| Flood frequency analysis typically assumes
that annual floods arise from a single distribution and are independent.
However, there is significant evidence for the existence of persistent climate
modes. Timescales associated with climate variability range from inter-annual
through to longer, multi-decadal time scales. In the case of the Australian
climate, previous studies of the Indian and Pacific Oceans have indicated marked
multi-decadal variability in both mean Sea Surface Temperatures (SST) and
typical circulation patterns. In this light, data from 40 stream gauges around
New South Wales are examined to determine whether flood frequency data are
indeed independent and distributed identically. Given likely correlation in
flood records between gauges, an assessment of the regional significance of
observed changes in flood frequency is required. To achieve this, flood
observations are aggregated into a regional index. A simple non-parametric test
is then employed to identify the timing and magnitude of any change in mean
annual flood. Finally, it is shown that the identified change in flood frequency
corresponds directly to an observed shift in SST and mean circulation. These
results demonstrate the role of natural variability in climate parameters and
the need for an improved conceptual framework for flood frequency estimation.
Keywords: Floods, flood frequency, climate variability, IPO, PDO, climate change |