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
Dec 22, 2015
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