Page 11 - APEC CLIMATE CENTER 2025 Annual Report

APEC CLIMATE CENTER 2025 ANNUAL REPORT

State of the State of the Torrential Rain in Southeast and South Asia

Asia-Pacific Asia-Pacific The city of Hat Yai in southern Thailand experienced record-breaking rainfall, with an ac-
cumulated 630 mm over a three-day period and a daily peak of 335 mm—the heaviest
Extreme Climate Extreme Climate rainfall in 300 years. In Sumatra, Indonesia, Cyclone Senyar made landfall and stalled,
triggering floods and landslides that resulted in hundreds of fatalities. Sri Lanka also en-
in 2025 in 2025 countered massive flooding due to Cyclone Ditwah. These events were characterized as
complex disasters, in which tropical cyclones and monsoon rains simultaneously struck
Thailand, Indonesia, Malaysia, and Sri Lanka.

According to the Thai Meteorological Department (TMD), the torrential rain resulted from
a combination of 1) strong low-level convergence and uplift caused by active cyclonic cir-
culation over Thailand and Malaysia (Fig. 6), and 2) strong northeasterly winds from high
pressure over the Asian continent gaining moisture over warm seas and hitting the topog-
raphy of southern Thailand, which intensified orographic lifting (Fig. 7).
Fig 4 Distribution of mean low-level wind field anomalies, February 1–7, 2025
Cyclone Senyar, which showed an unusual formation in the Strait of Malacca and north-
U.S. Heatwave western Sumatra in November, stalled and intensified, maintaining a persistent precipita-

In June 2025, an early-season heatwave originated in the U.S. Midwest and spread to the tion band over southern Thailand and Malaysia. Analysis suggests that the recent La Niña
East Coast. Between June 22 and 25 alone, over 100 million people were affected, and 726 resulted in relatively high sea surface temperatures and enhanced convective activity in
counties exceeded previous record highs. New York City reached 38.9°C, a record for June, the Western Pacific/Maritime Continent (MC) region. Additionally, the Madden-Julian Os-
while temperatures in Boston rose to 37.8°C and Baltimore hit 40.6°C. High humidity ele- cillation (MJO) index stalled near the Western Pacific/MC after mid-November, strengthen-
vated heat indices in major cities like New York and Philadelphia to exceed 43.3°C. ing upper-level divergence and low-level convergence, thereby increasing the likelihood
of tropical cyclone formation in Southeast Asia.
The extreme heat caused thermal expansion and buckling—often referred to as "blow-
ups"—of highway asphalt in parts of the Midwest and East, leading to significant traffic
disruptions. In New Jersey, approximately 150 people were treated for heat exhaustion
during a high school graduation ceremony as heat-related illnesses surged (Fig. 5).

A "heat dome" developed as a strong stationary high-pressure system stagnated over the
region, trapping hot air near the surface. This heat dome migrated from the Midwest to
the East, expanding the scope of the damage. This heatwave event was estimated to be
3 to 5 times more likely due to climate change. Reports indicate that humid heatwaves,
driven by the combination of high temperatures and humidity, are being intensified by
2)
human-induced climate change along the U.S. East Coast .

Fig 6 Distribution of mean Outgoing Longwave Radiation (OLR) anomalies, November 12–18, 2025

2) Faranda, D., and T. ALberti, 2025
High temperatures in the June 2025
Eastern USA heatwave exacerbated
by human-driven climate change.
CLimaMeter. Institut Pierre Simon
LapLace, CNRS.
https://doi.org/10.5281/zenodo. Fig 5 Distribution of mean low-level wind field anomalies, February 1–7, 2025
15746087. Fig 7 Distribution of mean low-level wind field anomalies, November 12–18, 2025

10
10 11 11