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Diurnal and Semidiurnal Variations of the Surface Wind Field over the Tropical Pacific Ocean

Clara DeserCIRES, University of Colorado, Boulder, Colorado

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Catherine A. SmithCIRES, University of Colorado, Boulder, Colorado

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Abstract

The climatological large-scale patterns of diurnal and semidiurnal near-surface wind variations over the tropical Pacific Ocean are documented using 3 yr of hourly measurements from the Tropical Atmosphere–Ocean moored buoy array. Semidiurnal variations account for 68% of the mean daily variance of the zonal wind component, while diurnal variations account for 82% of the mean daily variance of the meridional wind component. The spatially uniform amplitude (0.15 m s−1) and phase (0300 LT) of the semidiurnal zonal wind variations are shown to be consistent with atmospheric thermal tidal theory.

The diurnal meridional wind variations on either side of the equator are approximately out of phase. This pattern results in a diurnal variation of wind divergence along the equator, with maximum divergence in the early morning (∼0800 LT). The average amplitude of the diurnal cycle in zonal mean divergence is 0.45 × 10−6 s−1, which corresponds to a day–night change of 45% relative to the daily mean. The relative day–night changes in near-surface equatorial wind divergence are larger in the western Pacific (78%) than in the eastern Pacific (31%) due mainly to differences in the daily mean divergence. The diurnal amplitude of equatorial wind divergence changes seasonally and interannually in proportion to the strength of the mean divergence.

It is suggested that diurnal heating of the sea surface may contribute to the zonally symmetric diurnal cycle of equatorial wind divergence.

Corresponding author address: Dr. Clara Deser, Climate and Global Dynamics Division, NCAR, P.O. Box 3000, Boulder, CO 80307-3000.

Email: cdeser@ucar.edu

* Current affiliation: National Center for Atmospheric Research, Boulder, Colorado.

Abstract

The climatological large-scale patterns of diurnal and semidiurnal near-surface wind variations over the tropical Pacific Ocean are documented using 3 yr of hourly measurements from the Tropical Atmosphere–Ocean moored buoy array. Semidiurnal variations account for 68% of the mean daily variance of the zonal wind component, while diurnal variations account for 82% of the mean daily variance of the meridional wind component. The spatially uniform amplitude (0.15 m s−1) and phase (0300 LT) of the semidiurnal zonal wind variations are shown to be consistent with atmospheric thermal tidal theory.

The diurnal meridional wind variations on either side of the equator are approximately out of phase. This pattern results in a diurnal variation of wind divergence along the equator, with maximum divergence in the early morning (∼0800 LT). The average amplitude of the diurnal cycle in zonal mean divergence is 0.45 × 10−6 s−1, which corresponds to a day–night change of 45% relative to the daily mean. The relative day–night changes in near-surface equatorial wind divergence are larger in the western Pacific (78%) than in the eastern Pacific (31%) due mainly to differences in the daily mean divergence. The diurnal amplitude of equatorial wind divergence changes seasonally and interannually in proportion to the strength of the mean divergence.

It is suggested that diurnal heating of the sea surface may contribute to the zonally symmetric diurnal cycle of equatorial wind divergence.

Corresponding author address: Dr. Clara Deser, Climate and Global Dynamics Division, NCAR, P.O. Box 3000, Boulder, CO 80307-3000.

Email: cdeser@ucar.edu

* Current affiliation: National Center for Atmospheric Research, Boulder, Colorado.

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