Contractions of Tropical Precipitation Under Global Warming

 
Modes of tropical precipitation changes (variability)
 
---Focus on zonal mean here (can apply to zonal inhomogeneities too)
---Apply to simulated response to external forcing in coupled climate models
       4XCO
2
, Last Glacial Maximum, Mid-Holocene, Freshwater hosing
 
 
    Aaron Donohoe, Applied Physics Lab
 – University of Washington
An example : change in precipitation under 4XCO2
Initial
 and 
target
 precip.
 
with 
all modes optimized
 
SHIFT
 
CONTRACT
 
INTENSIFIED
Results: Shifting, contracting and intensifying tropical precip.
ITCZ Shifts explain very little of
tropical precipitation changes;
contractions  and intensification
explain more
 
Contraction and intensification
are highly coupled and there is a
robust contraction and
intensification in a warmer world
and expansion reduction in a
cooler world – with fixed ratio
 
Take the contraction and
intensifying ratio as empirical. We
can then describe the tropical
precipitation shift with two
modes: shifting and
intensifying/contract with little
loss of explanatory power
 
ITCZ Shifts are:
       (1) small in magnitude
       (2) explain little precip. Change
       (3) not robust with climate change
 
What determines the width of tropical precipitation in the annual mean?
 
Annual mean precipitation is the
statistical average of precipitation
during solsticial seasons
 
Distribution of tropical
precipitation is bimodal
         
 Like a sine wave
         
 ITCZ is seldom in
 annual mean location
Role of seasonal cycle in stretching out the annual
mean precipitation
GFDL aquaplanet with
seasonal insolation coupled
to slab ocean (Dargan)
Change depth of slab (50m
to 2.4 m --globally uniform)
Amplitude of seasonal ITCZ
migration off the equator
controls the width of the
tropics!
 
Width of tropical
precipitation controlled by
seasonal range of ITCZ
migration
 
Contraction of tropical
precipitation under global
warming is equivalent to
reduced seasonal
migration of ITCZ
 
CLAIM:
Seasonal amplitude (range) of
cross equatorial energy
transport dictates seasonal
migration of ITCZ off equator
 tropical width
Previous work has demonstrated that
annual mean ITCZ location is dictated by
annual mean inter-hemispheric heat
transport (Frierson Hwang  2012,
Donohoe et al. 2013 many others)
 
 
Reduced seasonal migration
of the ITCZ (tropical
contraction) under global
warming:
 
Is 
NOT 
a consequence of
reduced seasonality of
energy transport across
the equator (range on
abscissa)
 
Results from reduction of
slope between ITCZ
location and energy
transport
 
 ITCZ (and Hadley cell)
migrates less far from the
equator to achieve a given
energy transport
 
 
SUMMARY
 
Tropical precipitation robustly contracts and intensifies
in a warmer world and expands and reduces in a colder
world
      
 dominant mode of tropical precipitation
 
variability and changes
  under external forcing
 
Width of tropics set by seasonal migration of
ITCZ off the equator
         
 Amplitude of seasonal energy input the
 
atmosphere
 
Speculation: Under global warming tropopause
rising and increased gross moist stability of the
tropics means the ITCZ (and Hadley cell) don’t
have to move as far off the equator to satisfy
seasonal energy constraints
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Aaron Donohoe's research focuses on the contraction of tropical precipitation with global warming, emphasizing the zonal mean and its implications for climate models. The results show a robust contraction and intensification of tropical precipitation in a warmer world. The width of tropical precipitation in the annual mean is determined by the distribution and bimodal nature of precipitation. The role of the seasonal cycle in stretching out the annual mean precipitation is highlighted, emphasizing the control of the width of tropical precipitation by the seasonal range of ITCZ migration.


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  1. Contraction of the Tropical Precipitation with Global Warming Aaron Donohoe, Applied Physics Lab University of Washington Modes of tropical precipitation changes (variability) ---Focus on zonal mean here (can apply to zonal inhomogeneities too) ---Apply to simulated response to external forcing in coupled climate models 4XCO2, Last Glacial Maximum, Mid-Holocene, Freshwater hosing

  2. An example : change in precipitation under 4XCO2 SHIFT Initial and target precip. with all modes optimized CONTRACT INTENSIFIED

  3. Results: Shifting, contracting and intensifying tropical precip. ITCZ Shifts explain very little of tropical precipitation changes; contractions and intensification explain more Contraction and intensification are highly coupled and there is a robust contraction and intensification in a warmer world and expansion reduction in a cooler world with fixed ratio Take the contraction and intensifying ratio as empirical. We can then describe the tropical precipitation shift with two modes: shifting and intensifying/contract with little loss of explanatory power ITCZ Shifts are: (1) small in magnitude (2) explain little precip. Change (3) not robust with climate change

  4. What determines the width of tropical precipitation in the annual mean? Distribution of tropical precipitation is bimodal Like a sine wave ITCZ is seldom in annual mean location Annual mean precipitation is the statistical average of precipitation during solsticial seasons

  5. Role of seasonal cycle in stretching out the annual mean precipitation GFDL aquaplanet with seasonal insolation coupled to slab ocean (Dargan) Change depth of slab (50m to 2.4 m --globally uniform) Amplitude of seasonal ITCZ migration off the equator controls the width of the tropics!

  6. Width of tropical precipitation controlled by seasonal range of ITCZ migration

  7. Contraction of tropical precipitation under global warming is equivalent to reduced seasonal migration of ITCZ

  8. Previous work has demonstrated that annual mean ITCZ location is dictated by annual mean inter-hemispheric heat transport (Frierson Hwang 2012, Donohoe et al. 2013 many others) CLAIM: Seasonal amplitude (range) of cross equatorial energy transport dictates seasonal migration of ITCZ off equator tropical width

  9. Reduced seasonal migration of the ITCZ (tropical contraction) under global warming: Is NOT a consequence of reduced seasonality of energy transport across the equator (range on abscissa) Results from reduction of slope between ITCZ location and energy transport ITCZ (and Hadley cell) migrates less far from the equator to achieve a given energy transport

  10. SUMMARY Tropical precipitation robustly contracts and intensifies in a warmer world and expands and reduces in a colder world dominant mode of tropical precipitation variability and changes under external forcing Width of tropics set by seasonal migration of ITCZ off the equator Amplitude of seasonal energy input the atmosphere Speculation: Under global warming tropopause rising and increased gross moist stability of the tropics means the ITCZ (and Hadley cell) don t have to move as far off the equator to satisfy seasonal energy constraints

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