Amplification of the North American Dust Bowl drought

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CRU 2 1 Temp Anomaly oC SST Only SST DUST CROP,72oN 72oN 72oN 1 2. 60oN 60oN 60oN 0 6,48oN 48oN 48oN,36oN 36oN 36oN 0 3. 24oN 24oN 24oN,12oN 12oN 12oN 1 2, 140oW 120oW 100oW 80oW 60oW 40oW 140oW 120oW 100oW 80oW 60oW 40oW 140oW 120oW 100oW 80oW 60oW 40oW. CRU 2 1 Precip Rate Anomaly mm day SST Only SST DUST CROP 0 9. 72oN 72oN 72oN,60oN 60oN 60oN 0 5,48oN 48oN 48oN 0 1. 24oN 24oN 24oN 0 5,12oN 12oN o 12oN o, 140oW 120oW 100oW 80oW 60oW 40oW 140 W 120oW 100oW 80oW 60oW 40oW 140 W 120oW 100oW 80oW 60oW 40oW 0 9.
Fig 1 Temperature oC and precipitation mm day 1 anomalies for the Dust Bowl drought from the Climate Research Unit CRU version 2 1 dataset 11 and. 2 of our model experiments SST ONLY our control and SST DUST CROP full land degradation in the form of a Great Plains dust aerosol source and crop. removal The CRU data are composed of monthly climate grids for the world statistically interpolated from station observations to continuous 0 5o spatial. resolution Model grid spacing is 2o 2 5o Anomalies are for the period 1932 1939 relative to the 1920 1929 observed average for CRU data or an ensemble. average from a 5 member ensemble run using observed SSTs for 1920 1929 for the model plots The block rectangle in the CRU plot surrounds the Great Plains. region 105 E 95 E and 30 N 50 N used to calculate spatially averaged anomalies for Fig 3. currently available In SST CROP we simulated vegetation over the Great Plains region 105oE 95oE and 30oN 50oN high. losses associated with the crop failure by converting the crop lighted by the black rectangle in Fig 1 Vegetation feedbacks. areas over the Great Plains to bare soil Fig 2 Right leading to SST CROP shift the temperature distribution toward warmer. fractional vegetation reductions of almost 50 in some grid values and this is the only ensemble whose members reach the. cells Finally in our SST DUST CROP experiment the model extreme values seen in the CRU data The warm anomaly is. was forced with observed SSTs along with a full representation reduced when dust aerosols are added but a region of strong. of crop failure via inclusion of both a dust source over the Plains warming from crop devegetation remains in the northern Plains. and vegetation reductions The addition of a dust source SST DUST and. All model based anomalies are calculated relative to an SST DUST CROP however has a large impact on precipita. ensemble forced by using observed SSTs from 1920 to 1929 to tion drying out much of the Great Plains and shifting the precip. enable comparison with the observed anomalies from the CRU itation anomaly distribution over the Great Plains to more negative. 2 1 dataset 11 The model ensemble forced with SSTs alone values. produces only a modest drying and warming over the Great The feedbacks vegetation and dust and their impacts can be. Plains region Fig 1 Center When both land surface forcings best understood by isolating each factor in separate ensemble. are included SST DUST CROP the temperature and pre experiments Fig 4 Area averaged top of the atmosphere. cipitation anomalies are amplified to the observed level and the TOA radiation balance and surface fluxes for each of our. drought is now correctly centered over the central and northern simulations are shown in Table 1 Different mechanisms explain. Great Plains Fig 1 Right Fig 3 displays the annual tempera the temperature and precipitation anomalies Removal of veg. ture and precipitation anomalies for each ensemble averaged etation SST CROP reduces total evapotranspiration from the. Dust Emission g m 2 yr Dust Aerosol Loading g m 2 Devegetated Fraction from Crop Removal. 72oN 1500 72oN 72oN 50,60oN o 1 0 o,500 0 5 20,48 N 48oN 48oN 10. 36 N 36oN 0 5 36oN 10,1000 24 N 30,12 N 1500 o o,140oW 120oW 100oW 80oW 60oW 40oW 12 N. 140oW 120oW 100oW 80oW 60oW 40oW, Fig 2 Dust emission Top g m 2 y 1 and dust aerosol loading Middle g m 2 from the SST DUST experiment and devegetated fraction Bottom from. Downloaded by guest on June 28 2020, the SST CROP experiment relative to SST ONLY for 1932 1939 The devegetated fraction is identical in the SST DUST CROP experiment and the dust emissions. and loading are similar discussed further in the SI. 4998 www pnas org cgi doi 10 1073 pnas 0810200106 Cook et al. Temperature Anomaly oC CRU 2 1 and Model,Temperature Anomaly oC.
