A Relationship Between ITCZ Organization and Subtropical Humidity

Abstract

Motivated by the results of idealized studies on the self‐aggregation of convection, we investigate a potential relationship between the degree of organization of the Intertropical Convergence Zone (ITCZ) and humidity based on reanalysis data. We focus on the Atlantic ITCZ and use the number of long convergence lines occurring per month to define the degree of organization. The latter shows a weak enhancement during June to August (JJA) and a large interannual variability. On an interannual time scale and particularly during JJA, a relationship exists between organization and humidity: Years with more organized ITCZs are associated with a moister ITCZ region and drier subtropics. Even though we cannot demonstrate any causality and cannot rule out the presence of another agent, we show that these moisture anomalies are not incompatible with an effect of organization. We also note that the annual cycle in sea surface temperature (SST) gradient may contribute to the intra‐annual variability in organization.

Convection and Convective‐Organization in Hothouse Climates

“…In addition to spontaneously self‐aggregating, convection can also be “forced” to aggregate by an SST gradient (Hohenegger & Jakob, 2020; Lutsko & Cronin, 2023; Müller & Hohenegger, 2020). In idealized “mock Walker” simulations, a simple SST gradient is imposed, forcing a large‐scale overturning circulation (Bretherton et al., 2006; Grabowski et al., 2000; Lutsko & Cronin, 2023).…”

The inner life of the Atlantic Intertropical Convergence Zone

“…To summarize, while the structure of low‐level convergence has been discussed in a few studies, there is still no clear consensus on how best to describe it. Theoretical and modeling studies usually emphasize a single convergence line (see e.g., Hohenegger & Jakob, 2020, for a recent example), whereby observations tend to document a more complex structure, often better idealized as a double convergence line, as shown schematically in Figure 1a and b respectively. To the extent that convergence within the ITCZ is associated with several lines of convergence that extend only a few degrees of longitude (Weller et al ., 2017), our limiting cases in Figure 1 encapsulate the idea that these lines tend to form and dissipate in the center, or toward the edges, of the rain belt.…”

Sensitivity of the Horizontal Scale of Convective Self‐Aggregation to Sea Surface Temperature in Radiative Convective Equilibrium Experiments Using a Global Nonhydrostatic Model

“…These observational studies show that with an increase in the degree of CSA, there is an increase in aridity, a decrease in cloud cover, and an increase in radiative cooling. By contrast, some attempts have been made to quantify the degree of CSA at larger spatial scales, such as the inter‐tropical convergence zone (Hohenegger & Jakob, 2020; Popp & Bony, 2019). For CSA over such large spatial scales, convective clusters that are more aggregated strengthen the contrast between the dry subtropic and moist deep convective regions.…”