Review work on ENSO

Occurrence of floods and droughts is world-wide phenomenon associated with extremes of rainfall. For the Indian subcontinent, the occurrence of floods and droughts is closely linked with the activity of summer monsoon. The activity of Indian summer monsoon rainfall has many tele-connections, various aspects of Indian summer monsoon activity- ENSO relationship have been investigated by several research workers such as Elliot and Angell (1987), Parthasarthy et al. (1985), Rasmusson and carpenter (1983), Sikka (1980), Bhalme et al. (1983), Sukla (1987), Shukla and Paolino (1983), Walker (1923) and many others.

It is observed that in general occurrence of El-Nino during the negative phase of Southern Oscillation (SO) proves to be conductive for poor activity of the Indian summer monsoon. However, there are some drought years not associated with ENSO whereas not all El-Nino years are drought years. Somolar obseravation holds good for La-Nina year and flood year for India. Also, the relationship of Indian summer monsoon rainfall and Tropospheric circulation features is well documented in literature Alexander et al. (1978), Banerjee et al.

(1978), Mooley et al. (1986) , Mooley and Shukla (1972) are a few name.

How far the mid-season rainfall deficiency is made up at the end of the season

A qualitative assessment of the relationship between the rainfalls at the middle of the summer monsoon season vis-a-vis that of the end of the season is carried out to examine on the aspects of making up the mid-season deficiency at the end of the season. For this purpose the area receiving deficient/scanty rainfall and area receiving excess rainfall during the mid-season period and at the end of the season have been compared.

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The analysis is done for the period 1951-2010.

From Table 3.1 it is evident that the mid-season rainfall deficiency is made up at the end of the season for a reasonably good percentage of total numbers of years, which includes 13 drought years also. The percentage is obviously higher for flood years compared to the of drought years. Nonetheless, it is seen that in the case of drought years; the decrease in the area of deficient rainfall or increase in area of excess rainfall at the end of the season as compared to its value at middle of the season is observed for a considerable percentage of total number of cases. The percentage of total number of cases in which the mid-season rainfall deficiency is made up at the end of the season is higher for DW years as compared to DnW years. The case is opposite for flood years; the percentage being higher for FnC years as compared to FC years.

Circulation features of drought years (1979-2009)

Various causes are identified as regards occurrences of floods and droughts over India and warm and cold ENSO events from anomaly diagram. The Anomalous tropospheric circulation features for flood and drought years are examined on a monthly anomaly charts of June to September. These charts are examined for the presence of various circulation features such as trough, ridge, cyclonic or anticyclonic circulation etc. From the cases of described, it is evident that during the period of 1979-2009, there are five DW years viz. 1982, 1987, 2002, 2004 and 2009 and two DnW years viz. 1979 and 1986. Following are the salient features of anomaly circulation observed for some of these years.

Drought years associated with warm ENSO

1982- Fig.1 depicts the monthly circulation pattern for 1982. In June, at 850 hPa, easterly flow is observed implying weaker than normal lower Tropospheric monsoon westerlies. At 500hPa, an anticyclonic circulation (AC) is observed over central India and at 300 hPa, a trough is observed along 20-230 N which implies that upper tropospheric easterlies are weaker than normal. In July, a similar pattern persisted with a ridge running across the region of monsoon trough at 850 hPa, a ridge along 20-250 N at 500 hPa and westerly flow at 300 hPa. In august, a ridge is observed along 27-300 N at 850 hPa along 150 N at 500 hPa and a trough along 15-200 N at 300 hPa. In September, a ridge is observed along 18-250 N at 850 hPa, along 200 N at 500 hPa and a trough along 24-250 N at 300 hPa.

Thus, weaker than normal monsoon circulation persisted through the entire monsoon season of 1982.

2002- Fig.2 In June, The circulation pattern for June was characterized by anomalous north-south trough roughly along 75 E passing through Kochi, Bangalore and Patiala embedded with anomalous cyclonic circulation over south Peninsula. This anomalous cyclonic circulation extended upto 500 hPa level. An E-W ridge was seen over northern Bay and adjoining Orissa and Chhattisgarh region at lower and middle troposphere. An anomalous cyclonic circulation was seen at lower and middle levels over NE region of the country and it extended upto middle tropospheric levels. At the same time the central and northern part of the country was associated with an anomalous ridge extending upto 500 hPa level. An anomalous cyclonic circulation with stronger westerlies prevailed over central and southern portion of the country at lower and middle tropospheric levels.

In July, an anomalous anticyclone was observed over central Bay which extended upto middle tropospheric levels. This anticyclone was associated with an E-W ridge over the southern part of the country. However, at lower levels the northern part of the country was associated with an anomalous cyclonic circulation. During July the E-W ridge moved further northwards and lay over central part of the country and extended upto 500 hPa level. Anomalous E-W ridge persisted over central India and was prominent upto middle troposphere and the upper level easterlies and lower level westerlies were weaker than normal.

