Subtropical Indian Ocean Dipole:

Press Release:

 
Discovering a Dipole Phenomenon In the South Indian Ocean Subtropics

南 インド洋亜熱帯海域にダイポール現象を発見

Dipole mode events evolve in the subtropical southern Indian Ocean region during the austral summer (Behera and Yamagata 2001). This is found to be linked with the strengthening/weakening of the Mascarene High. The subtropical Indian Ocean dipole (SIOD) is remarkably well simulated in a coupled ocean-atmosphere general circulation model (Suzuki et al. 2004). In a positive dipole event, which is associated with above normal rainfall in parts of southern Africa and Australia, the subtropical high is strengthened along with the strengthening and west ward shift of the west coast trough observed in the western Australia. The resultant stronger than normal winds off the coast of Australia cause cooling of SST in the southeastern part mainly through the enhanced evaporation. On the other hand reduced evaporation due to reduced wind speed allows increased influx from the atmosphere thus warming the southwestern part. Ekman transport along with surface mixing processes also help the formation of SST dipole.

The increased moisture advection from the southeastern part by the stronger easterlies then helps in the enhanced convective activity over southern Africa (Reason,. 2006). The local and remote responses of the rainfall anomalies to the subtropical dipole pttern is investigated using the standalone atmospheric GCMs (Reason 2001; 2002). Terray et al. (2003) suggested that the SIOD related anomalies over the SEIO are highly persistent and affect the position of the Mascarene High and the subsequent Indian summer monsoon. When the amplitude of SSTA in the southern Indian Ocean is large enough, SSTA in the southern Indian Ocean can play an important role in controlling rainfall, which is evident in the late 1980s (Wang et al. 2006). Several paper discussed the SIOD impact on regional and global climate variabilities (e.g. Reason et al. 2005; Mapande and Reason 2005a, 2005b; Jia and Li 2005; England et al. 2005; Reason, 2006)

The subtropical dipole mode in the Indian Ocean is generally found to accompany similar dipole mode events in the subtropical southern Atlantic (e.g. Venegas et al. 1997; Fauchereau et al. 2003; Hermes and Reason 2004) and Pacific Oceans (Fig.1).

The SIOD is also shown to affect the seasonal ocean-atmosphere gas exchanges in the southern Indian Ocean; results from field experiments suggest that warm anomalies related to SIOD in the western Indian Ocean are responsible for reducing the oceanic carbon dioxide uptake (Jabaud-Jan et al. 2004).

 The SIOD Index

Some Related References:

Behera SK, Yamagata T. 2001. Subtropical SST dipole events in the southern Indian Ocean. Geophysical Research Letters 28: 327–330.

England, Matthew H., Caroline C. Ummenhofer and Agus Santoso. 2006: Interannual Rainfall Extremes over Southwest Western Australia Linked to Indian Ocean Climate Variability. Journal of Climate, 19, 1948–1969.

Fauchereau, N., S. Trzasaka, Y. Richard, P. Roucou and P. Camberlin, 2003: Sea-surface temperature co-variability in the southern Atlantic and Indian Oceans and its connections with the atmospheric circulation in the Southern Hemisphere, Int. Jr. of Climatology, 23, 663–677.

Hermes, J. C., and C. J. C. Reason. 2005: Ocean Model Diagnosis of Interannual Coevolving SST Variability in the South Indian and South Atlantic Oceans. Journal of Climate, 18, 2864–2882.

Jabaud-Jan, A., N. Metzl, C. Brunet, A. Poisson, and B. Schauer, 2004: Interannual variability of the carbon dioxide system in the southern Indian Ocean (20⁰S–60⁰S): The impact of a warm anomaly in austral summer 1998, Jr. Geop. Res. 18, doi:10.1029/2002GB002017.

Jia XL, CY Li, 2005: Dipole oscillation in the Southern Indian Ocean and its impacts on climate. Chinese J. Geop,  48, 1238-1249.

Mapande AT, CJC Reason, 2005a: Links between rainfall variability on intraseasonal and interannual scales over western Tanzania and regional circulation and SST patterns, Met. Atm. Phys., 89 215-234.

Mapande AT, CJC Reason 2005b: Interannual rainfall variability over western Tanzania. Int. Jr. of Climatology, 25, 1355-1368.

Qian, W. H., H.R. Hu, Deng Y, P. Roucou, P. Camberlin, 2002: Signals of interannual and interdecadal variability of air-sea interaction in the basin-wide Indian Ocean Atmosphere-Ocean 40 (3): 293-311.

Reason, C. J. C., 2001: Subtropical Indian Ocean SST dipole events and southern African rainfall, Geophys. Res. Lett., 28, 2225-2228, 10.1029/2000GL012735.

Reason. CJC. 2002: Sensitivity of the southern African circulation to dipole sea-surface-temperature patterns in the south Indian Ocean. International Journal of Climatology 22: 377-393.

Reason CJC, S Hachigonta, and RF Phaladi, 2005: Interannual variability in rainy season characteristics over the Limpopo region of southern Africa. Int. Jr. of Climatology, 25, 1835-1853

Reason CJC, F Engelbrecht, WA Landman, et al. 2006: A review of South African research in atmospheric science and physical oceanography during 2000-2005  S. Afri. J. Sci, 102, 35-45

Suzuki, R., S.K. Behera, S. Iizuka, and T. Yamagata, 2004a: The Indian Ocean subtropical dipole simulated using a CGCM, Jr. Geop. Res. 109, C09001, doi:10.1029/2003JC001974..

Terray, P.,  P. Delecluse, S. Labattu, and L. Terray, 2003: Sea surface temperature associations with the late Indian summer monsoon, Clim. Dynamics, 21, 593 – 618.

Venegas S, L. A. Mysak, and D. N. Straub, 1997. Atmosphere–ocean coupled variability in the South Atlantic. J. Climate 10, 2904–2920.

Wang X, CY Li, W Zhou, 2006: Interdecadal variation of the relationship between Indian rainfall and SSTA modes in the Indian Ocean. Int. Jr. of Climatology, 26, 595-606.

White, W.B. , 2000: Coupled Rossby waves in the Indian Ocean on interannual time scales. Jr of Phys. Oceanography, 30, 2972–2989.

Zinke J, Pfeiffer M, Timm O, Dullo WC, Davies GR, 2005: Atmosphere-ocean dynamics in the Western Indian Ocean recorded in corals Phil. Trans. Royal Soc. London  363 (1826): 121-142.

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