Abstract
Atmospheric moisture transport is crucial for understanding New Zealand's climate dynamics, particularly in influencing extreme precipitation events. While the majority of previous studies have focussed on Atmospheric Rivers (ARs), this study examines the entire spectrum of water vapour transport and its link to extreme precipitation using 40 years (1981–2020) of Integrated Water Vapour Transport (IVT) data over the region. Although ARs are important drivers of extreme precipitation, they are infrequent as they only account for less than 10% of total moisture transport at most coastal locations. Extreme water vapour transport (defined by the 90th percentile IVT threshold) corresponds more closely with precipitation extremes than ARs alone, even using an expanded AR detection range. Here, IVT is classified into strength categories from weak to strong. Over the study period, all but the weakest category of IVT has increased in frequency of occurrence over most of South Island, while decreasing in northern North Island. Similarly, monthly IVT anomaly trends show a positive trend in the South Island and negative trend in the northern North Island during warmer months. Separate analysis of windspeed and total column water (TCW) reveals the driving factors underpinning these IVT trends: TCW has increased universally across New Zealand, while windspeed has decreased in more northern regions. Correspondingly, these findings indicate the importance of analysis both dynamic and thermodynamic factors in seeking to understand hydrometeorological variation and when investigating the responses to climate change.