content.
Feedback of Tropical Instability-Wave-Induced Atmospheric Variability onto the Ocean
Feedback of Tropical Instability-Wave-Induced Atmospheric Variability
onto the Ocean
H
YODAE
S
EO
Scripps Institution of Oceanography, La Jolla, California
M
ARKUS
J
OCHUM
National Center for Atmospheric Research, Boulder, Colorado
R
AGHU
M
URTUGUDDE
ESSIC/DAOS, University of Maryland, College Park, College Park, Maryland
A
RTHUR
J. M
ILLER AND
J
OHN
O. R
OADS
Scripps Institution of Oceanography, La Jolla, California
(Manuscript received 27 September 2006, in final form 10 April 2007)
ABSTRACT
The effects of atmospheric feedbacks on tropical instability waves (TIWs) in the equatorial Atlantic
Ocean are examined using a regional high-resolution coupled climate model. The analysis from a 6-yr
hindcast from 1999 to 2004 reveals a negative correlation between TIW-induced wind perturbations and
TIW-induced ocean currents, which implies damping of the TIWs. On the other hand, the feedback effect
from the modification of Ekman pumping velocity by TIWs is small compared to the contribution to TIW
growth by baroclinic instability. Overall, the atmosphere reduces the growth of TIWs by adjusting its wind
response to the evolving TIWs. The analysis also shows that including ocean current (mean
TIWs) in the
wind stress parameterization reduces the surface stress estimate by 15%20% over the region of the South
Equatorial Current. Moreover, TIW-induced perturbation ocean currents can significantly alter surface
stress estimations from scatterometers, especially at TIW frequencies. Finally, the rectification effect from
the atmospheric response to TIWs on latent heat flux is small compared to the mean latent heat flux.
1. Introduction
Tropical instability waves (TIWs) are generated from
instabilities of equatorial zonal currents and are a com-
mon feature in both the tropical Atlantic (Düing et al.
1975) and Pacific Oceans (Legeckis 1977; Legeckis et
al. 1983). Observations reveal TIWs as westward propa-
gating wavelike oscillations of the sea surface tempera-
ture (SST) near the equator with a typical wavelength
of
10° longitude and a phase speed of
0.5 m s
1
(Weisberg and Weingartner 1988; Qiao and Weisberg
1995, and references therein). A detailed study of TIWs
is necessary because they are an important element in
the momentum balance (Weisberg 1984) and equatorial
ocean heat budget (Hansen and Paul 1984; Bryden
and Brady 1989; Baturin and Niiler 1997; Jochum and
Murtugudde 2006).
Numerous studies have discussed the generation
mechanisms and energetics of TIWs. Analytical studies
by Philander (1976, 1978) showed that meridional shear
of the zonal currents leads to a barotropic conversion of
mean kinetic energy to eddy kinetic energy (EKE),
which supports the growth of waves with wavelengths
and periods similar to those of the observed TIWs. Cox
(1980) showed that baroclinic instability, though less
important, is also a source of the EKE that is drawn
from the mean potential energy. In addition, frontal
instability (Yu et al. 1995) and KelvinHelmholtz insta-
bility (Proehl 1996) were shown to be important EKE
Corresponding author address: Hyodae Seo, Climate Research
Division, Scripps Institution of Oceanography, 9500 Gilman Dr.,
Mail Code 0224, La Jolla, CA 92093.
E-mail: hyseo@ucsd.edu
5842
J O U R N A L O F C L I M A T E
V
OLUME
20
DOI:10.1175/2007JCLI1700.1
JCLI4330 sources for the TIWs. A more comprehensive numeri-
cal study of the generation and the energetics of Pacific
TIWs has shown that the northern temperature front is
baroclincally unstable, while shear of the zonal currents
causes barotropic instability at the equator (Masina et
al. 1999, hereafter MPB). These two different instabili-
ties are phase locked and are both important energy
sources for TIWs. On the other hand, Jochum et al.
(2004, hereafter JMB) found that barotropic instability
in the Atlantic was dominant in their energy budget and
the baroclinic term was less important.
Chelton et al. (2004) and Xie (2004) demonstrated
that oceanatmosphere interactions involving the oce-
anic mesoscale occur throughout the World Ocean.
SST on this scale induces wind response in the atmo-
spheric boundary layer through modification of the ver-
tical turbulent mixing (Wallace et al. 1989; Hayes et al.
1989). Air over the warm water is destabilized, and
increased turbulent mixing of momentum accelerates
near-surface winds. Conversely, cold SST suppresses
the momentum mixing, decouples the near-surface
wind from wind aloft, and hence decreases the near-
surface wind. Small et al. (2003) and Cronin et al.
