Comparison of Techniques for Isolating Equatorial Rossby Waves in Synoptic Studies Carl J. Schreck,...

Comparison of Techniques for Isolating Equatorial Rossby Waves in Synoptic Studies

Carl J. Schreck, III Department of Earth and Atmospheric

SciencesUniversity at Albany, SUNY

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

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

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

•Tb 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

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

•Tb 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

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

•Tb 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

15-40 Day Hovmöller

•850 hPa u contoured every 1 m s-1, with easterly anomalies dashed

•Black line indicates objective wave tracks

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 r2 values

-3.480.71

-4.820.89

-4.670.92

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 r2 values

-5.400.99

-4.731.00

-4.531.00

Interaction with Background Convergence

Time-filtered variance

Background zonal wind

Background zonal divergence

Space-timefiltered variance

Unfiltered Map•850 hPa wind vectors

•850 hPa heights contoured every 10 m

•Tb below 290 K shaded in 25 K intervals

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

•Tb shaded every 5 K, with negative anomalies in warm colors and positive anomalies in cool colors

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

•Tb shaded every 2.5 K, with negative anomalies in warm colors and positive anomalies in cool colors

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

•Tb shaded every 2.5 K, with negative anomalies in warm colors and positive anomalies in cool colors

Summary

● Unfiltered Data– Strengths

● No concerns about artificial signals● Easy in real-time

– Weaknesses● Waves obscured by other features

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

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