Eddy and enhanced primary productivity captured in Moana Project model and satellite images
The Moana Project backbone model clearly illustrates the occurrence of a cyclonic eddy off the coast of Kaikoura on Saturday the 11th of September 2021 (see below-left figure). MetOcean scientists using the model came across the eddy where satellite images from the same day show elevated levels of chlorophyll-a concentrations in the centre of the eddy (see below figure right). Ocean currents are affected by multiple factors, including wind forcing, bathymetry and continental geography, which cause water movement that is different to the predominant current direction, driving waters in cyclonic (clockwise in the Southern Hemisphere) or anticyclonic directions (counterclockwise in the Southern Hemisphere).
The above figures show the Moana Project’s Moana Project’s ROMS (Regional Ocean Modeling System) national forecast model output of mean sea surface current (left figure) and satellite images of chlorophyll-a concentrations from the MODIS instrument (right figure) in waters off the coast of Kaikoura, New Zealand on Saturday the 11th September 2021
Kaikoura is famous for its abundant marine life due to the proximity of the coastal shelf to the coast. In fact, the beginning of Kaikoura Canyon sits only 500 m from the Kaikoura shoreline and the canyon extends 60 km seaward to depths of 1200 m [1]. This bathymetry causes the northward-flowing Southland Current to meander and form eddies.
“As eddies interact with boundary currents and the coastal seas, they provide a connection between inshore waters and the adjacent open ocean. In addition, cyclonic eddies pump the deep nutrient-rich waters closer to the surface in their centre making it available to primary producers in the photic zone,” explains Moana Project Science Lead Dr João Souza.
Primary producers such as microalgae and photosynthetic cyanobacteria can reproduce at rapid rates when conditions are right (nutrient and light availability) and are the basis of the oceanic food web.
“Eddy dynamics can have important consequences for the local ecosystem by increasing overall productivity to an originally nutrient-poor region,” adds Dr Souza.
The operational hydrodynamic model was developed as part of the Ngā Ripo o te Moana team’s research, and offers an unprecedented spatial and temporal resolution to forecast ocean behaviours around Aotearoa. This model is able to represent the main phenomena influencing ocean currents and structures in Aotearoa, resolving processes with important socio-economic impacts such as ocean eddies. Eddies may influence fishing activities, thus, the capability of our model to predict when and where eddies occur is likely to be of specific interest to stakeholders concerned with the associated increase in biological activity.
[1] Lewis, K. B., Barnes, P. M., (1999) Kaikoura Canyon, New Zealand: active conduit from near-shore sediment zones to trench-axis channel, Marine Geology, Volume 162, Issue 1, 1999, Pages 39-69, https://doi.org/10.1016/S0025-3227(99)00075-4.
[2] Anand, A., P. Krishnan, G. Grinson, Goutham Bharathi, M. P., Kaliyamoorthy, M., Hareef Baba Shaeb, K., Suryavanshi, A. S., Srinivasa Kumar, T., & Joshi, A. K., (2014) The influence of mesoscale eddies on a commercial fishery in the coastal waters of the Andaman and Nicobar Islands, India, International Journal of Remote Sensing, 35:17, 6418-6443, DOI: 10.1080/01431161.2014.958246