Remote vegetation feedbacks and the mid-holocene Green Sahara

Transcription

1 Remote vegetation feedbacks and the mid-holocene Green Sahara Abigail L.S. Swann University of Washington work with: Inez Fung, John Chiang, & Yuwei Liu

2 ~6 years ago, the Sahara more like the Sahel Mike Hettwer 2 from 1btravelling

3 Sahara more like the Sahel 6Kya Biome6 6Kya Biome Map temperate deciduous broadleaf forest warm-temperate evergreen broadleaf and mixed forest xerophytic woods/scrub steppe desert 3 Biome6 dataset, Prentice and Webb 1998

4 Precip. over Africa higher in mid-holocene Why? Biome6 6Kya Biome Map orbital forcing: e.g. Kutzbach and Street- Perrott, 198 local vegetation albedo: Charney et al., 197 temperate deciduous broadleaf forest warm-temperate evergreen broadleaf and mixed forest xerophytic woods/scrub steppe desert previous modeling studies: Joussaume et al. (199); Kutzbach et al. (1996); Claussen and Gayler (1997); Brovkin et al. (1998); Claussen et al. (1999); Zeng et al. (1999); Braconnot et al. (2); Bonfils et al. (21); Levis et al. (24); etc. NDVI July, 29 Previous modeling work has been unable to simulate enough precipitation over the Sahara with orbital forcing and local vegetation feedbacks 4

5 Mid-Holocene Green Sahara? Traditional view: Regional vegetation supports regional precipitation, orbital forcing shifts circulation New Hypothesis: Mid latitude forests remotely forced circulation to increase precipitation over Africa.

6 Model setup CAM 3. - CLM 3. - CASA Atmospheric CO2 is fixed at 28ppm run with slab ocean (fixed ocean heat flux) T42 resolution orbital forcing at either -6 yrs or 19 Also tried all runs with CAM 3 - CLM 3. - CASA with the same qualitative results 6

7 CASA CO2 Live Pools (3) Interactive Leaf Leaf area CO2 Photosynthesis Dead Pools (9) Wood Root 7

8 Adding grass and trees in CLM Grasslands added All bare ground in Sahara and Saudi Arabia turned to grass Forests added All C3 grass and agriculture in Eurasia turned to deciduous broadleaf trees %

9 Temperature Change Effect of Orbital forcing [Mid-Holocene control] - [Present Day control] Near Surface AirTemperature (K) TREFHT, casainitmh casainintpi annual mean, masked for sig K TREFHT, SEVMH SVMH annual mean, masked for sig K

10 Temperature Change Effect of Orbital forcing [Mid-Holocene control] - [Present Day control] Effect of all forcing (orbital + grass + trees) [orbital + sahara grass + eurasian trees] - [Present Day Control] Near Surface AirTemperature (K) TREFHT, casainitmh casainintpi annual mean, masked for sig TREFHT, SEVMH casainintpi annual mean, masked for sig TREFHT, SEVMH SVMH annual mean, masked for sig K 2 4 K. 1 K

11 Temperature Change Effect of Orbital forcing [Mid-Holocene control] - [Present Day control] Effect of all forcing (orbital + grass + trees) [orbital + sahara grass + eurasian trees] - [Present Day Control] Additive effect of Eurasian Trees [orbital + sahara grass + eurasian trees] - [orbital + sahara grass] Near Surface AirTemperature (K) TREFHT, casainitmh casainintpi annual mean, masked for sig TREFHT, SEVMH casainintpi annual mean, masked for sig K TREFHT, TREFHT, SEVMH SVMH SEVMH SVMH annual annual mean, mean, masked masked for for sig. sig. 9 K K

12 Precipitation Change Effect of Orbital forcing [Mid-Holocene control] - [Present Day control] Effect of all forcing (orbital + grass + trees) [orbital + sahara grass + eurasian trees] - [Present Day Control] Additive effect of Eurasian Trees [orbital + sahara grass + eurasian trees] - [orbital + sahara grass] Precipitation (mm/day) PRECIP, casainitmh casainintpi annual mean, masked for sig PRECIP, SEVMH casainintpi annual mean, masked for sig. mm/day PRECIP, SEVMH SVMH annual mean, masked for sig. PRECIP, SEVMH SVMH annual mean, masked for sig mm/day mm/day. mm/day

13 Trees cause more rain over Africa Precipitation PRECIP casainitmh casainitpi (mm/day) Effect of Orbital forcing [Mid-Holocene control] - [Present Day control] Effect of all forcing (orbital + grass + trees) [orbital + sahara grass + eurasian trees] - [Present Day Control] PRECIP casainitmh casainitpi Additive effect of Eurasian Trees [orbital + sahara grass + eurasian trees] - [orbital + sahara grass] PRECIP SEVMH SVMH

14 - Hadley Circulation Streamfunction Pressure (hpa) Mean meridional streamfunction Annual mean 8 O N 6 O N 4 O N 2 O N O 2 O S 4 O S 6 O S 8 O S kg/sec NORTH SOUTH ITCZ 14 ERA-4 Atlas

