The Extratropical 40-Day Oscillation in the UCLA General Circulation Model. Part II: Spatial Structure

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  • 1 Space Geodetic Science and Applications Group, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California
  • | 2 Department of Atmospheric Sciences and Institute of Geophysics and Planetary Physics, University of California, Los Angeles, Los Angeles, California
  • | 3 Space Geodetic Science and Applications Group, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California
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Abstract

Intraseasonal oscillations in a 3-yr, perpetual-January simulation are examined using a version of the UCLA GCM that produces no self-sustained Madden–Julian oscillation in the Tropics. A robust, 40-day oscillation is found to arise in the model's Northern Hemisphere (NH) extratropics when standard topography is present. Part I of this study addressed the zonally averaged component of the GCM oscillation, manifested in wind- and pressure-induced variations in atmospheric angular momentum (AAM). The focus here is on the spatial features of the oscillation as manifested in the variability of the 500-mb height field.

A standing, wavenumber-two pattern is found in the NH extratropics, which undergoes tilted-trough vacillation in conjunction with the model's AAM oscillation. High (low) values of AAM are associated with low (high) 500-mb heights over the northeast Pacific and Atlantic Oceans; the two centers' of action slightly different frequencies give rise to a long-period modulation (of about 300 days) in the amplitude of the intraseasonal oscillation. Global correlations with the leading empirical orthogonal functions of the NH extratropical 500-mb height field show northeast–southwest teleconnection patterns extending into the Tropics, similar to those found in observational studies. The zonally averaged latent heating in the Tropics exhibits no intraseasonal periodicity, but a 39-day oscillation is found in cumulus precipitation over the western Indian Ocean. The latter shows significant coherence with EOF 1 but is absent in three shorter no-mountain experiments (see Part I), indicating that it may be remotely forced by the intraseasonal oscillation that arises in the model's NH extratropics only in the standard-topography experiment.

Abstract

Intraseasonal oscillations in a 3-yr, perpetual-January simulation are examined using a version of the UCLA GCM that produces no self-sustained Madden–Julian oscillation in the Tropics. A robust, 40-day oscillation is found to arise in the model's Northern Hemisphere (NH) extratropics when standard topography is present. Part I of this study addressed the zonally averaged component of the GCM oscillation, manifested in wind- and pressure-induced variations in atmospheric angular momentum (AAM). The focus here is on the spatial features of the oscillation as manifested in the variability of the 500-mb height field.

A standing, wavenumber-two pattern is found in the NH extratropics, which undergoes tilted-trough vacillation in conjunction with the model's AAM oscillation. High (low) values of AAM are associated with low (high) 500-mb heights over the northeast Pacific and Atlantic Oceans; the two centers' of action slightly different frequencies give rise to a long-period modulation (of about 300 days) in the amplitude of the intraseasonal oscillation. Global correlations with the leading empirical orthogonal functions of the NH extratropical 500-mb height field show northeast–southwest teleconnection patterns extending into the Tropics, similar to those found in observational studies. The zonally averaged latent heating in the Tropics exhibits no intraseasonal periodicity, but a 39-day oscillation is found in cumulus precipitation over the western Indian Ocean. The latter shows significant coherence with EOF 1 but is absent in three shorter no-mountain experiments (see Part I), indicating that it may be remotely forced by the intraseasonal oscillation that arises in the model's NH extratropics only in the standard-topography experiment.

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