In August, the monsoon flow slightly improved from its very weak monsoon characteristic condition during July. The anomalous flow over peninsula was cyclonic associated with an anomalous cyclonic circulation over south peninsula thus indicating the revival of monsoon. The westerly flow remained stronger than normal upto middle tropospheric levels, associated with an anomalous cyclonic circulation over NW Bay and adjoining NE India, tilting southwards with height, thus, indicating strong monsoon condition. The anomalous cyclonic circulation over NE India persisted and extended upto upper troposphere. An anomalous N-S ridge persisted roughly along 750 E at lower levels. Thus, indicating subdued monsoon activity.

In September, there was an anomalous cyclonic circulation associated with an anomalous E-W trough over Orissa coast and neighbourhood at lower levels. Again, there existed an anomalous E-W ridge over peninsular India at lower and middle levels. The lower level westerlies were weaker than normal during the week. The anomalous cyclonic circulation over Orissa coast also persisted along with an anomalous E-W trough at lower and middle troposphere. Towards the southern parts of the country, the anomalous E-W ridge also persisted during the week. For the week ending 17 September, an anomalous E-W trough existed at lower levels over northern parts of the country. South of this trough the country was mainly covered by northwesterly wind. The upper level easterlies were also weaker during the week. During the week ending on 24 September an anomalous cyclonic circulation existed over central Bay, extending upto middle troposphere. An anomalous N-S ridge existed along around 75 E at middle tropospheric levels.

This is in conformity with the observation that there are some years of extreme events not associated with ENSO and some ENSO years are not the years of extreme events over India. Thus, it seems that, ENSO may have an impact to modify the circulation pattern over India, of which the rainfall activity is a manifestation. This must be different than of the zonal Walker circulation cells, Also, a probability of interaction (may be nonlinear) between the planetary scale and regional scale circulation changes cannot be ruled out. Any extreme event in association with ENSO may occur through the effect of ENSO to modify the circulation pattern over India to make it a typical of an extreme event. Some extreme events may also occur without being associated with ENSO.

This may be due to some other factors (some of which may even be independent of ENSO) to make the circulation pattern over India a typical of an extreme. Moreover, some ENSO events may not be the years, of extreme events if in those years, the interaction and/or phase locking between the planetary and the regional scale circulation changes have resulted into wiping off the effect of ENSO and/or the other factors competing with ENSO to modify the Indian circulation pattern are more dominant. Thus, the regional scale circulation pattern which is embedded in the planetary scale circulation seems to be the key feature. The rainfall activity is just a manifestation of this circulation pattern. This pattern gets modified due to many fold factors, of which ENSO is quite dominant. Hence, as brought out earlier studies, it is possible to say about non-occurrence of a drought (flood) over India association with Cold (warm) ENSO events.

While the inverse relationship between the eastern Pacific (EP) type of El Niño-Southern Oscillation (ENSO) and the Indian monsoon rainfall weakened significantly, the correlation between the central Pacific (CP) type of ENSO and the monsoon rainfall strengthened after the late 1970s. Moreover, the drought-producing El Niño warming pattern also exhibits a notable decadal modulation associated with the climate shift.

The climate system experienced notable decadal variation in the late 1970s, which is related to the phase transition of the Pacific Decadal Oscillation (Trenberth, 1990; Mantua and Hare, 2002) and human activity induced global warming. During this climate transition, the conventional inverse relationship between ENSO and AISMR has collapsed. Krishna Kumar et al. (1999) analyzed the 140-year historical record and found a drop in correlations between Indian summer monsoon rainfall and summer (June-July-August (JJA) mean) Nino-3 SST anomalies after the late 1970s. Although this weakening of the ENSO-Indian monsoon rainfall relationship may be due to natural variability (Mehta and Lau, 1997; Gershunov et al., 2001), some evidence (Krishna Kumar et al., 1999) suggests that anthropogenic global warming might be the root cause.

Further investigations (Krishna Kumar et al., 2006) of the detailed spatial distributions of eastern-to-central tropical Pacific SST anomalies related to the Indian monsoon droughts revealed that El Nino events characterized by CP warming are more effective in producing Indian monsoon failure than those accompanied by EP warming.

Fan et al., 2017 has analysed that CP warming plays an essential role in producing Indian monsoon droughts. However, the “flavor” of the drought-inducing El-Nino events is not necessarily of the CP type. In the earlier 1950–1978 sub period, the composite SST difference pattern between drought and drought-free El-Nino years is featured by consistently enhanced warming throughout the eastern-to-central equatorial Pacific, and the Indian monsoon drought-inducing El-Nino events are of canonical EP type with warming signals extended westward. By contrast, the composite SST difference pattern in the later 1979–2012 sub period is characterized by positive SST anomalies in the central Pacific and negative SST anomalies in the eastern Pacific, and the drought-producing El-Nino warming pattern may be of traditional EP type or the recently recognized CP type.

These findings refine the previous view of drought-producing El-Nino warming pattern and raise the possibility that the severe Indian monsoon droughts may be induced by different spatial distributions of El Nino warming associated with climate shifts.