Early warning of the Indian Ocean Dipole using climate network analysis

Abstract

SignificanceThe Indian Ocean Dipole (IOD), an air-sea coupled phenomenon over the tropical Indian Ocean, has substantial impacts on the climate, ecosystems, and society. Due to the winter predictability barrier, however, a reliable prediction of the IOD has been limited to 3 or 4 mo in advance. Our work approaches this problem from a new data-driven perspective: the climate network analysis. Using this network-based method, an efficient early warning signal for the IOD event was revealed in boreal winter. Our approach can correctly predict the IOD events one calendar year in advance (from December of the previous year) with a hit rate of higher than 70%, which strongly outperforms current dynamic models.

Keywords: Indian Ocean Dipole; climate network; prediction.

Fig. 1.

The constructed climate network in the TIO and its composite of node degree. (A) The regions are determined according to the definition of the IOD based on EOF2 of SSTA. The eastern and western regions are represented by blue and red nodes, respectively. (B) The spatial distribution of composite of node degree in the eastern region. (C) Same as B but in the western region. Regions A (black box from 50${}^{°}$E to 70${}^{°}$E and 10${}^{°}$S to 10${}^{°}$N) and B (black box from 90${}^{°}$E to 110${}^{°}$E and 10${}^{°}$S to the equator) are part of the eastern and western regions, respectively, used to calculate DMI.

Fig. 2.

Early warning signal based on climate network and 10-m zonal wind indicator. (A) The TD TD(t) shows the fluctuation of early warning signals for IOD (black dotted line). The gray bars show TD(y) derived from TD(t) in each year, and the red dotted line shows the threshold $T{D}^{TH}(y)$ for TD(y) with significance at the confidence level of 0.05. The green triangles represent the years when TD(y) exceeds $T{D}^{TH}(y)$, indicating the early warning signals are released 1 y in advance. (B) The green bars show the next years followed by early warning signals. The black curve is the 3-mo running mean DMI, and the pIOD and nIOD events are represented by red and blue areas above and below 0.5 ${}^{°}$C and –0.5 ${}^{°}$C (dotted lines), respectively, for consecutive 3 mo. The green bars with nothing, dots, and oblique lines represent correct alerts, false alerts, and an alert that needs to be confirmed in the future, respectively. (C) The relationship between the TD (the black line, which is the same as in A) and the number of negative links connecting regions A and B (the blue line) is highly negative correlated with a correlation coefficient of –0.98. (D) The diagram of 10-m zonal wind index (Materials and Methods) in December of years when early warning signals are released and the IOD amplitude in the next year. The blue and red circles represent nIOD and pIOD events, respectively, among the correct alerts, and the black ones represent the false alerts. The numbers in the circles stand for the year when early warning signals are released. The vertical dotted lines indicate ±0.5 SD of December wind anomalies. The horizontal dashed lines represent 0.5 ${}^{°}$C and –0.5 ${}^{°}$C for DMI index.

Fig. 3.

Composite analysis of 10-m wind and the depth of 20 ${}^{°}$C isotherm for pIOD and nIOD. (A–F) The composite of 10-m wind (arrows) and the depth of 20 ${}^{°}$C isotherm (colors) for the nIOD of correct alerts from the previous December in which the forecast starts initially to the current October in which IOD reaches its peak. (G–L) The same as A–F but for pIOD. The nodes with magenta arrows and green dots are significant at the confidence level of 0.05 for 10-m wind and the depth of 20 ${}^{°}$C isotherm, respectively.

Fig. 4.

Results of the network-based approach and the dynamical models. (A) Result of the network-based approach on IOD events forecast. The forecasted pIOD and nIOD events are represented by red and blue bars, respectively, and the real IOD events are represented by the red and blue areas based on its definition. The black curve is the 3-mo running mean DMI, and the dotted lines stand for 0.5 ${}^{°}$C and –0.5 ${}^{°}$C. The meanings of red or blue bars with dots and oblique lines are the same as in Fig. 2B. (B) The diagram of false alarm rate and hit rate (Materials and Methods) on the performance of the network-based approach and dynamical models. The circles, squares, asterisks, and triangles stand for performance of the network-based approach, the BCC_CSM, the ECs5, and the CFSv2, which forecast initially in the previous December, the previous December, the current May, and the current February, respectively. The black, blue, and red colors of the four marks represent the performance of these four approaches on the total, nIOD, and pIOD events.