Future changes in tropical rainfall and large-scale circulation - - PowerPoint PPT Presentation

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Future changes in tropical rainfall and large-scale circulation under global warming from CMIP5 model Projections

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The Yin-Yang (floods and droughts ) and the Chi (Hadley Circulation)

• f the Global Water Cycle

Lau and Kim, 2012: The Russian heat wave and Pakistan flood 2010: A teleconnection of hydroclimate extremes.. J. Hydro. Meteor. Lau et al. 2013: A Canonical Response of Rainfall Characteristics to Global Warming, Geophys. Res. Lett.

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Excerpts from IPCC AR5 : It is likely that since about 1950 the number of heavy precipitation events

• ver land has increased in more regions than it has decreased. Regional

trends vary but confidence is highest for central North America with very likely trends towards heavier precipitation events,……..the frequency and intensity of drought has likely increased in the Mediterranean and West Africa and likely decreased in central North America and north-west Australia since 1950. It is likely that circulation features have moved poleward since the 1970s, involving a widening of the tropical belt, a poleward shift of storm tracks and jet streams, and a contraction of the northern polar vortex. Evidence is more robust for the Northern Hemisphere. It is likely that the Southern Annular Mode has become more positive since the 1950s. Confidence is high that the increase in the northern mid-latitude westerly winds and the NAO index from the 1950s to the 1990s and the weakening of the Pacific Walker circulation from the late 19th century to the 1990s have been largely offset by recent changes. .

Monsoon Climate Change as inferred from CMIP5 Multi-Model Mean (MMM)

33 CMIP5 coupled models, future projection

• 1% per year increase in CO2 emission (140 yrs starting at

present climate)

Control = 1st 25 year of integration MMM anomaly = last-25-years minus Control (TCO2)

• 17 CMIP5 coupled models (1900-2005), 20th Century

Historical runs

a) Control (pre-industrial conditions + natural)) b) All ( GHGs + anthropogenic aerosols +natural) c) GHGs only (GHGs + natural) d) Nature run (solar change, volcanic eruption…)

Global temperature Global Rainfall

DCO2 Yr 71-97 TCO2 Yr 114-140

1.35± 0.38 % K-1 2–3.6 K century-1

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Central /Eastern Pacific Americas Asian-Australia Euro-Africa Meridional profiles of rainfall response (ensemble mean difference) to global warming at different longitudinal sectors from CMIP5

140E-130W 40W-130W 40W- 50E 50E-140E

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A canonical global rainfall response: more heavy rain , less moderate rain , more light rain and more no-rain occurrence NR LR MR HR VHR

Increased risk of heavy rain, and prolonged period of drought due to CO2 warming are connected, occurring in preferred geographical locations, through changes in the large-scale circulation.

A loaded climate dice

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Lau et al. 2013, GRL.

Heavy Rain (>98 perentile) anomaly Prolonged dry-month anomaly

A prevailing view about the changes in the tropical circulation under global warming

Globally E=P = qM

M= vertical mass flux at cloud base; q= moisture at cloud base

dM/M = dP/P - dq/q = 1.35 – 6.5 = -5.15 % K-1 < 0

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===> Weakening of the tropical circulation,

i.e., the Walker and Hadley Circulations (Held and Soden 2006, Vecchi and Soden 2008…..)

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A Conundrum : How can a weakened tropical circulation, particularly the Hadley Circulation compatible with increased latent heat due to enhance rainfall in the tropics under global warming? The fundamental question: Is the Hadley Circulation weakened or strengthened under global warming??

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Equatorial Region Deep Tropics Tropics Tropics/ Subtropics Tropics/ Subtropics/ Midlatitudes Tropics/ Subtropics/ Mid-, High-latitudes Near Global Global

Time variation of zonal mean W500 as a function of spatial scales CMIP5 1% per year CO2 increase

• Narrowing Convective Zone in deep tropics
• Expanding subtropics
• Poleward shift of Ferrel and polar cells

W TC Deeper Clouds Enhanced Ascent

Global warming raised the HC outflow region, intensifying the upper branch of the HC

Time-height cross-section of meridional winds , at 10N and 10S under global warming total (contour) and anomalies (color) Rate of total div. Mass outflow At 200-100 hPa +10% K-1

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∆P ∆U

MC/WPAC EPAC Walker Circulation (MC/WPAC Minus-EPAC) ~ 20% reduction

ΔC <0 , Δ q <0 Δ Sm<0, Δ E<0 +ve feedback over land

Pre-industrial Climate

ΔC >0 Δ q>0 ΔC >0 Δ q>0 Δ E >0 Δ T > Δ TS Δ HT >0

Global warming Climate Δ TS> 0 Δ

• V· q>o

δ Div Vq >0

Stronger, deeper, narrower Convective zone Weakening/polar shift of polar jet??

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Back -Up

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Zonal mean SSTA removed

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Deeper convection/clouds in near equatorial regions

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