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Bo Qiu, Shuiming Chen, Niklas Schneider, and Bunmei Taguchi

Abstract

Being the extension of a wind-driven western boundary current, the Kuroshio Extension (KE) has long been recognized as a turbulent current system rich in large-amplitude meanders and energetic pinched-off eddies. An important feature emerging from recent satellite altimeter measurements and eddy-resolving ocean model simulations is that the KE system exhibits well-defined decadal modulations between a stable and an unstable dynamic state. Here the authors show that the decadally modulating KE dynamic state can be effectively defined by the sea surface height (SSH) anomalies in the 31°–36°N, 140°–165°E region. By utilizing the SSH-based KE index from 1977 to 2012, they demonstrate that the time-varying KE dynamic state can be predicted at lead times of up to ~6 yr. This long-term predictability rests on two dynamic processes: 1) the oceanic adjustment is via baroclinic Rossby waves that carry interior wind-forced anomalies westward into the KE region and 2) the low-frequency KE variability influences the extratropical storm tracks and surface wind stress curl field across the North Pacific basin. By shifting poleward (equatorward) the storm tracks and the large-scale wind stress curl pattern during its stable (unstable) dynamic state, the KE variability induces a delayed negative feedback that can enhance the predictable SSH variance on the decadal time scales.

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Niklas Schneider, Arthur J. Miller, and David W. Pierce

Abstract

A systematic analysis of North Pacific decadal variability in a full-physics coupled ocean–atmosphere model is executed. The model is an updated and improved version of the coupled model studied by Latif and Barnett. Evidence is sought for determining the details of the mechanism responsible for the enhanced variance of some variables at 20–30-yr timescales. The possible mechanisms include a midlatitude gyre ocean–atmosphere feedback loop, stochastic forcing, remote forcing, or sampling error.

Decadal variability in the model is expressed most prominently in anomalies of upper-ocean streamfunction, sea surface temperature (SST), and latent surface heat flux in the Kuroshio–Oyashio extension (KOE) region off Japan. The decadal signal off Japan is initiated by changes in strength and position of the Aleutian low. The atmospheric perturbations excite SST anomalies in the central and eastern North Pacific (with opposing signs and canonical structure). The atmospheric perturbations also change the Ekman pumping over the North Pacific, which excites equivalent barotropic Rossby waves that carry thermocline depth perturbations toward the west. This gyre adjustment results in a shift in the border between subtropical and subpolar gyres after about five years. This process consequently excites SST anomalies (bearing the same sign as the central North Pacific) in the KOE region. The SST anomalies are generated by subsurface temperature anomalies that are brought to the surface during winter by deep mixing and are damped by air–sea winter heat exchange (primarily latent heat flux). This forcing of the atmosphere by the ocean in the KOE region is associated with changes of winter precipitation over the northwestern Pacific Ocean. The polarity of SST and Ekman pumping is such that warm central and cool eastern Pacific anomalies are associated with a deep thermocline, a poleward shift of the border between subtropical and subpolar gyres, and warm SST anomalies and an increase of rain in the KOE region.

The preponderance of variance at decadal timescales in the KOE results from the integration of stochastic Ekman pumping along Rossby wave trajectories. The Ekman pumping is primarily due to atmospheric variability that expresses itself worldwide including in the tropical Pacific. A positive feedback between the coupled model KOE SST (driven by the ocean streamfunction) and North Pacific Ekman pumping is consistent with the enhanced variance of the coupled model at 20–30-yr periods. However, the time series are too short to unambiguously distinguish this positive feedback hypothesis from sampling variability. No evidence is found for a midlatitude gyre ocean–atmosphere delayed negative feedback loop.

Comparisons with available observations confirm the seasonality of the forcing, the up to 5-yr time lag between like-signed central North Pacific and KOE SST anomalies, and the associated damping of SST in the KOE region by the latent heat flux. The coupled model results also suggest that observed SST anomalies in the KOE region may be predictable from the history of the wind-stress curl over the North Pacific.

