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.

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