# Presentation: 755034

## Impact of African dust on Western North Hemisphere

- Paul Ginoux
- NOAA-GFDL

## IPCC TAR and AR4, and AR5

- Is there any significant additional development for aerosols needed  for AR5?

## AR-4 GFDL CM2.1 aerosol distribution

- Ginoux et al., 2006

- Dust inter-annual variability

- Dust concentration at Barbados (Prospero and Lamb, 2003)

- Factor 4 increase

- Since 1970ies dust concentration in Caribbean (Prospero and Lamb, 2003), dust deposition in French Alps (De Angelis and Gaudichet, 1991), visibility records in West Africa (M’bourou have increased by a factor 4-5

**1970**

- Coupled Atm-Ocean-Ice-Land model run for IPCC AR-4 used constant dust load for their simulations (historical run+future scenarios)

- What could have been the results had the models considered the inter-annual variability of natural aerosol, in particular dust.

## Model experiment

- Set of 2 runs of 120 model-years:
- 	- **Control **(HIGHDUST): Run SM2.1 with the exact same forcing agents as used by the GFDL CM2.1 simulation. Dust monthly mean distribution was calculated with MOZART for the year 1992-1993.
- 	- **Perturbed** (LOWDUST): Run SM2.1 with exact same forcing agents as Control but with monthly mean dust distribution calculated with new MOZART simulation performed with same conditions but with dust emission divide by 5 over West Africa.

## Dust at Barbados

**LOWDUST**

**HIGHDUST**

- U. Miami data
- 1965-1969

- Std

- Mean

- U. Miami data
- 1970-1999

- 1965

- 1992

- Model

- Model

## TOA Instantaneous Forcing

| W/m2 | SW | LW | Net |
| --- | --- | --- | --- |
| TOA | 0.08 | 0.04 | 0.12 |

- Global annual mean

## SFC Instantaneous Forcing

| W/m2 | SW | LW | Net |
| --- | --- | --- | --- |
| TOA | -0.9 | 0.1 | -0.8 |

- Global annual mean

## D-J-F Sea Level Pressure

## Temperature Changes

- Cooling

- Warming

- -0.5oC

- +0.5oC

- 850 hPa

- 850 hPa

- Temperature at 850hPa

- Zonal Temperature profile

## Gill’s model

- Gill (QJRMS, 1980) solved the shallow water equation to study the response of tropical atmosphere to a given distribution of heating.

- (x,y) non-dimensional distance
- (u,v) proportional to horizontal velocity
- P proportional to pressure perturbation
- Q heating rate
- W vertical velocity

- Approximation:
- Horizontal length scale ~ 10o , the time scale ~ 1⁄4 day
-              replaces by              with epsilon small (0.1)
-  eastward flow is in geostrophic balance with pressure gradient

- Applications:
- 1 Symmetric equatorial heating over Indonesia -> Walker circulation;
- 2 Asymmetrical heating North of equator-> monsoon flow + low on western flank of Q

## SLP D-J-F

**Analytical solution of the shallow water equation with an atmospheric cooling over Western Sahara**

**Vorticity **shading** and Streamfunction **contours
**500 mb D-J-F**

## Positive Feedback

- HIGHDUST enhances surface winds over West Africa. It results an increase of dust emission, which will further accentuate the effects of dust on the circulation, producing a positive feedback. Also, the decrease of evaporation from the surface and the low clouds fraction may increase dust lifetime.

- More than 100% increase of dust emission in D-J-F

- Emission=S*u2*(u-ut)

- Ginoux et al., 2001

## Precipitation J-J-A

## Temperature Changes J-J-A

- Warming

- 1 to 1.5oC

- Temperature at 700hPa

- Zonal Temperature profile

- Warming

## SST departures J-A-S

| oC | ERSST
[1970to1999] – [1950to1969] | SM2.1
HIGHDUST
-
LOWDUST |
| --- | --- | --- |
| North Atlantic – South Atlantic | -0.37 | -0.13 |
| Indian Ocean | 0.38 | 0.15 |
| Mediterranean Sea | -0.15 | -0.38 |
| Black Sea | -0.24 | -0.29 |
| Red Sea | -0.13 | -0.25 |

## Dust impact on hurricane genesis

- Recent increase of hurricane activity in the Atlantic have been related to increase of SST and decrease in vertical wind shear
- Main development region (MDR): [85W-20W] and [10N-20N]

- 1971-1994: Low activity
- 1953-1970: High activity

## Parameterization of dust emission

- Q=C sp u*2(u*-u*t)

- F= Q

- The threshold velocity u*t is a function of particle radius, surface roughness height, temperature, and surface wetness

- Vertical Flux of clay and silt

- Horizontal flux of Sand

- Efficiency factor
- Marticorena and Bergametti (1995):

## Conclusions

- IPCC simulations from historical runs to future scenarios have been forced by constant post-1980s dust load.
- A set of experiments with coupled Atm-slab ocean has been realized: a control HIGHDUST case (corresponding to dust of the 70s-80-90s) and a perturbed LOWDUST case (corresponding to dust of the 50s-60s)
- Dust decadal variability may have:
- Reduced sea surface temperature over the North Atlantic
- Amplify the deficit in precipitation over Sahel and Amazon
- In Dec-Jan-Feb:
- Contributed as much as 50% of the anomalous positive phase of the NAO
- Enhanced downward branch of Hadley cell over Sahara
- Invigorate the easterlies (Harmattan) trade winds
- In Aug-Sep-Oct: Increased wind shear and atmospheric stability over MDR, potentially affecting the genesis of hurricanes

## Conclusions cont’d

- The same experiment will be repeated but with full ocean model
- The simulation of decadal variability of dust emission should include a dependency on vegetation.

## Acknowledgments

- Yi Ming: SM2.1 simulations
- Larry Horowitz: MOZART simulation with reduced dust
- Dan Schwarzkopf: radiative forcing calculation
- Kirsten Findell: statistical analysis (T-test)

## Slide 22