Twofold expansion of the Indo-Pacific warm pool warps the MJO life cycle

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Abstract

The Madden–Julian Oscillation (MJO) is the most dominant mode of subseasonal variability in the tropics, characterized by an eastward-moving band of rain clouds. The MJO modulates the El Niño Southern Oscillation1, tropical cyclones2,3 and the monsoons4,5,6,7,8,9,10, and contributes to severe weather events over Asia, Australia, Africa, Europe and the Americas. MJO events travel a distance of 12,000–20,000 km across the tropical oceans, covering a region that has been warming during the twentieth and early twenty-first centuries in response to increased anthropogenic emissions of greenhouse gases11, and is projected to warm further. However, the impact of this warming on the MJO life cycle is largely unknown. Here we show that rapid warming over the tropical oceans during 1981–2018 has warped the MJO life cycle, with its residence time decreasing over the Indian Ocean by 3–4 days, and increasing over the Indo-Pacific Maritime Continent by 5–6 days. We find that these changes in the MJO life cycle are associated with a twofold expansion of the Indo-Pacific warm pool, the largest expanse of the warmest ocean temperatures on Earth. The warm pool has been expanding on average by 2.3 × 105 km2 (the size of Washington State) per year during 1900–2018 and at an accelerated average rate of 4 × 105 km2 (the size of California) per year during 1981–2018. The changes in the Indo-Pacific warm pool and the MJO are related to increased rainfall over southeast Asia, northern Australia, Southwest Africa and the Amazon, and drying over the west coast of the United States and Ecuador.

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Fig. 1: A twofold expansion of the warm pool.
Fig. 2: Changes in the MJO life cycle.
Fig. 3: Correlation between MJO phase duration and ocean–atmosphere conditions.
Fig. 4: Changes in global rainfall in response to the changes in MJO phase duration.

Data availability

The MJO RMM index used in the study for the period 1981–2018 is available from the Australian Bureau of Meteorology (http://www.bom.gov.au/climate/mjo/). The monthly values of air temperature, specific humidity and winds, and the daily OLR and GPCP monthly precipitation can be obtained from the NOAA website (https://www.esrl.noaa.gov/psd/data/gridded/). HadISST data are available for download at the Met Office Hadley Centre website (https://www.metoffice.gov.uk/hadobs/hadisst/). The high-resolution daily OLR data can be acquired from the University of Maryland OLR CDR portal (http://olr.umd.edu/).

Code availability

The MJO events identified in this study, and the code for estimating the individual MJO phase duration and the Indo-Pacific warm pool area, are available at https://github.com/RoxyKoll/warmpool-mjo. The code for filtering the MJO component from the OLR data is available from C. Schreck at GitLab (https://k3.cicsnc.org/carl/carl-ncl-tools/blob/master/filter/filter_waves.ncl).

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Acknowledgements

M.K.R. acknowledges NOAA/PMEL for the National Research Council Senior Research Associateship Award by the US National Academy of Sciences (PMEL contribution no. 4975). P.D. was supported by the IITM Research Fellowship. D.K. was supported by the DOE RGMA program (DE-SC0016223), the NOAA CVP program (NA18OAR4310300), and the KMA R&D program (KMI2018-03110). We thank N. Bond and R. Murtugudde for their comments on an early draft of this manuscript.

Author information

M.K.R. conceived the study, performed the analysis and prepared the manuscript. P.D. performed the MJO detection and initial analysis. T.S. provided additional MJO tracking algorithm for verification. All co-authors contributed to the interpretation of the results and drafting of the manuscript for publication.

Correspondence to M. K. Roxy.

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Extended data figures and tables

Extended Data Fig. 1 Typical life cycle of the MJO.

Composite anomalies of 30–100 day OLR (W m−2) during November–April, for the period 1981–2018, showing the RMM phases 1–8. Typically, the MJO events are initiated over the Indian Ocean and move eastward over the Maritime Continent to the Pacific (ah). The region within the solid black lines highlight the west Pacific warm pool region (120° E to 160° E) where ocean–atmospheric changes related to the MJO lifespan are the largest. OLR values are based on the NOAA interpolated OLR dataset. The NCAR Command Language (NCL, http://ncl.ucar.edu) is used to plot the MJO life cycle on the map.

Extended Data Fig. 2 Annual average period of MJO events.

Time series of yearly average period of MJO events during November–April, 1981–2016 (phases 1–8). The grey shade overlaid on the time series represents ± two standard deviations of the MJO phase duration over a 10-year moving window.

