## Christopher Torrence and Peter J. Webster, [Interdecadal changes in the ENSO-Monsoon system (1998)](), clim_12_103.2679_2690.pdf

> The El Nino–Southern Oscillation (ENSO) and Indian monsoon are shown to have undergone significant interdecadal changes in variance and coherency over the last 125 years. Wavelet analysis is applied to indexes of equatorial Pacific sea surface temperature (Nin SST), the Southern Oscillation index, and all-India rainfall.

> Time series of 2–7-yr variance indicate intervals of high ENSO–monsoon variance (1875–1920 and 1960–90) and an interval of low variance (1920–60). The ENSO–monsoon variance also contains a modulation of ENSO–monsoon amplitudes on a 12–20-yr timescale.

> The annual-cycle (1 yr) variance time series of Nin SST and Indian rainfall is negatively correlated with the interannual ENSO signal. The 1-yr variance is larger during 1935–60, suggesting a negative correlation between annual-cycle variance and ENSO variance on interdecadal timescales.

> The method of wavelet coherency is applied to the ENSO and monsoon indexes. The Nin SST and Indian rainfall are found to be highly coherent, especially during intervals of high variance. The Nin SST and Indian rainfall are approximately 180Њ out of phase and show a gradual increase in phase difference versus Fourier period. All of the results are shown to be robust with respect to different datasets and analysis methods.

So wavelet analysis of time series with a 2-7 year variance distinguish the periods 1875-1920 and 1960-1990 of high ENSO-monsoon variance from the period 1920-1960 of low variance.

Q. What are th peak and minimal ENSO-monsoon variances on a 12-20 year time scale? TBD.

The low 2-7 year variance period 1920-1960 shows a larger 1 year variance time series of the Nino3 index that is negatively correlated with interdecadal ENSO variance.

> during March–May, the center of tropical convection migrates from the western Pacific warm pool to the northwest, announcing the arrival of both the Southeast Asian monsoon and the Indian summer monsoon (Meehl 1987). Every few years, an El Nino (or warm) event produces a warming of the sea surface temperature (SST) in the central and eastern Pacific, accompanied by diminished easterly trade winds and an eastward shift in tropical convection. The opposite La Nina (or cold) event, which sometimes follows a warm event, produces an anomalous westward shift in warm SSTs and convection, as well as enhanced easterly trades (Rasmusson and Carpenter 1982).

Q. Is it worth looking for a 120 degree change from NW to E in the movement of the Pacific warm pool? TBD

> The strength of the monsoon and the occurrence of warm or cold ENSO events depend on the location and magnitude of western Pacific SSTs and on tropical convection (Soman and Slingo 1997). A strong monsoon [heavy rains, low sea level pressure (SLP), strong easterlies] tends to inhibit warm events and favor cold

events (Yasunari 1990). Conversely, a warm ENSO event (decreased convection and high SLP in the west Pacific, weak easterlies) tends to suppress the monsoon (Webster 1995). No cause or effect is implied (or indeed warranted) in either case.

Lack of interdecadal analysis has inhibited decomposition of ENSO-monsoon interactions and changes in the global ocean atmosphere background state.

Data : Nino3 SST, SOI (anomalous SLP in the eastern Pacific) and all-India rainfall.

Q. As missing data is filled in using EOF what is the probability that subsequent analysis of what is supposed to be data just mirrors the parameter settings of that EOF interpolation? TBD.

Nino3 SST and SOI are clearly out of phase by 180 degrees.

...

Q. As missing data is filled in using EOF what is the probability that subsequent analysis of what is supposed to be data just mirrors the parameter settings of that EOF interpolation? TBD.

Nino3 SST and SOI are clearly out of phase by 180 degrees.

...

El Nino JJAS rainfall minima < -1 : 1877, 1899, 1918, 1952, 1965, 1972, 1982, 1986.

Q. In 1884, 1953, 1962 (very weak), 1969 (very weak) and 1994 there are posive rainfall anomalies in an El Nino year. What gives? Is there anything special about these El Nino years? TBD.

