Pacific decadal oscillation

There is evidence of reversals in the prevailing polarity (meaning, changes in cool surface waters versus warm surface waters within the region) of the oscillation that occurred around 1925, 1947, and 1977. The last two reversals corresponded with dramatic shifts documented in salmon production regimes in the North Pacific Ocean. This climate pattern also affects coastal sea and continental surface air temperatures from Alaska to California.

During a "warm", or "positive", phase, the west Pacific becomes cooler and part of the eastern ocean warms; during a "cool", or "negative", phase, the opposite pattern occurs. The Pacific decadal oscillation was named by Steven R. Hare, who noticed the pattern while he was studying salmon production pattern results in 1997.

The Pacific decadal oscillation index is the leading empirical orthogonal function (EOF) of monthly sea surface temperature anomalies (SST-A) over the North Pacific (poleward of 20°N) after the global average sea surface temperature has been removed. This PDO index is the standardized principal component time series.

A PDO 'signal' has been reconstructed that reaches as far back as 1661 using existing scientific records that exist through tree-ring chronologies in the Baja California area.

Mechanisms

Several studies have indicated that the PDO index may be reconstructed as the superposition of tropical forcing and extra-tropical processes. Thus, unlike the El Niño–Southern Oscillation (ENSO) that exists south of the equator, the PDO is not a single physical mode of ocean variability, but rather, the sum of several processes with different dynamic origins.

At inter-annual time scales the PDO index is reconstructed as the sum of random and ENSO induced variability in the Aleutian Low, whereas, on decadal timescales ENSO teleconnections, stochastic atmospheric forcing and changes in the North Pacific oceanic gyre circulation contribute approximately equally. Additionally, sea surface temperature anomalies have some winter to winter persistence due to the reemergence mechanism.

ENSO teleconnections, the atmospheric bridge

ENSO can influence the global circulation pattern thousands of kilometers away from the equatorial Pacific through the "atmospheric bridge". During El Niño events, deep convection and heat transfer to the troposphere is enhanced over the anomalously warm sea surface temperature, this ENSO-related tropical forcing generates Rossby waves that propagate poleward and eastward and, are subsequently refracted back from the pole to the tropics. The planetary waves form at preferred locations both in the North and South Pacific Ocean, and the teleconnection pattern is established within 2–6 weeks. ENSO driven patterns modify surface temperature, humidity, wind, and the distribution of clouds over the North Pacific that alter surface heat, momentum, and freshwater fluxes and thus induce sea surface temperature, salinity, and mixed layer depth (MLD) anomalies.

The atmospheric bridge is more effective during boreal winter, when the deepened Aleutian Low results in stronger and cold northwesterly winds over the central Pacific and warm-humid southerly winds along the North American west coast, the associated changes in the surface heat fluxes and, to a lesser extent Ekman transport creates negative sea surface temperature anomalies and a deepened MLD in the central Pacific and warm the ocean from Hawaii to the Bering Sea.

SST reemergence

Midlatitude SST anomaly patterns tend to recur from one winter to the next, but not during the intervening summer, this process occurs because of the strong mixed layer seasonal cycle. The mixed layer depth over the North Pacific is deeper, typically 100-200m, in winter than it is in summer and thus, SST anomalies that form during winter and extend to the base of the mixed layer are sequestered beneath the shallow summer mixed layer when it reforms in late spring and, are insulated effectively from the air-sea heat flux.

When the mixed layer deepens again in the following autumn-early winter the anomalies may again influence the surface. This process has been entitled "reemergence mechanism" by Alexander and Deser and the process is observed over much of the North Pacific Ocean although it is more effective in the west where the winter mixed layer is deeper and the seasonal cycle greater.

