Press Releases
Jan. 13, 2010

Reconstruction of 115 year - The Indian Ocean Dipole variability recorded in Kenya Coral

— Mode shift in the Indian Ocean climate under global warming stress —
Presenters
  • Nobuko Nakamura (Department of Earth and Planetary Science, University of Tokyo)
  • Hajime Kayanne (Department of Earth and Planetary Science, University of Tokyo)
  • Hiroko Iijima (Department of Earth and Planetary Science, University of Tokyo)
  • Timothy R. McClanahan (Wildlife Conservation Society, U.S.A.)
  • Swadhin K. Behera (Climate Variations Research Program, JAMSTEC)
  • Toshio Yamagata (Department of Earth and Planetary Science, University of Tokyo)

Abstract

The Indian Ocean Dipole (IOD), an oscillation of sea surface temperatures in the Indian Ocean, has become a major influence on the weather variations in the Indian Ocean region. During positive IOD events, abnormally warm sea surface temperatures in the western Indian Ocean are accompanied by severe droughts over the Indonesian region and heavy rainfall over east Africa. To learn more about IOD patterns, we studied a 115-year coral record from Kenya. We analyzed coral oxygen isotope ratios, which trace rainfall anomalies, to reconstruct IOD variability. The results add to evidence that IOD has been occurring more frequently in recent decades. We found that before 1924, IOD occurred approximately every 10 years, but since 1960, IOD events have been occurring approximately 18 months to 3 years apart. We suggest that global warming effects on the western Indian Ocean have driven the observed shift in IOD variability, and note that the IOD has replaced the El Nino/Southern Oscillation as the major driver of climate patterns over the Indian Ocean region.

Paper information

Nakamura, N., H. Kayanne, H. Iijima, T. R. McClanahan, S. K. Behera, and T. Yamagata (2009), Mode shift in the Indian Ocean climate under global warming stress, Geophyscal Research Letters, 36, L23708, doi:10.1029/2009GL040590.

Figure 1

Fig 1. : Precipitation anomalies in the Indian Ocean during a positive Indian Ocean Dipole mode. Green and gray areas show positive and negative precipitation anomalies, respectively. Red dot shows the sampling site (Malindi, Kenya).

Figure 2

Fig 2. : Drilling operation of a coral core underwater

Figure 3

Fig 3. : X-radiograph of the coral core. 115y- annual bands from the top (the upper left ; 2002 Oct) to the bottom (lower right : 1887)

Figure 4

Fig 4. : (a) Coral δ18O values for 1887-2002 at almost monthly scale (blue line, A set of high/low peak means seasonal cycle), running means at 12-month (purple line), and 5-year (black line) scale. The top of the vertical axis means high sea surface temperature and high precipitation. This result indicates 0.7°C increasing of the mean SST and increasing of local rainfall for 115 years.

(b) The coral IOD index derived from extracting the rainfall anomaly from coral δ18O. Positive and negative IOD occurred in the red and blue dotted years, respectively. Coral IOD years match well with observed IOD years in the last 40 years.

(c) The mode shift of coral IOD periodicity. Wavelet spectrum of the coral IOD index shows IOD periodicity shifted from decadal in the early, 5 to 7 years in the middle to quasi-bienniel in the late 20th century.

(d) Correlation coefficients between the coral IOD-index - Indian Summer Monsoon Rainfall (blue), and NINO3 SST - Indian Summer Monsoon Rainfall (red). Indian Summer Monsoon was mainly controlled by ENSO during the former half of the 20th century, but has been controlled by IOD since 1980s.