Warm and Cold Water Routes to the South Atlantic - a Lagrangian based comparison of two state-of-the-art climate models

Abstract

The South Atlantic Ocean plays a crucial role in the Atlantic Meridional Overturning Circulation (AMOC), linking it with the Indian and Pacific Oceans as part of the global overturning circulation. Waters from the Indian Ocean that enter the South Atlantic through Agulhas leakage, are referred to as the ”warm” route, while waters from the Pacific Ocean that arrive via the Drake Passage are referred to as the ”cold” route. The properties and relative contri- butions of these routes matter because they can affect the strength and stability of the AMOC. The relative contribution of both routes remains unclear both in observations and General Circulation Models (GCMs). Additionally, their sensitivity to model resolution is unclear. In this study, we run a Lagrangian analysis, where large sets of virtual particles are integrated offline within the three-dimensional, time-evolving velocity field output from a GCM. This is done from the southern boundary of the South Atlantic at 34.5oS to determine the route preference in two state-of-the-art CMIP6 GCM configurations. One model configuration is eddying (horizontal resolution 0.1o), while the other has parameterised eddies (horizontal resolution 1o). We find that both the Drake Passage contribution in terms of volume transport and water property are model-resolution dependent, with an increased contribution with increased resolution. Despite this, our results agree with recent literature that find the warm water route to be the dominant source for northern transport at 34.5oS, with it contributing 95% in the 1o model configuration, and 70% in the 0.1o model configuration. In the 1o model configuration, the cold water route is largely composed of surface water, while in the 0.1o model configuration, surface and intermediate waters are present, illustrating large inter-model dif- ferences in resolved processes. Moreover, our results for both experiments indicate that the majority of cold route waters experience large along track changes in thermohaline properties, with an increase in temperature and salin- ity, while the warm route waters remain largely unchanged. Further research is needed to understand inter-model resolution biases on heat and salt transport in the South Atlantic, especially for future climate projections.