Abstract The seasonal evolution of tropical meridional circulation varies across regions and plays a key role in monsoon onset and rainfall migration, with important impacts on agriculture and disaster preparedness. Observations show that these transitions often occur abruptly within days. We develop a new framework to diagnose zonally asymmetric abrupt seasonal changes (ASC) in tropical circulation, using a pseudoâstreamfunction (Ψpseudo) and a dualâcomponent vector index (ASCI). Ψpseudo recovers the traditional overturning streamfunction when zonally averaged and is decomposed into rotational and divergent parts via Helmholtz decomposition. Strong ASC is found over land regions containing lowâheatâinertia land area and intense deep convection, notably the Maritime Continent and South America. Unlike the zonally symmetric Hadley cell, rotational flow dominates ASC in these regions, with divergent flow contributing in regions with strong convection. These results highlight the essential role of tropical and extratropical eddies in shaping abrupt seasonal transitions in regional tropical circulation.
Observations from the Little Bunker
Last updated: February 24, 2026 10:22 PM
Atmospheric COâ
429.9 ppm
+17.3
ppm/year
Atmospheric CHâ
1946.47 ppb
+6.49
ppb/year
Global Temperature
+1.48 °C
Peak: +1.73°C
World Population
8.1 billion
+67
million/year