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  • American Meteorological Society  (4)
  • Nagura, Motoki  (4)
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  • American Meteorological Society  (4)
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  • 1
    Online Resource
    Online Resource
    The Shallow Overturning Circulation in the Indian Ocean
    American Meteorological Society ; 2018
    In:  Journal of Physical Oceanography Vol. 48, No. 2 ( 2018-02), p. 413-434
    Details
    In: Journal of Physical Oceanography, American Meteorological Society, Vol. 48, No. 2 ( 2018-02), p. 413-434
    Abstract: The number of in situ observations in the Indian Ocean has dramatically increased over the past 15 years thanks to the implementation of the Argo profiling float program. This study estimates the mean circulation in the Indian Ocean using hydrographic observations obtained from both Argo and conductivity–temperature–depth (CTD) observations. Absolute velocity at the Argo float parking depth is used so there is no need to assume a level of no motion. Results reveal previously unknown features in addition to well-known currents and water masses. Some newly identified features include the lack of an interior pathway to the equator from the southern Indian Ocean in the pycnocline, indicating that water parcels must transit through the western boundary to reach the equator. High potential vorticity (PV) intrudes from the western coast of Australia in the depth range of the Subantarctic Mode Water, which leads to a structure similar to a PV barrier. The subtropical anticyclonic gyre retreats poleward with depth, as happens in the subtropical Atlantic and Pacific. An eastward flow was found in the eastern basin along 15°S at the depth of the Antarctic Intermediate Water—a feature expected from property distributions but never before detected in velocity estimates. Meridional mass transport indicates about 10 Sv (1 Sv ≡ 10 6 m 3 s −1 ) southward flow at 6°S and 18 Sv northward flow at 20°S, which results in meridional convergence of currents and thermocline depression at about 16°–20°S. These estimated absolute velocities agree well with those of an ocean reanalysis, which lends credibility to the strictly databased analysis.
    Type of Medium: Online Resource
    ISSN: 0022-3670 , 1520-0485
    URL: Article
    DOI: 10.1175/JPO-D-17-0127.1
    Language: Unknown
    Publisher: American Meteorological Society
    Publication Date: 2018
    detail.hit.zdb_id: 2042184-9
    detail.hit.zdb_id: 184162-2
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    Online Resource
  • 2
    Online Resource
    Online Resource
    Dual-Frequency Wind-Driven Mixed Rossby–Gravity Waves in the Equatorial Indian Ocean
    American Meteorological Society ; 2023
    In:  Journal of Physical Oceanography Vol. 53, No. 6 ( 2023-06), p. 1535-1553
    Details
    In: Journal of Physical Oceanography, American Meteorological Society, Vol. 53, No. 6 ( 2023-06), p. 1535-1553
    Abstract: Frequency spectra of in situ meridional velocity measurements in the central equatorial Indian Ocean show two distinct peaks at “quasi biweekly” periods of 10–30 days. One is near the surface at frequencies of 0.06–0.1 cpd (periods of 10–17 days) and the other is in the pycnocline (∼100-m depth) at lower frequencies of 0.04–0.06 cpd (periods 17–25 days). Analysis of a wind-forced ocean general circulation model shows that variability in the two frequency bands represents wind-driven mixed Rossby–gravity waves. The waves share a similar horizontal structure, but the meridional scale of lower-frequency variability is about one-half of that of higher-frequency variations. Higher-frequency variability has its largest amplitude in the eastern basin while the lower-frequency variability has its largest amplitude in the central basin. The vertical wavelength of lower-frequency variability is smaller by a factor of 3–4 than that of higher-frequency variability. These results are consistent with expectations from linear mixed Rossby–gravity wave theory. Numerical simulations show that the primary driver of these waves is surface wind forcing in the central and eastern Indian Ocean and that dynamical instability does not play a major role in their generation. Significance Statement Spectra of meridional velocity in the central equatorial Indian Ocean from in situ measurements show two distinct peaks in the biweekly period band with different spatial structures. This study uses an ocean general circulation model to show that variability in these two bands is driven by surface winds that are themselves spatially structured. The variability in both period bands is consistent with linear mixed Rossby–gravity wave theory, but the spatial structures, including meridional trapping scale, vertical wavelength, and zonal distribution of energy, are very different.
