Transcript Document

Kelvin Waves
as Observed by the SABER Instrument
on the TIMED Spacecraft
Jeffrey M. Forbes, Xiaoli Zhang, Saburo Miyahara,
Scott E. Palo, James Russell, Christopher J. Mertens and Martin Mlynczak
This Paper:
• Main Focus on Equatorial Temperatures, 20-120 km
• Wavenumber vs. Period Spectra as a Function of Height
• Ultra-Fast Kelvin Waves (UFKW), Periods 2.5-4.5 days
• Intraseasonal Oscillation (ISO) of UFKW and
Zonal Mean Temperatures
Data processing
• Sliding 60-day window, 1 day at a time, covering all local times and longitudes
• Extract zonal mean, diurnal & semidiurnal solar & lunar tides,
& stationary planetary waves
• Analyze residuals from above fit
Raw Temperature Residuals at Equator
30 km: ~10-day eastward-propagating
structures (Kelvin waves) clearly visible
ascending
descending
October-November, 2005
30 km
90 km: Larger amplitudes, but no clear patterns.
Kelvin waves probably masked by day-today variability of tides, gravity waves, etc.
ascending
descending
October-November, 2005
90 km
Data processing: sliding fits performed
• zonal wavenumbers s = -6 (eastward) to s = +6 (westward)
• periods 2 to 20 days in increments of 0.5 days
• window length = 3 x wave period
• all data during 2002-2006
Multi-Year
Mean Spectrum
6-7 day oscillation
“Background
spectrum” due
to various
sources of
variability, e.g.,
tides, gravity
waves
3.0-3.5 day Ultra-fast
Kelvin Wave
~16-day normal mode
ISO, possibly
driven by UFKW &
diurnal tide EPFD
(e.g.,Miyoshi &
Fujiwara,
GRL, 2006)
Dominant Kelvin waves (s = -1, s = -2) transition from long-periods (5-10
days) and short-wavelengths (9-13 km) in the stratosphere, to shorter periods
(2-3 days) and longer wavelengths (35-45 km) in the MLT
2  x

z 

u


N T
z = vertical wavelength
35-42
47
21
14
x = zonal wavelength
T = wave period
N = buoyancy frequency
155
ubar = zonal mean u (= 0)
58
e.g. Holton et al. (2001)
30
18
10
116
38
Zonal phase speed ms-1
In Addition to Kelvin Waves,
Other Parts of the Spectrum also Vary with Height, e.g., s = 0
Slow Kelvin
waves 13
9
9
21
14
37
30 Ultra fast Kelvin waves
17
35-42
Results similar to the previous were obtained by examining the
symmetric component of the temperature residuals
No notable results were obtained when the anti-symmetric
component of the temperature residuals was examined.
We now concentrate on
• MLT Kelvin waves, periods 2.5-4.5 days, i.e., UFKW
• Characterizing IS variability of MLT UFKW, and possible
connections with IS variability of the zonal mean state
Ultra-Fast Kelvin Waves (UFKW), Diurnal Tides (DT)
and Intraseasonal Oscillations (ISO) in the MLT
• In the context of a full-atmosphere GCM, Miyoshi and Fujiwara (2006)
established connections between EPFD due to DT and UFKW, and 20-60
day ISO in zonal mean winds.
• Variations in DT and UFKW are connected with established troposphere
ISO’s at 20-25 days (Hartmann et al., 1992) and 40-60 days (Madden and
Julian, 1994) manifested in tropical convection, e.g., latent heating rates.
• Existence of UFKW are well-established in the tropical MLT:
Lieberman and Riggin (1997), Riggin et al. (1997), Yoshida et al. (1999)
• Previous similar suggestions and supportive observations relating
waves and ISO in the MLT provided by Eckerman et al. (1997), Isoda et al.
(2004), Lieberman et al. (1998).
The SABER data provide the first look at the above
that extends continuously from 20-120 km, -50o to +50o
latitude, 2002 to 2006
ISO of 2.5-4.5 day Wave Amplitudes, 90 km, Eastward s = -1
UFKW and Zonal Mean Variability at the Equator, 2003
3.3 km day-1
Filtered zonal mean 20-60 days ~± 2-4K
Spectra show some similarities, but not close correspondence. However,
the “UFKW” omits the effects of longer-period and s  -1 KW & DT
UFKW
Zonal Mean
SUMMARY & CONCLUSIONS
• SABER temperature data provide the first opportunity to “see” vertical
coupling from the lower stratosphere to lower thermosphere in the equatorial
region vis-à-vis vertically-propagating waves with periods > 2 days.
• The dominant waves responsible for this coupling are symmetric eastwardpropagating waves, i.e., Kelvin waves.
• Dominant Kelvin waves transition from long-periods (5-10 days) and shortwavelengths (9-13 km) in the stratosphere, to shorter periods (2-3 days) and
longer wavelengths (35-45 km) in the MLT.
• UFKW (periods 2.5-4.5 days) intermittently exist at similar amplitudes (3-10
K, 80-120 km) during all months of the year, with variability in the 20-60 day
range.
• An ISO of zonal mean temperatures also exists with periods 20-60 days that
may be driven by EPFD due, at least in part, to UFKW.
• The zonal mean ISO preferentially exists above 70 km, consistent with insitu generation at these altitudes.
• Possible F-region effects of UFKW vis-à-vis dynamo, similar to DE3?
(see Takahashi et al. Paper 4.2-10)