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A 6-Yr Climatology of Vertical Mean and Shear Components of Kinetic Energy for the Australian–South Pacific Jet Stream

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  • 1 Department of Earth and Atmospheric Sciences, Purdue University, West Lafayette, Indiana
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Abstract

The climatology of the kinetic energy associated with the subtropical jet over the Australian–South Pacific region is investigated for a 6-yr period, January 1985–December 1990, using monthly mean data. The total kinetic energy (TKE) is partitioned into vertically averaged mean kinetic energy (KM) and level-by-level departure from the mean, or so-called shear kinetic energy (KS). A comparison of the two components during the annual cycle reveals that KM within the region of the subtropical jet is usually greater than KS. An out-of- phase relationship between the annual cycle of TKE and the annual cycle of the percentage of TKE represented by KS is found. A higher percentage of KS occurs in the summer season, when the jet is weakest. During late summer, KS dominates in the entrance region of the jet over Australia and the western Pacific. This appears to coincide with the annual strengthening of the jet. During winter, when the jet reaches its maximum intensity, KM dominates. It also dominates throughout the year in the exit region of the jet.

In addition, a comparison of TKE during an El Niño–Southern Oscillation cycle is made. Results indicate an increase of kinetic energy during El Niño over the central Pacific coupled with a decrease over Australia, indicating eastward movement of the jet. Subsequently, during La Niña, an opposite pattern is observed as the jet moves westward. The results of this climatological study, which appear to be in good agreement with the previous seasonal studies of the subtropical jet, could be beneficial to seasonal or year-to-year forecasting.

* Current affiliation: Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado.

Corresponding author address: Dr. Dayton G. Vincent, Department of Earth and Atmospheric Sciences, Purdue University, 1397 Civil Engineering Building, West Lafayette, IN 47907-1397.

Email: dvincent@meteor.eas.purdue.edu

Abstract

The climatology of the kinetic energy associated with the subtropical jet over the Australian–South Pacific region is investigated for a 6-yr period, January 1985–December 1990, using monthly mean data. The total kinetic energy (TKE) is partitioned into vertically averaged mean kinetic energy (KM) and level-by-level departure from the mean, or so-called shear kinetic energy (KS). A comparison of the two components during the annual cycle reveals that KM within the region of the subtropical jet is usually greater than KS. An out-of- phase relationship between the annual cycle of TKE and the annual cycle of the percentage of TKE represented by KS is found. A higher percentage of KS occurs in the summer season, when the jet is weakest. During late summer, KS dominates in the entrance region of the jet over Australia and the western Pacific. This appears to coincide with the annual strengthening of the jet. During winter, when the jet reaches its maximum intensity, KM dominates. It also dominates throughout the year in the exit region of the jet.

In addition, a comparison of TKE during an El Niño–Southern Oscillation cycle is made. Results indicate an increase of kinetic energy during El Niño over the central Pacific coupled with a decrease over Australia, indicating eastward movement of the jet. Subsequently, during La Niña, an opposite pattern is observed as the jet moves westward. The results of this climatological study, which appear to be in good agreement with the previous seasonal studies of the subtropical jet, could be beneficial to seasonal or year-to-year forecasting.

* Current affiliation: Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado.

Corresponding author address: Dr. Dayton G. Vincent, Department of Earth and Atmospheric Sciences, Purdue University, 1397 Civil Engineering Building, West Lafayette, IN 47907-1397.

Email: dvincent@meteor.eas.purdue.edu

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