The Role of Internally Generated Megadroughts and External Solar Forcing in Long Term Pacific...
-
date post
20-Jan-2016 -
Category
Documents
-
view
215 -
download
0
Transcript of The Role of Internally Generated Megadroughts and External Solar Forcing in Long Term Pacific...
![Page 1: The Role of Internally Generated Megadroughts and External Solar Forcing in Long Term Pacific Climate Fluctuations Gerald A. Meehl NCAR.](https://reader035.fdocuments.us/reader035/viewer/2022062806/56649d3e5503460f94a16b79/html5/thumbnails/1.jpg)
The Role of Internally Generated Megadroughts and External Solar
Forcing in Long Term Pacific Climate Fluctuations
Gerald A. Meehl
NCAR
![Page 2: The Role of Internally Generated Megadroughts and External Solar Forcing in Long Term Pacific Climate Fluctuations Gerald A. Meehl NCAR.](https://reader035.fdocuments.us/reader035/viewer/2022062806/56649d3e5503460f94a16b79/html5/thumbnails/2.jpg)
Fig. 20
______
Medieval Warm Period (pre ~1300AD):Cool dry tropical eastern PacificWarm, wet, high sea level, tropical northwest and southwest PacificDry southwestern North AmericaGreater solar irradiance
![Page 3: The Role of Internally Generated Megadroughts and External Solar Forcing in Long Term Pacific Climate Fluctuations Gerald A. Meehl NCAR.](https://reader035.fdocuments.us/reader035/viewer/2022062806/56649d3e5503460f94a16b79/html5/thumbnails/3.jpg)
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
![Page 4: The Role of Internally Generated Megadroughts and External Solar Forcing in Long Term Pacific Climate Fluctuations Gerald A. Meehl NCAR.](https://reader035.fdocuments.us/reader035/viewer/2022062806/56649d3e5503460f94a16b79/html5/thumbnails/4.jpg)
What’s producing these patterns?
Two candidates (both are probably at work):
1. Internal multi-decadal climate variability
2.Changes in solar forcing
![Page 5: The Role of Internally Generated Megadroughts and External Solar Forcing in Long Term Pacific Climate Fluctuations Gerald A. Meehl NCAR.](https://reader035.fdocuments.us/reader035/viewer/2022062806/56649d3e5503460f94a16b79/html5/thumbnails/5.jpg)
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
![Page 6: The Role of Internally Generated Megadroughts and External Solar Forcing in Long Term Pacific Climate Fluctuations Gerald A. Meehl NCAR.](https://reader035.fdocuments.us/reader035/viewer/2022062806/56649d3e5503460f94a16b79/html5/thumbnails/6.jpg)
9 megadroughts in 1360 years (average of one roughly every 150 years)
southwest U.S.
![Page 7: The Role of Internally Generated Megadroughts and External Solar Forcing in Long Term Pacific Climate Fluctuations Gerald A. Meehl NCAR.](https://reader035.fdocuments.us/reader035/viewer/2022062806/56649d3e5503460f94a16b79/html5/thumbnails/7.jpg)
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
![Page 8: The Role of Internally Generated Megadroughts and External Solar Forcing in Long Term Pacific Climate Fluctuations Gerald A. Meehl NCAR.](https://reader035.fdocuments.us/reader035/viewer/2022062806/56649d3e5503460f94a16b79/html5/thumbnails/8.jpg)
Coupled model Correlation of low pass filtered (13 yr) area averaged precip with similarly filtered global SST
southwest U.S.
Indian monsoon
![Page 9: The Role of Internally Generated Megadroughts and External Solar Forcing in Long Term Pacific Climate Fluctuations Gerald A. Meehl NCAR.](https://reader035.fdocuments.us/reader035/viewer/2022062806/56649d3e5503460f94a16b79/html5/thumbnails/9.jpg)
Observed, 1901-2000Correlation of low pass filtered (13 yr) area averaged precip with similarly filtered global SST
Southwest U.S.
