Local variability of internal wave driven upwelling at four remote Pacific Reefs

Internal wave driven upwelling has been shown to deliver cool, nutrient-rich water (‘cold pulses’) to the shallow shores of remote reefs, providing potential thermal refugia and nutrient enrichment. However, the spatial variability within nearby islands is yet to be explored. Two methods were used to quantify the distribution of cold pulse events between two sets of remote tropical Pacific Islands: Kingman Reef and Palmyra Atoll (66 km apart), and Howland and Baker Islands (70 km apart); the two groups of islands are ~1700 km apart.

Using data from subsurface temperature loggers (STR’s) from 2008 to 2018, moored on the forereefs of our four Islands, we show that there were clear differences in upwelling behaviour. Around Palmyra Atoll, the northwest and west logger sites are <1 km apart and have a difference in degree cooling hours (DCH – hours in a day during which cold pulses were present) of up to 0.6 per day. The temperature drop at these sites differs by up to 1°C per cold pulse. Kingman Reef showed up to 0.5 DCH difference between sites (~4 km apart) per day, with temperature drop differences of up to 2°C per pulse. In contrast, Howland and Baker Islands showed up to 3 DCH difference per day between islands, whereas the temperature drop around Baker Island differed by up to 2°C per pulse. During the very strong 2015/2016 El Niño, Palmyra showed an increase of up to 1.3 DCH in a day, whereas Kingman reached <0.2 DCH per day. Howland and Baker Islands showed a similar response during this El Niño event but differed during normal ENSO phases. For example, during 2013/2014, Baker Island showed a maximum of up to 2.75 DCH per day, whereas Howland Island did not reach past 0.5 DCH per day.

We conclude that cold pulse behaviour varies between geographically close reefs and so one reef’s data cannot be used as a proxy for other islands and reefs. Subsequently, we hypothesise that the slope angle of the reef may be correlated to the presence of cold pulse activity, and that increased cold pulses may be able to mitigate the effects of a global warming on reefs with specific characteristics.