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An analysis of tropical instability waves in a numerical model of the Pacific Ocean - 1. Spatial variability of the waves
Language
English
Obiettivo Specifico
4A. Clima e Oceani
Status
Published
JCR Journal
JCR Journal
Peer review journal
Yes
Title of the book
Issue/vol(year)
C12/104 (1999)
ISSN
0148-0227
Publisher
American Geophysical Union
Pages (printed)
29613-29635
Issued date
December 15, 1999
Abstract
Unstable oscillations confined within the mixed layer close to the
equator are generated
in wind-forced
experiments
performed in a multilevel general
circulation model configured
for the tropical Pacific Ocean. The experiments
indicate that the waves develop preferentially in the eastern Pacific along the
northern temperature front. However, there is clear evidence of a second
unstable
region
along the southern
temperature
front in the central Pacific. In both regions
the instabilities propagate westward, but in the central Pacific their phase speed
is considerably
smaller. The differences
between the wave characteristics
in the
eastern and central Pacific are closely correlated to the differences
in the time
mean conditions of the flow. The eastern instabilities have a structure with two
peaks in amplitude: one located on the equator and the other a few degrees
north
of it. Their dispersion
characteristics
show many similarities to those of tropical
instability
waves
(TIWs) observed
in the Pacific
Ocean,
while the instabilities
which
grow in the central Pacific do not have any known observed
correspondents.
We
explore the spatial variability of the simulated waves through a wavelet analysis,
which provides
detailed results on how the period and wavelength of the instabilities
change
as a function
of longitude,
latitude, and depth. The wavelet
analysis
reveals
that in the eastern Pacific and close to the surface the TIWs have a phase speed
of-48 cm/s, while in the central
Pacific
they have
a phase
speed
of-11 cm/s. In
particular, the change
in the phase
speed
is due to a change
in the dominant period
of the TIWs: The period of the central Pacific instabilities is considerably
longer
than the period of the instabilities present in the eastern Pacific.
equator are generated
in wind-forced
experiments
performed in a multilevel general
circulation model configured
for the tropical Pacific Ocean. The experiments
indicate that the waves develop preferentially in the eastern Pacific along the
northern temperature front. However, there is clear evidence of a second
unstable
region
along the southern
temperature
front in the central Pacific. In both regions
the instabilities propagate westward, but in the central Pacific their phase speed
is considerably
smaller. The differences
between the wave characteristics
in the
eastern and central Pacific are closely correlated to the differences
in the time
mean conditions of the flow. The eastern instabilities have a structure with two
peaks in amplitude: one located on the equator and the other a few degrees
north
of it. Their dispersion
characteristics
show many similarities to those of tropical
instability
waves
(TIWs) observed
in the Pacific
Ocean,
while the instabilities
which
grow in the central Pacific do not have any known observed
correspondents.
We
explore the spatial variability of the simulated waves through a wavelet analysis,
which provides
detailed results on how the period and wavelength of the instabilities
change
as a function
of longitude,
latitude, and depth. The wavelet
analysis
reveals
that in the eastern Pacific and close to the surface the TIWs have a phase speed
of-48 cm/s, while in the central
Pacific
they have
a phase
speed
of-11 cm/s. In
particular, the change
in the phase
speed
is due to a change
in the dominant period
of the TIWs: The period of the central Pacific instabilities is considerably
longer
than the period of the instabilities present in the eastern Pacific.
Sponsors
This work was supported by the Department of Commerce/NOAA grant NA56GP0026. One of the authors (SM) was partially supported by a NASA Global Change Fellowship NGT-30288.
References
Bryan, K., and M.D. Cox, A numerical investigation of the
ocean general circulation, Tellus, 19, 54-80, 1967.
Combes, J.M., .A. Grossmann, and P. Tchamitchian, Wave-
lets: Time-Frequency Methods and Phase Space, 315 pp.,
Springer-Verlag, New York, 1989.
Cox, M.D., Generation and propagation of 30-day waves in
a numerical model of the Pacific, J. Phys. Oceanogr., 10,
1168-1186, 1980.
Cox, M.D., A primitive equation, 3-dimensional model of
the ocean, Ocean Group Tech. Rep. 1, 143 pp., Geophys.
Fluid Dyn. Lab., Princeton, N.J., 1984.
Eriksen, C.C., and J.G. Richman, An estimate of equatorial
wave energy flux at 9- to 90-day periods in the central
Pacific, J. Geophys.
Res., 93, 15,455-15,466, 1988.
