1. Comparison of Techniques for Isolating Equatorial Rossby Waves in Synoptic Studies Carl J. Schreck, III Department of Earth and Atmospheric Sciences University at Albany, SUNY

2. Introduction ● This study investigates Equatorial Rossby (ER) wave activity in the Central Pacific during the September-November 1997, the peak of a very strong El Nino. ● These waves might be associated with three events of twin tropical cyclogenesis. ● Waves are examined using three methods: – Unfiltered data – 15-40 day band-pass time filter – Space-time filter

3. Space-time Filters ● Wheeler and Kiladis (1999; hereafter WK99) – n = 1 ER wave filter that is bounded by the dispersion curves for U = 0 and 8 m < h < 90 m – Separates data into symmetric and antisymmetric components about the equator ● Roundy and Frank (2004) – ER wave filter expands beyond dispersion curves – Enforces no symmetry constraints ● Yang et al. (2003) – Data are broadly filtered into eastward and westward propagating components – Each component is then projected onto the meridional structure from shallow water modes – Allows for U ≠ 0

4. Unfiltered Hovmöller 850 hPa u contoured every 3 m s -1, with westerlies in red and easterlies in blue, averaged 2.5°S-2.5°N T b below 290 K shaded in 25 K intervals, averaged 10°S-10°N Locations of tropical cyclogenesis within 10.5° of equator, with open symbols for SH

5. 15-40 Day Hovmöller 850 hPa u contoured every 1 m s -1, with westerlies in red and easterlies in blue, averaged 2.5°S-2.5°N T b shaded every 5 K, with negative anomalies in warm colors and positive anomalies in cool colors, averaged 10°S-10°N Locations of tropical cyclogenesis within 10.5° of equator, with open symbols for SH

6. WK99 ER band filtered Hovmöller 850 hPa u contoured every 0.5 m s -1, with westerlies in red and easterlies in blue, averaged 2.5°S-2.5°N T b shaded every 2.5 K, with negative anomalies in warm colors and positive anomalies in cool colors, averaged 10°S-10°N Locations of tropical cyclogenesis within 10.5° of equator, with open symbols for SH

7. 15-40 Day Hovmöller 850 hPa u contoured every 1 m s -1, with easterly anomalies dashed Black line indicates objective wave tracks

8. 15-40 Day Hovmöller 850 hPa u contoured every 1 m s -1, with easterly anomalies dashed Black line indicates objective wave tracks Magenta line indicates linear best-fit of wave tracks Green numbers show best-fit phase speed (m s -1 ) Blue numbers show r 2 values -3.48 0.71 -4.82 0.89 -4.67 0.92

9. WK99 ER band filtered Hovmöller 850 hPa u contoured every 1 m s -1, with easterly anomalies dashed Black line indicates objective wave tracks Magenta line indicates linear best-fit of wave tracks Green numbers show best-fit phase speed (m s -1 ) Blue numbers show r 2 values -5.40 0.99 -4.73 1.00 -4.53 1.00

10. Interaction with Background Convergence Time-filtered variance Background zonal wind Background zonal divergence Space-time filtered variance

11. Unfiltered Map 850 hPa wind vectors 850 hPa heights contoured every 10 m T b below 290 K shaded in 25 K intervals

12. Time-Filtered Map 850 hPa wind anomaly vectors 850 hPa height anomalies contoured every 3 m, with positive anomalies in red and negative anomalies in blue T b shaded every 5 K, with negative anomalies in warm colors and positive anomalies in cool colors

13. Asymmetric Space-Time Filtered Map 850 hPa wind anomaly vectors 850 hPa height anomalies contoured every 2 m, with positive anomalies in red and negative anomalies in blue T b shaded every 2.5 K, with negative anomalies in warm colors and positive anomalies in cool colors

14. Symmetric Space-Time Filtered Map 850 hPa wind anomaly vectors 850 hPa height anomalies contoured every 2 m, with positive anomalies in red and negative anomalies in blue T b shaded every 2.5 K, with negative anomalies in warm colors and positive anomalies in cool colors

15. Summary ● Unfiltered Data – Strengths ● No concerns about artificial signals ● Easy in real-time – Weaknesses ● Waves obscured by other features

16. Summary ● 15-40 Day Band-pass Filtered Data – Strengths ● Wave packet shows clearly in Hovmöller ● Variance increases in a region of background zonal convergence ● Wind-height relationship resembles idealized mode – Weaknesses ● Difficult to determine phase speed

17. Summary ● WK99 Space-time Filter – Strengths ● Clearly separates each wave type ● Smooth westward propagation ● Waves can be clearly identified in maps, particularly with symmetry constraints – Weaknesses ● Low amplitude ● More difficult to examine the relationship between filtered variance and zonal divergence