Stochastic atmospheric forcing

Long term sea surface temperature variation may be induced by random atmospheric forcings that are integrated and reddened into the ocean mixed layer. The stochastic climate model paradigm was proposed by Frankignoul and Hasselmann, in this model a stochastic forcing, represented by the passage of storms, alter the ocean mixed layer temperature via surface energy fluxes and Ekman currents resulting in the system being damped due to the enhanced (reduced) heat loss to the atmosphere over the anomalously warm (cold) SST via turbulent energy and longwave radiative fluxes; in the simple case of a linear negative feedback the model may be written as the separable ordinary differential equation: ${\displaystyle \frac{\mathrm{d}y}{\mathrm{d}t}=v(t)-\lambda y}$

where v is the random atmospheric forcing, λ is the damping rate (positive and constant) and y is the response.

The variance spectrum of y is: ${\displaystyle G(w)=\frac{F}{w^2+{\lambda }^2}}$

where F is the variance of the white noise forcing and w is the frequency, an implication of this equation is that, at short time scales (w>>λ) the variance of the ocean temperature increase with the square of the period while at longer timescales(w<<λ, ~150 months) the damping process dominates and limits sea surface temperature anomalies so that the spectra became white.

Thus, an atmospheric white noise generates SST anomalies at much longer timescales, but without spectral peaks. Modeling studies suggest that this process contributes to as much as 1/3 of the PDO variability at decadal timescales.

Ocean dynamics

Several dynamic oceanic mechanisms and SST-air feedback may contribute to the observed decadal variability in the North Pacific Ocean. SST variability is stronger in the Kuroshio Oyashio extension (KOE) region and is associated with changes in the KOE axis and strength, that generates decadal and longer time scales SST variance, but without the observed magnitude of the spectral peak at ~10 years, and SST-air feedback. Remote reemergence occurs in regions of strong current such as the Kuroshio extension and the anomalies created near the Japan current may reemerge the next winter in the central pacific.

Advective resonance

Saravanan and McWilliams have demonstrated that the interaction between spatially coherent atmospheric forcing patterns and an advective ocean, shows periodicities at preferred time scales when non-local advective effects dominate over the local sea surface temperature damping. This "advective resonance" mechanism may generate decadal SST variability in the Eastern North Pacific that is associated with the anomalous Ekman advection and surface heat flux.

North Pacific oceanic gyre circulation

Dynamic gyre adjustments are essential to generate decadal SST peaks in the North Pacific, the process occurs via westward propagating oceanic Rossby waves that are forced by wind anomalies in the central and eastern Pacific Ocean. The quasi-geostrophic equation for long non-dispersive Rossby Waves forced by large scale wind stress may be written as the linear partial differential equation: ${\displaystyle \frac{\mathrm{\partial }h}{\mathrm{\partial }t}-c\frac{\mathrm{\partial }h}{\mathrm{\partial }x}=\frac{-\mathrm{\nabla }\times \overrightarrow{\tau }}{{\rho }_0{f}_0}}$

where h is the upper-layer thickness anomaly, τ is the wind stress, c is the Rossby wave speed that depends on latitude, ρ₀ is the density of sea water and f₀ is the Coriolis parameter at a reference latitude. The response time scale is set by the Rossby waves speed, the location of the wind forcing and the basin width, at the latitude of the Kuroshio Extension c is 2.5 cm s⁻¹, and the dynamic gyre adjustment timescale is ~(5)10 years if the Rossby wave was initiated in the (central)eastern Pacific Ocean.

If the wind white forcing is uniform zonally, it should generate a red spectrum in which h variance increases with the period and reaches a constant amplitude at lower frequencies without decadal and interdecadal peaks, however, low frequencies atmospheric circulation tends to be dominated by fixed spatial patterns so that wind forcing is not zonally uniform, if the wind forcing is sinusoidal zonally then decadal peaks occurs due to resonance of the forced basin-scale Rossby waves.

The propagation of h anomalies in the western pacific changes the KOE axis and strength and impact SST due to the anomalous geostrophic heat transport. Studies during the 2000s suggest that Rossby waves excited by the Aleutian low propagate the PDO signal from the North Pacific to the KOE through changes in the KOE axis, while Rossby waves associated with the NPO propagate the North Pacific Gyre oscillation signal through changes in the KOE strength.