generation of interannual climate variability in the tropical Atlantic

An oceanic mechanism for the

generation of interannual climate variability in the tropical Atlantic

PIRATA 22, Nov. 5-10, 2017

M. Claus, R. Greatbatch, P. Brandt, F. Tuchen, J.-D. Matthießen

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Interannual mode of climate variability

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Model setup

● MITgcm, z-coordinate

● 1/4° x 1/4°, 200 level

● Uniform salinity, linear EOS

● flat bottom, free-slip BBC, no-slip LBC

● Vertical mixing: Pacanowski and Philander (1981)

● Driven by zonally uniform steady wind stress

● Velocity decomposition with 70-day cut-off

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v’ at the center of the basin

Intraseasonal variability

➔ Pronounced intraseasonal variability of large vertical scale

➔ Spectrum is similar to observations

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Momentum flux from intraseasonal to interannual time scales

z

x Eq

x y

without EDJs

with EDJs

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Momentum flux from intraseasonal to interannual time scales

6 4 2 0 2 4 6

regression slope [s ¹]

0

1000

2000

3000

4000

5000

depth [m]

Model andObservations

➔ Equatorial deep jets are maintained by the convergence of intra-seasonal momentum flux

z

x Eq

x y

without EDJs

with EDJs

Greatbatch et al., in prep.

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Momentum flux from intraseasonal to interannual time scales

➔ Low frequency zonal flow variability leads momentum flux convergence

z

x Eq

x y

without EDJs

with EDJs

Greatbatch et al., in prep.

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Momentum flux from intraseasonal to interannual time scales

➔ The combination of small vertical scale u’

and large vertical scale v’ maintain the EDJs

z

x Eq

x y

without EDJs

with EDJs

vertical mode of u’

vertical mode of v’

Vertically integrated power input

Greatbatch et al., in prep.

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Surface signature and vertical energy propagation

➔ Interannual variability of NECC is linked to EDJs

Matthießen et al., 2015, OD

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Vertical energy flux 1e-8

Surface signature and vertical energy propagation

➔ Interannual variability of NECC is linked to EDJs

➔ Energy of EDJs is propagating upwards

Matthießen et al., 2015, OD

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➔ Distinct from other mechanisms in the climate system that generate low frequency variability

➔ Bears potential predictability on seasonal to decadal time scales

Summary and discussion

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Summary and discussion

➔ What sets the vertical scale of the EDJs?

➔ Large amplitude, short intraseasonal Yanai waves are subject to barotropic shear instability which produces stacked zonal jets (Hua et al., 2008, JFM; Gill, 1974, GFD)

➔ More stable Yanai waves flux momentum to low frequency small vertical scale motion through resonant triad interaction

(see Appendix of Hua et al., 2008, JFM)

➔ Why are most ocean models unable to generate EDJs?

➔ Vertical resolution?

➔ Presence of coastline?

➔ Wind stress variability?

➔ Topography?

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Summary and discussion

100 level 75 level

46 level 200 level, coastline

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Summary and discussion

Climatological wind stress realistic topography

idealized ridge

➔ Why are most ocean models unable to generate EDJs?

✔ Vertical resolution

✗ Presence of coastline

✗ Wind stress variability

✔ Topography

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➔ Distinct from other mechanisms in the climate system that generate low frequency variability

➔ Bears potential predictability on seasonal to decadal time scales

Summary and discussion