Editorial Type:
Article
Article Type:
Research Article

Origin of the Intraseasonal Variability over the North Pacific in Boreal Summer

Lu Wang The National Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics, Chinese Academy of Sciences, and National Satellite Meteorological Center, Beijing, China

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Tim Li International Pacific Research Center and Department of Meteorology, University of Hawaii at Manoa, Honolulu, Hawaii

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Tianjun Zhou The National Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China

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Xinyao Rong Chinese Academy of Meteorological Sciences, Beijing, China

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Print Publication:
15 Feb 2013
DOI:
https://doi.org/10.1175/JCLI-D-11-00704.1
Page(s):
1211–1229
Restricted access

Abstract

The spatial structure and temporal evolution of the intraseasonal oscillation (ISO) in boreal summer over the midlatitude North Pacific Ocean are investigated, through the diagnosis of NCEP reanalysis data. It is found that the midlatitude ISO has an equivalent-barotropic structure, with maximum amplitude at 250 hPa. Initiated near 120°W, the ISO perturbation propagates westward at a phase speed of about 2.4 m s−1 and reaches a maximum amplitude at 150°W. A diagnosis of barotropic energy conversion shows that the ISO gains energy from the summer mean flow in the ISO activity region. A center-followed column-averaged vorticity budget analysis shows that the nonlinear eddy meridional vorticity transport plays a major role in the growth of the ISO perturbation. There is a two-way interaction between ISO flows and synoptic eddies. While a cyclonic (anticyclonic) ISO flow causes synoptic-scale eddies to tilt toward the northwest–southeast (northeast–southwest) direction, the tilted synoptic eddies then exert a positive feedback to reinforce the ISO cyclonic (anticyclonic) flow through eddy vorticity transport. The reanalysis data and numerical simulations show that the midlatitude ISO is primarily driven by local processes and the tropical forcing accounts for about 20% of total intraseasonal variability in midlatitudes. However, 20% might be an underestimate given that the tropical intraseasonal forcing is not fully included in the current observational analysis and modeling experiment.

School of Ocean and Earth Science and Technology Contribution Number 8781 and International Pacific Research Center Contribution Number 928.

Corresponding author address: Tim Li, IPRC and Department of Meteorology, University of Hawaii at Manoa, 2525 Correa Rd., Honolulu, HI 96822. E-mail: timli@hawaii.edu

Abstract

The spatial structure and temporal evolution of the intraseasonal oscillation (ISO) in boreal summer over the midlatitude North Pacific Ocean are investigated, through the diagnosis of NCEP reanalysis data. It is found that the midlatitude ISO has an equivalent-barotropic structure, with maximum amplitude at 250 hPa. Initiated near 120°W, the ISO perturbation propagates westward at a phase speed of about 2.4 m s−1 and reaches a maximum amplitude at 150°W. A diagnosis of barotropic energy conversion shows that the ISO gains energy from the summer mean flow in the ISO activity region. A center-followed column-averaged vorticity budget analysis shows that the nonlinear eddy meridional vorticity transport plays a major role in the growth of the ISO perturbation. There is a two-way interaction between ISO flows and synoptic eddies. While a cyclonic (anticyclonic) ISO flow causes synoptic-scale eddies to tilt toward the northwest–southeast (northeast–southwest) direction, the tilted synoptic eddies then exert a positive feedback to reinforce the ISO cyclonic (anticyclonic) flow through eddy vorticity transport. The reanalysis data and numerical simulations show that the midlatitude ISO is primarily driven by local processes and the tropical forcing accounts for about 20% of total intraseasonal variability in midlatitudes. However, 20% might be an underestimate given that the tropical intraseasonal forcing is not fully included in the current observational analysis and modeling experiment.

School of Ocean and Earth Science and Technology Contribution Number 8781 and International Pacific Research Center Contribution Number 928.

Corresponding author address: Tim Li, IPRC and Department of Meteorology, University of Hawaii at Manoa, 2525 Correa Rd., Honolulu, HI 96822. E-mail: timli@hawaii.edu
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