A waveform skewness index for measuring time series nonlinearity and its applications to the ENSO–Indian monsoon relationship

Many geophysical time series possess nonlinear characteristics that reflect the underlying physics of the phenomena the time series describe. The nonlinear character of times series can change with time, so it is important to quantify time series nonlinearity without assuming stationarity. A common way of quantifying the time evolution of time series nonlinearity is to compute sliding skewness time series, but it is shown here that such an approach can be misleading when time series contain periodicities. To remedy this deficiency of skewness, a new waveform skewness index is proposed for quantifying local nonlinearities embedded in time series. A waveform skewness spectrum is proposed for determining the frequency components that are contributing to time series waveform skewness. The new methods are applied to the El Niño–Southern Oscillation (ENSO) and the Indian monsoon to test a recently proposed hypothesis that states that changes in the ENSO–Indian monsoon relationship are related to ENSO nonlinearity. We show that the ENSO–Indian rainfall relationship weakens during time periods of high ENSO waveform skewness. The results from two different analyses suggest that the breakdown of the ENSO–Indian monsoon relationship during time periods of high ENSO waveform skewness is related to the more frequent occurrence of strong central Pacific El Niño events, supporting arguments that changes in the ENSO–Indian rainfall relationship are not solely related to noise.

Solar activity as a potential factor for foreshadowing drought in Indian summer monsoon regime

An attempt bas been made to determine Influence o solar activity on drought incidence in India, 100 year's rainfall (1875-1974) for northwest and peninsular India, have been analysed with respect to sunspot numbers and Baur's solar index and their distribution during drought years determined. Frequency of drought per cycle of solar activity have been obtained. Association between drought and maximum/minimum spottedness was tested by chi-square technique. Correlation have been computed between sunspots, Baur's index and rainfall during different phases of solar activity. The results revealed that on an average drought occures once in a solar cycle. Occurrence of maxima or minima did not have any relationship with the drought, which was found to occur earlier as well as later than either of the epochs, Correlation analysis revealed that for the Peninsula, sunspots numbers and its values during middle of the waxing phase were significant. The Baur's index bears significant correlation with northwest Indian rainfall. It's value during the waxing phase as also during middle of waxing phase is also significantly correlated with northwest Indian rainfall. The index was significant for peninsula rainfall during middle of waxing phase.

Impact of the southern annular mode on extreme changes in indian rainfall during the early 1990s

The variability in rainfall amounts in India draws much attention because it strongly influences the country’s ecological and social systems. Indian rainfall is associated with climate factors, including El Niño/Southern Oscillation and the Indian Ocean Dipole. Here we identified the Southern Annular Mode (SAM), the primary pattern of climate variability in the Southern Hemisphere, as the ultimate forcing leading to decadal changes in Indian rainfall. Through statistical analyses using observational data covering the period from 1979 to 2015, we show an increase in the decadal rainfall amount in the early 1990s over the Indian region. Examining atmospheric environmental conditions, we demonstrate that conditions have become more favorable over the past few decades. Specifically, during the positive SAM phase since the early 1990s, changes in the atmospheric fields have evoked anomalous vertical motion over the continent and the Indian Ocean, enhancing the southerly cross-equatorial flow by increased land–sea thermal contrast, thereby increasing decadal rainfall in the region.

Enhanced Tropical Eastern Indian Ocean Rainfall Breaks down the Tropical Easterly Jet-Indian Rainfall Relationship

Previous studies found a tight connection between the tropical easterly jet (TEJ) and Indian summer monsoon rainfall (ISMR). Here we show that the TEJ-ISMR relationship is nonstationary and breaks down from 1994–2003 (epoch P2), in contrast to the significant positive correlation during the epoch P1 (1979–1993) and P3 (2004–2016). The breakdown of the TEJ-ISMR relationship concurs with the increased rainfall variability over the tropical eastern Indian Ocean (TEIO). The enhanced TEIO rainfall anomalies excite a significant lower-level cyclonic circulation that reduces the ISMR, meanwhile, strengthens the upper-level divergence and excites a pair of upper-level anticyclone to the west of the TEIO as Rossby wave responses, both accelerating the TEJ. Thus, the TEIO rainfall plays a more important role than the ISMR in the TEJ variability during P2, causing the breakdown of the TEJ-ISMR relationship. In contrast, a relatively weak amplitude of the TEIO rainfall during P1 and P3 was unable to change the positive TEJ-ISMR relationship. The changes in the TEIO rainfall variability is mainly attributed to the increased SST variability over the tropical southeastern Indian Ocean, but the cause of it remains elusive.

A Waveform Skewness Index for Measuring Time Series Nonlinearity and its Applications to the ENSO-Indian Monsoon Relationship

Many geophysical time series possess nonlinear characteristics that reflect the underlying physics of the phenomena the time series describe. The nonlinear character of times series can change with time, so it is important to quantify time series nonlinearity without assuming stationarity. A common way to quantify the time-evolution of time series nonlinearity is to compute sliding skewness time series, but it is shown here that such an approach can be misleading when time series contain periodicities. To remedy this deficiency of skewness, a new waveform skewness index is proposed for quantifying local nonlinearities embedded in time series. A waveform skewness spectrum is proposed for determining the frequency components that are contributing to time series waveform skewness. The new methods are applied to the El Niño/ Southern Oscillation (ENSO) and the Indian monsoon to test a recently proposed hypothesis that states that changes in the ENSO-Indian Monsson relationship are related to ENSO nonlinearity. We show that the ENSO-Indian rainfall relationship weakens during time periods of high ENSO waveform skewness. The results from two different analyses suggest that the breakdown of the ENSO-Indian monsoon relationship during time periods of high ENSO waveform skewness is related to the more frequent occurrence of strong central Pacific El Niño events, supporting arguments that changes in the ENSO-Indian rainfall relationship are not solely related to noise.

A Continuum Approach to Understanding Changes in the ENSO-Indian Monsoon Relationship

It is well-documented that the relationship between the El Niño/Southern Oscillation (ENSO) and the Indian summer monsoon changes on interdecadal timescales, yet an explanation for the variations is still a subject of debate. Here, using a continuum framework based on one-point partial correlation maps, we show that the ENSO-Indian rainfall relationship is influenced by the gradient of sea surface temperature anomalies (SSTA) across the Niño 3 region. Based on this identified SSTA pattern, a simple trans Niño 3 (TN3) index is created that explains up to 50% of All-India rainfall variability during the mid to late monsoon season after the 1960s. It is also shown that the influence of the TN3 pattern on the relationship between common ENSO metrics and All-India rainfall is strongest during the August-September (AS) monsoon sub-season and weakest during the June-July sub-season. The TN3 pattern accounts for up to 80% of the change and sign reversal in the AS Niño 1+2-All-India rainfall relationship in recent decades. The 1940s coincides with the intensification of the TN3 pattern and its influence. As the TN3 index is nearly orthogonal to the Niño 3 index, and both are strongly correlated with All-India rainfall, the strengthening TN3 influence must be systematically associated with the weakening Niño 3-All-India relationship in recent decades. This work supports arguments that recent changes in the ENSO-Indian rainfall relationship are not solely related to noise.