Increased stratification intensifies surface marine heatwaves north-east of Aotearoa New Zealand in New Zealand’s Earth System Model

Liv Cornelissen; Erik Behrens; Denise Fernandez; Phil Sutton

Just Accepted

This article has been peer reviewed and accepted for publication. It is in production and has not been edited, so may differ from the final published form.

Increased stratification intensifies surface marine heatwaves north-east of Aotearoa New Zealand in New Zealand’s Earth System Model

Liv Cornelissen , Erik Behrens, Denise Fernandez, Phil Sutton

Abstract

The Western Boundary Current system in the South Pacific is an important element of the climate system as it carries heat from the tropical regions poleward. The East Auckland Current flows along the continental shelf break of New Zealand’s North Island, transporting heat into this region. Sea surface temperatures (SST) increase about 0.15 to 0.2°C/per decade in this region, just above the global average, and marine heatwaves are projected to intensify despite a predicted decline in oceanic volume transport in this region. This study investigates the possible drivers of the extreme oceanic warming in a low (SSP1-2.6), medium (SSP2-4.5) and high (SSP3-7.0) emission scenario using New Zealand's Earth System model. Our projections suggest a mean decline of heat transport in the East Auckland Current of 5.3% in SSP1-2.6, 22% in SSP2-4.5 and 46% in SSP3-7.0 in the second half of this century compared to the historical period (1995-2014). While net heat transport (top 1000m) within the EAUC is projected to decline, the heat near the surface intensifies . This in turn leads to an increase in stratification, shallower mixed layers, by 5m in SSP1-2.6, 15m in SSP2-4.5 and 30m in SSP3-7.0 on average between 2050 and 2099 compared to the historical mean, and more intense surface MHWs, despite a net decline in heat transport into this region. Increased stratification in the top 250m contributes to the surface warming of the SSTs in all SSPs, which reach about 2°C in SSP1-2.6 to 4°C warming in SSP3-7.0 by the end of the century. Despite an overall decline in the oceanic heat transport into this region MHWs are projected to further intensify due to sustained surface warming and reduced wind induced vertical mixing.

ES23030 Accepted 12 February 2025