Indian monsoon
![Page 10: The Role of Internally Generated Megadroughts and External Solar Forcing in Long Term Pacific Climate Fluctuations Gerald A. Meehl NCAR.](https://reader035.fdocuments.us/reader035/viewer/2022062806/56649d3e5503460f94a16b79/html5/thumbnails/10.jpg)
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
![Page 11: The Role of Internally Generated Megadroughts and External Solar Forcing in Long Term Pacific Climate Fluctuations Gerald A. Meehl NCAR.](https://reader035.fdocuments.us/reader035/viewer/2022062806/56649d3e5503460f94a16b79/html5/thumbnails/11.jpg)
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
![Page 12: The Role of Internally Generated Megadroughts and External Solar Forcing in Long Term Pacific Climate Fluctuations Gerald A. Meehl NCAR.](https://reader035.fdocuments.us/reader035/viewer/2022062806/56649d3e5503460f94a16b79/html5/thumbnails/12.jpg)
Fig. 20
______
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.)
![Page 13: The Role of Internally Generated Megadroughts and External Solar Forcing in Long Term Pacific Climate Fluctuations Gerald A. Meehl NCAR.](https://reader035.fdocuments.us/reader035/viewer/2022062806/56649d3e5503460f94a16b79/html5/thumbnails/13.jpg)
Fig. 20
______
![Page 14: The Role of Internally Generated Megadroughts and External Solar Forcing in Long Term Pacific Climate Fluctuations Gerald A. Meehl NCAR.](https://reader035.fdocuments.us/reader035/viewer/2022062806/56649d3e5503460f94a16b79/html5/thumbnails/14.jpg)
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?
![Page 15: The Role of Internally Generated Megadroughts and External Solar Forcing in Long Term Pacific Climate Fluctuations Gerald A. Meehl NCAR.](https://reader035.fdocuments.us/reader035/viewer/2022062806/56649d3e5503460f94a16b79/html5/thumbnails/15.jpg)
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.)
![Page 16: The Role of Internally Generated Megadroughts and External Solar Forcing in Long Term Pacific Climate Fluctuations Gerald A. Meehl NCAR.](https://reader035.fdocuments.us/reader035/viewer/2022062806/56649d3e5503460f94a16b79/html5/thumbnails/16.jpg)
(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
![Page 17: The Role of Internally Generated Megadroughts and External Solar Forcing in Long Term Pacific Climate Fluctuations Gerald A. Meehl NCAR.](https://reader035.fdocuments.us/reader035/viewer/2022062806/56649d3e5503460f94a16b79/html5/thumbnails/17.jpg)
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.)
![Page 18: The Role of Internally Generated Megadroughts and External Solar Forcing in Long Term Pacific Climate Fluctuations Gerald A. Meehl NCAR.](https://reader035.fdocuments.us/reader035/viewer/2022062806/56649d3e5503460f94a16b79/html5/thumbnails/18.jpg)
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
![Page 19: The Role of Internally Generated Megadroughts and External Solar Forcing in Long Term Pacific Climate Fluctuations Gerald A. Meehl NCAR.](https://reader035.fdocuments.us/reader035/viewer/2022062806/56649d3e5503460f94a16b79/html5/thumbnails/19.jpg)
(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
![Page 20: The Role of Internally Generated Megadroughts and External Solar Forcing in Long Term Pacific Climate Fluctuations Gerald A. Meehl NCAR.](https://reader035.fdocuments.us/reader035/viewer/2022062806/56649d3e5503460f94a16b79/html5/thumbnails/20.jpg)
• 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
![Page 21: The Role of Internally Generated Megadroughts and External Solar Forcing in Long Term Pacific Climate Fluctuations Gerald A. Meehl NCAR.](https://reader035.fdocuments.us/reader035/viewer/2022062806/56649d3e5503460f94a16b79/html5/thumbnails/21.jpg)
Observed decadal pattern (1871-2000), “IPO”, >13 yr low pass
Observed interannual pattern (1871-2000), 2-7 yr band pass
![Page 22: The Role of Internally Generated Megadroughts and External Solar Forcing in Long Term Pacific Climate Fluctuations Gerald A. Meehl NCAR.](https://reader035.fdocuments.us/reader035/viewer/2022062806/56649d3e5503460f94a16b79/html5/thumbnails/22.jpg)
IPO correlated with observed low pass precip, 1901-2000