Clouds Amplify Mechanisms of Southern Ocean Heat Uptake
The Southern Ocean (SO) fundamentally helps regulate the global climate due to its ability to absorb, store, and transport heat. It has absorbed an estimated 93 percent of the excess heat associated with human-based warming, reducing the rate of global warming. But how does the atmosphere interact with SO heat gain and loss? Researchers used two observation-based reanalysis datasets to understand the roles of atmospheric processes in mediating heat exchange between the air and sea as well as assess the influence of clouds on mechanisms of SO heat gain and loss from 1979 – 2020.
The SO has warmed over the past 50 years. Much research has focused on the oceanic mechanisms of heat uptake. Ocean heat gain and loss occur via multiple processes related to ocean mixing, upwelling, and horizontal heat transport. However, the atmosphere can modulate SO heat gain and loss. This research found that changes in clouds and their associated atmospheric states amplify ocean heat uptake by increasing downward longwave radiation at the surface. More clouds are linked to a more stable atmosphere that increases ocean heat uptake by suppressing heat going out of the ocean surface.
The SO has absorbed most of the excess heat associated with anthropogenic greenhouse gas emissions. Since SO observations are sparse in certain regions and seasons, much scientific knowledge of ocean heat uptake is based on climate model simulations. However, climate models still inadequately represent some properties of SO clouds and have not identified mechanisms by which clouds may affect SO heat uptake (SOHU). Researchers use two sets of observation-based reanalysis datasets to assess the influence of clouds and other atmospheric processes on SOHU from 1979 to 2020. They find that years with ocean heat uptake anomalies during winter and spring, but not summer or fall, have the highest SOHU between 45° and 65°S. Cloud amounts during winter and spring can be seven percent higher when SOHU is up to 5.5 W/m2 higher than the climatological seasonal mean. Those clouds also contain more liquid water. These changes in cloud properties increase downwelling longwave radiation. The cloud changes also accompany a more stable lower atmosphere that suppresses turbulent heat loss to the atmosphere from the ocean surface. The changes in radiative and turbulent energy fluxes both lead to reduced heat loss from the SO, which retains more heat. A better understanding of how clouds and atmospheric processes impact ocean heat uptake may help improve scientific understanding of ocean heat uptake mechanisms in current climate models, leading to more accurate projections of climate change.