Coupling of ITCZ/Monsoons to ocean circulation (John Marshall, MIT) - - PowerPoint PPT Presentation

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coupling of itcz monsoons to ocean circulation
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Coupling of ITCZ/Monsoons to ocean circulation (John Marshall, MIT) - - PowerPoint PPT Presentation

Feb 05, 2024 288 likes •619 views

Coupling of ITCZ/Monsoons to ocean circulation (John Marshall, MIT) Hadley Cells Subtropical Cells STCs 1. Climatological position of the ITCZ is north of the equator because of cross-equatorial ocean heat transport. 2. ITCZ migrations are

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Coupling of ITCZ/Monsoons to ocean circulation

  • 1. Climatological position of the ITCZ is north of the equator

because of cross-equatorial ocean heat transport. (John Marshall, MIT)

Subtropical Cells STCs Hadley Cells

  • 2. ITCZ migrations are strongly damped

by coupling of trade winds to ocean’s subtropical cells. (Brian Green)

  • 3. Explore Strong coupling of the Indian monsoon with ocean circulation.

(Nick Lutzko)

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SLIDE 2
  • Kang et al, 2008

ITCZ resides in the warm hemisphere

Equator

Obvious explanation for relative warmth of the Northern Hemisphere might be albedo differences.

However…..

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p 0.298 p 0.299

Donohoe and Battisti, 2011, J. Climate

Note, a 0.01 difference in p implies a cross-equatorial heat transport of 0.2PW.

Symmetry of Hemispheric planetary albedos

Voigt et al, 2013: J. Climate Stevens et al, 2015, GRL

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SLIDE 4

Atlantic All oceans Atlantic

Lumpkin and Speer, 2007

Atmospheric heat transport is southward at the equator

Trenberth and Caron, 2001

Sv

To compensate, ITCZ shifts north to ensure that atmosphere transports heat south across Equator

NH is warmer than the SH because of ocean circulation

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SLIDE 5

Heat transport can be up-gradient in the ocean because the ocean is mechanically forced

  • Donohoe et al, 2013; Frierson et al, 2013;

Marshall et al, 2014; Schneider et al, 2014; Stevens et al, 2015

ITCZ is in the NH because of ocean circulation

Kang et al, 2008

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SLIDE 6

Coupling of ITCZ/Monsoons to ocean circulation

  • 1. Climatological position of the ITCZ is north of the equator

because of cross-equatorial ocean heat transport. (John Marshall, MIT)

Subtropical Cells STCs Hadley Cells

  • 2. ITCZ migrations are strongly damped

by coupling of trade winds to ocean’s subtropical cells. (Brian Green)

  • 3. Explore Strong coupling of the Indian monsoon with ocean circulation.

(Nick Lutzko)

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SLIDE 7

Overturning Circulation Surface Winds

ITCZ on the Equator

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SLIDE 8

Overturning Circulation Anomalous Surface Winds

Northward ITCZ shift

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Overturning Circulation Anomalous Surface Winds Anomalous Ocean Ekman Transport

Northward ITCZ shift

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Overturning Circulation Anomalous Surface Winds Asymmetric component

  • f overturning circulation

Northward ITCZ shift

Anomalous Ocean Ekman Transport

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SLIDE 11

Overturning Circulation Anomalous Surface Winds Asymmetric component

  • f overturning circulation

Northward ITCZ shift

Anomalous Ocean Ekman Transport

Anomalous heat transport

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SLIDE 12

Forced minus control

slab

Coupled, Double Drake

Green and Marshall, J. Climate, 2017

0.4 2.5

where ‘c’ is the compensation

MITgcm with Frierson moist physics

The ITCZ is ‘sticky’

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SLIDE 13

Dynamics of the Ocean’s Cross-Equatorial Cell

at depth

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SLIDE 14

Literature on the Cross-equatorial Cell of the Indian Ocean Fritz Schott (observationally) Jay McCreary (theoretically) and collaborators at depth

