Self‐Aggregation of Convection in Spatially Varying Sea Surface Temperatures

Abstract

The phenomenon of self-aggregation of convection was first identified in convection-permitting simulations of radiative convective equilibrium, characterized by homogeneous boundary conditions and in the absence of planetary rotation. In this study, we expose self-aggregation of convection to more complex, nonhomogeneous boundary conditions and investigate its interaction with convective aggregation, as forced by large-scale variations in sea surface temperatures (SSTs). We do this by conducting radiative convective equilibrium simulations on a spherical domain, with SST patterns that are zonally homogeneous but meridionally varying. Due to the meridional contrast in SST, a convergence line first forms, mimicking the Intertropical Convergence Zone. We nevertheless find that the convergence line breaks up and contracts zonally as a result of the self-aggregation of convection. The contraction is significant, being here more than 50% of the original extent. The stability of the convergence line is controlled by the strength of the meridional circulation, which depends upon the imposed SST contrast. However, the process of self-aggregation, once it is initiated, is insensitive to the strength of the SST contrast. The zonal contraction is accompanied by a slight meridional expansion and a moistening of the high latitudes, where SSTs are low. The moistening of the high latitudes can be understood from the fact that the convective cluster intensifies and expands its moist meridional low-level outflow when it self-aggregates zonally. Overall, our results suggest that the Intertropical Convergence Zone may be unstable to the self-aggregation of convection, that self-aggregation may serve as a precursor to the formation of atmospheric rivers, and that longer convergence lines are more likely to exist in regimes with strong SST gradients.Plain Language Summary Storm clouds over the tropical oceans are found to organize in large systems. They tend to form where the temperatures of the sea surface (SSTs) are highest and that is most often along the equator. With idealized computer model experiments we investigate how different sea surface temperature patterns, along with the natural tendency of clouds to aggregate, control the properties of the storm cloud system. We find that a contrast in SSTs immediately acts to organize the storm clouds. In our simulations, this means the formation of a long line of clouds, oriented along the equator. With time however, this line breaks up and then contracts just by itself. Here we find that the spatial difference in SSTs controls the stability of the cloud system against the break up and contraction: The stronger the difference in SST, the more stable is the line of clouds. Also we find that the storm clouds export moisture only in one direction: from the equator to the higher latitudes.

Self‐Aggregation of Convective Clouds With Interactive Sea Surface Temperature

“…Worth to mention that all the studies mentioned so far used fixed homogeneous surface temperature. Using a nonhomogeneous SST (constant in time but inhomogeneous in space) or an interactive SST (evolving in time) can also change the occurrence of self‐aggregation and the dominating feedback (Hohenegger & Stevens, 2016; Liu & Moncrieff, 2008; Müller & Hohenegger, 2020; Shamekh et al, 2019; Tompkins, 2001). Introducing an SST anomaly can dictate the preferred location of convection (Tompkins, 2001) and thus impact the self‐aggregation.…”

3D convection-resolving model of temperate, tidally-locked exoplanets

“…Another improvement of the model will be to take into account ocean circulation. Del Genio et al ( 2019) showed the impact of such circulation on the SST and the clouds self-aggregation (Müller and Hohenegger, 2020) and how SST anomalies can favor the aggregation of convection (Shamekh et al, 2020). The oceanic circulation is also believed to have a significant impact on the atmospheric circulation, with a planetary heat transport dominated by the ocean at moderate incident stellar fluxes and by the atmosphere near the inner edge of the habitable zone Yang et al (2019a).…”

Quantifying Convective Aggregation Using the Tropical Moist Margin's Length

“…Second, we have quantified organization in idealized simulations and reanalysis of different geometries to show the versatility of the moist margin's approach. However, this limits our ability to meaningfully compare idealized simulations to observations, as, for example, idealized simulations with weak rotation and meridional surface temperature gradients would be better analogs to the Atlantic ITCZ (e.g., Müller & Hohenegger, 2020).…”