Atmospheric Dynamics and Meteorology
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Understanding the El Nino—Southern Oscillation and Its Interactions with the Indian Ocean and Monsoon
Department of Earth System Science, University of California, Irvine, California, USA
The Pacific and Indian Oceans are closely linked to each other through atmospheric circulation and oceanic throughflow. Climate variations in one ocean basin often interact with those in the other basin. This includes phenomena such as the El Nino-Southern Oscillation (ENSO), biennial monsoon variability, and the Indian Ocean zonal/dipole mode. Increasing evidence suggests that these interbasin interactions and feedbacks are crucial in determining the period, evolution, and pattern of ENSO and its decadal variability. This article reviews recent efforts in using a series of basin-coupling CGCM (coupled atmosphere-ocean general circulation model) experiments to understand the physical processes through which ENSO interacts with the Indian Ocean and monsoon, the impacts of interbasin interactions on the characteristics of ENSO, the relative roles of the Pacific and Indian Oceans in monsoon variability, and ENSO's role in the Indian Ocean zonal/dipole mode.
The Pacific Ocean exhibits prominent sea surface temperature (SST) variations on time scales that range from interannual to inter-decadal. The interannual fluctuations are primarily associated with the El Niño-Southern Oscillation (ENSO), which results from the interactions between the tropical Pacific Ocean and the overlying atmosphere (Bjerknes, 1969). Warm (El Nino) and cold (La Niña) ENSO events occur quasi-periodically and are generally associated with significant anomalies in global and regional climate patterns. The fundamental physical processes that give rise to ENSO are believed to reside within the tropical Pacific. ENSO research over the past few decades has focused mainly on the tropical Pacific Ocean, and has resulted in a significant understanding of this climate phenomenon. Coupled atmosphere-ocean models that include only the tropical Pacific Ocean have obtained encouraging success in ENSO simulations and prediction (e.g. Cane and Zebiak, 1985; Cane et al., 1986; Schopf and Suarez, 1988; Battisti and Hirst, 1989; Delecluse et al., 1998; Latif et al., 2001; Yu and Mechoso, 2001).
Nevertheless, variations originating from the Indian Ocean and monsoons, such as weak and strong summer monsoons, interannual variability in the Indian Ocean SST, and volume fluctuations in the oceanic throughflow, are also potentially significant in the interaction of ENSO dynamics. The rising zone of the transverse and lateral circulation components of the Indian monsoon coincides with the ascending branch of the Walker circulation (Webster et al., 1992). Variations in the strength of the monsoon can affect Pacific trade winds and, consequently, the period and magnitude of ENSO (Barnett, 1984; Wainer and Webster, 1996). In the ocean, ENSO manifests itself as a zonal displacement of warm water between the western and eastern tropical Pacific. The warm water pool in the western Pacific is connected to the eastern Indian Ocean through the Indonesian throughflow. This throughflow is widely known, on average, to carry warm and fresh Pacific waters through the Indonesian archipelago into the Indian Ocean. This leakage of upper ocean waters from the western Pacific to the Indian Ocean acts as a major heat sink for the Pacific Ocean and a heat source for the Indian Ocean (Godfrey, 1996). There were studies suggesting that the throughflow could affect the mean climate state of the tropical Pacific, such as its mean thermocline depth and zonal temperature gradients (Hirst and Godfrey, 1993; Murtugudde et al., 1998; Lee et al., 2002). Since ENSO's period and growth rate depend on the mean climate state over the Pacific (Fedorov and Philander, 2000), the throughflow provides another possible mechanism for the Indian Ocean to affect ENSO activity.
On the other hand, it is known that ENSO events have profound influences on the Indian monsoon (see Shukla and Paolino, 1983; Klein et al., 1999). The relationship between ENSO and the Indian monsoon has long been an area of extensive research. The cause-and-effect relationship between ENSO and one of the strongest interannual signals of the monsoon variability, the tropospheric biennial oscillation (TBO), is still not yet fully understood. The TBO is referred to as the tendency for strong and weak monsoons to flipflop back and forth from year to year (Mooley and Parthasarathy, 1984; Yasunari, 1990; Clarke et al., 1998; Webster et al., 1998; Meehl and Arblaster, 2002). It has been associated with SST anomalies in both the tropical Pacific and the Indian ocean (Rasmusson and Carpenter, 1982; Meehl, 1987; Kiladis and van Loon, 1988; Ropelewski et al., 1992; Lau and Yang, 1996; Meehl and Arblaster, 2002). The associated SST anomalies in the Pacific Ocean are characterized by an ENSO-type pattern with a biennial (^2 years) periodicity. This association leads one to conclude that the TBO and the biennial ENSO component may be related. Some studies suggested that the TBO is forced by the biennial component of ENSO (e.g. Fasullo, 2004), but others argued that it has its own dynamics that are independent of ENSO (e.g. Li et al., 2006).
