Alice Della Penna

Aotearoa New Zealand's marine environment is heavily impacted by El Niño Southern Oscillation (ENSO). Little is known about the effect of ENSO on oceanographic properties in the area or their consequences on the distribution of marine organisms. Here we characterize the spatio‐temporal variability of fine‐scale fronts (<10 km) in the area of the Tīkapa Moana Te Moananui ā Toi Hauraki Gulf (HG) and investigate how it is impacted by dominant wind direction, seasonality, and ENSO phase. We processed satellite Ocean‐Land Color Instrument images from 2016 to 2022 with a fit‐for‐purpose version of the Belkin and O'Reilly frontal detection algorithm. We find coherent shifts in the position of fine‐scale features depending on the ENSO phase, with El Niño isolating the gulf sub‐regions and La Niña connecting them together. Overall, fronts tend to co‐locate with the 70 and 40 m isobaths in the outer and inner HG respectively, and their locations shift close or away from shore in response to changes in dominant wind direction. Furthermore, offshore frontal occurrences increase during winter and spring, and nearshore ones increase during summer and autumn. Our results sketch a first assessment of the distribution of fine‐scale features in a biologically important yet understudied region.