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  3. El Niño Modoki in the tropical Pacific Ocean was key to successfully predicting the 2019 Super Indian Ocean Dipole phenomenon
April 6, 2020
JAMSTEC

El Niño Modoki in the tropical Pacific Ocean was key to successfully
predicting the 2019 Super Indian Ocean Dipole phenomenon

1. Key points

An extremely strong positive Indian Ocean Dipole occurred in 2019.
The worst bushfires in Australian history, locust swarms that induced food shortages in East Africa, and the warmest winter on record in Japan may be attributed to this phenomenon.
Successful predictions were made from the autumn of the previous year using numerical simulations with supercomputers.
The occurrence of the El Niño Modoki phenomenon in the tropical Pacific Ocean was key in successfully predicting this strong dipole event.

2. Overview

Takeshi Doi and colleagues in the Research Institute for Value-Added Information Generation at the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) showed that the El Niño Modoki phenomenon in the tropical Pacific Ocean played a critical role in successfully predicting the extremely strong positive Indian Ocean Dipole (IOD; *1) that occurred in 2019. The IOD causes a variety of weather and climate abnormalities worldwide. For example, it increases precipitation above average in East Africa, while it decreases precipitation in Indonesia and Australia. For these reasons, accurately predicting the occurrence of such phenomena is important not only from a scientific perspective, but from a socioeconomic one as well. However, it is extremely difficult to predict IOD phenomena, which occur from the summer to autumn, from as early as the previous autumn and across the winter season.

Scientists at the JAMSTEC Application Laboratory distributed monthly quasi-real-time IOD prediction data up to 12 months in advance, based on the numerical prediction system “SINTEX-F” (*2), which uses the “Earth Simulator” supercomputer. While the current predictive accuracy of simulations conducted from the previous autumn remain low, the 2019 prediction of the strong IOD was accurate. Upon further investigation, it was found that the occurrence of the El Niño Modoki (*3) phenomenon in the tropical Pacific Ocean was a key factor that controlled the accuracy in this prediction. The results of this study are expected to advance our understanding of the mutual relationships between the IOD and El Niño Modoki phenomena, as well as the development of agricultural and infectious disease research based on related predictive data.

This study was published in the online version of Geophysical Research Letters on April 2, 2019 (JST).

Title:
Predictability of the super IOD event in 2019 and its link with El Niño Modoki
Authors:
Takeshi Doi1, Swadhin K. Behera1, Toshio Yamagata1
  1. Japan Agency for Marine-Earth Science and Technology, Yokohama, Japan

3. Introduction

An extremely strong positive IOD occurred in 2019. It grew rapidly from May onward, peaked in November, and remnants were still present in December despite its decay; it finally ended at the beginning of 2020. This event was several factors stronger and longer in duration relative to past examples, and as such has been referred to as a “super” IOD, along with those that occurred in 1994 and 1997 (Figure 1a). The worst bushfires in Australian history, locust swarms that induced food shortages in East Africa, and the warmest winter on record in Japan may be attributed to this phenomenon.

It has been shown that IODs are difficult to predict several months in advance, even with the latest simulation techniques. It is particularly difficult to predict these phenomena, which occur from the summer to autumn, from the autumn of the previous year (skipping the winter) – a challenge known as the “winter prediction barrier.” This may be due to the difficulty of passing on information from the previous year as a result of wind systems in the Indian Ocean reversing between the summer and winter. However, the 2019 super IOD was accurately predicted in the autumn of the preceding year (Figure 2a). Why was the 2019 IOD able to be predicted with such a long lead time? In order to answer this question, we focused on the covariance between prediction ensemble members. In other words, we investigated whether or not there was any covariance in the physical structures or control processes for fluctuations between each of the simulation results that used ensemble methods (i.e., methods that include several repeated simulations with slightly different conditions).

4. Results

We found that the El Niño Modoki phenomenon, which varies relatively slowly and is easy to predict, was important for predicting the presence of the IOD (Figures 1b, 2b). Predictive simulations in which a strong El Niño Modoki phenomenon appeared also showed strong IODs. This is thought to be due to the occurrence of the El Niño Modoki in the Pacific Ocean, which generates easterly wind deviations that form offshore of Sumatra due to the formation of an anticyclonic system over Indonesia, thus generating a positive IOD (Figure 3). The El Niño Modoki phenomenon in the tropical Pacific Ocean made the long-term prediction of the 2019 IOD possible, and its strong influence is thought to have broken through the “winter prediction barrier.”

5. Future Developments

Extreme seasonal anomalies have occurred globally and with increasing frequency in recent years. Many of these are thought to be due to the convolution of natural climatic variations, such as El Niño and IODs, which occur once every several years, and the further aridification or moistening of conventionally dry or wet lands due to progressive global warming. Although global warming and IODs are separate phenomena, reports have indicated that the former can amplify and increase the frequency of the latter. As such, it has become increasingly important to develop techniques that are capable of predicting the occurrence of climatic variations several months to a year in advance, as well as their accompanying seasonal anomalies (i.e., seasonal climate predictions) in the context of a progressively warming planet. Researchers at the JAMSTEC Application Laboratory seek to develop applied research based on seasonal climate predictions (e.g., for modeling agriculture and infectious diseases), and specifically contribute to social activities in order to improve human safety and security. To that end, the numerical simulation “SINTEX-F” used in this study has indicated that there is a relatively high possibility of a moderate, positive IOD occurring in the summer of 2020. The latest prediction information are now available at:http://www.jamstec.go.jp/aplinfo/sintexf/e/seasonal/outlook.html.

