Thesis or Dissertation Origin of Intraseasonal Variability in the Eastern Equatorial Indian Ocean : Intrinsic Variability, and Local and Remote Wind-Stress Forcings

李, チョロン

Eastern equatorial Indian Ocean(EEIO)experienceslarge intraseasonal variability(ISV), which arisesfrom intrinsic variabilityand alsois induced by local and remote atmospheric forcings. We investigate the relative contributions ofintrinsicvariabilityand local and remote forcings to ISV in EEIO byconducting a set of numerical model experiments using Regional Ocean Model System over the Indian Ocean with grid interval of 0.25° in longitudes and latitudes. In the control run, the model is forced year-to-year varying forcings for 20 years (1996–2015). We account the first 10 years as the necessary period to spin up. The control run can reproduce realistic ISV in EEIO.To distinguish ISV originating from intrinsic variabilityand that caused by atmospheric forcings, we examine three experiments with different perturbation at the beginningof 10-year integration (2006-2015). The ensemble-mean of these experimentsrepresents forced responses, while differences among the experiments indicate the intrinsic variability. In the central to eastern equatorial Indian Ocean, forced oceanic ISV is much larger than intrinsic ISV in the whole 500 m depth.For ISV of sea surface temperature, atmospheric forcings play a larger role than intrinsic variabilityin the most region of the Indian Ocean. Atmospheric forced ISVdistinctly penetrates to the subsurface in EEIO.To understand a relative role of atmospheric forcings, especially thatof wind-stress, four regional forcing experiments areconducted.In the first two experiments, wind-stress ISV is retained over western (west of 80°E)oreastern (east of 80°E)region, while in the other two experiments the latter forcing region is further subdivided into middle eastern (80°–100°E) and far eastern (east of 100°E) regions. At the surface, local forcing, especially inthemiddle eastern Indian Ocean, dominates ISV in EEIO. In the subsurface, remote forcing also plays an important role in ISV. Below the thermocline, the influence of wind forcing from the west of 80°Ecannot be neglected on ISV, even work constructivelywith that from the east of 80°E. The contributionof wind forcing east of 100°Eis small but cannot be ignored near the Java coast.
Hokkaido University(北海道大学). 博士(理学)

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