Objectively identified large-scale convergence lines and their role in monsoon variability

Convergence lines are regions where air accumulates, often leading to convection and rainfall. Prominent examples include the Intertropical Convergence Zone (ITCZ) and the South Pacific Convergence Zone (SPCZ). Despite being fundamentally defined by convergence, the ITCZ has traditionally been identified using related but distinct fields such as precipitation and outgoing longwave radiation. Here, we apply a convergence-based weather-object algorithm to examine the ITCZ and other large-scale convergence-line features globally, with an emphasis on monsoon regions and the role of the ITCZ in monsoon variability.

Low-level (1000-850 hPa), 6-day time-mean large-scale convergence lines are identified objectively in the ERA5 reanalysis and classified according to whether the local specific humidity is high (high-q) or low (low-q). High-q convergence lines are predominantly tropical features and are most commonly associated with the ITCZ, while low-q convergence lines largely correspond to heat troughs and mid- to high-latitude convergence features.

High-q convergence lines are a frequent component of major monsoon systems, including the Indian and Australian monsoons, where they are closely associated with precipitation and intraseasonal variability. Their occurrence increases during relatively wet monsoon years and during active phases of the monsoon. Over India, high-q convergence lines near the Tibetan Plateau tend to shift southward during wet years and active monsoon periods. A similar behaviour is found in Australia, where convergence lines extend southward from the Maritime Continent onto the Australian continent.

In contrast, convergence lines in the West African monsoon region are generally drier than those in the Indian and Australian monsoon regions. The frequency of high-q convergence lines shows relatively little increase during wet years or active phases. Instead, low-q convergence lines, which constitute much of the West African ITCZ, shift northward during wet years and intraseasonal monsoon bursts. Our current work extends this framework to CMIP6 simulations to assess how well climate models represent convergence-line structure and variability across different monsoon systems.