Understanding of the Atmospheric Dynamics of Extreme Heat Events in Tropical Cities of Indonesia: An Analysis from Observation Networks in Medan, Jakarta, and Surabaya

The rapid urbanization occurring in the Maritime Continent—an expansive geographical domain encompassing Southeast Asia and Oceania—has intensified the exposure of population centers to extreme heat. However, the seasonal drivers and physiological consequences of this thermal stress exhibit spatial heterogeneity. This study investigates the seasonal variation of 2-m air temperature and human biometeorological indices across three major Indonesian cities—Medan, Jakarta, and Surabaya—representing diverse geographical conditions, monsoon onsets, and other climatological regimes.

Meteorological surface observations reveal a distinct bimodal temperature distribution in all three cities, with peak heat stress intensities typically occurring during the monsoonal transition periods in April and October. However, the magnitude and primary peak month vary substantially due to regional climatological influences. In Medan (Northern Sumatra), April is identified as the hottest month, where high ambient temperatures are exacerbated by high specific humidity. In contrast, Surabaya (East Java) experiences its maximum thermal intensity in October, coinciding with the late dry season's clear-sky conditions. Meanwhile, Jakarta (Western Java) exhibits nearly symmetrical peaks during both transitional periods, reflecting its intermediate position relative to the shifting monsoonal air masses.

To quantify the localized impact, canyon layer Urban Heat Island (UHI) intensities are estimated by comparing urban-rural station pairs. Furthermore, human thermal comfort is evaluated using two robust biometeorological indices: the Universal Thermal Climate Index (UTCI) and the Physiologically Equivalent Temperature (PET). By employing both indices, the study estimates day and night-time heat stress levels and the resulting physiological strain on the urban population. Preliminary results indicate that high humidity levels in cities like Medan and Jakarta substantially sustain elevated UTCI and PET values into the night, effectively hindering nocturnal cooling. These findings underscore the importance of incorporating accurate meteorological factors for localized heat-health warning systems and urban planning to effectively mitigate the escalating risks of heat-related disasters in tropical environments.