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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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Shoshiro Minobe, Mio Terada, Bo Qiu, and Niklas Schneider

Abstract

To better understand coastal sea level variability and changes, a theory that predicts sea levels along a curved western boundary using interior ocean sea level information is proposed. The western boundary sea level at a particular latitude is expressed by the sum of contributions from interior sea levels propagating onto the western boundary by long Rossby waves between that latitude and a higher latitude, and from the western boundary sea level at the higher latitude. This theory is examined by using a linear, reduced gravity model. A comparison between the theory and the model shows good agreement. A simple scaling law (or rule of thumb) derived from the theory provides a measure of the higher-latitude sea level and ocean interior sea level contributions. The theory is then tested using data from 34 climate models in phase 5 of the Coupled Model Intercomparison Project (CMIP5) for dynamic sea level changes between the end of the twentieth and twenty-first centuries. The theory captures the nearly uniform sea level rise from the Labrador Sea to New York City (NYC), with a reduction in the increase of sea level farther south toward the equator, qualitatively consistent with the CMIP5 multimodel ensemble, even though the theory underestimates the equatorward reduction rate. Along the South American east coast, the theory successfully reproduced the spatial pattern of the sea level change. The theory suggests a strong link between a sea level rise hot spot along the northeastern coast of North America and the sea level increase in the Labrador Sea, consistent with the result that rates of NYC sea level rise are highly correlated to those in the Labrador Sea in CMIP5 models.

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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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Shota Katsura, Eitarou Oka, Bo Qiu, and Niklas Schneider

Abstract

Formation and subduction of the North Pacific Tropical Water (NPTW), its interannual variability, and its associated mechanisms were investigated by using gridded Argo-profiling float data and various surface flux data in 2003–11. The NPTW has two formation sites in the center of the North Pacific subtropical gyre, corresponding to two regional sea surface salinity maxima. Mixed layer salinity variations in these two NPTW formation sites were found to be significantly different. While seasonal variation was prominent in the eastern formation site, interannual variation was dominant in the western site. The mixed layer salinity variation in the eastern site was controlled mainly by evaporation, precipitation, and entrainment of fresher water below the mixed layer and was closely related to the seasonal variation of the mixed layer depth. In the western site, the effect of entrainment is small due to a small vertical difference in salinity across the mixed layer base, and excess evaporation over precipitation that tended to be balanced by eddy diffusion, whose strength varied interannually in association with the Pacific decadal oscillation. After subduction, denser NPTW that formed in the eastern site dissipated quickly, while the lighter one that formed in the western site was advected westward as far as the Philippine Sea, transmitting the interannual variation of salinity away from its formation region.

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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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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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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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Shayne McGregor, Axel Timmermann, Niklas Schneider, Malte F. Stuecker, and Matthew H. England

Abstract

During large El Niño events the westerly wind response to the eastern equatorial Pacific sea surface temperature anomalies (SSTAs) shifts southward during boreal winter and early spring, reaching latitudes of 5°–7°S. The resulting meridional asymmetry, along with a related seasonal weakening of wind anomalies on the equator are key elements in the termination of strong El Niño events. Using an intermediate complexity atmosphere model it is demonstrated that these features result from a weakening of the climatological wind speeds south of the equator toward the end of the calendar year. The reduced climatological wind speeds, which are associated with the seasonal intensification of the South Pacific convergence zone (SPCZ), lead to anomalous boundary layer Ekman pumping and a reduced surface momentum damping of the combined boundary layer/lower-troposphere surface wind response to El Niño. This allows the associated zonal wind anomalies to shift south of the equator. Furthermore, using a linear shallow-water ocean model it is demonstrated that this southward wind shift plays a prominent role in changing zonal mean equatorial heat content and is solely responsible for establishing the meridional asymmetry of thermocline depth in the turnaround (recharge/discharge) phase of ENSO. This result calls into question the sole role of oceanic Rossby waves in the phase synchronized termination of El Niño events and suggests that the development of a realistic climatological SPCZ in December–February/March–May (DJF/MAM) is one of the key factors in the seasonal termination of strong El Niño events.

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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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Jason C. Furtado, Emanuele Di Lorenzo, Niklas Schneider, and Nicholas A. Bond

Abstract

The two leading modes of North Pacific sea surface temperature (SST) and sea level pressure (SLP), as well as their connections to tropical variability, are explored in the 24 coupled climate models used in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) to evaluate North Pacific decadal variability (NPDV) in the past [twentieth century; climate of the twentieth century (20C3M) scenario] and future [twenty-first century; Special Report on Emissions Scenarios (SRES) A1B scenario] climate. Results indicate that the two dominant modes of North Pacific oceanic variability, the Pacific decadal oscillation (PDO) and the North Pacific Gyre Oscillation (NPGO), do not exhibit significant changes in their spatial and temporal characteristics under greenhouse warming. However, the ability of the models to capture the dynamics associated with the leading North Pacific oceanic modes, including their link to the corresponding atmospheric forcing patterns and to tropical variability, is questionable.

The temporal and spatial statistics of the North Pacific Ocean modes exhibit significant discrepancies from observations in their twentieth-century climate, most visibly for the second mode, which has significantly more low-frequency power and higher variance than in observations. The dynamical coupling between the North Pacific Ocean and atmosphere modes evident in the observations is very strong in the models for the first atmosphere–ocean coupled mode, which represents covariability of the PDO pattern with the Aleutian low (AL). However, the link for the second atmosphere–ocean coupled mode, describing covariability of an NPGO-like SST pattern with the North Pacific Oscillation (NPO), is not as clearly reproduced, with some models showing no relationship between the two.

Exploring the tropical Pacific–North Pacific teleconnections reveals more issues with the models. In contrast with observations, the atmospheric teleconnection excited by the El Niño–Southern Oscillation in the models does not project strongly on the AL–PDO coupled mode because of the displacement of the center of action of the AL in most models. Moreover, most models fail to show the observational connection between El Niño Modoki–central Pacific warming and NPO variability in the North Pacific. In fact, the atmospheric teleconnections associated with El Niño Modoki in some models have a significant projection on, and excite the AL–PDO coupled mode instead. Because of the known links between tropical Pacific variability and NPDV, these analyses demonstrate that focus on the North Pacific variability of climate models in isolation from tropical dynamics is likely to lead to an incomplete view, and inadequate prediction, of NPDV.

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