the role of internally generated megadroughts and external solar forcing in long term pacific...

The Role of Internally Generated Megadroughts and External Solar

Forcing in Long Term Pacific Climate Fluctuations

Gerald A. Meehl

NCAR

Fig. 20

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Medieval Warm Period (pre ~1300AD):Cool dry tropical eastern PacificWarm, wet, high sea level, tropical northwest and southwest PacificDry southwestern North AmericaGreater solar irradiance

Fig. 20

______Little Ice Age (post ~1300AD) :Warm wet tropical eastern Pacific, weaker tradesCool, dry, low sea level, tropical northwest and southwest PacificWeak south Asian monsoonWetter southwestern North America Lower solar irradiance

What’s producing these patterns?

Two candidates (both are probably at work):

1. Internal multi-decadal climate variability

2.Changes in solar forcing

Define “megadrought”:

• Southwest U.S. (32N - 42N, 118W - 106W)• Indian monsoon region (5N - 40N, 60E -

100E)

• 11-year running mean of regional area-

averaged precipitation anomalies less than zero for at least 20 consecutive years

9 megadroughts in 1360 years (average of one roughly every 150 years)

southwest U.S.

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, 1901-2000Correlation of low pass filtered (13 yr) area averaged precip with similarly filtered global SST

Southwest U.S.

Indian monsoon

EOF1 of low pass filtered (13 yr) SST

Multi-decadal (IPO)

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

Fig. 20

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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, 1605–1623.)

Fig. 20

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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.)

(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:10.1029/2006JD007378.)

The 11 year solar cycle shows a similar pattern of response coincident with the peaks in solar forcing

Two global coupled climate models show a similar La Nino-like 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

(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.)

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

• 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 ocean-atmosphere 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 solar-forced 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

Conclusions

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