Extended Data Fig. 3 Warm pool area in multiple datasets and breakpoint analysis.

a, Time series of the warm pool area during November–April, 1900–2018, based on HadISST, ERSST_v3b and COBE_SST2 datasets. Theil–Sen trend estimates computed based on HadISST (as in Fig. 1) are overlaid on the time series for the entire period (solid blue line) and for 1981–2018 (dashed blue line). b, Breakpoint analysis identifying the significant shifts in the mean of the Indo-Pacific warm pool time series, using HadISST. The breakpoint analysis shows two shifts in the time series, the first during 1945–1946 and the second during 1979–1980. Although the rate of change in warm pool area during 1900–1945 and 1946–1979 are different, the average warm pool area remains almost the same during both the periods. The breakpoint analysis confirms that the shifts to higher warm pool values occurred in the annual series during 1979–1980. c, Table showing the trend in warm pool area using a range of breakpoints, from 1976–1977 to 1982–1983. The rate of warming does not change substantially with different breakpoints. At the same time, the difference between the trends is significant for all breakpoints considered. The significance of the difference between the slopes is estimated based on a t-test58.

Extended Data Fig. 4 Correlation between MJO phase duration and ocean–atmosphere conditions, without removing the trends.

ac, Correlation between yearly average of MJO phase distribution (phases 5, 6 and 7) with (a) SST anomalies, (b) winds and vertical velocity and (c) air temperature (colours) and specific humidity (contours) over the Indo-Pacific basin for November–April, during 1981–2018 (n = 37). The correlation analyses are performed after removing the ENSO variability from the time series, but without removing the trends. PyFerret (http://ferret.pmel.noaa.gov/Ferret/) is used to generate the map and the plots.

Extended Data Fig. 5 Trend in specific humidity anomalies.

Trend in specific humidity anomalies (g kg−1 per 38 years) for November–April, during 1981–2018. The trends indicate an increase (red colours) in tropospheric moisture over the warm pool region and a reduction (blue colours) in tropospheric moisture over the Indian Ocean (900–400 hPa levels).

Extended Data Fig. 6 Schematics showing the changes in MJO life cycle and impact on the global climate.

a, As the Indo-Pacific warm pool expands with increasing SSTs, moist winds converge over the Maritime Continent–west Pacific, prolonging the MJO phase duration over this region by 5–6 days and shortening the MJO duration over the Indian Ocean by 3–4 days. b, As a response to the changes in the MJO phase duration, an increase in mean rainfall is observed over most of the Maritime Continent including southeast Asia, and over northern Australia, west Pacific, Amazon basin and southwest Africa. A decline in rainfall is observed over the central Pacific, Ecuador and California, and a slight decrease in rainfall over the Yangtze basin in China and Florida. The Generic Mapping Tools (GMT, https://github.com/GenericMappingTools/gmt) was used to create the map.

Extended Data Fig. 7 Relationship between MJO phase duration and global rainfall, without removing the trends.

Correlation between the MJO phase duration (phases 5, 6 and 7) and rainfall anomalies for November–April, during 1981–2018. The correlation analysis is performed after removing the ENSO variability from the time series, but without removing the trends. Rainfall values are based on the GPCP dataset. The Generic Mapping Tools (GMT, https://github.com/GenericMappingTools/gmt) was used to create the map.

Extended Data Fig. 8 Mann–Whitney U-test for testing the significance of the differences in MJO phase duration.

The difference in the mean of the MJO phase duration distributions is tested for different starting points. The P values are computed for different groups (1981–1999, 1982–1999 to 1990–1999) as the first sample and 2000–2018 as the second sample. a, According to the Mann–Whitney U-test, the difference in MJO phase duration (1, 2, 3) is statistically robust (P < 0.05, where we can reject the null hypothesis) for the most part of the varying first sample (1981–1999 to 1990–1999, except 1987–1999 where P = 0.07). b, For the MJO phase duration (5, 6, 7) the difference in mean is always statistically robust (where we can reject the null hypothesis) for the varying first sample (1981–1999 to 1990–1999, where P always <0.05).

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Roxy, M.K., Dasgupta, P., McPhaden, M.J. et al. Twofold expansion of the Indo-Pacific warm pool warps the MJO life cycle. Nature 575, 647–651 (2019) doi:10.1038/s41586-019-1764-4

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