Currents in the Tasman Sea in a changing climate

Abstract

Western boundary currents are an important element of the climate system since they control the oceanic meridional heat transport. Climate projections around Australia and New Zealand suggest an increase in volume transport of the Tasman Leakage but a decline for the Tasman Front, which carries water and heat towards New Zealand. Despite this decline, the region around the North Island of New Zealand has been identified as a region with extreme warming and this raises the question of what drives this warming. In this study we investigate climate projections of New Zealand’s Earth System Model to quantify changes to transports of the oceanic currents and possible driver for the warming using low (SSP1-2.6), medium (SSP2-4.5) and high (SSP3-7.0) emission scenarios in combination with Lagrangian particle tracking analyses. The direction of change in volume transport agrees with previous studies. Our projections reveal an increase between 16% and 76% in volume transport and a 29% and 122% increase in temperature transport of the Tasman Leakage between the low and high emission scenarios. In the Tasman Front, volume transport for SSP2 and SSP3 scenarios decline between 10% and 33%. The temperature transport increases in SSP2 with 4% and decreases with 20% in SSP3. Therefore, a possible driver for the intensifying marine heat waves around the North Island of New Zealand could be the decrease in volume transport that allows for heat accumulation in the region. The Lagrangian analyses reveal that the temperature along the trajectory is projected to increase in the high emission scenario. At the end of the 20th century only 1% of the volume weighted trajectories towards New Zealand are above 20°C, this increases to 5.5% by the end of the 21st century for the high emission scenario. Despite a decline in the overall Lagrangian temperature transport, the volume weighted fraction of trajectories which encounter higher temperatures from the EAC to the north of NZ increases which possibly widens or opens up a pathway for tropical species towards NZ.