The Great Paradox of Indian Monsoon Failure
K. Krishna Kumar1, Balaji Rajagopalan2,3, Martin Hoerling4,*, Gary Bates4 and
Mark Cane5
1Indian Institute of Tropical Meteorology, Pune, 411008, India
2Department of Civil Environmental and Architectural Engineering, University of Colorado, Boulder, CO 80309 USA
3Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO 80309 USA
4NOAA Earth Systems Research Laboratory, Boulder, CO 80305 USA
5Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY, 10964, USA
One Sentence Summary:
Severe droughts in India have always been accompanied by El Niño events but El Niño events have not always produced severe droughts, a paradox that is explained by the influence of different El Niño sea surface temperature patterns.
Submitted to SCIENCE - February, 2006
*Corresponding Author email:
The Great Paradox of Indian Monsoon Failure
K. Krishna Kumar1, Balaji Rajagopalan2,3, Martin Hoerling4,*, Gary Bates4 and
Mark Cane5
1Indian Institute of Tropical Meteorology, Pune, 411008, India
2Department of Civil Environmental and Architectural Engineering, University of Colorado, Boulder, CO 80309 USA
3Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO 80309 USA
4NOAA Earth Systems Research Laboratory, Boulder, CO 80305 USA
5Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY, 10964, USA
Examination of Tthe 132-year historical rainfall record srevealshows that severe droughts in India have always been accompanied by El Niño events. Yet, El Niño events have not always produced severe droughts. This paradox challenges the conventional wisdom that El Niño events lead tocauses a monolithic failure of the Indian summer monsoon rainfall. These seemingly contradictory qualities of El Niño impacts are explained by detailed analysis of observations. El Niño events with warmest Sea Surface Temperature (SST) anomalies in the central equitorial Pacific closer to the dateline are shown to be more effective in focusing drought producing subsidence over India than focus enhanced subsidence over the Indian subcontinent resulting in droughts and vice-versa during El Niño events with warmest SST in the eEastern equitorial tropical Pacific. The physical basis for such different impacts is established using atmospheric general circulation model (AGCM) experiments forced with idealized tropical Pacific warmings representing the above two El Niño flavours. These findings have significant implications forto Indian monsoon forecasting, economic planning, and risk mitigation.
Climate is the decisive influence on habitation and subsistance of India's burgeoning population. Spreading throughout the Ganges region, Indians pursue a thriving agricultural lifestyle in the fertile alluvial plain that by many accounts is the nation's lifeblood. India's wealth is measured by its agricultral output, and now even modest harvest failures result in exaggerated economic and societal consequences. These swings in crop abundances are propelled by the year-to-year succeses of the summer (June to September) monsoon rains (1). As a result, monsoon predictions are achieving new importance for setting into motion timely and effective preparedness and mitigation activities. Ironically, the predictions themselves can be as influential as the actual verified monsoon rainfall, as happened for Zimbabwe during 1997 when drought predictions led to curtailment of bank loans for agricultural development (2). A similar situation, also during 1997 occurred in India when a much touted prediction of poor monsoon rains proved false. A more painful scenario unfolded during 2002 and 2004 (3,4) when normal monsoon rains were predicted but severe drought materialized for which no contigencies were in place.
In most seasonal forecast tools Indian monsoon rains are predicted to vary in direct proportion to the strength of the El Niño Southern Oscillation (ENSO) phenomenon in the tropical Pacific (5,6,7), measured, for example, by the standardized NINO3 index (8). And, indeed, years with moderate to extreme cold states (NINO3 index < -1), have had abundant monsoon rains without exception. On the other hand, years of moderate to extreme warm states have not been reliably dry. As seen in Fig. 1, the 6 leading droughts since 1871 have occurred in tandem with a standardized NINO3 index exceeding +1, but the presence of El Niños has not guaranteed drought. No simple association describes the relation between the Indian monsoon and NINO3 SSTs when moderate to strong El Niño conditions exist. Indeed, almost the full range of monsoon rains have accompanied SST warmings. For example, 1997 was the century's strongest El Niño, though no drought occurred, while the moderate El Niño of 2002 was accompanied by one of the worst Indian droughts of the past century (4). Such ambiguity undermines the utility of monsoon predictions used for mitigation of drought's societal impacts.
