Looped In: A Metric for Predicting the Strength and Location of the Tropical Rain Belt
The tropical rain belt—also known as the Intertropical Convergence Zone (ITCZ)—is a key component of the global energy and hydrological cycle, but its response to warming can be hard to pin down. While some metrics can help predict how the ITCZ’s location changes with warming, more metrics are necessary to aid understanding of the ITCZ intensity’s response to warming. Scientists at the U.S. Department of Energy’s Pacific Northwest National Laboratory found a metric directly linked to rainfall intensity across a suite of climate models. The metric is based on the curvature of the surface moist entropy field. Moist entropy measures the state of the atmosphere, combining temperature and humidity, and the ITCZ is shown to exist where the strongest curving in the spatial structure of this field occurs. The study showed that this metric accurately predicted changes in rainfall intensity and location under a warming scenario, even when other ITCZ predictors, such as the entropy maximum, were less effective. By examining the flow of energy through the models, researchers found that energy transport by transient eddies could be why the metric accurately predicts ITCZ changes.
Determining the ITCZ’s location and intensity is crucial for climate prediction because of their importance in the global hydrological cycle. This work identified a metric based on the spatial distribution of surface moist entropy that is skilled at determining the ITCZ’s location and intensity on timescales of days to weeks. Researchers proposed a physical pathway—energy transport by transient eddies—that explained the metric’s skill and could help direct future research efforts to better understand the ITCZ’s response to warming.
The seasonal evolution of the ITCZ’s location and intensity is a key factor in tropical rainfall. Both the location and intensity of the ITCZ may change with warming. Previous studies established useful metrics for determining the change in the ITCZ’s location, such as the energy flux equator metric, which relates the ITCZ location to the cross-equatorial flow of energy. Additional metrics are needed to understand how the ITCZ’s intensity changes. To address that lack of metrics, researchers analyzed a suite of models from the Tropical Rain belts with an Annual cycle and a Continent Model Intercomparison Project (TRACMIP) using idealized aquaplanet (without land) experiments. They discovered that the curvature of the surface moist entropy field—a predictor for the onset of an overturning circulation—could be used to predict the strength of that circulation and the intensity of associated rainfall. The relationship between this curvature metric and the rainfall location and intensity holds at timescales less than a month. This makes the curvature metric a valuable tool for exploring changes in rainfall on short timescales, particularly during spring and fall when other metrics struggle. Researchers theorized energy transport by transient eddies as an explanation for this relationship based on a budget analysis of moist static energy (a quantity similar to the entropy used in the curvature metric). The role of transient eddies is important because it may help explain why the curvature metric is effective on the short timescales shown in this study.