frequency atmospheric anomalies through vorticity flux convergence (Fang & Yang, 2015;Kushnir et al, 2002;Lau & Nath, 1990;Xue et al, 2018;Zhou et al, 2017).…”

“…More relevant for the discussion here is the orientation of the accompanying 850-hPa geopotential height anomalies during the N-S phase (Figures 5d-5f), in which high (low) atmospheric pressure anomalies overlay warm (cold) SSTs, indicative of the well-known equivalent barotropic atmospheric response to underlying SST anomalies that takes place in the extratropics on seasonal to decadal timescales (e.g., Ferreira & Frankignoul, 2005;Frankignoul, 1985;Kushnir et al, 2002;Latif & Barnett, 1996;Lau & Nath, 1990;Namias & Cayan, 1981;Shutts, 1987). This equivalent barotropic atmospheric response leads to unstable growth of the oceanic and atmospheric anomalies (Goodman & Marshall, 1999;Ferreira et al, 2001;Farneti, 2007;Fang & Yang, 2011) through positive feedbacks in which the wind stress curl associated with the atmospheric anomaly augments the changes in SSTs via vertical and horizontal advection (Farneti, 2007;Yi et al, 2015;Kwon et al, 2010;Qiu et al, 2014;Fang & Yang, 2011, 2015. The SST anomalies subsequently augment the changes in the overlying atmosphere by altering the meridional temperature gradient that controls atmospheric baroclinicity.…”

“…The latter are particularly prevalent both at the large scale (~1,000 km and larger) in the vicinity of the North Pacific's subtropical gyre and at the mesoscale (~100 km) in the vicinity of the Kuroshio Current and Kuroshio Extension (KE) current. Coincidently (or not), KE structure and dynamics have a well-known quasi-decadal periodicity to them (e.g., Qiu & Chen, 2005;Pierini & Dijkstra, 2009;Qiu & Chen, 2010;Ceballos et al, 2009;Yi et al, 2015;Qiu et al, 2014;Pierini, 2014Pierini, , 2015Kawakubo et al, 2017), part of which has been linked to large-scale forcing by the NPO (Ceballos et al, 2009;Qiu et al, 2014;Sasaki et al, 2014;Pierini, 2014;Kawakubo et al, 2017). Given these previous works, along with evidence from our own previous analysis of the PDP, the motivation of this current research is to assess any potential links between the PDP and KE variability through empirical analyses.…”