Pacific
Studies:
Phenomena important for tropical Pacific climate variability
include interannual variability of the El Niņo-Southern Oscillation
(ENSO) and Pacific decadal variability. Since the Pacific Ocean
is the largest ocean on the Earth, its climate phenomena largely
affect weather around the world. Better description and understanding
of these Pacific phenomena and their associated mechanisms are
important steps toward finally providing reliable climate prediction
for the general public. AOML has been conducting observational,
numerical modeling, and theoretical studies of ENSO, Pacific
decadal variability, ocean-related processes in the inter-tropical
convergence zone/cold tongue region, impact of NCEP model initializations
on ENSO prediction, and response of greenhouse gases to El Niņo.
Observational
studies performed at AOML show that ENSO displays western Pacific
anomaly patterns in addition to eastern Pacific anomaly patterns
(Figure 8). During the warm phase
of ENSO, warm SST and low sea level pressure (SLP) anomalies
in the equatorial eastern Pacific and low outgoing longwave
radiation (OLR) anomalies in the equatorial central Pacific
are accompanied by cold SST and high sea level pressure anomalies
in the off-equatorial western Pacific and high outgoing longwave
radiation anomalies in the off-equatorial far western Pacific.
Also, while the zonal wind anomalies over the equatorial central
Pacific are westerly, those over the equatorial western Pacific
are easterly. The nearly out-of-phase behavior between the eastern
and western tropical Pacific is also observed during the cold
phase of ENSO, but with anomalies of opposite sign. The western
Pacific anomaly patterns are important for the evolution of
ENSO since equatorial easterly (westerly) wind anomalies in
the western Pacific produce ocean responses that proceed eastward
to terminate (initialize) El Niņo (Figure
9).
AOML
performed a theoretical study toward understanding why ENSO
occurs on interannual time scales, or why the Earth has the
interannual phenomenon of ENSO. A new, unified ENSO theory was
developed at AOML. This unified ENSO theory includes the physics
of the delayed oscillator, the western Pacific oscillator, the
recharge-discharge oscillator, and the advective-reflective
oscillator that have been previously proposed to interpret the
oscillatory nature of ENSO. All of these oscillator models assume
a positive ocean-atmosphere feedback in the equatorial eastern
and central Pacific. The delayed oscillator assumes that the
western Pacific is an inactive region, and wave reflection at
the western boundary provides a negative feedback for the coupled
system oscillate. The western Pacific oscillator emphasizes
an active role of the western Pacific in ENSO. The recharge-discharge
oscillator argues that discharge and recharge of equatorial
heat content make the coupled system oscillate. The advective-reflective
oscillator emphasizes the importance of zonal advections associated
with wave reflection at both the western and eastern boundaries.
Motivated by the existence of these different oscillator models,
a unified oscillator model is formulated and derived from the
dynamics and thermodynamics of the coupled ocean-atmosphere
system. All of the different oscillators can be extracted as
special cases of the unified oscillator. This unified oscillator
model shows an ENSO-like oscillation. As suggested by this new
ENSO theory, all of the previous ENSO mechanisms may be operating
in nature.
We
investigated the impact of tropical Pacific decadal/interdecadal
variability on ENSO variability and the interactions between
the tropical and extratropical Pacific Oceans. Coupled ocean-atmosphere
model runs performed at AOML show both tropical Pacific interannual
and decadal/interdecadal variability. The slow decadal/interdecadal
variation in the model mean thermocline affects the intensity
and frequency of ENSO events. The model ENSO shows modest-amplitude
oscillations between large-amplitude oscillations, an increase
in frequency of El Niņo, and an absence of La Niņa during some
periods. The studies at AOML pointed out that the atmospheric
meridional Hadley Circulation may also serve to link the tropical
and extratropical Pacific Oceans, in addition to the oceanic
linkage by the influx of ocean water from high latitude. Both
the oceanic and atmospheric processes, as well as local ocean-atmosphere
coupling, are responsible for the observed climate variability
of the coupled tropical-extratropical ocean-atmosphere system.
Scientists
at AOML will continue to investigate Pacific climate variability
by a combination of observational and numerical modeling studies.
We will be interested in seeing how coupled global climate models
perform in the western Pacific. If the western Pacific anomaly
patterns are not in the models, then our analyses will provide
directions for improving the coupled model and, hence, improving
dynamically-based predictions. If they are in the models, we
can quantify to what extent they play a role in ENSO evolution.
We
use observations to describe and diagnose the ocean-related
processes which control the evolution of heat content in the
mixed-layer (and therefore SST) for several regimes within the
inter-tropical convergence zone/cold tongue region. We also
make direct comparisons with the NCEP ocean reanalysis to assess
the ability of ocean global circulation models to adequately
simulate the observed processes
The
NCEP ENSO predictions rely on initializations of the ocean state.
We have found large differences between the initialized ocean
states and observations in the western tropical Pacific, usually
between about 150°E and 180°. These differences propagate eastward
from about 150°E in mid 1996 to 160°W in mid 1997 and to about
110°W by early 1998. Recall that mid 1996 precedes the development
of the 1997 El Niņo while mid 1997 corresponds to the time during
which the event developed into one of the largest events of
the century. We hypothesize that these large differences result
from times when westerly wind bursts cause the ocean to respond
in a way that the model system cannot capture. We need to check
this out by correlating the occurrence of westerly wind bursts
with the development of these strong model/observation differences.
Ultimately, we hope to assess the impact of this error on the
ENSO predictions.
The equatorial Pacific upwelling supplies approximately 0.3-1.2
PgC (=Gton C) CO2 to the atmosphere annually. During non-El
Niņo years, upwelling of waters enriched in nutrients and CO2
extends from the coastal waters west of South America to approximately
160°E. The large area affected by the upwelling process makes
this region the largest oceanic source of CO2 to the atmosphere.
A comprehensive set of atmospheric and surface ocean pCO2 measurements
and supporting hydrographic data were obtained from 1992 through
1999 on NOAA research ships servicing the Tropical Atmosphere
Ocean (TAO) array. The 1992-1994 cruises occurred during a prolonged
mild El Niņo, the 1995-1996 cruises occurred during the well
developed cold tongue conditions, and the 1997-1998 cruises
occurred during the strongest El Niņo of this century (Figure
10). This was followed by a strong La Niņa contrasting the
effect of the ENSO on the CO2 cycling. During El Niņos, the
subsurface supply of CO2 diminishes and fluxes decrease dramatically,
while during La Niņas enhanced upwelling brings more waters
enriched with CO2 to the surface. Quantification of these fluxes
is critical if we are to improve understanding of atmospheric
CO2 trends and projection of future atmospheric CO2 levels.
Future
work will include increasing data coverage by utilizing floats
and additional ships of opportunity, and improved spatial and
temporal interpolation techniques to better quantify the flux.
Remote sensing of SST, chlorophyll, and surface wind will be
a critical component. To improve our mechanistic understanding,
shipboard measurements of pCO2 will be augmented with those
of chlorophyll, oxygen, and total carbon. This work will continue
in strong collaborative fashion with Dr. R. Feely at PMEL, Dr.
F. Chavez at the Monterey Bay Aquarium Research Institute, and
investigators in Japan (Drs. Inoue and Ishii) and France (Drs.
Boutin, Etcheto, and Dandonneau).