Moana Ocean Future Climate V1.0: High Resolution Marine Climate Futures For The New Zealand Region

Abstract. We have produced a series of 5 km resolution future climate dynamic downscalings for the ocean surrounding New Zealand covering CMIP6 reference conditions; SSP2-4.5 and SSP3-7.0 emissions trajectories. These downscalings combine the Moana Backbone 5 km resolution ROMS configuration with lateral boundary forcing from the 15 km resolution New Zealand Earth System Model (NZESM) and atmospheric forcing from the New Zealand Regional Climate Model 12 km atmospheric model.

We validated our reference period downscaling against the Moana Ocean Hindcast and find reasonable agreement to the west and north of New Zealand, but significant disagreement in the region of the Sub-Tropical Front to the east and southeast of the domain. This disagreement is consistent with known issues with the version of NZESM used as forcing in this study.

We see similar relative rates of increase in Ocean Heat Content in the upper ocean and mode waters all around New Zealand, but in the deeper ocean the rate of warming is stronger in the Tasman Sea and Antarctic Circumpolar Current than in the Sub Tropical Front East of New Zealand. We examine the occurrence of Marine Heat Waves (MHWs) and find that the use of a “fixed" baseline or one that takes into consideration a long-term warming based on the historical period results in important differences in the estimated number of days under MHWs for mid and end-of-the-century scenarios.