Longitude (degree)

Figure 13. Time-lag correlation coefficients between the principal component of the leading M-SSA mode of the interannual Indian Ocean SST variability and the low-pass filtered SST anomalies in the Indo-Pacific Run. Values are averaged between 4°S and 4°N. Contour intervals are 0.1. Positive values are shaded. (From Yu and Lau, 2004.)

ENSO pattern peaks about 3.5 months before the IOZM peaks. This discrepancy may be due to the fact that the model ENSO tends to peak in early fall, rather than in winter as observed (Yu and Mechoso, 2001). The close connection between the IOZM and ENSO in the correlation analyses suggests that the IOZM-like oscillatory mode in the Indo-Pacific Run is related to ENSO. The correlation between the time series of the NINO3 index and the IOZM index from the Indo-Pacific Run is also examined. Following Saji et al. (1999), the IOZM index is defined as the SST anomaly difference between the western Indian Ocean (50°E-70°E and 10°S-10°N) and the eastern Indian Ocean (90°E-110°E and 10°S-0°). It is found that major IOZM and ENSO events coincide with each other during the simulation (not shown), although the simultaneous correlation coefficient between them is only 0.5.

No oscillatory mode can be found in the Indian Ocean when the ENSO influence is excluded in the Indian Ocean Run. However, IOZM-like features can still be found in the leading variability modes of the Indian Ocean SST. Examinations of these IOZM-like features in the Indian Ocean Run reveal similar ocean-atmosphere coupling associated with enhanced and weakened Indian summer monsoon circulations, as in the Indo-Pacific Run. Our modeling results indicate that IOZM-like features can occur even in the absence of large ENSO events in the Pacific. However, the oscillatory feature of the IOZM is forced by ENSO. ENSO acts as a strong pacemaker to the IOZM. The IOZM may be considered a coupled mode that is weakly damped and cannot be self-sustained when lacking external forcing, such as ENSO and the monsoon (Li et al., 2003).

8. Conclusions

Climate changes and variations have strong impacts on human society, and are of common concern to people across national and regional boundaries. In the Indo-Pacific sector, ENSO and the monsoon are two of the most important climate features. These two features occur on opposite sides of the Indo-Pacific basin but have strong interactions with each other. A better understanding of their complex interactions and feedbacks is crucial for successful forecasts of ENSO and monsoon variability. This article reports a unique modeling effort toward that goal. By turning on and off the atmosphere-ocean coupling in various regions of the Indo-Pacific Ocean, the interactions between ENSO and the Indian Ocean and monsoon can be isolated in different CGCM experiments to study the ENSO-monsoon interaction and the ENSO-Indian Ocean interactions individually.

The results obtained from this series of basin-coupling CGCM experiments suggest that the Indian Ocean-monsoon system plays an active role in affecting the amplitude, frequency, and evolution of ENSO and in modulating their decadal variations. The Indian Ocean-monsoon system should be considered a crucial part of the ENSO dynamics. It is known that the Indian Ocean has been experiencing a gradual but significant warming trend in the past few decades (Nitta and Yamada, 1989; Terray, 1994; Wang, 1995). This trend may change the importance of the Indian Ocean to ENSO, and may be an additional reason for the decadal ENSO variability. In particular, the modeling results reported here indicate that the Indian Ocean-monsoon system is crucial to the selection of the dominant frequency of ENSO. An active Indian Ocean may favor a shorter period of ENSO, i.e. the biennial ENSO component. For the low-frequency ENSO component, influence from the Indian Ocean may be less important. The interactions between the Pacific and the Indian Ocean may be different between the decades that have strong biennial ENSO and the decades that have strong low-frequency ENSO. Much more can be learned about the decadal variability of ENSO by looking into the ENSO-Indian Ocean-monsoon interactions from the perspective of biennial and low-frequency components of ENSO. The basin-coupling modeling strategy pioneered in these CGCM experiments will be a useful and effective tool for the investigations.


Support from NOAA (NA03OAR4310061), NSF (ATM-0638432), and NASA (NAG5-13248 and NNX06AF49H) is acknowledged. Model integrations and analyses were performed at the San Diego Supercomputer Center (SDSC) and the University of California Irvine's Earth System Modeling Facility, which is funded by the National Science Foundation (ATM-0321380).

[Received 15 December 2005; Revised 9 May 2007; Accepted 4 June 2007.]


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Ocean—Atmosphere—Land Feedbacks on the Western North Pacific—East Asian Summer Climate

Chia Chou

Research Center for Environmental Changes, Academia Sinica and Department of Atmospheric Sciences, National Taiwan University, Taipei,, Taiwan

[email protected]. tw

Jien-Yi Tu

Department of Atmospheric Sciences, Chinese Culture University, Yang-Ming Shan, Taipei,, Taiwan

In this review article, we summarize mechanisms limiting the poleward extent of the summer monsoon rain zones, particularly for the Asian summer monsoon. They include local processes associated with net heat flux into the atmosphere and soil moisture, ventilation by cross-continental flow, and the interactive Hodwell-Hoskins (IRH) mechanism, defined as the interaction between monsoon convective heating and baroclinic Rossby wave dynamics. The last two mechanisms, ventilation and the IRH mechanism, also induce an east-west asymmetry of the summer monsoon rain zones. Processes that change land-ocean heating contrast, and differences in net heat flux into the atmosphere rather than in surface temperature, are also discussed. Convection associated with the Asian summer monsoon is initiated by net heat flux into the atmosphere and modified by soil moisture via an evaporation process. In Asia, ventilation by moisture advection is particularly important, and the IRH mechanism tends to favor interior arid regions and east coast precipitation. Land surface conditions, such as surface albedo and topography, and ocean heat transport tend to modify land-ocean heating contrast, in terms of a tropospheric temperature gradient, and then change the Asian summer monsoon circulation and its associated rain zone. The stronger meridional gradient of tropospheric temperature tends to enhance the summer monsoon rainfall and extend the rain zone farther northward. Local SST, such as the warm SST anomalies in the western North Pacific during El Nino, is also important in the summer monsoon rainfall and its position.

1. Introduction

Land-ocean heating contrast is known to be fundamental to summer monsoon circulations (Webster, 1987; Young, 1987). However, examining the relation between solar heating of the continent and the associated monsoon rainfall, the rain zone does not extend as far poleward as the maximum heating would seem to indicate. This suggests that other mechanisms besides land-ocean heating contrast determine the poleward extent of summer monsoons. Many studies (Lofgren, 1995; Meehl, 1994; Nicholson, 2000; Xue and Shukla, 1993; Xue et al., 2004; Yang and Lau, 1998) have discussed the importance of land processes, such as surface albedo, soil moisture and vegetation, in affecting the magnitude and position of the monsoon. Topography, such as the Tibetan Plateau, also affects the monsoon circulation (Flohn, 1957; He et al., 1987; Murakami, 1987; Meehl, 1992; Wu and Zhang, 1998; Yanai and

Li, 1994; Ye, 1981). Other effects, such as changes of the westerly jet stream in the upper troposphere, persist from the previous winter and spring into summer, and then affect the Asian summer monsoon (Chang et al., 2001; Yang et al., 2004). In this study, we examine dynamical mechanisms that mediate land-ocean contrasts and affect the poleward extent of summer monsoons. Figure 1 shows precipitation climatology in the boreal summer and winter. The associated summer monsoon rain zones, such as the Asia summer monsoon, tend to

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