(2003) reported that the pressure gradient mechanism
of Lindzen and Nigam (1987) is likely to be an impor-
tant mechanism as well. Furthermore, Chelton et al.
(2001) showed that undulating SST fronts by TIWs fur-
ther affect the perturbation wind stress derivatives in
the atmosphere, suggesting a possible feedback from
the atmosphere to the TIWs through Ekman dynamics.
The lack of simultaneous measurements of ocean cur-
rents and wind stresses on the TIW scale makes it dif-
ficult to quantify in great detail the feedbacks from the
perturbation wind field on the TIWs.
Pezzi et al. (2004) modeled this SSTwind coupling
and showed that it reduces variability of TIWs. Their
simple coupling parameterization included the effect of
TIW-induced SST variations directly on the wind fields
and through the modification in wind stress derivatives.
Seo et al. (2007) used a full-physics high-resolution re-
gional coupled model to explore several aspects of the
tropical Pacific TIWs, reproducing the observed cou-
pling strength as a function of SST gradient.
Recent findings on this close coupling between the
ocean and the atmosphere at the oceanic mesoscale
raise new questions that have been largely unexplored
in the aforementioned studies. What is the role of the
wind response in the energy budget of TIWs? How do
the atmospheric feedbacks amplify or dampen the
TIWs? What is the rectification effect on the mean sur-
face heat flux from the atmospheric response to the
TIWs? These questions will be addressed in the present
study, which is among the first of its kind using a re-
gional coupled oceanatmosphere model at eddy-
resolving resolution.
In the present study, the regionally coupled high-
resolution model of Seo et al. (2007) is used to quantify
the contribution of tropical Atlantic oceanatmosphere
covariability to the energetics of the TIWs. It is shown
that the direct response of winds to the TIW-induced
SST imposes a negative feedback on the growth of
TIWs. It is also shown that perturbation Ekman pump-
ing due to TIWs (Chelton et al. 2001) is a very small
forcing effect compared to baroclinic instability in the
equatorial ocean.
It is also argued that ocean currents (mean
TIWs)
substantially reduce the surface stress estimation by
15%20% over the large area of the South Equatorial
Current. Moreover, TIW-induced perturbation ocean
currents can significantly alter the local surface stress
estimate during the active TIW season. This suggests
that numerical studies of TIWs will suffer from a con-
sistency problem when the model is forced with the
observed winds such as scatterometer wind stresses.
Last, perturbation latent heat flux generated at the
sea surface by evolving TIWSST is small compared to
the contribution from the mean component, indicating
only a weak rectification effect on the ocean from these
high-frequency perturbations.
In section 2, the model and experiment designed for
this study are explained. In section 3, the main results of
the study are discussed, followed by the conclusions
and summary in section 4.
2. Model and experiment
The coupled model used for the present study is the
Scripps Coupled OceanAtmospheric Regional
(SCOAR) model (Seo et al. 2007). It combines two
well-known, state-of-the-art regional atmosphere and
ocean models using a fluxSST coupling strategy. The
atmospheric model is the Experimental Climate Predic-
tion Center (ECPC) Regional Spectral Model (RSM)
and the ocean model is the Regional Ocean Modeling
System (ROMS).
The RSM, originally developed at the National Cen-
ters for Environmental Prediction (NCEP) is described
in Juang and Kanamitsu (1994) and Juang et al. (1997).
The code was later updated with greater flexibility and
much higher efficiency (Kanamitsu et al. 2005; Kana-
maru and Kanamitsu 2007). Briefly, it is a limited-area
primitive equation atmospheric model with a perturba-
tion method in spectral computation, and utilizes a ter-
rain-following sigma coordinate system (28 levels). The
model physics are the same as for the NCEP global
seasonal forecast model (Kanamitsu et al. 2002a) and
1 D
ECEMBER
2007
S E O E T A L .
5843 NCEPNational Center for Atmospheric Research
(NCAR) reanalysis model (Kalnay et al. 1996) except
for the parameterization of convection and radiative
processes.
The ROMS solves the incompressible and hydro-
static primitive equations with a free surface on hori-
zontal curvilinear coordinates and utilizes stretched
generalized sigma coordinates in order to enhance ver-
tical resolution near the sea surface and bathymetry.
The details of the model can be found in Haidvogel et
al. (2000) and Shchepetkin and McWilliams (2005).
A fluxSST coupler bridges the atmospheric (RSM)
and ocean (ROMS) models. The coupler works in a
sequential fashion; th