Behavior of One to Two Week Summertime Subtropical Wind Maxima over the South Pacific during an ENSO Cycle

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Abstract
The behavior of subtropical jet streaks over the Australian–South Pacific region is investigated for four consecutive 6-month summer seasons, November to April, during the period 1985–89. The study is an extension of a recent one by Ko and Vincent (hereafter referred to as KV), in which jet streaks during the summer season of 1984–85 were examined. Ko and Vincent found that the upper-tropospheric zonal wind behaved in a quasi-periodic manner, with maxima recurring over the western South Pacific at intervals between one and two weeks. In the present study, wind maxima are found to exhibit a 1–2-week periodic oscillation over the South Pacific in all four years, thus substantiating the finding by KV. In addition, a second set of jet streaks, with a similar periodicity, is found over the southern Australian region. It is also seen that the mean summertime position of the South Pacific jet and its accompanying jet streaks are much farther cast during the El Niño event of 1986–87 than they are in the other three years. The jet is also strongest in 1986–87. Based on point-to-point correlations of the 5–20-day filtered winds, it appears that the South Pacific jet streak form over eastern and sometimes northern Australia, and propagate east-southeastward at about 10° longitude per day (12 m s−1). The jet streaks over southern Australia originate over the eastern Indian Ocean and propagate eastward at about the same phase speed as the South Pacific jets. These two tracks of wind maxima appear to repeat their patterns approximately every 12 days. In some years, the South Pacific and southern Australian jet streaks appear to be distinctly separate, while in other years it seems as though the South Pacific jets are a continuation of the jets that originated over the Indian Ocean and propagated across southern Australia. Finally, it is found that the jet streaks in both regions are advected eastward in close proximity to upper-tropospheric midlatitude troughs, which also have a statistically significant period between one and two weeks. Consequently, these results could impact on medium-range, as well as short-term intraseasonal, forecasting. In the concluding remarks, the similarity is noted between the results found in this study and those found by other authors who have investigated jet streak activity in the Northern Hemisphere. A hypothesis is suggested for the behavior of the jet streaks diagnosed in this study.

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