> The El Nino–Southern Oscillation (ENSO) and Indian monsoon are shown to have undergone significant interdecadal changes in variance and coherency over the last 125 years. Wavelet analysis is applied to indexes of equatorial Pacific sea surface temperature (Nin SST), the Southern Oscillation index, and all-India rainfall.

> Time series of 2–7-yr variance indicate intervals of high ENSO–monsoon variance (1875–1920 and 1960–90) and an interval of low variance (1920–60). The ENSO–monsoon variance also contains a modulation of ENSO–monsoon amplitudes on a 12–20-yr timescale.

> The annual-cycle (1 yr) variance time series of Nin SST and Indian rainfall is negatively correlated with the interannual ENSO signal. The 1-yr variance is larger during 1935–60, suggesting a negative correlation between annual-cycle variance and ENSO variance on interdecadal timescales.

> The method of wavelet coherency is applied to the ENSO and monsoon indexes. The Nin SST and Indian rainfall are found to be highly coherent, especially during intervals of high variance. The Nin SST and Indian rainfall are approximately 180Њ out of phase and show a gradual increase in phase difference versus Fourier period. All of the results are shown to be robust with respect to different datasets and analysis methods.

So wavelet analysis of time series with a 2-7 year variance distinguish the periods 1875-1920 and 1960-1990 of high ENSO-monsoon variance from the period 1920-1960 of low variance.

Q. What are th peak and minimal ENSO-monsoon variances on a 12-20 year time scale? TBD.

The low 2-7 year variance period 1920-1960 shows a larger 1 year variance time series of the Nino3 index that is negatively correlated with interdecadal ENSO variance.

> during March–May, the center of tropical convection migrates from the western Pacific warm pool to the northwest, announcing the arrival of both the Southeast Asian monsoon and the Indian summer monsoon (Meehl 1987). Every few years, an El Nino (or warm) event produces a warming of the sea surface temperature (SST) in the central and eastern Pacific, accompanied by diminished easterly trade winds and an eastward shift in tropical convection. The opposite La Nina (or cold) event, which sometimes follows a warm event, produces an anomalous westward shift in warm SSTs and convection, as well as enhanced easterly trades (Rasmusson and Carpenter 1982).

Q. Is it worth looking for a 120 degree change from NW to E in the movement of the Pacific warm pool? TBD

> The strength of the monsoon and the occurrence of warm or cold ENSO events depend on the location and magnitude of western Pacific SSTs and on tropical convection (Soman and Slingo 1997). A strong monsoon [heavy rains, low sea level pressure (SLP), strong easterlies] tends to inhibit warm events and favor cold

events (Yasunari 1990). Conversely, a warm ENSO event (decreased convection and high SLP in the west Pacific, weak easterlies) tends to suppress the monsoon (Webster 1995). No cause or effect is implied (or indeed warranted) in either case.

Lack of interdecadal analysis has inhibited decomposition of ENSO-monsoon interactions and changes in the global ocean atmosphere background state.

Data : Nino3 SST, SOI (anomalous SLP in the eastern Pacific) and all-India rainfall.

Q. As missing data is filled in using EOF what is the probability that subsequent analysis of what is supposed to be data just mirrors the parameter settings of that EOF interpolation? TBD.

Nino3 SST and SOI are clearly out of phase by 180 degrees.

...

Q. As missing data is filled in using EOF what is the probability that subsequent analysis of what is supposed to be data just mirrors the parameter settings of that EOF interpolation? TBD.

Nino3 SST and SOI are clearly out of phase by 180 degrees.

...

El Nino JJAS rainfall minima < -1 : 1877, 1899, 1918, 1952, 1965, 1972, 1982, 1986.

Q. In 1884, 1953, 1962 (very weak), 1969 (very weak) and 1994 there are posive rainfall anomalies in an El Nino year. What gives? Is there anything special about these El Nino years? TBD.