BAROCLINIC AND BAROTROPIC ANNULAR MODES Lina Boljka Ted

BAROCLINIC AND BAROTROPIC ANNULAR MODES Lina Boljka, Ted Shepherd and Mike Blackburn Unified Model Training Course, 12/4/2018 1 Copyright University of Reading

CONTENTS • Introduction • Baroclinic vs barotropic • Barotropic & baroclinic annular modes • Motivation • Unified Model • Held-Suarez Model Configuration • Results • Cross-spectra • Conclusions 2

INTRODUCTION 3

LINKED Baroclinic • Baroclinic instability • Strong meridional temperature gradients vs Barotropic • Horizontal wind shear important (d[u]/dy) • Uniform vertical structure BAROCLINIC • Momentum flux [v*u*] LIFE • Vertical wind shear important (d[u]/dz) important CYCLES (CYCLONE DEVELOPMENT) • Heat flux [v*θ*] important • Decay of storm tracks > cyclones • Growth of storm tracks > cyclones • Acceleration of zonal mean flow (jet stream)

ANNULAR MODES • Name: from “annulus” (“a ringshaped object - a region bounded by two concentric circles. ’’) • Are ‘’hemispheric scale patterns of climate variability’’ • Internal mid-latitude dynamics cause them (not air-sea coupling etc. ) • Most important modes of variability in mid-high latitudes • Explain 20 -30% of variability in field

BAROTROPIC ANNULAR MODE • Based on EOF of wind/geopotential/pressure fields • We know Southern (SAM, Antarctic oscillation) and Northern (NAM, Arctic oscillation) annular modes • Represent the jet shift poleward (positive Linked to phase)/equatorward (negative phase) Momentum flux > Impact weather • Timescale: 10 days Thompson and Woodworth (2014)

BAROCLINIC ANNULAR MODE • BAM: based on EOF of EKE • Represents pulsing (variations in amplitude) of EKE rather than a shift impacts to be determined • Related to oscillator model, periods 20 -30 days • Linked to heat flux 8 Thompson and Woodworth (2014)

(DE)COUPLING OF SAM & BAM • Thompson & Woodworth (2014) • SAM only affected by momentum fluxes (weak heat flux signal) • BAM only affected by heat fluxes (weak momentum flux signal) • No correlation between SAM and BAM • Baroclinic and barotropic modes of variability decoupled SAM & [v*u*] BAM & [v*θ*]

(DE)COUPLING OF SAM & BAM • Barotropic and baroclinic parts of the flow decoupled • Counter-intuitive compared to the general perspective from baroclinic life cycles (cyclones) • Baroclinic and barotropic responses linked through eddy growth and decay (momentum and heat fluxes linked) • I aim to understand this paradox theoretically & through modelling/modes of variability

UNIFIED MODEL 11

HELD-SUAREZ (HS) MODEL • After Held & Suarez (1994) (use: Unified Model vn 8. 6) • Resolution: 1. 25 o x 1. 875 o x 63 levels (32 km top level) & daily output • Run for 26 years + 4 years for spin-up • Dry model – no moisture • Very idealised: • Usually perpetual conditions, e. g. equinox > no seasonal cycle • Imposed relaxation to equilibrium temperature • Prescribed bottom friction • No asymmetries (only synoptic eddies/cyclones) • Results robust to more complex configurations

CLIMATOLOGY & FORCING Height (m) TEMPERATURE Equillibrium Temperature CLIMATOLOGY latitude Height (m) ZONAL MEAN WIND latitude

HELD-SUAREZ & MODEL HIERARCHIES • Simple model • no moisture, seasonal cycle, asymmetries • Can add complexity (easily) 14 Shaw et al. (2016)

RESULTS 15

SIMPLIFIED EKE BUDGET • Motivated by decoupling between baroclinic & barotropic modes • Model EKE using heat flux only & represent the rest as dissipation α, τ : constants Tendency Heat Flux Thompson et al. (2017) Dissipation 16

CROSS-SPECTRUM ANALYSIS • Follow Lorenz and Hartmann (2001) • Similar simplification for zonal momentum equation • Use spectral analysis to test EKE relationship FFT ω: angular frequency EKE Heat Flux Im Re Boljka et al. , JAS (2018) 17

CROSS-SPECTRUM ANALYSIS • EKE at 300 mb • Heat Flux at 850 mb • Average EKE & Heat Flux over a 20 o latitude band & take zonal mean • Plot imaginary and real parts of cross spectrum cross-spectrum decoupling INTERMEDIATE FREQUENCY coupling SYNOPTIC TIMESCALE frequency (cycles/day) Period = 10 days Im Re Cross Spectrum Boljka et al. , JAS (2018)

CONCLUSIONS • 2 timescales of annular mode variability • Synoptic timescale baroclinic life cycles (~10 days) • Cyclones; baroclinic & barotropic processes linked • Intermediate frequency oscillator model (~20 -30 days) • Baroclinic & barotropic processes decoupled • Implications: better understanding of climate variability • e. g. impacts on precipitation patterns, subseasonal prediction etc. • Unified Model can be used for hierarchies of models studies! Boljka et al. , JAS (2018) 19

THANK YOU FOR YOUR ATTENTION! ANY QUESTIONS? REFERENCES: Boljka, L. , T. G. Shepherd and M. Blackburn, 2018: On the coupling between barotropic and baroclinic modes of extratropical atmospheric variability. J. Atmos. Sci. , in press. 20