CRU 2 1 20 29 CRU 2 1 32 39 SST Only 20 29 SST Only 32 39 SST CROP SST DUST SST DUST CROP. Precipitation Anomaly mm day 1 CRU 2 1 and Model,Precipitation Anomaly mm day 1. CRU 2 1 20 29 CRU 2 1 32 39 SST Only 20 29 SST Only 32 39 SST CROP SST DUST SST DUST CROP. Fig 3 Box and whisker plots for precipitation and temperature anomalies from the CRU 2 1 data and each ensemble member SST Only SST CROP. SST DUST SST DUST CROP Anomalies in mm day are averaged over the Great Plains region 105 E 95 E and 30 N 50 N the same area as in Fig 1. Anomalies are for the period 1932 1939 relative to the 1920 1929 observed average for CRU data or an ensemble average from a 5 member ensemble run. using observed SSTs for 1920 1929 for the model plots The boxes indicate the lower quartile median and upper quartile values and the whiskers mark the. most extreme values within 1 5 times the interquartile range Outliers values falling beyond the whiskers are marked with the red crosses. ENVIRONMENTAL, land surface by severely limiting transpiration the flux of water simultaneously warming the surface and reducing precipitation. from the soil to the atmosphere through plants during photo The reduction in transpiration by the removal of crops in the. synthesis Over the main region of impact at the surface 100oW combined experiment causes warming that is only slightly re. 90oW 35oN 50oN decreased summertime evapotranspiration duced compared with the effect of crops alone because the. mostly compensated by increased sensible heating raises the reduction in net TOA radiation by dust causes only a slight offset. Bowen ratio from 0 52 SST ONLY to 0 59 SST CROP This to surface temperature Similarly precipitation is reduced in the. leads to increased soil and near surface air temperatures Warm combined experiment as in the dust only case because the TOA. ing during the summer is carried over into the fall and winter radiative anomaly is dominated by the effect of dust Our. seasons by positive soil temperature anomalies when these improved simulation of temperature and precipitation anoma. warmer soils release this heat to the atmosphere SI lies when more realistic land surface boundary conditions are. The addition of a dust source and subsequent increase in dust included suggests that land surface feedbacks from the human. aerosol loading SST DUST reduces net radiation at TOA induced land degradation are a necessary ingredient to explain. the atypical nature of the Dust Bowl drought, and at the surface largely by shortwave reflection Table 1 and. As with many simulations of historical climate we are limited. SI To balance a compensatory increase in atmospheric energy. by observational uncertainties during the period Climate. input is required manifesting as upper level convergence and. records over North America are fairly reliable 11 but few. low level divergence reflected in geopotential height anomalies quantitative estimates of dust emission aerosol loading or even. at 850 hPa 500 hPa and 300 hPa SI This indicates anomalous magnitude of crop failure are available Spatial extent of the. subsidence inhibiting convection and cloud development and Great Plains dust source area and crop removal are based on a. reducing moisture convergence into the region SI leading to map of soil erosion from ref 10 These issues are discussed. a subsequent reduction in precipitation and a shift of the drought further in the SI The forcing from the dust aerosols tends to. center northward over the central Great Plains near the center ameliorate the warming associated with crop failure as seen in. of the dust aerosol cloud Note that the precipitation anomaly is experiments SST DUST CROP Shortwave reflection may be. slightly positive in the SST CROP experiment consistent with overestimated in our model and cooling associated with the dust. the net radiative anomaly at TOA aerosols may be too high Still the precipitation pattern is well. When the effects of dust and crop removal are combined resolved in both SST DUST and SST DUST CROP and the. Downloaded by guest on June 28 2020, SST DUST CROP feedbacks from the separate experi temperature anomaly suggests that the crop failure contributed. ments act in concert to augment the impact of SST forcing to the anomalous warmth during the period. Cook et al PNAS March 31 2009 vol 106 no 13 4999,SST CROP SST DUST 1 2.