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Elena Yulaeva, Niklas Schneider, David W. Pierce, and Tim P. Barnett

Abstract

Potential predictability of low-frequency climate changes in the North Pacific depends on two main factors. The first is the sensitivity of the atmosphere to ocean-induced anomalies at the sea surface in midlatitudes. The second is the degree of teleconnectivity of the tropical low-frequency variability to midlatitudes. In contrast to the traditional approach of prescribing sea surface temperature (SST) anomalies, the response of a coupled atmospheric general circulation (CCM3)–mixed layer ocean model to oceanic perturbations of the mixed layer heat budget is examined. Since positive oceanic heat flux perturbations partially increase SST anomalies (locally), and partially are vented directly into the atmosphere, expressing boundary forcing on the atmosphere by prescribing upper-ocean heat flux anomalies allows for better understanding of the physical mechanism of low-frequency variability in midlatitudes. In the framework of this approach SST is considered to be a part of the adjustment of the coupled system rather than an external forcing. Wintertime model responses to mixed layer heat budget perturbations of up to 40 W m−2 in the Kuroshio extension region and in the tropical central Pacific show statistically significant anomalies of 500-mb geopotential height (Z500) in the midlatitudes. The response to the tropical forcing resembles the well-known Pacific–North American pattern, one of the leading modes of internal variability of the control run. The amplitude of the Z500 geopotential height reaches 40 m in the region of the Aleutian low. The response of Z500 to forcing in the Kuroshio Current extension region resembles the mixture of western Pacific and Pacific–North American patterns, the first two modes of the internal variability of the atmosphere. In midlatitudes this response is equivalent barotropic, with the maximum of 80 m at (60°N, 160°W). Examination of the vorticity and thermodynamic budgets reveals the crucial role of submonthly transient eddies in maintaining the anomalous circulation in the free atmosphere.

At the surface the response manifests itself in changes of surface temperature and the wind stress. The amplitude of response to the tropical forcing in the SST field at the Kuroshio Current extension region is up to 0.3 K (in absolute value) that is 2 times weaker than SST anomalies induced by midlatitude forcing of the same amplitude. In addition, the spatial structures of the responses in the SST field over the North Pacific are different. While tropical forcing induces SST anomalies in the central North Pacific, the midlatitude forcing causes SST anomalies off the east coast of Japan, in the Kuroshio–Oyashio extension region. Overall, remote tropical forcing appears to be effective in driving anomalies over the central North Pacific. This signal can be transported westward by the oceanic processes. Thus tropical forcing anomalies can serve as a precursor of the changes over the western North Pacific.

In the case of midlatitude forcing, the response in the wind stress field alters Ekman pumping in such a way that the expected change of the oceanic gyre, as measured by the Sverdrup transport, would counteract the prescribed forcing in the Kuroshio extension region, thus causing a negative feedback. This response is consistent with the hypothesis that quasi-oscillatory decadal climate variations in the North Pacific result from midlatitude ocean–atmosphere interaction.

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Niklas Schneider, Emanuele Di Lorenzo, and Pearn P. Niiler

Abstract

Hydrographic observations southwestward of the Southern California Bight in the period 1937–99 show that temperature and salinity variations have very different interannual variability. Temperature varies within and above the thermocline and is correlated with climate indices of El Niño, the Pacific decadal oscillation, and local upwelling. Salinity variability is largest in the surface layers of the offshore salinity minimum and is characterized by decadal-time-scale changes. The salinity anomalies are independent of temperature, of heave of the pycnocline, and of the climate indices. Calculations demonstrate that long-shore anomalous geostrophic advection of the mean salinity gradient accumulates along the mean southward trajectory along the California Current and produces the observed salinity variations. The flow anomalies for this advective process are independent of large-scale climate indices. It is hypothesized that low-frequency variability of the California Current system results from unresolved, small-scale atmospheric forcing or from the ocean mesoscale upstream of the Southern California Bight.

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Bunmei Taguchi, Niklas Schneider, Masami Nonaka, and Hideharu Sasaki

Abstract

Generation and propagation processes of upper-ocean heat content (OHC) in the North Pacific are investigated using oceanic subsurface observations and an eddy-resolving ocean general circulation model hindcast simulation. OHC anomalies are decomposed into physically distinct dynamical components (OHCρ) due to temperature anomalies that are associated with density anomalies and spiciness components (OHCχ) due to temperature anomalies that are density compensating with salinity. Analysis of the observational and model data consistently shows that both dynamical and spiciness components contribute to interannual–decadal OHC variability, with the former (latter) component dominating in the subtropical (subpolar) North Pacific. OHCρ variability represents heaving of thermocline, propagates westward, and intensifies along the Kuroshio Extension, consistent with jet-trapped Rossby waves, while OHCχ variability propagates eastward along the subarctic frontal zone, suggesting advection by mean eastward currents. OHCχ variability tightly corresponds in space to horizontal mean spiciness gradients. Meanwhile, area-averaged OHCχ anomalies in the western subarctic frontal zone closely correspond in time to meridional shifts of the subarctic frontal zone. Regression coefficient of the OHCχ time series on the frontal displacement anomalies quantitatively agree with the area-averaged mean spiciness gradient in the region, and suggest that OHCχ is generated via frontal variability in the subarctic frontal zone.

