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

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 (EAUC) flows along the continental shelf break of Aotearoa New Zealand’s North Island, transporting heat into this region. Sea surface temperatures (SSTs) increase ~0.15–0.2°C per decade in this region, just above the global average, and marine heatwaves (MHWs) 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. Although net heat transport (top 1000 m) within the East Auckland Current is projected to decline, the heat near the surface intensifies. This in turn leads to an increase in stratification, shallower mixed layers, by 5 m in SSP1–2.6, 15 m in SSP2–4.5 and 30 m in SSP3–7.0, and more intense surface MHWs, despite a net decline in heat transport into this region. Increased stratification in the top 250 m contributes to the surface warming of the SSTs in all SSPs, which reach ~2°C in SSP1–2.6 to 4°C warming in SSP3–7.0. Despite an overall decline in oceanic heat transport into this region, MHWs are projected to further intensify owing to sustained surface warming and reduced wind-induced vertical mixing.