Abstract

The surface winds over the Arabian Sea and South China Sea are meaningful indicators for the strength of the Indian monsoon and East Asian monsoon, respectively. Paleo-monsoon variability has been studied through analysis of sediment records from these two monsoon regions. To facilitate interpretation of these records, we focus on the impacts of `internal' and `external' forcing of the monsoon system by contrasting the annual cycle and interannual variability of two subsystems: the monsoon over the Indian sector (40–105°E) and over the East Asian sector (105–160°E). Differences in the annual cycle within these subsystems arise primarily from the different land–ocean configurations that determines atmospheric response to the solar forcing. Thus factors that drive intensities of the monsoonal annual cycle share common features with the external (geographic and orbital) forcing that controls paleo-monsoon variability. We show that the differences in interannual variations between the two monsoon subsystems are primarily due to internal factors of the coupled atmosphere–ocean–land system, such as remote impacts of El Niño/La Niña and local monsoon–ocean interactions. The mechanisms that operate on interannual to interdecadal timescales may differ fundamentally from that on geologic/orbital timescales. The low-level flows over the East Asia and Australia are essentially established by geographic forcing. The amplification of the Australia summer monsoon during increased solar precession is likely caused by an enhanced East Asian winter monsoon, rather than following an enhanced Indian summer monsoon as on the interannual timescale. It is also found that El Niño influences the low-level flow moderately over the Arabian Sea but to a greater extent over the South China Sea. As such, large changes in the Pacific thermal conditions may significantly alter the intensity of the East Asian monsoon but not the Indian monsoon.

Author Keywords: Indian monsoon; East Asian–Australian monsoon; geographic forcing; orbital forcing; monsoon–ocean interaction; ENSO forcing

Article Outline

1. Introduction

2. Data and analysis methods

3. The contrasting annual cycles in the Indian and East Asia sectors

4. Differences in interannual variability between Indian and East Asian monsoons

5. Discussion: causes of the differences between the two A–AM subsystems

5.1. Annual cycle: geographic and orbital forcing

5.2. Interannual variation: local monsoon–ocean interaction and El Niño forcing

6. Conclusion and discussion

Acknowledgements

References

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Fig. 1. Climatological July–August mean precipitation rates (shading in mm/day) and 925 hPa wind vectors (arrows) in the A–AM region. The precipitation and wind climatology are derived from CMAP ([Xie and Arkin, 1997]) (1979–2000) and NCEP/NCAR reanalysis (1951–2000), respectively. The three boxes define major summer precipitation areas of the Indian tropical monsoon (5–27.5°N, 65–105°E), WNP tropical monsoon (5–22.5°N, 105–150°E), and the East Asian subtropical monsoon (22.5–45°N, 105–140°E).

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Fig. 2. Climatological pentad (5-day) mean precipitation rate (mm/day) averaged over (a) the Indian sector (65–105°E), and (b) the western Pacific sector (105–145°E). The data used are derived from CMAP ([Xie and Arkin, 1997]) for the period of 1979–2000.

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Fig. 3. Climatological pentad mean precipitation rate (mm/day) averaged over three regions: Indian summer monsoon (5–27.5°N, 65–105°E), WNP summer monsoon (5–22.5°N, 105–150°E), and East Asian summer monsoon (22.5–45°N, 105–140°E). The abscissa runs from pentad 1 (January 1–5) to pentad 73 (December 27–31). The data used are derived from CMAP ([Xie and Arkin, 1997]) for the period of 1979–2000. The thick, thin, and long-dashed horizontal lines indicate, respectively, the annual mean rainfall rates averaged for the WNP, East Asian, and Indian monsoon regions.

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Fig. 4. Same as in Fig. 1 except for January–February mean.

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Fig. 5. Time series of monthly mean Niño 3 (black) and Niño 3.4 (gray) sea surface temperature anomalies from January 1951 to December 2000. The data used are from Reynold's reconstructed SST ([Reynolds and Smith, 1994]). Anomalies are departures from 1951–2000 climatology. Solid boxes outline the 10 El Niño cases used in the composite. Two dashed boxes indicate two El Niño events that are not selected.

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Fig. 6. Composite seasonal mean SST anomalies for selected 10 El Niño episodes: (a) JJA(0), (b) D(0)/JF(1), and (c) JJA(1), where 0 denotes the year in which El Niño develops and 1 as the year El Niño decays. The data used are from Reynold's reconstructed SST ([Reynolds and Smith, 1994]).

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Fig. 7. Seasonal mean precipitation rate (mm/day, contour) and 925 hPa wind anomalies composite for the 10 selected El Niño episodes shown in Fig. 6: (a) JJA (0), (b) SON(0), (c) D(0)/JF(1), (d) MAM(1), and (e) JJA(1), where year 0 denotes the year in which El Niño develops and 1 as the year El Niño decays. The data used are derived from NCEP/NCAR reanalysis (1951–2000) ([Kalnay et al., 1996]).

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Fig. 8. Correlation map of seasonal (DJF) mean meridional wind anomalies with reference to the Australian summer monsoon (DJF) index that is defined by the zonal wind anomalies averaged over the region (2.5–12.5°S, 120–150°E). The correlation coefficients are computed for the period of 1951–2000.

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Corresponding author. Present address: International Pacific Research Center, University of Hawaii, Honolulu HI, 96822. Fax: 1-808-956-9425

¹ Also affiliated with LASG/Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China, 100029. The International Pacific Research Center is sponsored in part by the Frontier Research System for Global Change.