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Chapter 1 Scientific Basis of Global Climate Change

ence global weather patterns through ocean-atmosphere coupling.
During El Niño events, warm water in the equatorial Pacific shifts eastward, leading to
increased precipitation in the tropical Pacific region, while drought conditions may occur in
Australia, Indonesia, and Southeast Asia. This shift in precipitation distribution affects global
atmospheric circulation patterns, thereby altering storm tracks and the frequency of extreme
weather events. For example, El Niño events are typically associated with heavy rainfall in
the southwestern United States and the western coast of South America, while being linked
to droughts in Australia and Southeast Asia.
La Niñaevents exhibit opposite characteristics to El Niño, with cooler sea surface
temperatures in the equatorial Pacific causing altered precipitation patterns. During La
Niñaevents, Australia and Southeast Asia may experience heavy rainfall, while the western
coast of South America and the southwestern United States may face drought conditions.
(II) Pacific Decadal Oscillation (PDO)
The Pacific Decadal Oscillation (PDO) is a long-term fluctuation phenomenon in sea
surface temperatures across the Pacific region, typically spanning 20-30 years. The posi-
tive phase of PDO manifests as elevated sea temperatures in the northern Pacific, while the
negative phase shows reduced sea temperatures. By influencing oceanic and atmospheric
circulation patterns in the Pacific region, PDO significantly impacts both regional and global
climate systems.
During the positive phase of PDO, increased sea temperatures in the northern Pacific
may lead to heightened precipitation along the west coast of North America, while drought
conditions could emerge in Southeast Asia. Conversely, during the negative phase, reduced
sea temperatures in the northern Pacific might result in decreased precipitation along North
America’s west coast, with Southeast Asia experiencing heavy rainfall events.
(III) North Atlantic Oscillation (NAO)
The North Atlantic Oscillation (NAO) is a major atmospheric circulation variability
pattern in the North Atlantic region, characterized by shifts in the pressure gradient between
the Icelandic Low and the Azores High. The positive phase of NAO typically correlates with
an intensified Icelandic Low and a weakened Azores High, driving alterations in wind speeds
and precipitation patterns across the North Atlantic region.
During the NAO positive phase, enhanced wind speeds in the North Atlantic region
may lead to increased precipitation in northwestern Europe, while drought conditions may
occur in the Mediterranean region. Conversely, during the NAO negative phase, weakened
wind speeds in the North Atlantic region may result in reduced precipitation in northwestern
Europe, while heavy rainfall may occur in the Mediterranean region.
(4) Impact of Internal Variability on Climate Predictions
Although internal atmospheric and oceanic variability are not fundamental drivers of
long-term climate change, they are crucial for seasonal and annual climate predictions. Inter-
nal variability such as ENSO, PDO, and NAO significantly alters short-to-medium-term cli-

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