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August 8, 2022

A Cause of the Indian Ocean Dipole Elucidated
-Triggered by cold water upwelling on the south coast of Java -

1. Key Points

Observational data analysis indicates that cold water brought up by coastal upwelling south of Java can trigger the onset of Indian Ocean Dipole (IOD).
A method to accurately determine coastal upwelling signal based on satellite chlorophyll-a data was developed and used for analysis in areas with limited observations.
As IOD affects the global climate, including summer weather in Japan, the findings of this study are expected to help improve the predictability of both the global climate and IOD.

2. Overview

Researchers from the Center for Coupled Ocean-Atmosphere Research (CCOAR), Research Institute for Global Change (RIGC), Japan Agency for Marine-Earth Science and Technology (JAMSTEC), led by Senior Researcher HORII Takanori found that local coastal upwelling south of Java in the eastern Indian Ocean can trigger an onset of the Indian Ocean Dipole (IOD; Figure 1), a large-scale short-term climate variability*1 in the Indian Ocean.

IOD is a short-term climate variability in which sea surface temperatures (SST) in the tropical Indian Ocean change year-to-year over several thousand kilometers. This widespread change in the SST is accompanied by changes in global atmospheric circulation, which affects weather patterns in the Indian Ocean region and all over the world, including Japan. Therefore, predicting IOD occurrence early and accurately is critical in our climate research. Previous research has linked coastal upwelling south of Java to the IOD, but the causal relationship has been unclear due to a lack of ocean observation data.

The current study created a new index of the occurrence of the coastal upwelling by combining chlorophyll-a concentration data collected by satellite with Indonesian sea level data collected along the coast of Java. Using this index, we discovered that, in years when coastal upwelling occurred about one to two months earlier than usual, positive IOD event occurred several months later (Figure 2). We also discovered that the cold water brought up by the early coastal upwelling was spread over a large area of the eastern Indian Ocean by stronger-than-normal westward current over the next several weeks to a month, creating favorable conditions for IOD development (Figure 3).

Figure 4 depicts the study’s background and new findings in a schematic diagram. Previous research has found that IOD influences the intensity of coastal upwelling on the south coast of Java. On the other hand, the current study found that cold water brought up by coastal upwelling south of Java can trigger the subsequent onset of IOD (Leftward arrow in Figure 4).

This study found that understanding the early signals of coastal upwelling south of Java can predict the onset of IOD. It is critical to improve observations and understanding of the eastern Indian Ocean in order to improve predictions of global climate change, including the Indian Ocean.

This accomplishment will be published on August 8 in Geophysical Research Letters, a journal published by the American Geophysical Union (Japan Standard Time). This study was supported by the Japan Society for the Promotion of Science (JSPS) Grants-in-Aid for Scientific Research, 18K03753 and 18H03731.

Can coastal upwelling trigger a climate mode? A study on intraseasonal-scale coastal upwelling off Java and the Indian Ocean Dipole
T. Horii1, E. Siswanto1, I. Iskandar2, I. Ueki1 and K. Ando1
1.JAMSTEC  2.Sriwijaya University (Indonesia)

Figure 1. Sea surface temperature (SST) anomalies during the peak of positive Indian Ocean Dipole (IOD) events. Anomalies mean differences from the climatology. The SST data were averaged during the peak of 10 IOD events that occurred during 2003-2020. SST anomalies in the western tropical Indian Ocean increase while that in the eastern Indian Ocean decrease.


Figure 2. (a) Scatterplot between the timing of the first upwelling signal and subsequent IOD peak. The IOD peak was defined as the maximum Dipole Mode Index (DMI) during August–October. The numerals in circles indicate the year (e.g., ‘06’ means ‘2006’). The correlation coefficient is 0.87. (b) Sea surface chlorophyll-a distribution in the Indian Ocean and that south of Java when the first coastal upwelling occurred. The observed values before the 2006 IOD onset are shown as an exampl. Chlorophyll-a signals more than 0.5 mm/m3 indicating the occurrence of upwelling are observed south of Java. (c) Sea surface temperature (SST) anomalies during IOD and the area where the Dipole Mode Index (DMI) are defined. The DMI is defined by the difference in SST anomalies in the western and eastern areas (gray squares).


Figure 3. (a) Contribution of ocean currents on SST changes when coastal upwelling occurs several months before the IOD onset. The integrated values (°C) from the onset of coastal upwelling to one month later are shown. White circles indicate statistically significant temperature changes. (b) Observed SST anomalies during the same period as (a). The white arrows schematically show the stronger-than-normal westward current observed during the period. The black box is the area of Figure 3a. The patterns of SST changes are similar in (a) and (b), indicating the temperature changes due to ocean currents are effective for the observed SST changes.


Figure 4. Schematic illustrating the context of this study and its accomplishment.

【Supplemental information】

Short-term climate variability: Climate variability generally refers to variations in climate, such as temperature and precipitation, over time periods ranging from decades to thousands of years, whereas short-term climate variability refers to changes in climate over a period of a few years. El Niño events in the tropical Pacific Ocean and Indian Ocean Dipole events in the tropical Indian Ocean are typical examples.


(For this study)
Takanori Horii, Senior Researcher, Research Institute for Global Change (RIGC) Center for Coupled Ocean-Atmosphere Research (CCOAR) Ocean-Atmosphere Climate Research Group, JAMSTEC
(For press release)
Press Office, Marine Science and Technology Strategy Department, JAMSTEC
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