The Tasman Box: Warming in the Tasman Sea

The temperature of the Tasman Sea largely determines New Zealand’s air temperatures because
New Zealand lies downstream of this region in terms of both the atmosphere with its prevailing
westerly winds and the ocean with its southward and eastward flowing boundary currents. The
Tasman Sea heat content is in turn set by the temperatures and speeds of currents flowing into and
out of the region together with the local air-sea heat fluxes. Warm, subtropical water flows into the
area in the East Australian Current and flows out in the Tasman Front and East Auckland Current. The relative transports of these current systems determine whether warm water converges in or is
removed from the region.

The Tasman Sea upper ocean temperature has a strong influence on New Zealand’s air-sea climate system. The increasing heat content of the Tasman since the 1990s has resulted in a significant sea level rise due to thermal expansion of seawater. In addition, Marine Heat Waves (MHWs) are becoming of increased interest because of their impact on weather, ecosystems and fisheries and recent work has found a link between intensity and occurrence of MHWs and ocean heat content. This report uses all available data to describe interannual and decadal variability in the Tasman Sea and, where possible, to describe the forcing mechanisms.