Study of the formation of North Pacific Intermediate Water by a general circulation model and the particle‐tracking method: 1. A pitfall of general circulation model studies

Kobayashi, Taiyo

doi:10.1029/1998jc900084

Cited by 16 publications

(15 citation statements)

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“…Previous studies suggest that when the Kuroshio overshoots too far to the north, a relaxation boundary correction at the sea surface can cause excessive cooling due to the large temperature difference between subtropical and subarctic waters. Such cooling can result in the formation of an over deep mixed layer in winter and could affect the NPIW formation [e.g., Kobayashi , 1999]. Thus the separation point of the Kuroshio is shown using maps of the velocity field at 50m depth in the Ctrl case (Figure 15a).…”

Section: Summary and Discussionmentioning

confidence: 99%

Effects of tidal mixing at the Kuril Straits on North Pacific ventilation: Adjustment of the intermediate layer revealed from numerical experiments

et al. 2006

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The effects of tidal mixing at the Kuril Straits on the North Pacific intermediate layer are investigated using an ocean general circulation model. A comparison of numerical experiments with and without a tidal mixing effect suggests that tidal mixing at the Kuril Straits enhances the ventilation of the North Pacific intermediate layer. The enhanced ventilation results in both freshening and cooling down to ∼27.6 σθ. In particular, the simulated North Pacific Intermediate Water (NPIW) becomes fresher and denser (by 0.3 psu and 0.1 σθ at the maximum) and hence more realistic. The enhanced ventilation is caused both through the supply of the ventilated water from the Kuril Straits, which subsequently spreads along the subarctic and subtropical gyres, and through a modification of the circulation there. The ventilation of the supplied water in turn originates from a combination of tidally enhanced convection in the Okhotsk Sea and downward diffusion at the Kuril Straits as discussed in a previous paper. The former affects the upper part of the NPIW, while the latter is dominant in the denser layers. The circulation is modified through a dynamical adjustment to the mass input into an intermediate layer that is produced by the convergence of diapycnal transport due to the tidally enhanced convection and diffusion. The dynamical adjustment is conducted mainly through Kelvin waves which have the ability to induce intergyre flow along the western boundary and also by eastward moving long Rossby waves. The latter can be present under the influence of the wind‐driven gyres for the second and higher baroclinic modes and act to spread information into the interior directly from the western boundary. As a direct consequence of this adjustment, transport of the ventilated water from the Kuril Straits to the subtropical gyre is enhanced by the intergyre flow along the western boundary, which appears as the southward intrusion of the Oyashio Current. The transported water leaves the coast to encircle the interior, and returns to the western boundary, eventually flowing into the equatorial region. Such equatorward transport associated with mass convergence in the intermediate layer is compensated by transport toward the Kuril Straits in the shallower and deeper layers, thereby enhancing both shallow and deep meridional overturning cells by 2–3 Sv. The above dynamical adjustment is thus central to our understanding of the ventilation of the intermediate layer and provides a basis for the analytical model developed in an accompanying paper.

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Section: Summary and Discussionmentioning

confidence: 99%

Effects of tidal mixing at the Kuril Straits on North Pacific ventilation: Adjustment of the intermediate layer revealed from numerical experiments

et al. 2006

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“…However, the models do not adequately reproduce the water-mass properties of the real ocean; in the numerical models, SAMW and AAIW are generally lighter and saltier, respectively, than the observed water masses. These discrepancies between the simulation and the observations probably reflect model inadequacies, which result in model predictions that are implausible or, in the worst case, completely wrong (Kobayashi 1999). Thus, it is necessary to use observations to verify model results.…”

Section: Introductionmentioning

confidence: 89%

Long-term variations of surface and intermediate waters in the southern Indian Ocean along 32°S

2012

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Variations of water properties in surface and intermediate layers along 32°S in the southern Indian Ocean were examined using a 50-year (1960-2010) time series reproduced from historical hydrographic and Argo data by using optimum interpolation. Salinity in the 26.7-27.3r h density layer decreased significantly over the whole section, at a maximum rate of 0.02 decade -1 at 26.8-26.9r h , for the 50-year average. Three deoxygenating cores were identified east of 75°E, and the increasing rate of apparent oxygen utilization in the most prominent core (26.9-27.0r h ) exceeded 0.05 ml l -1 decade -1 . The pycnostad core of Subantarctic Mode Water (SAMW) and the salinity minimum of Antarctic Intermediate Water shifted slightly toward the lighter layers. Comparisons with trans-Indian Ocean survey data from 1936 suggest that the tendencies found in the time series began before 1960. Interestingly, cores of many prominent trends were located just offshore of Australia at 26.7-27.0r h , which is in the SAMW density range. Spectrum analysis revealed that two oscillation components with time scales of about 40 and 10 years were dominant in the subsurface layers. Our results are fairly consistent with, and thus support, the oceanic responses in the southern Indian Ocean to anthropogenic climate change predicted by model studies.