The relationship between ENSO and the monsoon variations is further complicated by the existence of significant interannual variability in the Indian Ocean SST. The variability may be intrinsic to the coupled Indian Ocean-monsoon system, or remotely forced by ENSO. An increasing number of recent studies suggest that the Indian Ocean may play an active role in influencing ENSO characteristics (e.g. Yu et al., 2000; Yu et al., 2003; Yu, 2005; Wu and Kirtman, 2004; Kug and Kang, 2005; Kug et al., 2006; Terray and Dominiak, 2005). It was suggested that the ENSO-forced basin-scale warming/cooling in the Indian Ocean might affect atmospheric circulation in the western Pacific to fasten the turnabout of the ENSO cycle and result in biennial ENSO (Kug and Kang, 2005). It was also postulated that the southeastern Indian Ocean SST anomalies act as persistent remote forcing to promote wind anomalies in the western equatorial Pacific and modulate the regional Hadley circulation in the south Pacific, both of which then affect the evolution of ENSO (Terray and Dominiak, 2005). These recent studies imply that the Indian Ocean may be a necessary part of the ENSO dynamics.
A thorough study of the interactions between ENSO and the Indian Ocean and monsoon is crucial for better understandings and successful forecasts of ENSO and monsoon activity. The complex coupling nature of these ENSO-Indian Ocean-monsoon interactions makes it difficult to determine their cause-and-effect relationships using observational analyses alone. Numerical model experiments that can isolate the coupling processes within and external to the Pacific Ocean are useful means for this purpose. In the past few years, a basin-coupling modeling strategy has been developed (Yu et al., 2002) to perform these types of experiments with coupled atmosphere-ocean general circulation models (CGCM's). This article summarizes the findings obtained from these basin-coupling CGCM experiments.
The article is organized as follows. Section 2 describes the University of California, Los Angeles (UCLA) CGCM used for the experiments and the setup of the basin-coupling experiments. The ENSO cycle simulated by the model and its teleconnection patterns in the Indo-Pacific Oceans are presented in Sec. 3. The influences of the Indian Ocean on the ENSO cycle are then discussed in Sec. 4. ENSO's interactions with the biennial monsoon variability (TBO) are examined in Sec. 5. Section 6 reports the findings on the importance of ENSO influence on Indian Ocean SST variability. Section 7 concludes with a review of new understandings of the interactions between ENSO and the Indian Ocean and monsoon.
The CGCM used in this study consists of the UCLA global atmospheric GCM (AGCM) (Suarez et al., 1983; Mechoso et al., 2000) and the Geophysical Fluid Dynamics Laboratory (GFDL) Modular Ocean Model (MOM) (Bryan, 1969; Cox, 1984; Pacanowski et al., 1991). The AGCM includes the schemes of Deardorff (1972) for the calculation of surface wind stress and surface fluxes of sensible and latent heat, Katayama (1972) for short-wave radiation, Harshvardhan et al. (1987) for longwave radiation, Arakawa and Schubert (1974) for parametrization of cumulus convection, and Kim and Arakawa (1995) for parametrization of gravity wave drag. The model has 15 layers in the vertical (with the top at 1 mb) and a horizontal resolution of 5° longitude by 4° latitude. The MOM includes the scheme of Mellor and Yamada (1982) for parametrization of subgrid-scale vertical mixing by turbulence processes. The surface wind stress and heat flux are calculated hourly by the AGCM, and the daily averages passed to the OGCM. The SST is calculated hourly by the OGCM, and its value at the time of coupling is passed to the AGCM. The ocean model has 27 layers in the vertical with 10 m resolution in the upper 100 m. The ocean has a constant depth of about 4150 m. The longitudinal resolution is 1°; the latitudinal resolution varies gradually from 1/3° between 10°S and 10°N to almost 3° at 50°N. No flux corrections are applied to the information exchanged by model components.
A series of basin-coupling experiments were performed with the UCLA CGCM. In each of the experiments, the atmosphere-ocean couplings were restricted to a certain portion of the Indo-Pacific Ocean by including only that portion in the ocean model component of the CGCM. Three CGCM experiments were conducted in this study: the Indo-Pacific Run, the Pacific Run, and the Indian Ocean Run. In the Pacific Run, the CGCM includes only the tropical Pacific Ocean in the domain of its ocean model component. The Indian Ocean Run includes only the tropical Indian Ocean in the ocean model domain. The Indo-Pacific Run includes both the tropical Indian and Pacific Oceans in the ocean model domain. Outside the oceanic model domains, SST's and sea-ice distributions for the AGCM are prescribed based on a monthly varying climatology. Figure 1 shows the ocean model domain and the sea/land masks used in each of these experiments and the long-term (^50 years) means of their simulated SST's. All three CGCM runs produce a reasonable SST climatology in the Pacific and Indian Oceans, with a warm pool (SST greater than 28° C) that covers both the tropical western Pacific and eastern Indian Oceans.
(a) Indian-Ocean CGCM Run (b) Indo-Pacific CGCM Run (c) Pacific CGCM Run
(a) Indian-Ocean CGCM Run (b) Indo-Pacific CGCM Run (c) Pacific CGCM Run
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