Deep MIP Early Eocene Indian Gyre comparison

In this study, we analyze the outputs of the Deep-Time Model Intercomparison Project (DeepMIP) for
the Early Eocene, focusing on the representation of the southwestern Indian Ocean gyre and investigating
the sources of inter-model variability. Specifically, we address three key aspects : (1) biases associated with
resolution through a comparison of realistic reanalyses with PI (pre-industrial) simulations, (2) the impact of
paleogeography by comparing PI with x1 (Early Eocene simulations under the same CO2 concentration as PI),
and (3) the sensitivity of paleo simulations to increased CO2 through a comparison of x1 and x3 scenarios.
(1) Our findings reveal that biases in the current representation of circulation in the southwestern Indian
Ocean particularly those linked to coarse resolution—persist across the seven DeepMIP models when compared
to realistic reanalyses such as GLORYS (ocean) and ERA5 (atmosphere). While certain patterns, like the
position of fronts, are well captured by the models, others, including stratification, western boundary currents,
and Agulhas leakage, display significant inter-model variability.
(2) Next, we evaluate the present-day/paleo sensitivity of these models by comparing PI and Early Eocene
(47–56 Ma) outputs under identical CO2 concentrations (x1). Consistent with existing literature, all seven
models exhibit a weaker wind stress curl and the absence of the ACC (Agulhas Circumpolar Current). Especially
at depth, temperatures in the region are generally 0.5°C to 4°C higher (except in COSMOS), and north-south
temperature gradients are weaker. On average, the frontal positions are located 5° farther south in the Early
Eocene. The first baroclinic Rossby deformation radius shows limited changes relative to the inter-model spread.

(3) Finally, we investigate the sensitivity of Early Eocene simulations to increased CO2 levels by comparing
x1 and x3 scenarios. Across all models, higher CO2 concentrations lead to slightly weaker wind stress and
transport. Water temperatures increase by 4–8°C, depending on the model, and the Rossby deformation radius
decreases slightly at mid-to-low latitudes.