Farge, M., Wavelet transform and their application to tur-
bulence, Ann. Rev. Fluid Mech., 2•, 395-457, 1992.
Halpern, D., Detection of 17.5-day period meridional current
oscillations
in the equatorial western
Pacific Ocean during
1985, Geophys. Res. Left., 16, 499-502, 1989.
Halpern, D., R.A. Knox, and D.S. Luther, Observations of
20-day period meridional current oscillations in the upper
ocean along the Pacific equator, J. Phys. Oceanogr., 18,
1514-1534, 1988.
Kuroda, Y., and M.J. McPhaden, Variability in the west-
ern equatorial Pacific Ocean during japanese Pacific cli-
mate study cruises in 1989 and 1990, J. Geophys. Res.,
98, 4747-4759, 1993.
Legeckis, R., Long waves in the eastern equatorial Pacific
Ocean: A view from a geostationary satellite, Science,
197, 1179-1181, 1977.
Levitus, S., Climatological atlas of the world ocean,
NOAA Prof. Pap. 13, U.S. Govt. Print. Office, Wash-
ington D.C., 1982.
Masina, S., Tropical instability waves in the Pacific Ocean,
Ph.D. thesis, 242 pp., Princeton Univ., Princeton, N.J.,
1996.
Masina, S., S.G.H. Philander, and A.B.G. Bush, An analysis
of tropical instability waves in a numerical model of the
Pacific Ocean, 2, Generation and energetics
of the waves,
J. Geophys. Res., this issue.
McCreary, J.P., Jr, and Z. Yu, Equatorial dynamics in a
2•-layer model, Prog. Oceanogr.,
29, 61-132, 1992.
McPhaden, M.J., Monthly period oscillations in the Pacific
North Equatorial Countercurrent, J. Geophys. Res., 101,
6337-6359, 1996.
McPhaden, M.J., and B.A. Taft, Dynamics of seasonal and
intraseasonal variability in the eastern equatorial Pacific,
J. Phys. Oceanogr., 18, 1713-1732, 1988.
Meyers, S.D., B.G. Kelly, and J.J. O'Brien, An introduc-
tion to wavelet analysis in oceanography and meteorol-
ogy: With application to dispersion
of Yanai waves, Mon.
Weather Rev., 121, 2858-2866, 1993.
Orlanski I., Localized baroclinicity: A source for meso-c,
cyclones, J. Atmos. Sci., •3, 2857-2885, 1986.
Orlanski I., and M.D. Cox, Baroclinic instability in ocean
currents, Geophys. Fluid Dy., •, 297-332, 1973.
Pacanowski, R.C., Effect of equatorial currents on surface
stress, J. Phys. Oceanogr., 11, 1443-1451, 1987.
Pacanowski, R.C., and S.G.H. Philander, Parameterization
of vertical mixing in numerical models of tropical oceans,
J. Phys. Oceanogr., 11, 1443-1451, 1981.
Pacanowski, R., K. Dixon, and A. Rosati, The G.F.D.L.
modular ocean model users guide, Ocean Group Tech.
Rep. 2, 1993.
Philander, S.G.H., Instabilities of zonal equatorial currents,
J. Geophys. Res., 81, 3725-3735, 1976.
Philander, S.G.H., Instabilities of zonal equatorial currents,
2, J. Geophys.
Res., 83, 3679-3682, 1978a.
Philander, S.G.H., Forced oceanic
waves,
Rev. Geophys.,
16,
15-46, 1978b.
Philander, S.G.H., Equatorial waves in the presence of the
equatorial undercurrent, J. Phys. Oceanogr., 9, 254-262,
1979.
Philander, S.G.H., and R.C. Pacanowski, A model of the
seasonal
cycle in the tropical Atlantic Ocean, J. Geophys.
Res., 91, 14,192-14,206, 1986.
Philander, S.G.H., D. Halpern, D. Hansen, R. Legeckis,
L.Miller, C. Paul, R. Watts, R. Weisberg, and M. Wim-
bush, Long waves in the equatorial Pacific Ocean, Eos
Trans. A GU, 66, 14, 154-156, 1985.
Philander, S.G.H., W.J. Hurlin, and R.C. Pacanowski,
Prop-
erties of long equatorial waves in models of the seasonal
cycle in the tropical Atlantic and Pacific Oceans, J. Geo-
phys. Res., 91, 14,207-14,211, 1986.
Philander, S.G.H., W.J. Hurlin and R.C. Pacanowski, Ini-
tial condition for a general circulation model of tropical
oceans, J. Phys. Oceanogr., 17, 147-157, 1987.