Two hypotheses are examined to understand this ambiguity in the El Niño-Indian monsoon relationship. One is that the Indian monsoon system exhibits sufficient in situ variability on seasonal time scales, to mask the remote effect of El Niño. Accordingly, the failure (abundance) of monsoon rains during 2002 (1997) would be viewed as the accidental behavior of an inherently noisy monsoon system, and the poor forecasts for these particular cases were the consequence of an only marginally predictable system. The other is that the Indian monsoon is highly sensitive to the details of tropical east Pacific sea surface warming. It is widely believed that El Niño’s impact on the Indian monsoon is through the east-west displacement of the ascending and descending branches of the Walker circulation that link Indo-Pacific climates (9,10). Unusually warm waters during El Niño cause an increased ascent associated with increased rainfall. Mass continuity requires increased descent broadly over south east AsiaIndia, supressesing monsoon rains. The hypothesis we explore is that the strength and position of these branches vary coherently with the details of El Niño warming.
We begin by examining the 23 strong El Niño years for atmosphere and ocean conditions that distinguish the 10 Indian monsoon droughts (red asterisks in Fig. 1) from the 13 drought-free years (green asterisks in Fig. 1). Figure 2A illustrates their contrasting temperatures. The most notable difference in the tropical Pacific SSTs is the greater central Pacific warming during failed Indian monsoon years (Fig. 2A). These analyses suggest India to be more prone to drought when the ocean warming signature of El Niño extends westward.
Figure 2B displays the difference in tropical rainfall for the drought versus drought-free El Niño years. Though based on a smaller sample of cases for which satellite rainfall estimates are available, a physical consistency with the underlying SST anomalies in Fig. 2A is apparent. Increased rainfall occurs over the aforementioned enhanced warmth of central Pacific Ocean waters, and the satellite estimates confirm dryness over India, the Indian Ocean, and other portions of Southeast Asia, indicating a wide reach to the drought signal. These rainfall anomalies form a dynamical couple that is linked via an Indo-Pacific anomalous mass circulation, as seen in the velocity potential at 200 hPa (Fig. 2B, contours). The pattern of mass divergence (convergence) over the central Pacific (Indian) Ocean regions is a classic Walker Circulation.
The composite anomaly differences highlighted by shading in Fig. 2(A,B) are statistically significant (11) and are physically consistent with the expected rainfall-SST relationship. While this empirical analysis does not establish causal (i.e. predictive) linkages, it does suggest that the two flavours of El Niño may orchestrate significantly different responses in the Indian monsoon.
The SST patterns of these two flavours appear to be described by the two leading, preferred patterns of tropical Pacific SST variability of the last half century (12), shown in Fig. 3. The first leading pattern (Fig. 3A) represents the overall strength of the ENSO events and its associated temporal pattern is highly correlated with fluctuations in the NINO3 index (Fig. 3C). The second pattern (Fig. 3B) has polarity of opposite sign between the tropical Central and Eastern Pacific and its temporal pattern is highly correlated with fluctuations of an index that measures the SST contrast across the Pacific basin (13, Fig. 3D). (do we need to mention the correlation values for these two series eitehr here or in the figure caption?) The first leading pattern is similar to the drought-free composite SST pattern (Fig. 2A, contours) and the second leading pattern closely resembles the SST difference between severe drought and drought-free monsoon years (Fig. 2A, shaded).
While it is not surprising that the Indian monsoon climate of drought versus drought-free years are distinct, as shown by the separability of their respective rainfall probability density functions (PDFs, 14) in Fig. 2C, the question of great importance is whether the ocean is principally responsible for this separation. If so, then predictability of drought must be judged to be high to the extent the flavor of El Niño is itself predictable.
The hypothesis is that during “westward shifted” Pacific Ocean warm events the subsidence limb of the anomalous Walker circulation is intense and focused over the Indian region, initiating severe drought. During “eastward –shifted” Pacific Ocean warm events this subsidence limb is weaker and circumscribed in its reach over the Indian subcontinent, thus resulting in a relatively mutedleading to a less severe impact on the monsoon rainfall. These two El Niño flavours have also been shown to have opposite teleconnections over the Thailand monsoon region (15)
General circulation model experiments (GCM, NCAR CCM3.10 at T42 resolution, 16), forced with SST patterns resulting from a suite of linear combinations of the first two leading patterns of tropical Pacific SST variability in Fig. 3A, are used to test the validity of this theory. We explore the sensitivity of Indian monsoon rains to the detailed pattern and strength of equatorial Pacific Ocean warmth. Four ensemble sets of experiments are performed: (i) a 150-year control run of the GCM forced by monthly evolving global climatological SST, (ii) an SST pattern resulting from the addition of the first two leading tropical Pacific SST patterns superimposed on the monthly evolving climatological SSTs globally, (iii) same as (ii) but by subtracting the second leading tropical Pacific SST pattern from the first and, (iv) an SST pattern corresponding to the first leading pattern (i.e. Fig. 3A) alone. The model experiments for (ii) (iii) and (iv) are performed for a range of imposed SST warmth from 0 to +3 standard deviations (SD), with results available at an interval of 0.2SD. 10-members with different atmospheric initial conditions are analyzed for each of these incremental warmings. Climatological SSTs were prescribed outside the tropical Pacific in these experiments. (do we need to mention that the runs are for 14 months starting 1st Nov?)
