Statistics of the subtropical jet stream over the Australian region

The position and structure of the lowest-latitude maximum wind speed in the westerlies over the Australian region at the 200 mb level (referred to here as the subtropical jet axis) are investigated for all months for the period 1956 to 1961. Classification is made according to month at each 10 degree meridian from 110°E to 170°E, of mean speed, mean latitude, and 60 and 80 percent departures on either side of means. The mean latitude of the subtropical jet-stream is found to vary from month to month, with a latitude range of 26°S to 32°S. This north-south shift does not always occur simultaneously at all longitudes, and the mean latitude may differ by as much as 4 degrees in the one month at different meridians. The day to day fluctuations along any longitude wer e g reatest during the summer months (standard deviations of about 6 or more degrees), being almost twice the deviation during the winter months. July is the month of highest mean core speed (140 kt), and January and February the months of lowest core speed (70 kt), with a period of steady rise or fall between these extremes. May is the month of steepest rise and November that of steepest decline in the mean speed. Standard deviations of speed are greatest in May (35 kt) and only in the low twenties during the summer months.