A Lagrangian Analysis of Heat Uptake in the Tropical and Subtropical Atlantic During the Global Warming Hiatus

The recent slowdown in the rate of increase of global mean surface temperature has been accompanied by a concurrent increase in ocean heat content. While it has been hypothesized that the reduced surface warming is a consequence of the cool phase of the Pacific Decadal Variability during this period, the increase in ocean heat content has not been limited to the Pacific basin, with some of the most pronounced trends in heat content being observed in the subsurface tropical and subtropical Atlantic and Indian Oceans. This raises two important questions: (1) What changes occurred in ocean-atmosphere heat exchange at locations remote from the Pacific that allowed these parts of the ocean to gain heat? And (2) What are the mechanisms that facilitated the subduction of heat from the surface at these locations within a timescale of a few years?

In this study, we attempt to answer these two questions for the subtropical South Atlantic, which has undergone a large increase in ocean heat content in the 300-700 m depth range. We apply a Lagrangian particle tracking software (the Connectivity Modeling System) to velocities from ocean reanalysis products in order to model the pathways that connect the surface ocean to regions of subsurface warming by initializing particles in the regions of heat content increase and simulating their trajectories backwards in time. We then compare these results with estimates of surface heat fluxes. Our findings indicate that the source waters for the region of warming include a significant contribution from the surface equatorial Atlantic. The increase in heat content at 300-700 m is thus consistent with an observed decrease in latent heat loss in the equatorial Atlantic during the hiatus period with respect to the previous decade. The influence of ocean mixing on this result is also explored.