Missions

This postdoctoral position is part of the ANR PUMP research project (2025–2028), which aims to better understand the mechanisms of ocean mixing in the surface layers of the equatorial Atlantic and their contribution to energy exchanges (i.e., momentum and heat) between the ocean and the atmosphere. The equatorial basins of the Atlantic and Pacific, which are particularly sensitive to climate variability, exhibit significant diurnal variability. This variability, which directly influences sea surface temperature, results from complex interactions between solar radiation, currents, stratification, and vertical mixing processes. Vertical mixing plays a crucial role in the redistribution of heat: it determines whether heat remains trapped at the surface to warm the atmosphere at the end of the day, or whether it is transferred to deeper layers of the ocean. A fundamental component of this mechanism is the vertical shear between the eastward-flowing Equatorial Undercurrent (EUC), located between 150 m and 50 m depth, and the westward-flowing surface South Equatorial Current. This shear acts against the stratification — maintained by solar heating and the presence of cold equatorial waters — in a way that sustains marginally unstable conditions (Richardson number close to 1/4).

In this context, gaining a better understanding of how these local processes — particularly wind forcing and the diurnal cycle — interact with larger-scale, lower-frequency phenomena, such as waves and zonal currents associated with variability in the EUC and thermocline, is essential. For instance, it has been shown that seasonal fluctuations linked to the vertical displacement of the EUC affect the depth of penetration of diurnal variability. Moreover, several studies highlight that the apparent dependence of this diurnal variability on local wind forcing may in fact (partially) reflect control by larger-scale processes.

A better understanding of multi-scale interactions is critical for deepening our knowledge of oceanic processes in the tropics and for improving their representation in numerical models, ultimately helping to reduce persistent biases in climate models in these regions. Since the vertical transport of nutrients relies on the same physical mechanisms as the downward transfer of heat, vertical mixing — and its intermittency — plays also a key role in regulating primary productivity.

Activités

The goal of this postdoctoral position is to explore the regimes of vertical mixing variability in the Tropical Atlantic using a dataset derived from the PIRATA network and deployments of Argo floats with appropriate sampling frequencies. This work will be complemented by multi-year regional simulations that will allow for the quantification of turbulent fluxes and potentially disentangle their respective drivers through sensitivity experiments.

More specifically, the main tasks will be:
1. Characterize the variability of the Atlantic equatorial system:
- Study the seasonal and interannual fluctuations of the Equatorial Undercurrent (EUC), the thermocline, and the surface layer using observational data.
- Compare numerical simulations (regional model) with observations (PIRATA moorings, Argo floats).

2. Quantify the effects of local wind and large-scale conditions:
- Define different circulation and forcing regimes (seasons, thermocline structure, climate modes).
- Study the responses of vertical dynamics and mixing to local wind forcing.
- Assess the respective contributions of local and remote mechanisms to ocean mixing using numerical experiments if necessary.

Compétences

Education:
PhD in Physical Oceanography or Atmospheric Sciences.

Skills:
- Strong experience in processing oceanographic data (in situ and satellite).
- Proficiency in numerical modeling (e.g., ROMS, NEMO, CROCO or equivalent).
- Mastery of data analysis tools (Python, MATLAB, or equivalent).
- Knowledge of equatorial dynamics and/or mixing processes.
- Good writing and scientific communication skills (in both French and English).

Contexte de travail

The postdoctoral researcher will join a multidisciplinary team involved in several regional and international projects (PIRATA, NEMO, etc.). International collaborations are anticipated.