*1
Indian Ocean Dipole:
The Indian Ocean Dipole is a climate change phenomenon that is observed in the tropical Indian Ocean once every several years from summer to autumn. This phenomenon has both positive and negative phases. When a positive Indian Ocean Dipole occurs, sea surface temperatures become cooler than in an average year on the southeastern side of the tropical Indian Ocean and warmer than in an average year on the western side. These changes in ocean temperatures cause vigorous convection that usually occurs in the eastern Indian Ocean to move westward; east Africa receives more rain, while Indonesia receives less. These changes also tend to result in less rain and higher temperatures in Japan. Conversely, when a negative Indian Ocean Dipole occurs, sea surface temperatures are warmer than in an average year in the southeastern tropical Indian Ocean and colder than in an average year in the west, causing convection in the eastern Indian Ocean to be more intense than usual, and more rain falls in Indonesia and Australia, and there is generally more rain and temperatures are lower in Japan.
*2
SINTEX-F Seasonal Climate Prediction System:
In order to gain greater insights into and to predict climate change phenomena generated on a scale of several months to several years, the JAMSTEC Application Laboratory (formerly the Global Frontier Research Systematic Climate Change Prediction Group) used a global simulator based on Japanese & European research to develop and improve a dynamic, seasonal climate prediction system based on the SINTEX-F climate model. A climate model was created using a group of differential equations pertaining to the physics of the atmosphere, ocean, land, and sea ice. The globe was divided into a three-dimensional grid, and a group of programs that integrates formulae over time was assigned to each square. Current observation data were incorporated into the climate model; for seasonal climate predictions, data on water temperature abnormalities in the oceans, which are massive heat sinks, are particularly important. A supercomputer was used to calculate how these data developed over time, making it possible to predict seasonal climatic abnormalities (i.e., divergences from an average year) several months in advance. JAMSTEC has a world-class climate simulating supercomputer and is working to develop and expand its oceanographic observation network. This two-pronged approach of observations and numerical calculations is capable of producing highly accurate advance predictions of phenomena that cause natural disasters. Seasonal climate prediction research makes a vital contribution to public safety and security.
SINTEX-F Website:
http://www.jamstec.go.jp/aplinfo/sintexf/e/seasonal/outlook.html
*3
El Niño Modoki:
A phenomenon of climatic variation seen in the tropical Pacific Ocean that is similar to an El Niño event but its global influence varies considerably by region; it is currently the subject of active research. El Niño phenomena increase sea surface temperatures (SSTs) average in the eastern side of the tropical Pacific Ocean, while El Niño Modoki decrease SSTs in both the eastern and western sides and increase them in the central area of the tropical Pacific Ocean. This phenomenon is known to influence variations in climate and sea level worldwide. For example, reports have indicated that El Niño events tend to cause cooler summers in Japan, whereas El Niño Modoki events induce extreme summer heat. El Niño Modoki events were discovered when investigating the causes of the extreme summer heat experienced in Japan in 2004, when a cooler summer was predicted due to an El Niño event.
El Niño Modoki Website:
http://www.jamstec.go.jp/aplinfo/sintexf/e/elnmodoki/about_elnm.html
fig.1

Figure 1:(a) Indian Ocean Dipole indices calculated with satellite observation data from 1983–present according to east-west deviations in the tropical Indian Ocean and focusing on deviations in SSTs relative to average years. Additional details are shown in Figure 2a. A positive event occurs once a value of 0.5ºC is exceeded. The occurrence of an extremely strong positive event can be seen in 2019, along with other events in 1994 and 1997. (b) El Niño Modoki indices according to the tripole structure values when the SSTs of the tropical Pacific Ocean are lower in the eastern or western sides and higher in the central area relative to an average year. Additional details are shown in Figure 2b. A positive event occurs once a value of 0.5ºC is exceeded. The occurrence of a positive event can be seen from 2018–2019.

fig.2

Figure 2:(a) Indian Ocean Dipole index. The black line is the observed value and colored lines are the predictions made on November 1, 2018 (light blue lines: predicted values of each ensemble member; dark blue lines: averaged value of the ensemble members). The trajectories of the dark blue line and the black line are very similar, and the prediction is thought to have been broadly successful. From the light blue lines, it can be seen that there is variation between the ensemble members, with some overestimating and others underestimating the occurrence of a dipole event. (b) El Niño Modoki index. The colors and lines have the same meaning as in Figure 2a. Additionally, there is variation between the ensemble members. This study revealed that these fluctuations covaried with fluctuations in the IOD predictions.

fig.3

Figure 3: Mechanism by which the El Niño Modoki phenomenon induces a positive IOD (schematic): (1) when the El Niño Modoki phenomenon occurs, the central area of the tropical Pacific Ocean becomes warmer and cyclonic; (2) simultaneously, the area near Indonesia becomes cooler and anticyclonic. Deviations in easterly winds subsequently become more likely offshore of Sumatra in the eastern Indian Ocean; (3) these easterly deviations generate a positive IOD. In other words, easterly wind deviations induce lower temperatures with upwelling in the eastern Indian Ocean, while driving warm water into the western Indian Ocean and increasing its temperature. The cooler and warmer temperatures in the eastern and western areas respectively promote anticyclonic and cyclonic conditions, which in turn strengthen the deviations in easterly winds, and enhance the contrast between high western and low eastern water temperatures (i.e., generate a positive feedback that acts on mutual interactions between the ocean and atmosphere).

Contacts

(For this study)
Takeshi Doi, Researcher, Application Laboratory, JAMSTEC
(For press release)
Public Relations Section, Marine Science and Technology Strategy Department, JAMSTEC
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