La Niña confirmed for New Zealand

Numerical modelling by MetOcean Solutions Research and Development team is being used to track the impact of the predicted La Niña conditions on New Zealand’s waters.  La Niña brings a tendency for warmer than average ocean and air temperatures for New Zealand once it becomes established. But in contrast, is characterised by cooler sea temperatures in the Eastern equatorial Pacific Ocean (see Figure 1).

Figure 1. Global Sea Surface Temperature anomaly (Source https://www.ospo.noaa.gov/Products/ocean/sst/anomaly/)

Earlier in September 2020 the United States National Oceanic and Atmospheric Administration (NOAA) issued a La Niña Advisory confirming that La Niña conditions have been observed and expected to continue through the coming months (with a 75% certainty) and into 2021. Equatorial sea surface temperatures below average across the eastern and central Pacific Ocean have been observed - and tropical atmospheric circulation consistent with La Niña is beginning to dominate.  Analysis by MetService indicate that the NIÑO 3 index likewise hints at the development of La Niña conditions (see Figure 2).

Figure 2. The NIÑO 3 index is a measure of how abnormally cold/warm the seas are along the equator, over the Eastern Tropical Pacific. Values from 1982 to 2020 are shown. Large positive values denote El Niño conditions while negative values denote La Niña conditions.

While every event is different, La Niña can make certain outcomes more likely.

Dr Joao de Souza, MetOcean Solutions’ Senior oceanographer explains, “Typically, during La Niña years stronger north-easterly winds are produced to the north of New Zealand, often leading to larger waves on the north coast from Cape Reinga to East Cape.

“These increased north-easterly winds often bring moist, humid, rainy conditions to the north–east of the North Island, while reduced rainfall can be experienced to the south and south–west of the South Island. 

“Increases in the frequency of occurrence of storms were observed in the past for the north coast during La Niña years.  The sequence of storms with a short time interval in between significant events can result in increased beach erosion due to the insufficient time for the beach to recover – for example.

“There is also the potential for more frequent storm surge events, particularly along the north-easterly facing coastlines of New Zealand.  These events have the potential to cause significant damage to coastal infrastructure around susceptible low-lying areas of New Zealand.”

On the waning cusp of the last La Niña period (summer period 2018) a significant Marine Heatwave (MHW) developed off the west coast of the South Island, with a surface water anomaly (temperatures above the long term average) of as much as 6˚C developing (Figure 3). Significantly, this MHW coincided with the collapse of the Hoki Fishery in the region of the Tasman Sea, a body of water warming at 3-4x the global average (https://www.nature.com/articles/ncomms16101).  Importantly, Seafoods New Zealand note that such dramatic shifts have profound implications for the marine environment as heatwaves become more frequent and more intense, while currently sea surface temperatures round New Zealand are ~1˚C warmer than the long term average (Figure 1).

Figure 3. Sea surface temperature anomaly offshore from the West Coast of the South Island, New Zealand December 2018. (Source https://www.nnvl.noaa.gov/view/globaldata.html#SSTA)

Improving our understanding of the coastal ocean circulation, connectivity and marine heatwaves to provide information that supports sustainable growth of the seafood industry (Māori, fisheries and aquaculture) are key deliverables of the Moana Project (https://www.moanaproject.org/project). Now entering its third year, the project funded through  Ministry of Business, Innovation and Employment’s (MBIE’s) Endeavour fund, aims to help the New Zealand seafood sector retain its competitive edge in a rapidly changing ocean impacted by marine temperature extremes and shifting currents. A marine heatwave machine learning based forecast is one of the tools under development in the Moana Project.

About La Niña

La Niña represents the cool phase of the naturally occurring global climate cycle, known as El Niño Southern Oscillation (ENSO).  La Niña refers to the periodic cooling of ocean surface temperatures in the central and east-central equatorial Pacific that occurs every 3 to 5 years or so (Source: Climate Prediction Center, NOAA).