60oN 60oN 0 6,36oN 36oN 0 3,12oN 12oN 1 2, 140oW 120oW 100oW 80oW 60oW 40oW 140oW 120oW 100oW 80oW 60oW 40oW. SST CROP SST DUST 0 9,60oN 60oN 0 5,36oN 36oN 0 1,12oN o 12oN o 0 7. 140 W 120oW 100oW 80oW 60oW 40oW 140 W 120oW 100oW 80oW 60oW 40oW. Fig 4 Spatial pattern for temperature C and precipitation mm day 1 anomalies for the Dust Bowl drought from our individual surface feedback. experiments SST CROP crop removal and SST DUST Great Plains dust aerosol source Anomalies are relative to an ensemble average from a 5 member. ensemble run using observed SSTs for 1920 1929, The results from this study suggest a mechanism that could Model Description. explain some of the anomalous drought patterns during the last All modeling experiments were conducted with the Goddard. thousand years as seen in proxy reconstructions from tree ring Institute for Space Studies GISS ModelE at 2o latitude by 2 5o. records 20 21 The Dust Bowl drought was likely unique during longitude horizontal resolution and with 40 vertical layers 25. the instrumental era but similar drought patterns can be found ModelE is a state of the art atmospheric general circulation. during the Medieval Climate Anomaly MCA 4 Typical model incorporating significant updates to the physics com. North American droughts during the MCA were longer lasting pared with previous versions and capable of calculating the. on the order of decades and more intense 21 and were evolution of several aerosol and chemical tracers as a function of. accompanied by large scale dune mobilization over parts of the the model climate 25 26 Simulations of modern day climate. Great Plains 22 This movement of dunes implies a near in ModelE compare favorably with observations with some. complete loss of vegetation cover in this case induced naturally notable biases particularly in the subtropical marine stratocu. by an intense and persistent drought and the possibility of a mulus regions ModelE is unusually successful at simulating the. productive dust source and subsequent aerosol and vegetation observed annual cycle of precipitation over the Great Plains and. feedbacks Additionally we note there are several areas in the Mexico along with interannual variations in precipitation during. world today where human land degradation manifesting as loss the second half of the 20th century 27 We use a version of. of vegetation cover and increased vulnerability to wind erosion ModelE coupled to a model of soil dust aerosols 28 Given. and drought potentially worsened by the subtropical drying that natural dust sources i e excluding sources created by anthro. is projected to occur as a consequence of global warming 23 24 pogenic land degradation 29 and forced by present day Mod. have the potential to interact leading to future Dust Bowl droughts elE climate the dust model reproduces the seasonal atmospheric. in some developing regions 14 Both issues will require an dust cycle as well as the magnitude and pattern of atmospheric. integrated modeling approach similar to the current study dust loading 28 30 Dust within the model interacts with. Table 1 Area averaged surface fluxes and TOA radiation balance for each ensemble simulation and difference from SST ONLY for. June August period,Bowen Net radiation,Simulation Mean SH mean LH SH mean ratio TOA. SST ONLY 107 34 55 47 162 81 0 517 72 84,SST CROP 101 72 59 62 161 34 0 586 73 17.
SST DUST 98 79 50 04 148 83 0 507 67 72,SST DUST CROP 94 40 51 62 146 03 0 547 67 77. Differences relative to SST ONLY,SST CROP 5 63 4 15 1 48 0 069 0 334. SST DUST 8 56 5 42 13 98 0 010 5 122,Downloaded by guest on June 28 2020. SST DUST CROP 12 94 3 85 16 79 0 030 5 066, 5000 www pnas org cgi doi 10 1073 pnas 0810200106 Cook et al. radiation in ModelE absorbing emitting and reflecting long Program administered by the University Corporation for Atmospheric Re. wave and shortwave but does not impact cloud microphysics search B I C along with the Climate Dynamics Program of the National. Science Foundation NSF through Grant ATM 06 20066 R S was supported. by NOAA Grants NA03OAR4320179 and NA06OAR4310151 and NSF Grant. ACKNOWLEDGMENTS We thank two anonymous reviewers for their com ATM 05 01878 R L M was additionally supported by the National Aeronau. ments This work was supported by the National Oceanic and Atmospheric tics and Space Administration Atmospheric Composition Program This is. Administration NOAA Climate and Global Change Postdoctoral Fellowship Lamont contribution 7242. 1 Schubert SD Suarez MJ Pegion PJ Koster RD Bacmeister JT 2004 On the cause of the 18 Zeng N Neelin JD Lau M L Tucker CJ 1999 Enhancement of interdecadal climate. 1930s Dust Bowl Science 303 1855 1859 variability in the Sahel by vegetation interaction Science 286 1537 1540. 2 Schubert SD Suarez MJ Pegion PJ Koster RD Bacmeister JT 2004 Causes of long term 19 Koven CD 2006 On the sources composition and climatic effects of mineral dust in. Amplification of the North American Dust Bowl drought through human induced land degradation Benjamin I Cooka b 1 Ron L Millerb and Richard Seagera aLamont Doherty Earth Observatory 61 Route 9W Palisades NY 10964 and bNASA Goddard Institute for Space Studies 2880 Broadway New York NY 10024 Edited by James E Hansen Goddard Institute for Space Studies New York NY

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