Ocean Circulation and Climate Raffaele Ferrari Earth, Atmospheric - - PowerPoint PPT Presentation

Ocean Circulation and Climate

Raffaele Ferrari

Earth, Atmospheric and Planetary Sciences, MIT Brown University, May 5 2015

Deep Ocean and Climate

Ocean and Climate

Ocean Heat Uptake Ocean Carbon Uptake

IPCC, 2015, third chapter IPCC, 2015, sixth chapter

Ocean Heat Uptake

Observed temperature trends Pathways of ocean heat uptake

IPCC, 2015, third chapter Durack and Wijffels (2010)

Ocean Carbon Uptake

Observed anthropogenic carbon inventory

IPCC, 2015, sixth chapter

Southern Ocean

A Theory of the Abyss

Latitude Depth [m] −80 −60 −40 −20 20 40 60 80 −4000 −3000 −2000 −1000 Neutral density [kg/m3] 22 23 24 25 26 27 27.2 27.4 27.6 27.8 28 28.1 28.2 28.3 28.4

Zonally averaged neutral density from WOCE climatology Zonally averaged circulation from Lumpkin and Speer (2007)

Southern Ocean

Ocean sea surface height (Jason and GRACE satellite missions)

Southern Ocean Dynamics

Movie generated by Ryan Abernathey

Idealized model of Southern Ocean circulation (Abernathey et al., 2011)

Antarctica Ocean Basins

Southern Ocean Dynamics

• Nonlinear momentum budget of the Southern Ocean

Marshall and Speer (2012)

ψ = wind − macroturbulence =

wind stress 2×days×sin(latitude)

− K × density slope =

τ |f|

− Ks

Future Climate

Changes in Roaring Forties

Swart and Fyfe (2012)

• Positive trend in Southern Annular Mode over last 30 years
• Strength of Southern Hemisphere westerlies has increased

Changes in Roaring Forties

• Le Quéré et al. (2007) speculated that increase in wind strength would
• strengthen upper cell and increase ocean release of deep carbon
• weaken lower cell and slightly decrease ocean release of abyssal carbon

ψupper = τ |f| − Ks > 0 ψlower = τ |f| − Ks < 0

Eddy compensation

Abernathey et al. et al. (2012)

• Increase in winds is accompanied by an increase in macro turbulence
• Increase in macro turbulence results in an increase in eddy diffusivity

ψupper = τ |f| − Ks > 0 ψlower = τ |f| − Ks < 0

ψwind = τ |f|

Ocean macro turbulence

• Need theories of nonlinear turbulent equilibration of Southern Ocean

MIT General Circulation Model DIMES Experiment Theory for interaction of macro turbulence with mean circulation

K = K(τ, f, topography, bottom friction, ...)

Eddy-mean flow interactions

• Strong jets reduce mixing by macro turbulence across jets

Ferrari & Nikurashin (2010)

150oW 145oW 140oW 135oW 130oW 125oW 64oS 56oS 48oS 40oS 32oS

−1.2 0.8

150oW 145oW 140oW 135oW 130oW 125oW 64oS 56oS 48oS 40oS 32oS

−0.2 0.2

500 1000 1500 2000 2500 3000 3500 4000 4500 −65 −60 −55 −50 −45 −40 −35 −30 Effective diffusivity, (m

Latitude Eddy Full Theory

Mean flow Eddy flow Diffusivity K

— ``Observed’’ K — No eddy mean interactions

• - With eddy mean interactions

K ' `eddy ⇣ |u0|2 ⌘3/2 8¯ u2

Macro turbulence models

• Quasi-Geostrophic dynamics
• Quasi-Geostrophic stochastic models of ocean macro-turbulence

∂t¯ q + J( ¯ ψ, ¯ q) = J(ψ0, q0), ¯ q = f + r2 ¯ ψ + ∂ ∂z ✓ f 2 N 2 ∂ ¯ ψ ∂z ◆ ∂tq0 + J( ¯ ψ, q0) + J(ψ0, ¯ q) = F(x, y, z)R(t) λq0 q0 = r2ψ0 + ∂ ∂z ✓ f 2 N 2 ∂ψ0 ∂z ◆

∂tq + J(ψ, q) = 0 q = f + r2ψ + ∂ ∂z ✓ f 2 N 2 ∂ψ ∂z ◆

Ferrari and Nikurashin (2010); Fitzgerald et al. (2015)

Conclusions

• The deep ocean circulation and stratification is controlled by
• winds and air-sea fluxes acting on the Southern Ocean
• air-sea fluxes in the North Atlantic
• macro turbulence in the Southern Ocean
• micro turbulence in the Atlantic, Indian and Pacific Oceans

Ferrari (Nature, 2014)