Average Circulation Conditions that Precede River Flooding in Eastern Catchments of New Zealand

Flooding is a frequent and costly hazard in New Zealand (NZ). With two-thirds of NZ’s population residing on the flood plains of large rivers, recent and significant rainfall events have resulted in many NZ cities being subjected to severe river flooding. Therefore, NZ is continuously (and increasingly) facing the challenge of how to best manage flood risks to minimise their distress and costs. Existing research has largely focused on understanding the meteorological drivers of flooding in NZ’s West Coast regions, despite the higher proportion of population residing on the flood plains, and the frequency of damaging and costly floods, in NZ’s eastern regions. To better understand NZ’s current and future flood risk, this research aims to determine the anomalous large-scale and regional atmospheric circulation patterns associated with river flooding in 17 catchments in four of NZ eastern regions. By developing an event database of historical high flows through flood frequency analysis and employing composite analysis techniques, the key regional weather patterns and their association with large-scale modes of climate variability that cause river flows exceeding a 1-in-5-year threshold for each of the 17 catchments were identified. The primary features were an anomalous low pressure to the west or north of NZ coupled with high pressure to the east or south, which together draw deep tropical moisture down from the low latitudes, with locally enhanced low-level convergence where the flow intercepts the catchments. Overall, relationships with large-scale climate modes were generally weak. However, a key feature for the Upper North Island and Lower North Island catchments was positive sea surface temperature anomalies (SSTs) to the north of NZ over the region of moisture transport, generally associated with La Niña conditions. Additionally, an asymmetric Zonal Wave 4 atmospheric circulation pattern was a key feature for the Upper North Island and South Island catchments. The findings of this research provide a foundation for the best methods to use for flood frequency analysis for eastern NZ catchments and shed light on the meteorological drivers of NZ’s ever increasing flood risk. This knowledge, alongside projections of how NZ weather systems and regional SST changes will change with climate change, can be used to aid in preparation and risk management associated with river flooding in eastern NZ regions.