Precipitation response to climate change over tropical oceans: - - PowerPoint PPT Presentation

Precipitation response to climate change over tropical oceans: Importance of changes in surface convergence driven by near-surface temperature gradients

Margaret L. Duffy (MIT EAPS)

Paul A. O’Gorman (MIT EAPS) Larissa E. Back (UW Madison AOS)

Funding by NSF

1 photo from NASA GOES satellite

Model (dis)agreement on mean precipitation response to climate change

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The response of tropical precipitation to climate change will be impactful. However, GCMs do not agree on the response

Precipitation response to climate change in two GCMs

△P = PRCP 8.5 - Phistorical PRCP 8.5 2080 - 2099 Phistorical 1980 - 1999

Three proposed mechanisms for precipitation response over tropical

• ceans

1. Wet-get-wetter: Historical pattern of precipitation enhanced by warming (Chou and Neelin 2004, Held and Soden 2006) 2. Warmer-get-wetter: Precipitation response corresponds to SST response, relative to tropical mean (Xie et al. 2010) 3. Surface convergence important for convection (and thus precipitation) and SST gradients are important for setting this convergence via pressure gradients (Lindzen and Nigam 1987, Back and Bretherton 2009)

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Three proposed mechanisms for precipitation response over tropical

• ceans

1. Wet-get-wetter: Historical pattern of precipitation enhanced by warming (Chou and Neelin 2004, Held and Soden 2006) 2. Warmer-get-wetter: Precipitation response corresponds to SST response, relative to tropical mean (Xie et al. 2010) 3. Surface convergence important for convection (and thus precipitation) and SST gradients are important for setting this convergence via pressure gradients (Lindzen and Nigam 1987, Back and Bretherton 2009)

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Mechanism for observed precipitation

Approach: 1. Use a simple model of precipitation (adapted from the “two-mode model” of Back and Bretherton 2009) to estimate the response of precipitation to climate change 2. Compare annual-mean precipitation response between the two-mode model and GCMs from CMIP5 3. Evaluate the relative importance of terms corresponding to proposed mechanisms of precipitation response

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What is the relative contribution of the different proposed mechanisms to the tropical precipitation response to climate change over oceans?

The two-mode model uses dry static energy budget to relate monthly-mean precipitation to monthly-mean SST and surface convergence

• Key approximation: Two modes of vertical motion (ω)

variability, one shallow and one deep

• Shallow mode amplitude is related to surface convergence via

mass continuity

• Deep mode amplitude is empirically related to surface

convergence and relative SST

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Expression for monthly-mean precipitation from two-mode model

χ

𝑀𝑄 = 𝐼 𝜓 𝑁(),(𝑏(𝑇𝐷 + 𝑁(),/ 𝑏/ 𝑇𝑇𝑈1)2 + 𝑐/ 𝑇𝐷 + 𝑑/ − 𝑆7

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LP Precipitation (W m-2) SC Surface convergence SSTrel Anomaly from tropical mean SST Mse,s Shallow dry effective stability Mse,d Deep dry effective stability as, ad, bd, cd Coefficients R0 Clear sky radiation Heaviside function

H [χ]

Precipitation from two-mode model compares favorably to observations

Mean precipitation (2000- 2008) from GPCP

• bservations (top) and from

the two-mode model using coefficients from reanalysis and inputs from observations (bottom)

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Two-mode model accurately reproduces GCM precipitation response

Precipitation response to climate change in different GCMs (top) and from two-mode model using coefficients and inputs from the GCM (bottom)

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Evaluate relative contribution of each mechanism

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Relatively warmer-get-wetter Surface convergence important Wet-get-wetter Wet-get-wetter: Mse,s and Mse,d change Relatively warmer-get-wetter: SSTrel changes Surface convergence important: SC changes All other coefficients remain fixed

LP = H [χ]{Mse,sasSC + Mse,d (adSC + bdSSTrel + cd) − R0}

GFDL-CM3 decomposition

• f precipitation response

11 Precipitation response to climate change in GFDL-CM3 from GCM (top) and from two-mode model (bottom) Contribution of each proposed mechanism to precipitation response to climate change

MPI-ESM-MR decomposition

• f precipitation response

12 Precipitation response to climate change in MPI-ESM-MR from GCM (top) and from two-mode model (bottom) Contribution of each proposed mechanism to precipitation response to climate change

• The pattern of precipitation response differs between the GCMs

studied, but the relative contributions are robust

• The surface convergence contribution is surprisingly strong and the

“warmer-get-wetter” contribution is surprisingly weak What is the relative contribution of different proposed mechanisms to the tropical precipitation response to climate change over oceans?

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The surface convergence response is key to the precipitation response; to what extent is it set by changes in SST gradients?

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Relationship between boundary layer convergence and surface temperature gradients

• Boundary layer winds are related to boundary layer pressure gradients,

and thus temperature gradients by the hydrostatic relationship (Lindzen and Nigam 1987)

• We use the mixed layer model from Back and Bretherton 2009 to

derive approximations for SC

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• Proportional to the Laplacian of boundary layer virtual temperature

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• Proportional to the Laplacian of SST

SC(r2SST) = ps✏iΦ850 R⇢0 (✏2

i + f 2) SST 2 r2SST

ps Surface pressure g acceleration due to gravity parameter proportional to mixing and friction R gas constant reference density f coriolis parameter Tv virtual tempearature vertical integral over boundary layer geopotential at 850 hPa

✏i

ρ0

Z

BL

¯ Φ850

Surface convergence approximations from mixed layer model

SC

• r2TBL
• =

psg✏i R⇢0 (✏2

i + f 2)

Z

BL

r2Tv T 2

v

dz

Changes in surface convergence are related to changes in the Laplacian of low- level temperatures

Response of surface convergence to climate change in the ensemble mean of 8 GCMs (top), approximated as proportional to the Laplacian

• f boundary layer virtual temperature

(middle), and approximated as proportional to the Laplacian of SST (bottom)

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Response of precipitation to climate change (a) in ensemble mean of 8 GCMs, (b) approximated using two-mode model, (c) using two-mode model with SC(▽2TBL), and (d) using two-mode model with SC(▽2SST).

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Precipitation response is captured by two-mode model when surface convergence is approximated as proportional to the Laplacian of low-level temperature

Only spatially- varying inputs are low-level Tv and SST Only spatially- varying input is SST

Conclusions

• All three proposed mechanisms contribute to precipitation response

to climate change over tropical oceans

• Surprisingly, surface convergence contribution is stronger than

warmer-get-wetter contribution

• Surface convergence contribution is approximated well by the

Laplacian of boundary layer temperature and, to a lesser extent, by the Laplacian of SST

• Implication: pattern of SST change affects precipitation via

both warmer-get-wetter and its effect on changes in surface convergence

19 Margaret L. Duffy, MIT EAPS, mlduffy@mit.edu