Revised and Resubmitted to Journal of Climate November 2005 Abstract

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1) Tropical ocean processes

Several recent studies have discussed various roles the ocean dynamics play in the evolution of the Indian Ocean coupled phenomenon (Vinayachandran et al. 1999, 2002; Murtugudde et al. 2000; Feng et al. 2001; Li and Mu 2001; Rao et al. 2002b; Reason et al. 2002; Xie et al. 2002; Huang and Kinter 2002; Saji and Yamagata 2003a; Guan et al. 2003; Masson et al. 2003b; Ashok et al. 2003a; Annamalai et al. 2003; Shinoda et al. 2004a) using observed data and ocean model simulations.

The oceanic condition in the eastern Indian Ocean was first reported by Meyers (1996); the study based on a repeated XBT section near the outlet for the Indonesian throughflow showed unusually cold anomalies off Java in close association with the zonal wind anomalies in the eastern equatorial Indian Ocean and their frequent occurrence in the past. This is consistent with the study by Yamagata et al. (1996), which suggested the arrival of equatorial Kelvin waves generated in the Indian Ocean contributing to the throughflow variability.

The dipole mode originally introduced using the SST anomalies is coupled strongly with subsurface temperature variability (Murtugudde et al. 2000; Feng et al. 2001; Rao et al. 2002a; Vinayachandran et al. 2002; Shinoda et al. 2004b; Feng and Meyers 2003). In fact, the dipole mode emerges as the first dominant mode in the subsurface temperature variability (Rao et al. 2002a). The close link between the surface signal and the subsurface signal is very striking when we calculate correlation between the zonal wind index from the central Indian Ocean and the heat content/ SST anomalies (Fig. 13). The high correlation with the equatorial wind anomalies shows the close coupling between the ocean and the atmosphere. Rao et al. (2002a) discussed how the evolution of the dominant dipole mode in the subsurface is controlled by equatorial ocean dynamics forced by zonal winds in the equatorial region. This evolution appears to be explained by a kind of delayed oscillator mechanism (cf. Schopf and Suarez 1988); the phase of the surface dipole reverses in the following year through propagation of oceanic Rossby/Kelvin waves (Rao et al. 2002a; Feng and Meyers 2003), which is also confirmed from coupled model studies (Gualdi et al. 2003; Yamagata et al. 2004). Thus, the turnabout of the subsurface dipole leads to the quasi-biennial oscillation (QBO) of the tropical Indian Ocean (Rao et al. 2002a; Feng and Meyers 2003). The ocean dynamics may play an important role in the QBO in the Indo-Pacific sector through changes of the Asian monsoon (cf. Meehl 1987). This is another challenging problem.

Interannual Rossby waves (from 3 to 5 years) are reported in the southern Indian Ocean by Perigaud and Delecluse (1993), Masumoto and Meyers (1998), Chambers et al. (1999) and White (2000). Particularly, Masumoto and Meyers (1998) concluded that these waves are primarily forced by the wind stress curl along the Rossby wave characteristic. Xie et al. (2002) have suggested that Rossby waves play a important role in the air-sea interaction of southern Indian Ocean where the doming mean thermocline allows subsurface anomalies to affect SST. These Rossby wave-induced SST anomalies modulate convection in the Indian Ocean ITCZ and may further influence the onset of the Indian summer monsoon (Annamalai et al. 2005b). While Xie et al. (2002) argued that these Rossby waves are dominantly forced by ENSO, a more detailed study of Rao and Behera (2005) shows that the relative importance of ENSO and IOD varies with latitude. The wind stress curl associated with the positive IOD forces the westward propagating downwelling long Rossby waves north of 10oS, increasing the heat content of the upper layer in the central and western Indian Ocean (e.g. Huang and Kinter 2002). The heat content anomaly maintains the SST anomaly; this SST anomaly, in turn, influences the wind stress anomaly, thereby completing the feedback loop. In contrast, the ENSO influence dominates over the upwelling dome south of 100S (cf. Schott et al. 2002b) in the southern Indian Ocean, as discussed by Xie et al. (2002) and Jury and Huang (2004). A similar response of sea level to wind forcing is found in the study by Wijffels and Meyers (2004). The cause of this is not very clear at this stage but the ENSO-related variation of the southern trade winds is one possible candidate. Another possible candidate is the Indonesian throughflow which intrudes into this region; the oceanic anomaly of Pacific origin may propagate westward and enhance local air-sea coupling south of 100S (cf. Masumoto and Meyers 1998).

The precondition for IOD evolution is another issue that requires more research. Several studies indicate the presence of a favorable mechanism in the eastern Indian Ocean that combines cold SST anomalies, anomalous southeasterlies and suppression of convection into a feedback loop (e.g. Saji et al. 1999; Behera et al. 1999). However, recent studies suggest a few alternatives: atmospheric pressure variability in the eastern Indian Ocean (e.g. Gualdi et al. 2003; Li et al. 2003), favorable changes in winds in relation to the Pacific ENSO and the Indian monsoon (e.g. Annamalai et al. 2003), oceanic conditions in the Arabian Sea related to the Indian monsoon (Prasad and McClean 2004; Suzuki et al. 2004b) and influences from the southern extratropical region (e.g. Lau and Nath 2004). All those studies fall short on more than one occasion to answer the failure (or success) in IOD evolution in spite of favorable (or unfavorable) precondition. For example, Gualdi et al. (2003) reported the failure of their proposed favorable mechanism to excite the IOD event in 1979. We also find several instances (e.g. the aborted 2003 event) when an IOD event is aborted abruptly by intraseasonal disturbances (Rao and Yamagata 2004). The out-of-phase ENSO variability further complicates the IOD preconditioning (Behera et al. 2005b). This indicates that the evolution of the IOD is more complex than thought and further studies of scale-interactions are needed, which will be discussed briefly in the next subsection.

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