Convective Aggregation and the Amplification of Tropical Precipitation Extremes

Abstract

It is widely believed that precipitation extremes will increase in response to a warming climate, as inferred from both observations and numerical simulations. In the absence of changes in atmospheric circulations, extreme precipitation is expected to increase in already-moist regions along a thermodynamical Clausius-Clapeyron scaling. However, within the tropics, the sensitivities inferred from observations of interannual variability are roughly twice as large, implying an unknown contribution from atmospheric dynamics. In this paper, we use satellite observations and climate model simulations to investigate the relationship between convective aggregation and precipitation and the role that convective aggregation plays in amplifying the response of tropical precipitation extremes to interannual surface warming. We find that the occurrence of heavier precipitation coincides with a higher degree of convective aggregation. Extreme precipitation events and convective aggregation tend to increase during warm, El Niño events compared to colder, La Niña events. Although both the frequency and intensity of heavy to extreme precipitation can increase with increased aggregation during El Niño, the changes in frequency are more consistent among the observations and models than changes in intensity. In both the observations and models, increases in large-scale convective aggregation contribute to roughly one third of the increase in extreme precipitation frequency due to interannual warming by shifting moderate-to-heavy precipitation events to more extreme precipitation intensities. The linkages between convective aggregation and precipitation extremes considered here offer insights into their potential response to anthropogenic warming. Plain Language Summary As climate warms, one of the most prominent changes in the global water cycle is more common extreme rainfall events in the tropics. A mechanism that enhances the convection process and its related precipitation could increase the rate of occurrence and strength of these events rising with the temperature. Here, we investigate how a phenomenon known as convective aggregation contributes to more common tropical extreme rainfall events on a year-to-year basis. We find that heavier rainfall is closely tied to a more aggregated state of atmospheric water vapor. Both observations and models show that more extreme rainfall events and the aggregation of moisture occur during warm, El Niño events relative to colder, La Niña events. The enhanced aggregation process creates extra moisture in the already-moist regions leading to more common heavier precipitation events and less common lighter precipitation ones. As a result, the increase in convective aggregation amplifies the increase in extreme rainfall events related to year-to-year surface warming by roughly one third. Our findings provide new insight on projections of future rainfall extremes, which have strong implications for disaster risk management.

Mesoscale convective clustering enhances tropical precipitation

“…Examples of organized tropical convective systems include mesoscale convective systems (1), tropical cyclones, and the Madden-Julian Oscillation (2). Among these organized systems, mesoscale systems of clustered convection are of first-order importance to the tropical hydrological cycle; they produce more than 60% of the total precipitation in the tropics (3,4) and are the main drivers of extreme precipitation events (5)(6)(7). Moreover, changes in the frequency of mesoscale organized convective systems have been ascribed as the cause of changes in tropical rainfall patterns observed in recent decades (8) and have been suggested as a potential cause of the future changes to global precipitation patterns predicted in simulations of greenhouse gas-induced global warming (9,10).…”

Investigating the Causes and Impacts of Convective Aggregation in a High Resolution Atmospheric GCM

“…Different metrics have been used to quantify the degree of aggregation under different circumstances such as the “organization index” that detects organized convective features using satellite observed infrared brightness temperatures (Bony et al., 2020; Tompkins & Semie, 2017), subsidence fraction (Coppin & Bony, 2015), the spatial variance of column relative humidity (Wing & Cronin, 2016) and the spatial variance of column integrated water vapor (Dai & Soden, 2020; Wing et al., 2020). In this study, we characterize the spatial organization of convection using subsidence fraction, which is computed as the fractional area covered by subsidence based on daily vertical pressure velocity at 500 hPa .…”

“…Additionally, the change in extreme precipitation in response to increases in greenhouse gases is found to depend on the magnitude of warming (Pendergrass et al, 2015) and the change in convective aggregation (Muller, 2013;Pendergrass et al, 2016). Recently, the impact of convective aggregation on extreme rainfall events are confirmed in observations (Dai & Soden, 2020;.…”

“…This aggregation of convection occurs even in the presence of uniform boundary conditions (self‐aggregation) and not only impacts the intensity of precipitation events (Bao & Sherwood, 2019), but also modulates the larger‐scale thermodynamic and radiative properties of the tropics (e.g., Bony et al., 2020). Increased aggregation is associated with increased spatial variance of moisture; dry regions become drier and moist regions become moister (Dai & Soden, 2020). The increased spatial variance of moisture directly impacts both the intensity of precipitating systems and the larger‐scale radiative fluxes (Bony et al., 2016, 2020; Bretherton et al., 2005; Wing et al., 2020).…”

“…become drier and moist regions become moister (Dai & Soden, 2020). The increased spatial variance of moisture directly impacts both the intensity of precipitating systems and the larger-scale radiative fluxes (Bony et al, 2016(Bony et al, , 2020Bretherton et al, 2005;Wing et al, 2020).…”