Impact of high-cloud radiative effects on monsoons

Understanding how high-level clouds shape the global energy balance is critical for characterizing the physical processes driving monsoon systems, which serve as a primary engine of the global water cycle and support billions of people. High-level clouds significantly influence Earth’s energy balance, not only by modulating top-of-atmosphere fluxes, but also their radiative interactions within the atmosphere itself. This high-level cloud radiative effect (HCRE) represents the internal atmospheric heating or cooling caused by high-level clouds. By modifying temperature gradients, the HCRE serves as a key component of the global energy balance and has been shown to influence circulation patterns and precipitation. While such findings suggest that the HCRE also modulates monsoon systems, its specific impact has not yet been investigated. The impact of the HCRE on monsoons involves two pathways: a pathway linked to changes in atmospheric temperatures, and a surface pathway linked to changes in surface temperatures. To date, research has primarily focused on the atmospheric pathway, and has neglected interactions with the ocean surface that are known to be central to monsoon dynamics.

In this study, we aim to quantify how the HCRE modulates monsoon systems when the temperature of the surface layer of the ocean responds to changes in the atmosphere. Specifically, we address how HCRE impacts the seasonal thermodynamic structure of the troposphere, circulation patterns, and the spatial extent and magnitude of monsoon rainfall. To achieve this, we use the Icosahedral Non-hydrostatic Earth System Model (ICON-ESM). Simulations are performed using both prescribed sea surface temperatures and an interactive slab ocean that allows sea surface temperatures to adjust to cloud-driven surface flux changes. For each ocean setup, a control simulation is compared to a simulation in which high-level clouds are made radiatively transparent but remain physically present. Simulations with prescribed sea surface temperatures, are used to isolate the atmospheric pathway. We then identify the surface pathway by subtracting the atmospheric pathway from the total impact of the slab ocean setup. We anticipate stronger and more spatially coherent shifts in the Intertropical Convergence Zone and Hadley-circulation, when the surface pathway is included. This is hypothesized to drive a northward expansion of the northern hemisphere monsoon, even as increased atmospheric stability suppresses mean tropical precipitation.