What effects might El Niño have this year?
There are indications that an El Niño will develop by spring. Many of the usual signs for El Niño are now visible in the warming waters of the Pacific Ocean, according to NIWA’s National Climate Centre.
During past El Niño events New Zealand has experienced enhanced southwesterly and westerly weather conditions, with low pressure systems centred below the country. As a result, the south and west of New Zealand experienced more frequent and larger storm rainfall events.
In those El Niño events river flows have been lower than normal in the northern North Island (especially Bay of Plenty), higher than normal in Southland and the Southern Alps (including inflows to hydroelectricity reservoirs), and near normal elsewhere. In addition, Hawke’s Bay, Canterbury, and Otago have experienced very low river flows in some, but not all, El Niño events.
In the northern, and possibly eastern, parts of New Zealand, where we expect lower flows and fewer floods than normal in an El Niño, water quality is likely to be different too. During an El Niño in those areas we would expect to see higher than average slime levels (i.e., nuisance periphyton) in rivers that have abundant nutrients in the water due to effects of land use, effluent discharges, or geological sources.