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Bo Qiu, Shuiming Chen, and Niklas Schneider

Abstract

Rather than a single and continuous boundary current outflow, long-term satellite observations reveal that the Oyashio Extension (OE) in the North Pacific Subarctic Gyre comprises two independent, northeast–southwest-slanted front systems. With a mean latitude along 40°N, the western OE front exists primarily west of 153°E and is a continuation of the subarctic gyre western boundary current. The eastern OE front, also appearing along 40°N, is located between 153° and 170°E, whose entity is disconnected from its western counterpart. During 1982–2016, both of the OE fronts exhibit prominent decadal fluctuations, although their signals show little contemporaneous correlation. An upper-ocean temperature budget analysis based on the Estimating the Circulation and Climate of the Ocean, phase II (ECCO2), state estimate reveals that the advective temperature flux convergence plays a critical role in determining the low-frequency temperature changes relating to the OE fronts. Specifically, the western OE front variability is controlled by the decadal mesoscale eddy modulations in the upstream Kuroshio Extension (KE). An enhanced eddy activity increases the poleward heat transport and works to strengthen the western OE front. The eastern OE front variability, on the other hand, is dictated by both the meridional shift of the KE position and the circulation intensity change immediately north of the eastern OE. Different baroclinic adjustment speeds for the KE and OE are found to cause the in-phase changes between these latter two processes. Lack of contemporaneous correlation between the decadal western and eastern OE variability is found to be related to the interaction of the meridionally migrating KE jet with the Shatsky Rise near 159°E.

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Yoshi N. Sasaki, Shoshiro Minobe, and Niklas Schneider

Abstract

This study examines interannual to decadal variability of the Kuroshio Extension (KE) jet using satellite altimeter observations from 1993 to 2010. The leading empirical orthogonal function (EOF) mode of sea level variability in the KE region represents the meridional shift of the KE jet, followed by its strength changes with a few month lag. This shift of the KE jet lags atmospheric fluctuations over the eastern North Pacific by about three years. Broad sea level anomalies (SLAs) emerge in the eastern North Pacific 3–4 years before the upstream KE jet shift, and propagate westward along the KE jet axis. In the course of the propagation, the meridional scale of the SLAs gradually narrows, and their amplitude increases. This westward propagation of SLAs with a speed of about 5 cm s−1 is attributed to the westward propagation of the meridional shift of the jet, consistent with the thin-jet theory, whose importance has been suggested by previous numerical studies. In addition, the westward-propagating signals tend to conserve their quasigeostrophic potential vorticity anomaly, which may explain the characteristic changes of SLAs during the propagation. After the westward-propagating signals of positive (negative) SLAs reach at the east coast of Japan, the upstream KE jet strengthens (weakens) associated with the strength changes of the northern and southern recirculation gyres. Interestingly, this strength change of the KE jet propagates eastward with a speed of about 6 cm s−1, suggesting an importance of advection by the current.

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Alexander Gershunov, Niklas Schneider, and Tim Barnett

Abstract

Running correlations between pairs of stochastic time series are typically characterized by low-frequency evolution. This simple result of sampling variability holds for climate time series but is not often recognized for being merely noise. As an example, this paper discusses the historical connection between El Niño–Southern Oscillation (ENSO) and average Indian rainfall (AIR). Decades of strong correlation (∼−0.8) alternate with decades of insignificant correlation, and it is shown that this decadal modulation could be due solely to stochastic processes. In fact, the specific relationship between ENSO and AIR is significantly less variable on decadal timescales than should be expected from sampling variability alone.

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Bo Qiu, Niklas Schneider, and Shuiming Chen

Abstract

Air–sea coupled variability is investigated in this study by focusing on the observed sea surface temperature signals in the Kuroshio Extension (KE) region of 32°–38°N and 142°E–180°. In this region, both the oceanic circulation variability and the heat exchange variability across the air–sea interface are the largest in the midlatitude North Pacific. SST variability in the KE region has a dominant time scale of ∼10 yr and this decadal variation is caused largely by the regional, wind-induced sea surface height changes that represent the lateral migration and strengthening/weakening of the KE jet. The importance of the air–sea coupling in influencing KE jet is explored by dividing the large-scale wind forcing into those associated with the intrinsic atmospheric variability and those induced by the SST changes in the KE region. The latter signals are extracted from the NCEP–NCAR reanalysis data using the lagged correlation analysis. In the absence of the SST feedback, the intrinsic atmospheric forcing enhances the decadal and longer time-scale SST variance through oceanic advection but fails to capture the observed decadal spectral peak. When the SST feedback is present, a warm (cold) KE SST anomaly works to generate a positive (negative) wind stress curl in the eastern North Pacific basin, resulting in negative (positive) local sea surface height (SSH) anomalies through Ekman divergence (convergence). As these wind-forced SSH anomalies propagate into the KE region in the west, they shift the KE jet and alter the sign of the preexisting SST anomalies. Given the spatial pattern of the SST-induced wind stress curl forcing, the optimal coupling in the midlatitude North Pacific occurs at the period of ∼10 yr, slightly longer than the basin-crossing time of the baroclinic Rossby waves along the KE latitude.