Proehl, J.A., Linear stability of equatorial flows, J. Phys.
Oceanogr., 26, 601-621, 1996.
Qiao, L., and R.H. Weisberg, Tropical instability wave kine-
matics: Observations from the Tropical Instability Wave
Experiment, J. Geophys. Res., 100, 8677-8693, 1995.
Rioul, O., and M. Vetterli, Wavelets and signal processing,
IEEE Signal Process. Mag., 14-38, 1991.
Schopf,
P.S., and M.A. Cane, On equatorial dynamics, mixed
layer physics and sea surface temperature,
J. Phys. Oceanogr., 13, 917-935, 1983.
Seigel,
A.D., A comment on long waves
in the Pacific Ocean,
J. Phys. Oceanogr., 15, 1881-1883, 1985.
Semtner, A.J., Jr, and W.R. Holland, Numerical simulation
of equatorial ocean circulation, I, A basic case
in turbulent
equilibrium, J. Phys. Oceanogr., 10, 667-693, 1980.
Weisberg, R.H., Equatorial waves during GATE and their
relation to the mean zonal circulation, Deep Sea Res. Part
A, 26, suppl. 2, 179-198, 1979.
Weisberg, R.H., and A.M. Horigan, Low-frequency variabil-
ity in the equatorial Atlantic, J. Phys. Oceanogr.,
11, 913-
920, 1981.
Weisberg, R.H., A. Horigan, and C. Colin, Equatorially
trapped Rossby-gravity wave propagation in the Gulf of
Guinea, J. Mar. Res., 37, 67-86, 1979.
Yu, Z., J.P. McCreary Jr., and J.A. Proehl, Meridional
asymmetry and energetics of tropical instability waves,
J. Phys. Oceanogr., 25, 2997-3007, 1995.
ocean general circulation, Tellus, 19, 54-80, 1967.
Combes, J.M., .A. Grossmann, and P. Tchamitchian, Wave-
lets: Time-Frequency Methods and Phase Space, 315 pp.,
Springer-Verlag, New York, 1989.
Cox, M.D., Generation and propagation of 30-day waves in
a numerical model of the Pacific, J. Phys. Oceanogr., 10,
1168-1186, 1980.
Cox, M.D., A primitive equation, 3-dimensional model of
the ocean, Ocean Group Tech. Rep. 1, 143 pp., Geophys.
Fluid Dyn. Lab., Princeton, N.J., 1984.
Eriksen, C.C., and J.G. Richman, An estimate of equatorial
wave energy flux at 9- to 90-day periods in the central
Pacific, J. Geophys.
Res., 93, 15,455-15,466, 1988.
Farge, M., Wavelet transform and their application to tur-
bulence, Ann. Rev. Fluid Mech., 2•, 395-457, 1992.
Halpern, D., Detection of 17.5-day period meridional current
oscillations
in the equatorial western
Pacific Ocean during
1985, Geophys. Res. Left., 16, 499-502, 1989.
Halpern, D., R.A. Knox, and D.S. Luther, Observations of
20-day period meridional current oscillations in the upper
ocean along the Pacific equator, J. Phys. Oceanogr., 18,
1514-1534, 1988.
Kuroda, Y., and M.J. McPhaden, Variability in the west-
ern equatorial Pacific Ocean during japanese Pacific cli-
mate study cruises in 1989 and 1990, J. Geophys. Res.,
98, 4747-4759, 1993.
Legeckis, R., Long waves in the eastern equatorial Pacific
Ocean: A view from a geostationary satellite, Science,
197, 1179-1181, 1977.
Levitus, S., Climatological atlas of the world ocean,
NOAA Prof. Pap. 13, U.S. Govt. Print. Office, Wash-
ington D.C., 1982.
Masina, S., Tropical instability waves in the Pacific Ocean,
Ph.D. thesis, 242 pp., Princeton Univ., Princeton, N.J.,
1996.
Masina, S., S.G.H. Philander, and A.B.G. Bush, An analysis
of tropical instability waves in a numerical model of the
Pacific Ocean, 2, Generation and energetics
of the waves,
J. Geophys. Res., this issue.
McCreary, J.P., Jr, and Z. Yu, Equatorial dynamics in a
2•-layer model, Prog. Oceanogr.,
29, 61-132, 1992.
McPhaden, M.J., Monthly period oscillations in the Pacific
North Equatorial Countercurrent, J. Geophys. Res., 101,
6337-6359, 1996.
