Atlantic overturning: new observations and challenges.

This paper provides an introduction to the special issue of the Philosophical Transactions of the Royal Society of London of papers from the 2022 Royal Society meeting on 'Atlantic overturning: new observations and challenges'. It provides the background and rationale for the meeting, briefly summarizes prior progress on observing the Atlantic overturning circulation and draws out the new challenges that papers presented at the meeting raise, so pointing the way forward for future research. This article is part of a discussion meeting issue 'Atlantic overturning: new observations and challenges'.

Spatially implicit plankton population models: Transient spatial variability.

Ocean plankton models are useful tools for understanding and predicting the behaviour of planktonic ecosystems. However, when the regions represented by the model grid cells are not well mixed, the population dynamics of grid cell averages may differ from those of smaller scales (such as the laboratory scale). Here, the 'mean field approximation' fails due to 'biological Reynolds fluxes' arising from nonlinearity in the fine-scale biological interactions and unresolved spatial variability. We investigate the domain-scale behaviour of two-component, 2D reaction-diffusion plankton models producing transient dynamics, with spatial variability resulting only from the initial conditions. Failure of the mean field approximation can be quite significant for sub grid-scale mixing rates applicable to practical ocean models. To improve the approximation of domain-scale dynamics, we investigate implicit spatial resolution methods such as spatial moment closure. For weak and moderate strengths of biological nonlinearity, spatial moment closure models generally yield significant improvements on the mean field approximation, especially at low mixing rates. However, they are less accurate given weaker transience and stronger nonlinearity. In the latter case, an alternative 'two-spike' approximation is accurate at low mixing rates. We argue that, after suitable extension, these methods may be useful for understanding and skillfully predicting the large-scale behaviour of marine ecosystems.

Assimilating satellite ocean-colour observations into oceanic ecosystem models.

The effectiveness of ocean-colour data assimilation in providing robust biological-parameter estimates for basin-scale ecosystem models is investigated for a phytoplankton-zooplankton-nutrient model using North Atlantic satellite chlorophyll data. The model is forced by annual cycles of mixed-layer depth, day length, photosynthetically available radiation and a temperature-dependent phytoplankton maximum growth rate. Although ocean-colour data are potentially limited in their ability to constrain model parameters because they provide information about the phytoplankton component only, this limitation is offset by the volume of data available covering the range of possible biogeochemical responses to similar and widely varying physical conditions. The results are improved by applying wintertime nutrient estimates based on in situ observations as an additional constraint. Repeatability of parameter estimates obtained from independent samples is examined. Results obtained using regional and basin-wide sampling strategies for obtaining the optimization dataset are compared and the geographic applicability of the calibrated models is assessed.

Satellite observations of the Agulhas Current system.

The Agulhas Current system is a complex interplay of currents and eddies with the bathymetry. Components such as the East Madagascar Retroflection and the Agulhas Return Current evolve significantly over a month, and they are thus not adequately resolved by infrequent research-ship cruises. This paper contrasts the abilities of three different spaceborne sensors for monitoring these complex regimes. A key parameter is sea-surface temperature, measured by both infrared and microwave radiometers. Ocean colour observations of chlorophyll can also be used to distinguish between water masses.