The Role of Internally Generated
Megadroughts and External Solar
Forcing in Long Term Pacific
Climate Fluctuations
Gerald A. Meehl
NCAR
______
Medieval Warm Period (pre ~1300AD):
Cool dry tropical eastern Pacific
Warm, wet, high sea level, tropical northwest and southwest Pacific
Dry southwestern North America
Greater solar irradiance
______
Little Ice Age (post ~1300AD) :
Warm wet tropical eastern Pacific, weaker trades
Cool, dry, low sea level, tropical northwest and southwest Pacific
Weak south Asian monsoon
Wetter southwestern North America
Lower solar irradiance
Fig. 20
What’s producing these patterns?
Two candidates (both are probably at work):
1. Internal multi-decadal climate variability
2. Changes in solar forcing
• Southwest U.S.
(32N - 42N, 118W - 106W)
• Indian monsoon region (5N - 40N, 60E 100E)
Define “megadrought”:
• 11-year running mean of regional areaaveraged precipitation anomalies less than
zero for at least 20 consecutive years
southwest U.S.
9 megadroughts
in 1360 years
(average of one
roughly every
150 years)
For period 1210-1249, 32 years out of 40 have
negative precipitation anomalies
Not every year has large negative precipitation
anomalies, but the majority do
Coupled model
Correlation of low
pass filtered (13
yr) area averaged
precip with
similarly filtered
global SST
southwest U.S.
Indian monsoon
Observed, 1901-2000
Correlation of low pass
filtered (13 yr) area
averaged precip with
similarly filtered
global SST
Southwest U.S.
Indian monsoon
Multi-decadal (IPO)
EOF1 of
low pass
filtered (13
yr) SST
The sign of this pattern with positive values in the eastern tropical
Pacific would be consistent with the Little Ice Age;
the opposite, with negative values in the eastern tropical Pacific,
would be consistent with the MCA
Correlation of
multidecadal
EOF1 SST with
Precipitation
The sign of this
pattern (top) with
positive values in
the eastern tropical
Pacific would be
consistent with the
Little Ice Age; the
opposite, with
negative values in
the eastern tropical
Pacific, would be
consistent with the
MCA
Sea level
pressure
______
Megadroughts in the Indian monsoon region and
southwest North America and a mechanism for
associated multi-decadal Pacific sea surface temperature
anomalies. (Meehl, G. A., and A. Hu, 2006, Journal of Climate, 19, 1605–
Fig. 20
1623.)
______
What about solar forcing?
Greater solar input to climate system during
MWP associated with La Nina-like pattern
Less solar input during LIA associated with
El Nino-like pattern
Could solar forcing produce these patterns?
For increased solar input during the first half of the
20th century, model simulations indicate a La Nina-like
response
(Meehl, G.A., W.M.
Washington, T.M.L.
Wigley, J.M. Arblaster,
and A. Dai, 2003: Solar
and greenhouse gas
forcing and climate
response in the 20th
century. J. Climate, 16,
426--444.)
The 11 year solar
cycle shows a
similar pattern of
response
coincident with
the peaks in solar
forcing
(van Loon, H., G. A.
Meehl, and D. J. Shea,
2007: Coupled air-sea
response to solar forcing
in the Pacific region
during northern winter.
Journal of Geophysical
Research, 112, D02108,
doi:10.1029/2006JD007
378.)
Two global
coupled climate
models show a
similar La Ninolike response to
peaks in the 11
year solar
forcing
(Meehl, G.A., J.M.
Arblaster, G. Branstator,
and H. Van Loon, 2007:
A coupled air-sea
response mechanism to
solar forcing in the
Pacific region. J.
Climate, in press.)
Though global solar forcing from solar max to min is on
the order of 0.2 Wm-2, regionally the solar forcing can
be an order of magnitude larger in the subtropics
The mechanism involves increased solar over cloud-free regions of the
subtropics translating to greater evaporation, and moisture convergence and
precipitation in the ITCZ and SPCZ, stronger trades, and cooler SSTs in
eastern equatorial Pacific
(Meehl, G.A., J.M.
Arblaster, G. Branstator,
and H. Van Loon, 2007:
A coupled air-sea
response mechanism to
solar forcing in the
Pacific region. J.
Climate, in press.)
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Conclusions
A La Nina-like SST pattern in the Pacific associated
with MWP, El Nino-like with LIA
This is a major mode of inherent multi-decadal climate
variability in climate models, and such climate
anomalies are connected with a mechanism that
produces inherent low frequency Pacific SST
variability through coupled tropical-midlatitude oceanatmosphere interactions
Models and observations suggest that the response
to increased solar forcing has a La Nina-like pattern
Past large-scale multi-decadal fluctuations of SST and
precipitation across the Pacific region are likely
connected to both inherent multi-decadal and solarforced variability with similar patterns
When both the inherent multi-decadal variability and
the response to solar forcing are both acting in the
same direction: an additive response and a rapid
(order 100 years) apparent shift
Observed decadal pattern (1871-2000), “IPO”, >13 yr low pass
Observed interannual pattern (1871-2000), 2-7 yr band pass
IPO correlated with observed low pass precip, 1901-2000