Long-term trends in the frequency and magnitude of upwelling along the West Coast of the South Island, New Zealand, and the impact on primary production

ABSTRACT

Satellite-derived estimates of sea surface temperature (SST), chlorophyll (Chl), and fluorescence line height (FLH), from 22 sites ∼20 km offshore from the West Coast of the South Island, New Zealand (WCSI) are used to investigate long-term trends in SST, the frequency and magnitude of cool and warm events, and their impacts on SSC and FLH. WCSI trends have been towards higher temperature and less chlorophyll, with a 0.83°C rise in mean along-shore SST since 1982, and a 0.1 mg Chl m⁻³ drop in mean along-shore SSC since 2002. With respect to the trend, there were 13 extreme cool and 14 extreme warm events between 1982 and 2019, with mean values of −2.55 and 2.62°C, respectively. There was no measurable change in the frequency of extreme events, but there is a suggestion that extreme events are becoming warmer. During extreme events, Chl and FLH show inverse relationships with SST, suggesting that there is an underlying response of increased primary production to lower temperatures. Likely mechanisms are that upwelling brings nutrients up from depth, or that Ekman flow forces freshwater farther offshore, simultaneously introducing nutrients into the offshore region and providing a more stable water column allowing production.

KEYWORDS:

• West Coast South Island
• Sea surface temperature
• Sea surface chlorophyll
• upwelling
• primary production
• long-term trends

Acknowledgements

We thank all who participated in the cruises in 2005 and 2014 for their efforts in obtaining the oceanographic data. We thank the NZ Meteorological Service for providing Farewell Spit winds. The NOAA 1/4° daily Optimum Interpolation Sea Surface Temperature (daily OISST) analyses are made freely available by the US National Centers for Environmental Information (https://www.ncdc.noaa.gov/oisst). The MODIS ocean colour data are made freely available by NASA’s Ocean Color Program supported by the Ocean Biology Processing Group (OBPG) at NASA’s Goddard Space Flight Center (https://oceancolor.gsfc.nasa.gov/). National Centers for Environmental Prediction (NCEP) wind field reanalyses are made freely available by NCEP (https://psl.noaa.gov/data/gridded/data.ncep.reanalysis.html). The Southern Oscillation Index (SOI) data were made freely available by the Australian Bureau of Meteorology. We thank J. Kämpf and an anonymous reviewer for their conscientious reviews.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by the Ministry of Business, Innovation and Employment under a grant to Niwa's Coast and Oceans Programme.