More realistic Parameterizations of Upper ocean Mixing Processes – PUMP

Equatorial regions, known for their vigorous air-sea coupled processes, exert a global influence on the Earth's climate. Cold tongue regions in the eastern Atlantic and Pacific Oceans constitute the main ocean heat uptake, the main oceanic source of carbon to the atmosphere and a site with significant air-sea oxygen exchanges. Through redistribution of subsurface momentum, heat and other properties, ocean mixing plays a key role in regulating fluxes between ocean and atmosphere.

PUMP is based on the three interconnected challenges:
1. Systematic biases in equatorial cold tongues are a common issue in successive generations of Coupled Model Intercomparison Project (CMIP) models, often resulting in a cold-bias Pacific cold tongue and a warm-bias Atlantic cold tongue. These biases may be attributed to the models' inability to realistically constrain turbulent mixing.
2. Models with high vertical resolution can technically represent upper ocean mixing processes, but many mixing parameterizations are based on unproven analogies with the atmosphere. A serious challenge is thus to develop parameterizations that can realistically represent the turbulent fluxes of momentum, heat, and other properties.
3.The gap in our understanding of mixing processes is exacerbated by limitations in observational capabilities lacking sufficient vertical resolution in the near surface. We thus need temperature, salinity, currents in the upper layer at high vertical resolution associated with turbulence measurements.

PUMP tackles these questions from the angle of upper ocean diurnal processes in the eastern equatorial Atlantic. In fact, a unique aspect of equatorial cold tongues is the penetration of diurnally forced mixing beneath the mixed layer, as a powerful communication channel between the atmosphere and ocean. In addition, the diurnal cycle is a key test for parameterization schemes since it involves vertical processes on short time scales, avoiding time-integrated effects. Importantly, unprecedented observations have been collected based on the meteo-oceanic buoys under the PIRATA program, including two of the three turbulence measurement sites. Long time series of turbulence/mixing data are not available elsewhere making the Atlantic cold tongue an invaluable site for addressing mixing-related scientific questions.

The overarching objective of PUMP is to investigate the diurnal pumping mechanism as an essential upper ocean mixing process for the Atlantic cold tongue and to achieve a realistic representation of its ultimate impact in ocean models through three objectives.

The first objective is to characterize the temporal and spatial modulation of oceanic diurnal cycle. Thanks to recent observation efforts since 2021, we aim to accurately describe the diversity of diurnal process features that modulate heat and momentum transport into the surface layer. The second objective is to determine the sensitivity of diurnal processes to existing and more-advanced model schemes. Based on a unidimensional approach, we will assess the sensitivity of diurnal processes to numerical parameterizations, and suggest improvements. The third objective is to study the large-scale effects of numerical choices in a regional configuration. Using numerical experiments, we will extend unidimensional model analyses into a realistic context and enhance the understanding of their impacts on key phenomena in the tropical Atlantic.

PUMP covers many aspects, from small-scale understanding to impacts on ocean dynamics, from observations to numerical modeling, and therefore involves several different teams. Based on recent observational efforts, the ambition of the PUMP project is to contribute to improving our collective understanding of the dynamics of mixing processes to increase confidence in models, and will be directly transferable within the NEMO community and to the operational centers of the European Copernicus program.