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Niklas Schneider, Tim Barnett, Mojib Latif, and Timothy Stockdale

Abstract

The physics of the Indo–Pacific warm pool are investigated using a coupled ocean atmosphere general circulation model. The model, developed at the Max-Planck-Institut fair Meteorologic, Hamburg, does not employ a flux correction and is used with atmospheres at T42 and T21 resolution. The simulations are compared with observations, and the model's mean and seasonal heat budgets and physics in the Indo–Pacific warm pool region are explored for the T42 resolution run.

Despite the simulation of a split intertropical convergence zone, and of a cold tongue that extends too far to the west, simulated warm pool temperatures are consistent with observations at T42 resolution, while the T21 resolution yields a cold bias of 1K. At T42 resolution the seasonal migration of the warm pool is reproduced reasonably well, as are the surface heat fluxes, winds, and clouds. However, simulated precipitation is too small compared to observations, implying that the surface density flux is dominated by fluxes of heat.

In the Pacific portion of the warm pool, the average net heat gain of the ocean amounts to 30–40 W m−2. In the northern branch, this heat gain is balanced by vertical advection, while in the southern branch, zonal, meridional, and vertical advection cool the ocean at approximately equal rates. At the equator, the surface heat flux is balanced by zonal and vertical advection and vertical mixing. The Indonesian and Indian Ocean portions of the warm pool receive from the atmosphere 30 and 50 W m−2, respectively, and this flux is balanced by vertical advection. The cooling due to vertical advection stems from numerical diffusion associated with the upstream scheme, the coarse vertical resolution of the ocean model, and near-inertial oscillations forced by high-frequency atmospheric variability.

The seasonal migration of the warm pool is largely a result of the seasonal variability of the net surface heat flux, horizontal and vertical advections are of secondary importance and increase the seasonal range of surface temperature slightly everywhere in the warm pool, with the exception of its southern branch. There, advection reduces the effect of the surface flux. The seasonal variability of the surface heat flux in turn is mainly determined by the shortwave radiation, but evaporation modifies the signal significantly. The annual cycles of reduction of solar radiation due to clouds and SST evolve independently from each other in the Pacific portion of the warm pool; that is, clouds have little impact on SST. In the Indian Ocean, however, clouds limit the maximum SST attained during the annual cycle.

In the western Pacific and Indonesian portion of the warm pool, penetrative shortwave radiation leads to convective mixing by heating deeper levels at a greater rate than the surface, which experiences heat losses due to turbulent and longwave heat fluxes. In the deeper levels, there is no mechanism to balance the heating due to penetrative radiation, except convection and its attendant mixing. In the Indian Ocean, however. the resulting vertical heating profile due to the surface fluxes decreases monotonically with depth and does not support convective mixing. Concurrently, the warm pool is shallower in the Indian Ocean compared with the western Pacific, indicating that convective mixing due to penetrative radiation is important in maintaining the vertical structure of the Pacific portion of the warm pool.

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

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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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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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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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Bo Qiu, Shuiming Chen, Niklas Schneider, Eitarou Oka, and Shusaku Sugimoto

Abstract

Decadal modulations of the Kuroshio Extension (KE) system between a stable and an unstable dynamic state in the western North Pacific have prevailed in the past three decades. This dominance of decadal variations is controlled by the negative feedback loop involving the wind-forced KE variability and its feedback onto the overlying extratropical storm tracks and the basin-scale surface wind field. The wind-forced decadal KE modulations were disrupted in August 2017 due to the development of the Kuroshio large meander south of Japan. By forcing the inflow KE paths northward and by avoiding overriding the shallow Izu Ridge, the Kuroshio large meander was able to compel the KE to change rapidly from the wind-forced, pre-existing, unstable state to a stable state. Following the large meander occurrence in late 2017, the stabilized KE change is found to affect the overlying storm tracks and the basin-scale wind field the same way as those generated by the wind-forced KE change prior to 2017. Given the consistent atmospheric response to both the large-meander-induced and wind-forced KE variability, we expect that the KE dynamic state will resume its decadal modulation after the phase reset relating to the 2017 large meander event.

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