Changes in surface climate of
the tropical Pacific
Based on…….
Outline and key messages
• Main features of region’s climate system
• We are affecting the climate system - climate is
already changing
• Future will be warmer
• Extreme weather likely to be more extreme
Our climate will be changing for foreseeable future
Main climate features of the region
Pacific atmospheric circulation
• Trade winds
• Convergence zones
• Walker and Hadley circulations
Sea surface temperature
El Niño-Southern Oscillation (ENSO)
• Major source of year-to-year climate variations
Typical El Niño and La Niña rainfall patterns
Wetter (green) or drier (orange)
Typical El Niño and La Niña rainfall
Annual rainfall – Pehnryn
Typical El Niño and La Niña temperature
Annual temperature – Rarotonga
Effects of El Niño and La Niña on SST
Warmer (red) or cooler (blue)
ENSO shifts SPCZ and tropical cyclones
El Niño
• Further north El Niño
• Further south La Niña
• Fewer cyclones further east El Niño
• More cyclones further west La Niña
La Niña
Cyclones in Cook Islands
• Number of cyclones passing
within 400 km of Rarotonga
Projected changes in climate
Redistributing sun’s energy = climate system
• Without the atmosphere the Earth would be ~30oC cooler
• More greenhouse gases trap more energy in climate system
Why are climate scientists so sure climate is
changing due to human activities?
• Theory
• Modelling
• Evidence:
 instrumental measurements
 changes in the physical world
 changes in the biological world
 paleoclimate archives
The climate system appears to be changing faster
than earlier thought likely
Steffen 2009
Measured increase in carbon dioxide
18th century = 280ppm
air bubbles in ice cores
2011= 391ppm
Mauna Loa
Observed warming of global temperatures
Projecting future climates
Emissions
Scenario
IPCC-AR4
(2007)
Low (B1)
450-500 ppm CO2
High (A2)
750-800 ppm CO2
Temperature (oC)
2035
2050
0.5-1.0
?
2100
Rainfall
2035
2050
2100
1.0-1.5 5-15% 10-20% 10-20%
0.5-1.0 1.0-1.5 2.5-3.0
5-20% 10-20% 10-20%
• Predicting future forcing – how much more greenhouse
gases?
• Range of possible futures but the future will be WARMER
Spatial variation in warming
Cook Island’s temperature projections
• Northern Group - increase in temperature of
0.5 – 0.9oC
• Southern Group - increase in temperature of
0.4 –1.0oC
• More hot days and warm nights
• Decline in cooler weather.
Spatial variation in rainfall
Cook Island’s rainfall projections
• Uncertainty around rainfall because model
results are not consistent but generally
projected to increase
• For the Southern Group, average rainfall
during the wet season is expected to
increase due to intensification of SPCZ
• Droughts are projected to become less
frequent throughout this century
Cyclones
• Cook Islands projections indicate
there is likely to be a decrease in
the number of tropical cyclones
by the end of the 21st century
• But there is likely to be an
increase in average maximum
wind speed of cyclones by
between 2% and 11%
Extremes
The answer to the oft-asked question of whether an event is
caused by climate change is that it is the wrong question
All weather events are affected by climate change because the
environment in which they occur is warmer and moister than it
used to be
Trenberth 2012
Important points to note
• Extremes will become more extreme
• Unclear how ENSO will change – continued
influence
Summary
•Not just a ‘new climate’ to which we can adapt…….
For foreseeable future, climate will be CHANGING