    Type of Medium: Online Resource
    ISSN: 0022-3670 , 1520-0485
    URL: Article
    DOI: 10.1175/JPO-D-22-0222.1
    Language: Unknown
    Publisher: American Meteorological Society
    Publication Date: 2023
    detail.hit.zdb_id: 2042184-9
    detail.hit.zdb_id: 184162-2
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    Online Resource
  • 3
    Online Resource
    Online Resource
    Interannual Variability in Sea Surface Height at Southern Midlatitudes of the Indian Ocean
    American Meteorological Society ; 2021
    In:  Journal of Physical Oceanography Vol. 51, No. 5 ( 2021-05), p. 1595-1609
    Details
    In: Journal of Physical Oceanography, American Meteorological Society, Vol. 51, No. 5 ( 2021-05), p. 1595-1609
    Abstract: This study examines interannual variability in sea surface height (SSH) at southern midlatitudes of the Indian Ocean (10°–35°S). Our focus is on the relative role of local wind forcing and remote forcing from the equatorial Pacific Ocean. We use satellite altimetry measurements, an atmospheric reanalysis, and a one-dimensional wave model tuned to simulate observed SSH anomalies. The model solution is decomposed into the part driven by local winds and that driven by SSH variability radiated from the western coast of Australia. Results show that variability radiated from the Australian coast is larger in amplitude than variability driven by local winds in the central and eastern parts of the south Indian Ocean at midlatitudes (between 19° and 33°S), whereas the influence from eastern boundary forcing is confined to the eastern basin at lower latitudes (10° and 17°S). The relative importance of eastern boundary forcing at midlatitudes is due to the weakness of wind stress curl anomalies in the interior of the south Indian Ocean. Our analysis further suggests that SSH variability along the west coast of Australia originates from remote wind forcing in the tropical Pacific, as is pointed out by previous studies. The zonal gradient of SSH between the western and eastern parts of the south Indian Ocean is also mostly controlled by variability radiated from the Australian coast, indicating that interannual variability in meridional geostrophic transport is driven principally by Pacific winds.
    Type of Medium: Online Resource
    ISSN: 0022-3670 , 1520-0485
    URL: Article
    DOI: 10.1175/JPO-D-20-0279.1
    Language: Unknown
    Publisher: American Meteorological Society
    Publication Date: 2021
    detail.hit.zdb_id: 2042184-9
    detail.hit.zdb_id: 184162-2
    Location Call Number Limitation Availability
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    Online Resource
  • 4
    Online Resource
    Online Resource
    Zonal Propagation of Near-Surface Zonal Currents in Relation to Surface Wind Forcing in the Equatorial Indian Ocean
    American Meteorological Society ; 2016
    In:  Journal of Physical Oceanography Vol. 46, No. 12 ( 2016-12), p. 3623-3638
    Details
    In: Journal of Physical Oceanography, American Meteorological Society, Vol. 46, No. 12 ( 2016-12), p. 3623-3638
    Abstract: Zonal propagation of zonal velocity along the equator in the Indian Ocean and its relationship with wind forcing are investigated with a focus on seasonal time scales using in situ observations from four acoustic Doppler current profilers (ADCPs) and an ocean reanalysis dataset. The results show that the zonal phase speed of zonal currents varies depending on season and depth in a very complicated way in relation to surface wind forcing. Surface layer zonal velocity propagates to the west in northern spring but to the east in fall in response to zonally propagating surface zonal winds, while in the pycnocline zonal phase speed is related to wind-forced ocean wave dynamics. In the western half of the analysis domain (78°–83°E), zonal phase speed in the pycnocline is eastward all year, which is attributed to the radiation of Kelvin waves forced in the western basin. In the eastern half of the domain (80°–90°E), zonal phase speed is westward at 50- to 100-m depths in northern fall, but eastward above and below, most likely due to Rossby waves generated at the eastern boundary.
    Type of Medium: Online Resource
    ISSN: 0022-3670 , 1520-0485
    URL: Article
    DOI: 10.1175/JPO-D-16-0157.1
    Language: Unknown
    Publisher: American Meteorological Society
    Publication Date: 2016
    detail.hit.zdb_id: 2042184-9
    detail.hit.zdb_id: 184162-2
    Location Call Number Limitation Availability
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    Online Resource
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