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)

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: DJF

Methodology: Composite MJO Events MJO Index: Derived similarly to RMM except using days filtered OLR only The Western Hemisphere Zonal Wind Index (WHZI): days 200hPa zonal wind averaged 2.5 ° N to 2.5 ° S and 140 ° W to 40 ° W WHZI 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

At 200 hPa: 10°S-5°N, 110°W-80°W

10°S-5°N, 110°W-80°W Averaged Budget Terms: 200 hPa Kelvin wave in Shallow Water System Time Lag (Days)

Methodology: Zonal Momentum Budget Time linear decomposition of zonal momentum equation : Background State (Seasonal Cycle + Periods longer than 100 days) : Intraseasonal Timescale ( days) : Transient Timescale (Periods shorter than 20 days)

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

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 Shading: Variance of 200hPa Intraseasonal Zonal Wind Black contour: Zonal Convergence of 200hPa Background Zonal Wind (2 x 10 6 s -1 interval)

10°S-10°N Averaged Longitude-Pressure Cross Sections Shading: intraseasonal zonal wind anomaly Black 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 H Kelvin Suppressed MJO H H

Additional Figures