The Dynamics of Western Hemisphere Circulation

Evolution in the MJO

Naoko Sakaeda and Paul Roundy

Dept. Atmospheric and Environmental Sciences

From Roundy (2014, J. Atmos. Sci)

From Roundy (2014, J. Atmos. Sci)

Data

• Wind data from CFS Reanalysis data (Saha et al. 2010)

• Interpolated Outgoing Longwave Radiation (OLR) as proxy for tropical convection (Liebmann and Smith 1996)

• Period: 1980-2010 DJF

Methodology: Composite MJO Events

• MJO Index: Derived similarly to RMM except using 20-100days filtered OLR only

• The Western Hemisphere Zonal Wind Index (WHZI): 20-100 days 200hPa zonal wind averaged 2.5°N to 2.5°S and 140°W to 40°W

WHZI MJO

MJO

MJO

Methodology: MJO Events

• Select MJO Phase 1 events with amplitude greater than or equal to 0.5 during DJF

• WHZI<0: Easterly Wind Events

• WHZI>0: Westerly Wind Events

Methodology: Zonal Momentum Budget

Xfvx

guvt

u

Methodology: Zonal Momentum Budget

At 200 hPa: 10°S-5°N, 110°W-80°W

10°S-5°N, 110°W-80°W Averaged Budget Terms: 200 hPa

xg

t

u

Kelvin wave in

Shallow Water System

Time Lag (Days)

Methodology: Zonal Momentum Budget

• Time linear decomposition of zonal momentum equation

Xfvx

guvt

u

'* uuuu : Background State (Seasonal Cycle + Periods longer than 100 days)

: Intraseasonal Timescale (20-100 days)

: Transient Timescale (Periods shorter than 20 days)

u*u'u

**

''*****

fvx

guvuvuvuvt

u

10°S-5°N, 110°W-80°W Averaged Budget Terms 200 hPa

acts to maintain and amplify but it is offset by the pressure gradient force

x

*

10°S-5°N, 110°W-80°W Averaged Budget Terms 200 hPa

10°S-5°N, 130°W-100°W Averaged Budget Terms, 100 hPa

10°S-5°N, 130°W-100°W Averaged Budget Terms, 100 hPa

10°S-5°N, 110°W-80°W Averaged Budget Terms

p

uωu

y

uvu

x

uu

uvut

u

t

uC

*****

***

2

1,

2

1

2

22

Shading: Variance of 200hPa Intraseasonal Zonal WindBlack contour: Zonal Convergence of 200hPa Background Zonal Wind (2 x 106 s-1 interval)

10°S-10°N Averaged Longitude-Pressure Cross Sections

Shading: intraseasonal zonal wind anomalyBlack contour: intraseasonal geopotential height anomaly at 2 m interval

Eastward and Westward-Moving OLR, 100hPa Wind, and Geopotential Height Anomalies

R

R

R

R

A

A

A

A

Summary

• Upper-tropospheric intraseasonal wind over the Western Hemisphere cannot simply be explained as a “free” Kelvin wave generated by MJO convection

MJO

H

HKelvin KelvinSuppressed

MJO

H

H

Additional Figures