Multi-scale oceanographic numerical modelling in support of regional marine science

Understanding the oceanographic conditions that underpin conservation efforts in the Indian Ocean
Indian Ocean reefs viewed from a drone

Multi-scale oceanographic numerical modelling in support of regional marine science

Understanding the oceanographic conditions that underpin conservation efforts in the Indian Ocean
Title: Multi-scale oceanographic numerical modelling in support of regional marine science
Funding amount: £479,222
Location: Indian Ocean
Dates: 1 April 2022 – 31 March 2026
University of Plymouth PI: Dr Philip Hosegood 
University of Plymouth staff: Vasyl Vlasnenko, Nataliya Stashchuk, Harvey Cairns 
 

Overview

Animals throughout the marine ecosystem exploit ocean currents and changes in water properties to improve foraging efficiency, as prey respond to energetic currents and increased food supply. Particles, including larvae, microplastics and contaminants, are suspended in the water and drift at the mercy of these currents. 
The ocean is a tremendously complex environment; currents arise from a myriad of forcing mechanisms such as tides, wind, sea surface elevation and internal density differences that each evolve at different timescales. As these flows interact with topography, the currents become even more complex at progressively smaller scales. Added complexity arises through natural global climate patterns such as the El Niño Southern Oscillation and anthropogenic change. Such complexity cannot be adequately measured everywhere with in-situ instrumentation across this range of scales, demanding a different approach to understanding how the ecosystem responds to changes in the physical environment.
To bridge this barrier, we will combine oceanographic numerical models that simulate ocean conditions at progressively smaller scale. Beginning with the basin scale, we will simulate over multiple years the effects of processes that act over the whole Indian Ocean including the monsoon and Indian Ocean Dipole. Then, we will progressively 'zoom in' to smaller scales, from a regional British Indian Ocean Territory (BIOT) scale simulation to one that resolves the complexity arising from interaction of currents with individual atolls and seamounts. By combining these simulations with satellite observations, we will provide the comprehensive understanding of oceanographic conditions that underpin the conservation efforts underway throughout the BIOT and wider Indian Ocean.

Objectives

Our overall aim is to provide the fundamental, underpinning tools for scientists operating throughout the BIOT and the wider Indian Ocean to understand the oceanographic context to their observations.
Our specific objectives are: 
  1. Develop multiple numerical models operating at a variety of spatiotemporal scales to identify the hydrodynamic processes driving habitat use and larval dispersal throughout the BIOT and wider Indian Ocean
  2. Identify likely biological 'hotspots' throughout the BIOT and Indian Ocean based on model-derived oceanographic parameters including upwelling, turbulence intensity and primary production, and assessment of conditions at known hotspots identified in other projects
  3. Use offline particle tracking in all models to enable the tracking of passively advected particles, including larvae
  4. Assess the impact of regional, climate-driven changes on BIOT oceanographic conditions over multiple years through analysis of publicly-available remote sensing products and basin-scale numerical model output
  5. Integrate model results into partner projects.

Physical oceanography plays a critical role in providing the 'why' for 'what' marine biologists observe. By applying the results from both observations and numerical models, we are identifying the critical oceanographic processes responsible for biological response throughout the Indian Ocean marine ecosystem, including how whirlpools concentrate zooplankton, leading to huge reef manta ray aggregations, or how freshwater inflow in the Indonesian Throughflow modulates primary production throughout the central Indian Ocean.

Philip HosegoodDr Philip Hosegood
Associate Professor in Physical Oceanography

Researchers snorkeling on the surface with equipment in the Indian Ocean
Researchers launching drone from a boat in the Indian Ocean
Reef manta rays swimming in the Indian Ocean

Context of the issue

Dynamical oceanographic processes are increasingly understood as being instrumental in creating biophysical niches that are exploited by animals to increase foraging efficiency and conversely avoid predation, while also transporting larvae between seemingly spatially discrete locations. Internal waves encourage fish schooling over submarine banks and seamounts, attracting foraging porpoises and sharks. In the open ocean, basking sharks track thermal fronts whose convergent currents accumulate zooplankton. Coherent structures arising through instability of ocean currents are tracked by birds due to increased food abundance within them, a dynamical process similar to the generation of warm core eddies preferred by great white sharks due to richer prey fields within them compared to cold-core eddies.
The background circulation throughout the ocean, but particularly near the numerous reefs and atolls within the BIOT where dynamic processes are intensified and particulate matter originates, is further critical to the transport of suspended particles including microscopic larvae from animals and corals but also plastic fragments. The benthic community is directly impacted by internal waves which are implicated in the relief of thermal stress experienced by corals as they are flushed with cooler water upwelled from depth over slopes surrounding atolls and seamounts.
Understanding and modelling the spatiotemporal variability in oceanographic processes responsible for generating these biologically-important features is thus critical to the identification of key locations throughout the BIOT ecosystem, within which specific species aggregate, rendering them especially susceptible to exploitation. The practical and financial issues surrounding their observation using traditional, ship-based methods currently hinder our understanding, especially over the required scales that vary from the mm-scale of turbulence to the 100 km typical of mesoscale eddies; it is simply not possible for a ship to simultaneously monitor the necessary range of spatial scales to identify their impact on the ecosystem and the locations likely to host high species abundance and biodiversity. 
Similarly, remote sensing, which can only give us a view of the surface of the ocean, can be limited by spatial resolution that cannot resolve the smaller scales, in particular those associated with the Island Mass Effect generated by the flow past the many small islands and atolls in the BIOT. Throughout the tropical Indian Ocean, cloud cover further confounds measurements of chlorophyll and, at high resolution, sea surface temperature.

How the project addresses the issue

To address these shortcomings and limitations that hinder the ability of field experiments to place their findings in context, we are developing a multi-scale numerical modelling approach to identify the dynamical oceanographic processes and resulting flow fields responsible for shaping the regional ecosystem throughout the BIOT and the wider Indian Ocean. The project will identify biophysical 'hotspots' and trajectories of particles including larvae, enabling the design of surveys to detect the enhancement in biological activity and informing the results of the observational studies conducted by our partners.
The output of the models, which will also be accompanied by an analysis of the remote sensing products where they are not compromised by cloud or resolution, will be fed to the wider consortium through a data-sharing portal, enabling a direct assessment of the locations most susceptible to biomass concentration at multiple trophic levels and thereby demanding of enhanced policing and monitoring.
Researchers freediving with equipment in the Indian Ocean
 
 
 

Centre for Coastal and Ocean Processes and Engineering (C-COPE)

C-COPE brings together strength areas from across the University's Faculty of Science and Engineering with a research focus on the physical and chemical processes in coastal, ocean and marine environments, and their human impacts.
The Centre's sphere of interest stretches from the head of tidal estuaries to the bottom of the ocean, and includes the disciplines of physical oceanography, marine biogeochemistry, coastal engineering and marine geology.
 
Tuvalu Tepuka atoll