Dynamics of the Ocean’s Cross-Equatorial Cell

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SLIDE 15

Coupling of Monsoon with Ocean Circulation

Asymmetric component of overturning circulation in Indian Sector Schematic from Peter Webster

  • 1. Coupling of Hadley Cell with ocean’s subtropical cells
  • 2. Damping of ITCZ migrations (Brian Green)
  • 3. Coupling of monsoons with ocean circulation (Nick Lutsko)

Ekman transport Loschnigg and Webster,

  • J. Climate, 2000
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SLIDE 16

Ocean heat transport:

𝐺 ϕ = 2π𝑏cosϕ𝑑𝑞 τ𝑦 𝑔 Δ𝑈

Minimalist model of coupling of Monsoon to the ocean

Prescribed and held constant GFDL dynamical core, Frierson physics

40 15 Equator

Surface stress from atmospheric model

0.5 m deep swamp

24m mixed layer with interactive q-flux Temperature difference across the thermocline. Seasonal Cycle

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Surface winds, precipitation and Moist Static Energy

Swamp Interactive ocean

Summer Δ𝑈 = 10𝐿

MSE MSE

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Swamp Interactive ocean

(time) (time)

Land Sector Ocean Sector

Precipitation

Δ𝑈 = 10𝐿

Land Sector Ocean Sector

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SLIDE 19

Observed Surface wind and precipitation

Animation courtesy of Simona Bordoni

Precipitation

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Increasing Δ𝑈

Enhancing role of ocean circulation

MSE SST Precip Surface wind

LAND LAND LAND OCEAN OCEAN OCEAN OCEAN

Summer

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SLIDE 21

MSE SST Precip Surface wind

LAND LAND LAND OCEAN OCEAN OCEAN OCEAN

Summer Increasing Δ𝑈

Enhancing role of ocean circulation

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SLIDE 22

MSE SST Precip Surface wind

LAND LAND LAND OCEAN OCEAN OCEAN OCEAN

Summer Increasing Δ𝑈

Enhancing role of ocean circulation

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SLIDE 23

MSE SST Precip Surface wind

LAND LAND LAND OCEAN OCEAN OCEAN OCEAN

Summer Increasing Δ𝑈

Enhancing role of ocean circulation

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SLIDE 24

Overturning circulation

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Overturning circulation and zonal wind

ci=2m/s

  • westerlies
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Monsoon Indices

  • Increasing Δ𝑈

Increasing Δ𝑈

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SLIDE 27

Ocean Heat Transport

LAND

OHT

(PW)

OHT at EQ

(PW)

(time)

Summer

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SLIDE 28

Peter Webster and collaborators

Western boundary currents in the Atmosphere and Ocean

Summary

Ekman Transport Net downward surface heat flux

W/m2

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Many discussions with Brian Green, David McGee, Alan Plumb, Nick Lutsko, MIT Aaron Donohoe, Dargan Frierson, UW

Coupling of the trade winds with ocean’s subtropical cells in the deep tropics has a profound effect on rainfall patterns:

Strongly damps ITCZ migrations Fundamental to the dynamics of the Indian Monsoon

Peter Webster, Georgia Tech Simona Bordoni, Tapio Schneider, Caltech

Conclusions

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SLIDE 30

In present climate there is a small 0.2 PW net (A+O) northward transport of heat across the equator

Donohoe et al, 2013, J. Climate

If this transport was achieved by atmosphere, ITCZ would be south of equator!

Marshall et al, 2014; Climate Dynamics

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SLIDE 31

In present climate there is a small 0.2 PW net (A+O) northward transport of heat across the equator

AHT OHT 0 AHT 0

and so, atmospheric heat transport must be southward

OHT 0

If this transport was achieved by atmosphere, ITCZ would be south of equator!

ITCZ is ‘pushed northward’ by OHT

Donohoe et al, 2013, J. Climate Marshall et al, 2014; Climate Dynamics

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SLIDE 32
  • Donohoe et al, 2013; Frierson et al, 2013;

Marshall et al, 2014; Schneider et al, 2014; Stevens et al, 2015

ITCZ is in the NH because of ocean circulation

Kang et al, 2008