climate dynamics 2011/01/10 r98229014 馮培寧 kirsten feng
TRANSCRIPT
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Climate Dynamics
2011/01/10 R98229014 馮培寧 Kirsten Feng
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Decadal Variability of the Kuroshio Extension: Observations and an Eddy-Resolving Model
Hindcast
BUNMEI TAGUCHI, SHANG-PING XIE, NIKLAS SCHNEIDER, MASAMI NONAKA,
HIDEHARU SASAKI, AND YOSHIKAZU SASAI
In final form 17 October 2006
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Model and Data
• Eddy-resolving model– Multidecadal hindcast by
the OGCM for the Earth Simulator (OFES)
– Domain: • 75°S to 75°N• 0.1°x0.1°x54(levels)
– Condition: • NCEP–NCAR reanalysis
– This paper:• (1950 spinup)1962-2003• 28°N~45°N, 140°E~180°E
• Observation – SSH:
• Ocean Topography Experiment (TOPEX)/ Poseidon (T/P)
• European Remote Sensing satellite (ERS-1/2) altimeter observations
– Monthly Ocean Temperature: • expendable bathythermograph
(XBT) • 1995~2004• 2° lat x 5°lon
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Method Examines the spatiotemporal structure/
• Variance• Leading mode and
comparison with satellite altimetry
• The KE jet’s signature in modal structure
• Multidecadal analysis• Recirculation variability
Determines this structure/• Baroclinic Rossby wave• Large- versus frontal-scale
variability• Intrinsic variability
• OGCM, Linear Rossby Wave Model are added here.
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Variance
• Although the OFES simulation misses some variance, the overall agreement of the mean and interannual variance with observations encourages to look further.
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Leading mode and comparison with satellite altimetry
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The KE jet’s signature in modal structure
• 2D EOF for each section.• The front gets southward
and broader from its upstream to downstream.
• EOF1 shows a clear dependency with the mean flow.
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Multidecadal analysis• The first mode primarily
represents the north–south shift of the KE jet.
• The second mode represents an intensification of the KE jet.
PC-1 corresponds well with the zonal mean of the KE’s latitudinal position determined as the maximum u velocity at 100 m.
PC-2 tracks very well the SSH difference, the latitudinal band for the KE jet core.
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Baroclinic Rossby wave• The linear Rossby wave model
captures the temporal evolution of the OFES’s leading modes very well except a 1–2-yr phase lag behind the OFES PC-2.
• The meridional structure of the Rossby wave solution, however, is quite different from that of the OFES hindcast.
• The linear Rossby wave models fail to reproduce the narrow modal structure as OFES and observation but display a rather broad meridional structure reflecting large-scale wind forcing.OGCM: Xie et al., 2000
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Large- versus frontal-scale variability
• The small, frontal-scale component of the SSH difference features zonally elongated, meridonally tight recirculations along the mean fronts.
• (c) = (b) –(a).• The broadscale
component of the difference field in OFES represents the spinup of both the subtropical and subpolar gyres, in broad agreement with the linear Rossby wave model. (R=0.67)
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West: positiveEast: negative
The negative SSH from the east extends in to the west and replace the positive SSH at lag -6yr.
The positive SSH in the east is growing and arrive to the Japanese coast (west boundary) at lag 6yr.
Broad scale Frontal scaleLow-passed PC-2
A pair of zonally elongated recirculations with the KE front in between.
This pair of anomalous recirculations alters the sign from lag 6 to 0, and again from lag 0 to 6 yr.
The frontal-scale recirculation adjustments to the large-scale environmental changes are initiated by the arrival of Rossby waves.
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Intrinsic variability
• The leading EOF modes of frontal-scale variability in the hindcast bear a remarkable resemblance to those in the climatological run.
• The second EOF that differs in phase near the Oyashio Extension north of 39°N, presumably due to the difference in the mean flow structures.
• The two sets of PCs are nearly identical, and it suggests the following scenario: basin-scale wind variability excites broadscale Rossby waves, which propagate westward, triggering intrinsic modes of the KE jet and reorganizing SSH variability in space.
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Prediction Experiment
• The slow propagation of baroclinic Rossby waves is a source of predictability.
• The resultant time series (blue curve) closely follows the OFES hindcast as well as T/P observations for up to 9 months.
• This simulation allows the prediction of the meridional position and intensity of the KE front.
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Summary • The first mode represents a southward shift and to a lesser degree, an
acceleration of the KE jet in the early 1980s that separates two periods.
• The second EOF mode represents intensity variations in the KE jet and displays quasi-decadal oscillations.
• The broadscale component indeed resembles the linear Rossby wave response to wind variability while the frontal-scale variability bears a remarkable similarity to intrinsic modes of the climatological run in spatial structure.
• Basin-scale wind variability excites broadscale Rossby waves, which propagate westward, triggering intrinsic modes of the KE jet and reorganizing SSH variability in space.
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Thank You