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“…Therefore the studies with numerical models will supplement the observational studies. Recently, some studies with a general circulation model (GCM) have been carried out in order to simulate NPIW and examine its formation processes [Yamanaka et al, 1998;Kobayashi, 1999]. However, Kobayashi [1999] clarified that most of the previous coarse GCMs cannot reproduce NPIW at all.…”

confidence: 99%

“…Recently, some studies with a general circulation model (GCM) have been carried out in order to simulate NPIW and examine its formation processes [Yamanaka et al, 1998;Kobayashi, 1999]. However, Kobayashi [1999] clarified that most of the previous coarse GCMs cannot reproduce NPIW at all. The reason is that the less saline water, "false NPIW," appears just above NPIW density range in the GCMs where the northwestern North Pacific subtropical gyre extends northward beyond the Subarctic Front, and such a circulation is expected in the coarse GCMs driven by the annual mean wind [e.g., Hellerman and Rosenstein, 1983].…”

confidence: 99%

“…The reason is that the less saline water, "false NPIW," appears just above NPIW density range in the GCMs where the northwestern North Pacific subtropical gyre extends northward beyond the Subarctic Front, and such a circulation is expected in the coarse GCMs driven by the annual mean wind [e.g., Hellerman and Rosenstein, 1983]. Thus Kobayashi [1999] concluded that the Kuroshio Separation must be realistically reproduced to simulate NPIW.…”

confidence: 99%

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Study of the formation of North Pacific Intermediate Water by a general circulation model and the particle‐tracking method: 2. Formation mechanism of salinity minimum from the view of the “critical gradient” of the Oyashio mixing ratio

Kobayashi¹

2000

J. Geophys. Res.

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Abstract. In order to avoid the "false North Pacific Intermediate Water (NPIW)" problem a general circulation model is driven by the winter-like North Pacific wind that can separate the modeled Kuroshio from the coast around 36øN. In the model a salinity minimum appears around 26.8o-0 in the North Pacific subtropical region, which does not outcrop in the North Pacific. Formation processes of the salinity minimum are examined. Comparing them with observations and discussing them sufficiently, this study clarified that NPIW salinity minimum is formed just above the less saline water influenced largely by the Oyashio water. The formation of NPIW is basically controlled by the rapid increase of the Oyashio layer thickness around 26.5-26.8o-0, which layer structure is determined by Okhotsk Sea Mode Water pycnostad at 26.8-26.9o-0 and the winter sea surface density of -26.5o. 0. For NPIW salinity minimum formation it is most important that the gradient of the Oyashio mixing ratio exceeds the "critical gradient" around 26.4-26.7o-0. NPIW salinity minimum density is almost determined by the heaviest density of this range (26.7o.0), which is -0.1o-0 less dense than the bottom of the rapid increase of the Oyashio layer thickness. Then cabbeling leads to an increase in its density to 26.8o-0. The Sea of Okhotsk is very important for the formation of NPIW through the formation of the Oyashio thick layers, and it practically determines NPIW salinity minimum density. Recently, some studies examined the Okhotsk influence on NPIW formation from different viewpoints of the water property. Yasuda et al. [1996] showed that the low-salinity Oyashio water seen along the Oyashio branches and the Kuroshio Extension has its low potential vorticity (fp-•Op/Oz) and suggested that NPIW formation is primarily density-driven because of sinking in the Okhotsk. Yasuda [1997] showed that NPIW salinity minimum is formed by the Oyashio transport maximum at 26.8o. o, which is attributed to the Oyashio potential vorticity minimum derived from the mode water in the Sea of Okhotsk. However, it is difficult to separate the wind-driven oceanic circulation from the density-driven oceanic circulation. Also, both studies are based on the hydrographic data obtained almost simultaneously. Around the mixed water region the 1055

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Study of the formation of North Pacific Intermediate Water by a general circulation model and the particle‐tracking method: 1. A pitfall of general circulation model studies

Cited by 16 publications

References 50 publications

Effects of tidal mixing at the Kuril Straits on North Pacific ventilation: Adjustment of the intermediate layer revealed from numerical experiments

Effects of tidal mixing at the Kuril Straits on North Pacific ventilation: Adjustment of the intermediate layer revealed from numerical experiments

Long-term variations of surface and intermediate waters in the southern Indian Ocean along 32°S

Study of the formation of North Pacific Intermediate Water by a general circulation model and the particle‐tracking method: 2. Formation mechanism of salinity minimum from the view of the “critical gradient” of the Oyashio mixing ratio

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