Evidence of eddy-related deep ocean current variability in the North-East Tropical Pacific Ocean induced by remote gap winds

Abstract

Abstract. There has been a steady increase of interest in mining of deep-sea minerals in the Clarion-Clipperton Zone (CCZ) in the eastern Pacific Ocean during the last decade. This region is known to be one of the most eddy-rich regions in the world. Typically, mesoscale eddies are generated by intense wind bursts channelled through gaps in the Sierra Madre mountains in Central America. Here, we use a combination of satellite and in situ observations to evaluate the relationship between deep-sea current variability at the region of potential future mining and Eddy Kinetic Energy (EKE) in the vicinity of gap winds. A geometry-based eddy detection algorithm has been applied to altimetry sea surface height data for a period of 24 years from 1993 to 2016 in order to analyse the main characteristic parameters and the spatiotemporal variability of mesoscale eddies in the North-East Tropical Pacific Ocean (NETP). Significant differences between the characteristics of eddies with different polarity (cyclonic vs. anti-cyclonic) were found. For eddies with lifetimes longer than one day, cyclonic polarity is more numerous that anticyclonic rotation. However, anticyclonic eddies are larger in size, show stronger in vorticity, and survive longer in the ocean than cyclonic eddies (often 90 days or more). Besides the polarity of eddies, the location of eddy formation should be taken into consideration when investigating the impacted deep ocean region, as we found eddies originating from the Tehuantepec (TT) gap wind lasting longer in the ocean and travelling farther distances in a different direction compared to eddies produced by the Papagayo (PP) gap wind. Long-lived anticyclonic eddies generated by the TT gap wind are observed to travel distances up to 4500 km offshore, i.e. as far as west of 110 W. EKE anomalies observed in the surface of the central ocean at distances of ca. 2500 km from the coast correlate with the seasonal variability of EKE in the region of the TT gap winds with a time lag of 5–6 months. A significant seasonal variability of deep ocean current velocities at water depths of 4100 m was observed in multiple year time-series data likely reflecting the energy transfer of the surface EKE generated by the gap winds to the deep ocean. Furthermore, the influence of mesoscale eddies on deep ocean currents is examined by analyzing the deep ocean current measurements when an anticyclonic eddy crosses the study region. Our findings suggest that despite the significant modulation of dominant current directions driven by the bottom-reaching eddy, the current magnitude intensification was not strong enough to trigger local sediment resuspension in this region. A better insight of annual variability of ocean surface mesoscale activity in the CCZ and their effects on deep ocean current variability are of great help to mitigate the impact of the benthic ecosystem from future potential 1deep-sea mining activities. On an interannual scale, a significant relationship between cyclonic eddy characteristics and El-Niño Southern Oscillation (ENSO) was found, whereas no robust correlation was detected for anticyclonic eddies.

Numerical Simulation of Deep-Sea Sediment Transport Induced by a Dredge Experiment in the Northeastern Pacific Ocean

“…Using long-term sea surface height anomaly data, Purkiani et al (2020) show that between 4 and 5 long-lived anticyclonic eddies with a life-time longer than 90 d are formed annually in the northeastern Pacific Ocean. The study shows that most of the long-lived mesoscale eddies pass the GLA and depending on their strength may increase the mean current velocity at the abyssal seabed by a factor of 3-4 (Aleynik et al, 2017) or at least change the prevailing southeastward current directions to a northward current direction for a period of a few weeks (Purkiani et al, 2020). Strong deep-sea current velocities e.g., at the northwestern Bermuda Rise related to sea surface eddies caused by the meandering Gulf stream are able to episodically erode seafloor sediment and develop a benthic nepheloid layer (Gardner et al, 1985(Gardner et al, , 2017.…”

Developing best environmental practice for polymetallic nodule mining - a review of scientific recommendations

“…The scale of horizontal dispersal of the operational sediment plume generated by mining along the seafloor depends on several factors: (a) composition and shape of the sediments (e.g. Muñoz-Royo et al, 2022), (b) the degree of flocculation and formation of a turbidity current (Muñoz-Royo et al, 2022), (c) seabed morphology (Peukert et al, 2018;Gausepohl et al, 2020;Purkiani et al, 2021), (d) background currents (Muñoz-Royo et al, 2022), (e) tidal cycles that will keep bottom currents in motion (Baeye et al, 2021); (f) occasional strong events, such as benthic storms or the propagation of surface storms or eddies to the ocean floor resuspend sediments in at least the benthopelagic mixing zone (Aleynik et al, 2017;Purkiani et al, 2020). The latter may be a regular process which will significantly contribute to the spreading of mining-generated plumes both horizontally and vertically.…”