Understanding Global Environmental
Trends and Projections
Ants Leetmaa
Geophysical Fluid Dynamics
Laboratory
Princeton, NJ 08542
Climate Scenarios Used for Attribution Studies of
Climate Variability and Change
Talk will focus on Climate of 20th Century (C20C) runs – model ensembles
starting in 1860 driven by radiative forcing changes resulting from
volcanic, solar, anthropogenic effects.
Global Trends Result from Natural and Anthropogenic Effects
Credible assessments and predictions come from synergistic use of
simulations, observations, and numerical diagnostic experiments.
Natural Modes
of Variability
Annular
Modes
Unknown
Others
Tropical
Coupled
oceanatmosphere
interactions
- ENSO
Radiative forcing - natural
and anthropogenic
Changes to mean
circulations (tropics and
related changes)
Changes
to modes
Surprises abrupt change
A firm basis is developing for understanding tropically
forced climate variability. Small changes in tropical
ocean temperatures can have big global impacts
Direct
effects
Changes to
oceanic thermohaline
circulation
Preliminary Results from GFDL C20C Studies
(uncertainties are “least” for this period)
Understanding origins of recent droughts and heat waves –
importance of sea surface temperature trends in tropical Indo-Pacific
Well- simulated global trends - a central role for natural and
anthropogenic radiative forcings changes
Changing tropical convection helps define regional impacts simulations plus diagnostics lead to physical understanding of:
•Sahelian drought - role of changes in radiative forcings
•North American trends - a start towards attribution
•Oceanic temperature, salinity and atmospheric height field trends
support picture of slowly changing tropical rainfall patterns
Global Average Sea Surface Temperature Trends
observed
3 member
model ensemble
Volcanic forcing by Krakatoa in late 1980’s probably overestimated
Equatorial Trends in Temperature and Rainfall
(observed T and model 8 member mean T & P)
Gradual warming
of tropics
Eastward shift
of mean rainfall
Is the eastward rainfall shift real? Agreement with “related” fields suggests
that it is, e.g. a real change in the global circulation has occurred.
Changes in Tropical Convection on Variety of
Timescales Alter Global Circulation Patterns
Multi-year to decadal time scales
implicated in global droughts, heat
waves, other environmental impacts
“small” but persistent trend in
eastward shift of convection
Seasonal to Interannual
Time Scales – ENSO
related impacts
Large interannual east-west
shifts in convection
Explaining Recent Major Climate Anomalies:
Warm and Dry Conditions during 1998-2002
Major Climate Anomalies can be Linked to Regional Sea
Surface Temperature Changes Through Model Experiments
GOGA
Observed anomalies
prescribed for global ocean
EPOGA
Observed anomalies
prescribed for ENSO region
IWPOGA
Observed anomalies prescribed
for Indo-Pacific region
Explaining Origins of Air Temperature
Anomalies - 1998-2004
GOGA
With specified
global SSTAs a
good simulation
Observed
EPOGA
ENSO impacts
provide minor
contribution
IWPOGA
Forcing by SSTA
in Indo-Pacific
region of dominant
importance
Explaining Origin of Rainfall Anomalies - 1998-2002
GOGA
With global SSTAs
model does good
simulation
Observed
EPOGA
ENSO a factor
IWPOGA
Indo-Pacific
SSTA’s critical
Observed Standardized SST Anomaly
June 1998 – May 2002
Indo - Pacific
Eastern Pacific
Recent Years from a Persistent
Warming Trend
Adapted from Hoerling & Kumar (Science, 2002.)
Central Africa is Undergoing Persistent Drought Conditions:
Sahel rainfall: (1980-2000) minus (1960-1940)
Model
mm/month
8 member
ensemble
Simulations and Diagnostic Studies are Leading to an
Understanding of the Origins of Central African Drought
Simulations with observed SSTs
A “prediction” starting in 1860
observed
observed
Spread from 10 member ensemble
One ensemble
member
•Diagnostic studies show the trends to be driven primarily by warming trends in
the Indo-Pacific region (results from many climate models)
•These simulation suggest an anthropogenic impact; previously the observed
trends were thought to be result of natural variability.
•Idealized runs show similar drought development (1% per year increase of CO2)
U.S. Temperature Trends:
Observations vs. GFDL Runs
Historical forcing runs of
CM2.0 and CM2.1 show fair
agreement with the US (48state) mean series (using 18801920 as reference period)
CM2.0 Ensemble Mean
CM2.1 Ensemble Mean
Simulation and “Prediction” of North American
and Surrounding Seas Temperature Trends
Model forced with observed SSTs
Ocean specified - land predicted
Specified radiative forcing from 1860
Ocean and Land predicted
Observed and Modeled Temperature and Salinity
(0-500m) Trends Last 50 Years
Salinity
Temperature
Model
Obs
Global Mean Atmospheric Temperature Trends:
MSU and GFDL GCM
Troposphere
Stratosphere
Linear Trends (1981-1999)
MSU: 0.14 K/decade
GCM: 0.16 +/- 0.02 K/decade
Satellite and Model Estimates of Atmospheric
Temperature Trends are Being Reconciled
Comments
•
•
•
•
The new generation of climate models are more credible – having
been tested and scored/verified against current climate variability
and trends
Some regional trends over the past 100 years can be attributed with
more confidence to changes in natural and anthropogenically
induced radiative forcing changes
The resulting small trends in tropical ocean temperatures have
already lead to shifts in tropical convection and consequently
impacted ocean and atmospheric circulation patterns
Climate projections for the next 30-50 years, representing partly
continuations of existing trends, will be more credible