McPhaden, M.J., and B.A. Taft, Dynamics of seasonal and
intraseasonal variability in the eastern equatorial Pacific,
J. Phys. Oceanogr., 18, 1713-1732, 1988.
Meyers, S.D., B.G. Kelly, and J.J. O'Brien, An introduc-
tion to wavelet analysis in oceanography and meteorol-
ogy: With application to dispersion
of Yanai waves, Mon.
Weather Rev., 121, 2858-2866, 1993.
Orlanski I., Localized baroclinicity: A source for meso-c,
cyclones, J. Atmos. Sci., •3, 2857-2885, 1986.
Orlanski I., and M.D. Cox, Baroclinic instability in ocean
currents, Geophys. Fluid Dy., •, 297-332, 1973.
Pacanowski, R.C., Effect of equatorial currents on surface
stress, J. Phys. Oceanogr., 11, 1443-1451, 1987.
Pacanowski, R.C., and S.G.H. Philander, Parameterization
of vertical mixing in numerical models of tropical oceans,
J. Phys. Oceanogr., 11, 1443-1451, 1981.
Pacanowski, R., K. Dixon, and A. Rosati, The G.F.D.L.
modular ocean model users guide, Ocean Group Tech.
Rep. 2, 1993.
Philander, S.G.H., Instabilities of zonal equatorial currents,
J. Geophys. Res., 81, 3725-3735, 1976.
Philander, S.G.H., Instabilities of zonal equatorial currents,
2, J. Geophys.
Res., 83, 3679-3682, 1978a.
Philander, S.G.H., Forced oceanic
waves,
Rev. Geophys.,
16,
15-46, 1978b.
Philander, S.G.H., Equatorial waves in the presence of the
equatorial undercurrent, J. Phys. Oceanogr., 9, 254-262,
1979.
Philander, S.G.H., and R.C. Pacanowski, A model of the
seasonal
cycle in the tropical Atlantic Ocean, J. Geophys.
Res., 91, 14,192-14,206, 1986.
Philander, S.G.H., D. Halpern, D. Hansen, R. Legeckis,
L.Miller, C. Paul, R. Watts, R. Weisberg, and M. Wim-
bush, Long waves in the equatorial Pacific Ocean, Eos
Trans. A GU, 66, 14, 154-156, 1985.
Philander, S.G.H., W.J. Hurlin, and R.C. Pacanowski,
Prop-
erties of long equatorial waves in models of the seasonal
cycle in the tropical Atlantic and Pacific Oceans, J. Geo-
phys. Res., 91, 14,207-14,211, 1986.
Philander, S.G.H., W.J. Hurlin and R.C. Pacanowski, Ini-
tial condition for a general circulation model of tropical
oceans, J. Phys. Oceanogr., 17, 147-157, 1987.
Proehl, J.A., Linear stability of equatorial flows, J. Phys.
Oceanogr., 26, 601-621, 1996.
Qiao, L., and R.H. Weisberg, Tropical instability wave kine-
matics: Observations from the Tropical Instability Wave
Experiment, J. Geophys. Res., 100, 8677-8693, 1995.
Rioul, O., and M. Vetterli, Wavelets and signal processing,
IEEE Signal Process. Mag., 14-38, 1991.
Schopf,
P.S., and M.A. Cane, On equatorial dynamics, mixed
layer physics and sea surface temperature,
J. Phys. Oceanogr., 13, 917-935, 1983.
Seigel,
A.D., A comment on long waves
in the Pacific Ocean,
J. Phys. Oceanogr., 15, 1881-1883, 1985.
Semtner, A.J., Jr, and W.R. Holland, Numerical simulation
of equatorial ocean circulation, I, A basic case
in turbulent
equilibrium, J. Phys. Oceanogr., 10, 667-693, 1980.
Weisberg, R.H., Equatorial waves during GATE and their
relation to the mean zonal circulation, Deep Sea Res. Part
A, 26, suppl. 2, 179-198, 1979.
Weisberg, R.H., and A.M. Horigan, Low-frequency variabil-
ity in the equatorial Atlantic, J. Phys. Oceanogr.,
11, 913-
920, 1981.
Weisberg, R.H., A. Horigan, and C. Colin, Equatorially
trapped Rossby-gravity wave propagation in the Gulf of
Guinea, J. Mar. Res., 37, 67-86, 1979.
Yu, Z., J.P. McCreary Jr., and J.A. Proehl, Meridional
asymmetry and energetics of tropical instability waves,
J. Phys. Oceanogr., 25, 2997-3007, 1995.
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