15 Winter Cell Does the Transport NH Summer 1 2 Mean meridional streamfunction Summer -1 - June-August Winter Pressure (hpa) Pressure (hpa) Mean meridional streamfunction Annual mean 8 O N 6 O N 4 O N 2 O N O 2 O S 4 O S 6 O S 8 O S NORTH SOUTH kg/sec NORTH Mean meridional streamfunction O N 6 O N 4 O N 2 O N O 2 O S 4 O S 6 O S 8 O S Winter December-February -1 SOUTH Summer Pressure (hpa) NH Winter ERA-4 Atlas 1 NORTH 3 8 O N 6 O N 4 O N 2 O N O 2 O S 4 O S 6 O S 8 O S SOUTH

16 Hypothesized Mechanism 1. dark trees = NH warm grass eq HOT trees eq

17 Hypothesized Mechanism 1. dark trees = NH warm 2. Energy gradient (N-S) grass between the Hemispheres increases eq HOT trees eq

18 Hypothesized Mechanism 1. dark trees = NH warm 2. Energy gradient (N-S) grass eq between the Hemispheres increases 3. Hadley Cell moves nice review on this: Chiang et al. 212 north to increase southward heat transport trees eq

19 Circulation shift driven by trees moves energy Southward 3% change at the Equator (pval =.8) 19 Swann et al., 211

20 Hypothesized Mechanism 1. dark trees = NH warm 2. Energy gradient (N-S) grass between the Hemispheres increases eq Hadley Cell moves north to increase southward heat transport trees 4. ITCZ shifts North eq

21 Hypothesized Mechanism 1. dark trees = NH warm 2. Energy gradient (N-S) grass between the Hemispheres increases eq Hadley Cell moves north to increase southward heat transport trees 4. ITCZ shifts North Tropical productivity eq follows changes in 21 precipitation

22 Productivity changes locally and remotely Effect of Orbital forcing FNPP, casainitmh casainintpi annual mean, masked for sig., modified FNPP, veg outlined EVMH casainintmh annual mean, masked for sig., modified veg outlined SVMH - CMH CMH - CPI Net Primary Production (gc/m 2 /yr) SEVMH - SVMH EVMH - CMH FNPP, SVMH casainintmh annual mean, masked for sig., modified veg FNPP, outlined SEVMH SVMH annual mean, masked for sig., modified veg outlined g/C/m /yr 7g/C/m 2 /yr FNPP, SVMH casainintmh annual mean, masked for sig., modified veg outlined g/c/m 2 1 /yr g/c/m 2 /yr 3 Additive effect of Effect of Orbital forcing + grass + trees g/c/m 2 /yr gc/m 2 /yr Effect of Eurasian trees trees

23 Hypothesized Mechanism 1. dark trees = NH warm 2. Energy gradient (N-S) grass between the Hemispheres increases eq Hadley Cell moves north to increase southward heat transport trees 4. ITCZ shifts North Tropical productivity eq follows changes in 23 precipitation

24 mid-holocene green Sahara mid-latitude trees in Eurasia can shift the ITCZ and increase rainfall over Africa in the mid-holocene paleo-proxy records based on plants may have information about climate forcing in addition to local climate conditions 24

25 2

26 .3 Precipitation over Africa ( to 3 E, 1 N to 3 N) Precipitation (mm/day) CMH CPI EVMH CPI SVMH CPI SEVMH CPI SEVMH SVMH Annual Mean DJF DJF MAM MAM JJA JJA SON SON

27 Energy Transport (PW) CMH CPI SVMH CPI EVMH CPI SEVMH CPI Total Northward Atmospheric Energy Transport 9S 6S 3S Eq 3N 6N 9N Latitude (deg)

28 Energy Flux (Wm 2 ) Total (SEVMH CMH) Non cloud (SEVMH CMH) Cloud (SEVMH CMH) Total (SEVMH SVMH) Non cloud (SEVMH SVMH) Cloud (SEVMH SVMH) Grass in the Sahara Trees in Eurasia 2 9S 6S 3S Eq 3N 6N 9N Latitude (deg)

29 Excess Precipitation estimated from pollen record Precipitation relative to present 3 Biome6 6Kya Biome Map temperate deciduous broadleaf forest warm-temperate evergreen broadleaf and mixed forest xerophytic woods/scrub steppe desert Latitude (deg N) Excess Precip mm/yr Jolly & Prentice, unpublished

30 Orbital and Dynamic Veg don t get enough precip Precipitation relative to present 3 Biome6 6Kya Biome Map temperate deciduous broadleaf forest warm-temperate evergreen broadleaf and mixed forest xerophytic woods/scrub steppe desert Latitude (deg N) Excess Precip. colored lines: PMIP2 model results mm/yr Braconnot et al. 27

31 Expand Mid-Latitude Forests Percent of gridcell afforested % of gridcell 31