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Niklas Schneider, Arthur J. Miller, Michael A. Alexander, and Clara Deser

Abstract

Observations of oceanic temperature in the upper 400 m reveal decadal signals that propagate in the thermocline along lines of constant potential vorticity from the ventilation region in the central North Pacific to approximately 18°N in the western Pacific. The propagation path and speed are well described by the geostrophic mean circulation and by a model of the ventilated thermocline. The approximate southward speed of the thermal signal of 7 mm s−1 yields a transit time of approximately eight years. The thermal anomalies appear to be forced by perturbations of the mixed layer heat budget in the subduction region of the central North Pacific east of the date line. A warm pulse was generated in the central North Pacific by a series of mild winters from 1973 to 1976 and reached 18°N around 1982. After 1978 a succession of colder winters initiated a cold anomaly in the central North Pacific that propagated along a similar path and with a similar speed as the warm anomaly, then arrived in the western tropical Pacific at 18°N around 1991. Tropical Ekman pumping, rather than further propagation of the midlatitude signal, caused the subsequent spread into the equatorial western Pacific and an increase in amplitude. Historical data show that anomalous sea surface temperature in the equatorial central Pacific is correlated with tropical Ekman pumping while the correlation with thermal anomalies in the North Pacific eight years earlier is not significant. These results indicate no significant coupling in the Pacific of Northern Hemisphere midlatitudes and the equatorial region via advection of thermal anomalies along the oceanic thermocline.

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Lina I. Ceballos, Emanuele Di Lorenzo, Carlos D. Hoyos, Niklas Schneider, and Bunmei Taguchi

Abstract

Recent studies have identified the North Pacific Gyre Oscillation (NPGO) as a mode of climate variability that is linked to previously unexplained fluctuations of salinity, nutrient, and chlorophyll in the northeast Pacific. The NPGO reflects changes in strength of the central and eastern branches of the subtropical gyre and is driven by the atmosphere through the North Pacific Oscillation (NPO), the second dominant mode of sea level pressure variability in the North Pacific. It is shown that Rossby wave dynamics excited by the NPO propagate the NPGO signature in the sea surface height (SSH) field from the central North Pacific into the Kuroshio–Oyashio Extension (KOE), and trigger changes in the strength of the KOE with a lag of 2–3 yr. This suggests that the NPGO index can be used to track changes in the entire northern branch of the North Pacific subtropical gyre. These results also provide a physical mechanism to explain coherent decadal climate variations and ecosystem changes between the North Pacific eastern and western boundaries.

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Karl Stein, Axel Timmermann, Niklas Schneider, Fei-Fei Jin, and Malte F. Stuecker

Abstract

One of the key characteristics of El Niño–Southern Oscillation (ENSO) is its synchronization to the annual cycle, which manifests in the tendency of ENSO events to peak during boreal winter. Current theory offers two possible mechanisms to account the for ENSO synchronization: frequency locking of ENSO to periodic forcing by the annual cycle, or the effect of the seasonally varying background state of the equatorial Pacific on ENSO’s coupled stability. Using a parametric recharge oscillator (PRO) model of ENSO, the authors test which of these scenarios provides a better explanation of the observed ENSO synchronization.

Analytical solutions of the PRO model show that the annual modulation of the growth rate parameter results directly in ENSO’s seasonal variance, amplitude modulation, and 2:1 phase synchronization to the annual cycle. The solutions are shown to be applicable to the long-term behavior of the damped model excited by stochastic noise, which produces synchronization characteristics that agree with the observations and can account for the variety of ENSO synchronization behavior in state-of-the-art coupled general circulation models. The model also predicts spectral peaks at “combination tones” between ENSO and the annual cycle that exist in the observations and many coupled models. In contrast, the nonlinear frequency entrainment scenario predicts the existence of a spectral peak at the biennial frequency corresponding to the observed 2:1 phase synchronization. Such a peak does not exist in the observed ENSO spectrum. Hence, it can be concluded that the seasonal modulation of the coupled stability is responsible for the synchronization of ENSO events to the annual cycle.

