The link between our increased ocean temperatures and cyclonic activity
The increased energy in the oceans due to warmer ocean temperatures is one of the factors creating the intense weather systems we have been seeing in New Zealand this summer. More heat from the ocean means more moisture in the atmosphere, more convection, and less slowing of storms due to meeting cooler oceans. The link between our increasing ocean temperatures and volatile weather systems is the focus of cutting-edge weather prediction and climate science research, says Moana Project Lead and Ocean Temperature Researcher, Dr. Joao De Souza.
January 2023 was record-breaking in terms of rainfall, with Auckland Airport receiving around 6 times its average January rainfall, including recording the wettest day since records began, with 245 mm of precipitation in a 24-hour period (Source: MetService, see here). The rainfall experienced by Auckland/Northland at the end of January was exceptional, with subtropical moisture streaming down from tropical latitudes. In February, Cyclone Gabrielle arrived in New Zealand as a powerful storm after transitioning from a tropical cyclone. Napier Airport has had its second wettest day since 1950, with 175.8mm of rainfall over a 24-hour period from February 13-14. This value is three times the average amount of rain received in all of February (56mm) (Source: MetService).
New Zealand's oceans have played their part in exacerbating these impactful events, a warmer ocean means a extra fuel for storms; extra heat and moisture creates instability and energy in the atmosphere. There has likely been 10 - 25% more moisture in the atmosphere for storms to uptake then release on nearby land (See here). Oceans represent the greatest energy sink for the greenhouse effect; they capture 90% of the excess heat generated by climate change emissions (Source: United Nations).
When a storm hits a cooler surface, like a cooler ocean, that surface absorbs and slows the energy of the storm. Normally we would see this when a cyclone hits the cooler waters around New Zealand, however the coastal waters around New Zealand are warmer than normal, most of them under marine heatwave conditions. This means this normal cooling and slowing effect is decreased. With the passage of Cyclone Gabrielle, the ocean surface mixing and general extraction and additional uptake of energy from the ocean by Gabrielle, sea surface temperatures around the country decreased.
According to Joao de Souza, with Cyclone Gabrielle moving away, ocean surface temperatures are returning to their pre-cyclone states, responding to large scale forcings. A spell of calm weather could then rapidly increase ocean temperatures above the marine heatwave threshold. Such temperatures may intensify future tropical weather systems which head our way. These forcings are forecast to prevail at least until April-May. For daily updated coastal ocean temperatures in New Zealand, check out the New Zealand marine heatwave forecast published by the Moana Project.
Satellite view of Cyclone Gabrielle (Himawari8 - Japan Meteorological Agency) on Monday 15 February from MetOceanView.