How will atmospheric moisture transport from ocean to land

change with anthropogenic climate change?

Michael Previdi

CICAR ACCWW Symposium22 May 2013

Trenberth et al., 2007

Trenberth et al., 2007

CMIP5 Analysis• Initially examine 5 models: CCSM4, CSIRO-Mk3-6-0,

GFDL-CM3, IPSL-CM5A-LR, and NorESM1-M

• For each model, consider four different experiments: 1) historical (1850-2005), 2) historicalGHG (well-mixed GHG forcing only), 3) historicalAA (anthropogenic aerosol forcing only), and 4) RCP8.5 (2006-2100)

• In addition to availability of “single forcing” GHG and aerosol simulations, these models were chosen because they close the global atmospheric moisture budget (Liepert and Lo, 2013), and because they likely differ significantly in their anthropogenic aerosol forcing during the historical period (e.g., see Forster et al., 2013).

Atmospheric Moisture Balance

∫ −=⋅∇+∂∂ sp

PEqdpvgt

w

0

1 v

• The four terms, from left to right, are the time rate of change of atmospheric moisture storage (including vapor, liquid and solid), the horizontal divergence ofthe atmospheric moisture flux, the surface evapotranspiration flux, and thesurface precipitation flux.

• Atmospheric moisture storage, E and P are obtained directly from monthly meanCMIP5 output, with the moisture flux divergence computed as a residual.

• The atmospheric moisture transport from ocean to land is obtained by integratingthe moisture flux divergence over the global land area.

Climatological mean ocean to land moisture transport

Observations from ERA-Interim reanalysis (Trenberth and Fasullo, 2013)

Annual mean global landQCONV :

CMIP5 multimodel mean:2.9 PW

Observed:2.7 PW

Ocean to land moisture transport in different reanalyses

ERA-40 CFSR

ERA-I

JRA

NCEP-1

Trenberth and Fasullo, 2013

Ocean to land moisture transport in different reanalyses

ERA-40 CFSR

ERA-I

JRA

NCEP-1

Trenberth and Fasullo, 2013

El ChichónPinatubo

‘97-’98 El Niño

Ocean to land moisture transport in different reanalyses

ERA-40 CFSR

ERA-I

JRA

NCEP-1

Trenberth and Fasullo, 2013

El ChichónPinatubo

‘97-’98 El Niño

Are multidecadal changes in moisture transport due to changes in theobserving system, internal variability, and/or external forcing?

CMIP5 historical simulations

CMIP5 historical simulations

Moisture transport sensitivity to GHG and aerosol forcing

dQ/dT ~ 3.6 %/K

dQ/dT ~ 2.0 %/K

More evidence for increased sensitivity to aerosols

O’Gorman et al., 2011

Trenberth et al., 2009

Trenberth et al., 2009

RATM

LH+SHNet atmospheric radiative cooling, RATM , is balanced by the turbulent transfer of latentand sensible heat from the surface to the atmosphere (LH+SH).

Trenberth et al., 2009

RATM

LH+SHNet atmospheric radiative cooling, RATM , is balanced by the turbulent transfer of latentand sensible heat from the surface to the atmosphere (LH+SH).Due to the atmosphere’s small heat capacity, perturbations to RATM (e.g., resulting fromexternal forcing) require a “fast” compensating response of the surface turbulent fluxes.

Trenberth et al., 2009

RATM

LH+SHNet atmospheric radiative cooling, RATM , is balanced by the turbulent transfer of latentand sensible heat from the surface to the atmosphere (LH+SH).Due to the atmosphere’s small heat capacity, perturbations to RATM (e.g., resulting fromexternal forcing) require a “fast” compensating response of the surface turbulent fluxes.To first order: ΔRATM ~ ΔLH = LΔP, where P is global mean precipitation.

Vertical distribution of radiative forcing

Data from Andrews et al., 2010

Slow (temperature dependent) response of the global hydrological cycle

Previdi, 2010

21st century anthropogenic GHG/aerosol forcing in CMIP5 RCP scenarios

Meinshausen et al., 2011

CMIP5 RCP8.5 simulations

Implications: land carbon storage

Friedlingstein et al., 2006

Implications: ice sheet mass balance

Bindschadler et al., 2013

Conclusions• Analysis of CMIP5 “single forcing” experiments suggests that

anthropogenic aerosol changes were the dominant driver of long-term (multidecadal) forced changes in ocean to land moisture transport during the 20th century (although forced changes still small compared to internal variability).

• The sensitivity of the moisture transport to aerosol forcing was estimated to be nearly twice that of GHG forcing (per degree of warming).

• In the 21st century, CMIP5 models project a significant (~ 0.4 PW, or 14%) increase in ocean to land moisture transport in response to rising GHG amounts and decreasing anthropogenic aerosol forcing (would be good to examine alternative future scenarios for aerosol emissions).

• The projected increase in moisture transport would have important implications for land precipitation, and also potentially land carbon storage and ice sheet mass balance (relevant for paleoclimatic change as well).