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Emanuele Di Lorenzo, Arthur J. Miller, Niklas Schneider, and James C. McWilliams

Abstract

Long-term changes in the observed temperature and salinity along the southern California coast are studied using a four-dimensional space–time analysis of the 52-yr (1949–2000) California Cooperative Oceanic Fisheries Investigations (CalCOFI) hydrography combined with a sensitivity analysis of an eddy-permitting primitive equation ocean model under various forcing scenarios. An overall warming trend of 1.3°C in the ocean surface, a deepening in the depth of the mean thermocline (18 m), and increased stratification between 1950 and 1999 are found to be primarily forced by large-scale decadal fluctuations in surface heat fluxes combined with horizontal advection by the mean currents. After 1998 the surface heat fluxes suggest the beginning of a period of cooling, consistent with colder observed ocean temperatures. Salinity changes are decoupled from temperature and appear to be controlled locally in the coastal ocean by horizontal advection by anomalous currents. A cooling trend of –0.5°C in SST is driven in the ocean model by the 50-yr NCEP wind reanalysis, which contains a positive trend in upwelling-favorable winds along the southern California coast. A net warming trend of +1°C in SST occurs, however, when the effects of observed surface heat fluxes are included as forcing functions in the model. Within 50–100 km of the coast, the ocean model simulations show that increased stratification/deepening of the thermocline associated with the warming reduces the efficiency of coastal upwelling in advecting subsurface waters to the ocean surface, counteracting any effects of the increased strength of the upwelling winds. Such a reduction in upwelling efficiency leads in the model to a freshening of surface coastal waters. Because salinity and nutrients at the coast have similar distributions this must reflect a reduction of the nutrient supply at the coast, which is manifestly important in explaining the observed decline in zooplankton concentration. The increased winds also drive an intensification of the mean currents of the southern California Current System (SCCS). Model mesoscale eddy variance significantly increases in recent decades in response to both the stronger upwelling winds and the warmer upper-ocean temperatures, suggesting that the stability properties of the SCCS have also changed.

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Kettyah C. Chhak, Emanuele Di Lorenzo, Niklas Schneider, and Patrick F. Cummins

Abstract

An ocean model is used to examine and compare the forcing mechanisms and underlying ocean dynamics of two dominant modes of ocean variability in the northeast Pacific (NEP). The first mode is identified with the Pacific decadal oscillation (PDO) and accounts for the most variance in model sea surface temperatures (SSTs) and sea surface heights (SSHs). It is characterized by a monopole structure with a strong coherent signature along the coast. The second mode of variability is termed the North Pacific Gyre Oscillation (NPGO). This mode accounts for the most variance in sea surface salinities (SSSs) in the model and in long-term observations. While the NPGO is related to the second EOF of the North Pacific SST anomalies (the Victoria mode), it is defined here in terms of SSH anomalies. The NPGO is characterized by a pronounced dipole structure corresponding to variations in the strengths of the eastern and central branches of the subpolar and subtropical gyres in the North Pacific. It is found that the PDO and NPGO modes are each tied to a specific atmospheric forcing pattern. The PDO is related to the overlying Aleutian low, while the NPGO is forced by the North Pacific Oscillation. The above-mentioned climate modes captured in the model hindcast are reflected in satellite altimeter data.

A budget reconstruction is used to study how the atmospheric forcing drives the SST and SSH anomalies. Results show that the basinwide SST and SSS anomaly patterns associated with each mode are shaped primarily by anomalous horizontal advection of mean surface temperature and salinity gradients (∇ Tand ∇ S) via anomalous surface Ekman currents. This suggests a direct link of these modes with atmospheric forcing and the mean ocean circulation. Smaller-scale patterns in various locations along the coast and in the Gulf of Alaska are, however, not resolved with the budget reconstructions. Vertical profiles of the PDO and NPGO indicate that the modes are strongest mainly in the upper ocean down to 250 m. The shallowness of the modes, the depth of the mean mixed layer, and wintertime temperature profile inversions contribute to the sensitivity of the budget analysis in the regions of reduced reconstruction skill.

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