Weather types and hourly to multi-day rainfall characteristics in Tropical Australia.

Six weather types (WTs) are computed for tropical Australia during the wet season (November–March 1979–2015) using cluster analysis of 6-hourly low-level winds at 850 hPa. The WTs may be interpreted as a varying combination of at least five distinct phenomena operating at different time scales: the diurnal cycle, fast and recurrent atmospheric phenomena such as transient low pressure, the intraseasonal Madden–Julian oscillation, the annual cycle, and interannual variations mostly associated with El Niño–Southern Oscillation. The WTs are also strongly phase-locked onto the break/active phases of the monsoon; two WTs characterize mostly the trade-wind regime prevalent either at the start and the end of the monsoon or during its breaks, while three monsoonal WTs occur mostly during its core and active phases. The WT influence is strongest for the frequency of wet spells, while the influence on intensity varies according to the temporal aggregation of the rainfall. At hourly time scale, the climatological mean wet intensity tends to be near-constant in space and not systematically larger for the monsoonal WTs compared to other WTs. Nevertheless, one transitional WT, most prevalent around late November and characterized by weak synoptic forcings and overall drier con- ditions than the monsoonal WTs, is associated with an increased number of high hourly rainfall intensities for some stations, including for the interior of the Cape York Peninsula. When the temporal aggregation exceeds 6–12 h, the mean intensity tends to be larger for some of the monsoonal WTs, in association with more frequent and also slightly longer wet spells.

Figure 13. Anomalous (lower triangle 5 negative anomaly, upper triangle 5 positive anomaly) frequency (expressed as a percentage vs the expected value) of (top row) wet hourly time steps and (rows 2–5) very short (1–2 h), short (3–6 h), medium (7–10 h), and long (more than 10 h) wet spells. The significance is computed as the difference between observed frequency and the expected frequency beyond the seasonal cycle, estimated by 1000 random permutation of the WT sequences by blocks of 3 consecutive days in nonoverlapping 15 days. Black triangles indicate significant anomalies at the two-sided 95% level.