The two SST patterns corresponding to the +2SD amplitude shown in Fig. 4A should be compared to the observed patterns in Fig. 2A. Ensemble mean rainfall and velocity potential (200hPa) computed as difference betwen experiments (ii) and (iii). This difference effectively responds to the second leading pattern shown by shading in Fig. 4A. s of these two runs are shown in Fig. 4B. Notice a large-scale enhanced drought over the Indian region consistent with enhanced subsidence and vice-versa in the tropical central Pacific. The similarity of this figure to that from the observations (Fig. 2B) is striking and confirms that the 'westward shifted' El Nino events weaken the Indian monsoon more severely. .
The behavior of Indian monsoon rainfall under climatological sea surface temperature conditions (control) and also under the anomalous conditions described by the three different SST patterns of experiments ii, iii and iv is assessed through the construciton of PDFs (14) that sample all the realizations of Indian rainfall drawn from the separate members of the GCM experiments (in the caption we are mentioning that the PDFs are based on 30 memebers but for let us say at 1SD or 2 SD we should only have 10members but Marty and I decided to say 30 considering the members on either side of this (for 1SD the members corresponding to .8, 1 and 1.2 and like wise for 2SD they correspond to 1.8,2 and 2.2). Please check if we need to clarify this 30 number either here or in the caption). The top panel in Fig. 4C shows the PDF corresponding to the control experiment (green curve), and the other two panels show the PDFs of Indian rainfall corresponding to two snap shots at about +1 and +2 SD of imposed SST anomalies as described above for different idealized experiments (ii, iii and iv). The effect resulting from a sum (difference) of the two leading tropical Pacific SST patterns are shown in red (blue) curves, whereas we show only the median value for the effect of the first leading SST pattern on the Indian monsoon (dashed black line).
While the experiments of moderate SST warming show Indian rainfall to be slightly less than the control experiment, the PDFs themselves are not separated. In contrast, at the stronger warming, the PDFs of the simulated rainfall become well separated and the median rainfall values are far below those of the control experiment. Under the influence of stronger SST forcing, the PDFs of the experiments summing and differencing the two leading tropical Pacific SST patterns fall on the dry and wet side of the median rainfall from the experiment using only the leading SST pattern, respectively. This indicates that the leading SST pattern in itself produces droughts in India when of sufficient amplitude, but depending on the sign of the superposed second leading SST pattern, the droughts in India are either strong or weak, with a clear separation in the PDFs.
We emphasize that the SST forcings, which elicited a strong response in the monsoon were entirely within the Pacific Ocean basin. It should be noted that Indian Ocean (17,18) and Equatorial Indian Ocean coupled ocean-atmospheric features (19) have also been shown to be associated with variations in both the monsoon and the teleconnection linking ENSO with the monsoon (20,21,22). Recent studies indicate that the ENSO and Indian Ocean SST play a combined role in the ENSO-monsoon linkage (23), though almost all of these Indian Ocean features have occurrred in conjunction with ENSO. We have clearly demonstrated the strong dependence of the Indian monsoon on the precise tropical Pacific SST anomaly pattern associated with different El Niños. These results do not rule out a role for the Indian Ocean, as suggested in work by us (24) and others.
Our results demonstrate that the spatial configuration of the tropical Pacific SST anomalies has a significant impact on the Indian monsoon teleconnection. Therefore, traditional statistical monsoon forecast methods using only the strength of the ENSO (mainly NINO3 or other such indices) are likely to be unsuccessful (25), with drastic consequences, in years when the spatial configuration of the SST anomalies is inconsistent with the strength – 1997, 2002 and 2004 are some of the recent years that attest to this. Incorporationn of the SST configuration information in the statistical models should improve monsoon forecast skill (supporting online material why the first graph looks different from our science paper fig? Did you use kaplan nino3 only? If so, does this figure uses jjas or jja like we did in science before? Please check this as this is given as a support material.If you have problems with this you may delete this online stuff!). There is also the intriguing question of whether either of these flavours of tropical Pacific warmings will become preferred as a consequence of the ocean's response to human-induced changes in the Earth's atmospherre’s chemical composition. Whereas the consensus of climate change models points to a so-called "El Niño" like warming pattern of the tropical Pacific (266), the results of this study indicate that details of that human-induced ocean warming could have material consequences for the monsoon intensity over India.