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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).