Storylines for Future Projections of Precipitation Over New Zealand in CMIP6 Models

Abstract

Large uncertainty exists in the sign of long‐term changes in regional scale mean precipitation across the current generation of global climate models. To explore the physical drivers of this uncertainty for New Zealand, here we adopt a storyline approach applying cluster analysis to spatial patterns of future projected seasonal mean precipitation change across CMIP6 models (n = 43). For the winter precipitation change signal, the models split roughly into two main groups: both groups have a very robust wet signal across the west coast of the South Island but differ notably in terms of the sign of precipitation change across the north of the North Island. These far north winter precipitation differences appear related to how far the Hadley cell edge and regional eddy‐driven jet shift across the models relative to their historical positions. In contrast, for summer, most models have a markedly weaker and spatially non‐uniform response, where internal variability often plays a large role. However, a small group of models predict a robust wet signal across most of the country in summer. This “wet model” group is characterized by a regional La Niña‐like increase in high pressure shifted further to the south‐east of New Zealand, associated with more frequent north‐easterly flow over the country and accompanied by significant warming of local sea surface temperatures. This regional circulation response appears related to changes in stationary Rossby wave paths as opposed to changes in La Niña occurrence frequency itself.

Robust changes to the wettest and driest days of the year are hidden within annual rainfall projections: a New Zealand case study

“…Figure 1 shows ensemble mean changes in annual rainfall across the country, comparing projections of a modelled 3 • C world (hereafter '3 K experiment') relative to simulations of the current climate (hereafter 'Current Climate' experiment). Consistent with evidence from past studies (MFE 2018, Bird et al 2023, Gibson et al 2024, annual mean rainfall is found to increase by up to 10% along the western coastline of both islands (particularly the South Island), while comparable decreases in annual rainfall can also be detected over drier inland regions of the South Island, particularly 'rain shadow' regions on the leeward side of the Southern Alps, like South Canterbury and Central Otago. Both patterns of change are consistent with a strengthening of rain-bearing weather systems arriving from the west of the country, while…”