Rainfall trends during the past century

Figure 1a,b shows the trend in precipitation over the South Asian region, during summer monsoon for the years 1901–2012, using gridded datasets from the India Meteorological Department (IMD) and the Climate Research Unit (CRU). The trends are similar in both the datasets, with a weakening trend over the central-east and northern regions of Indian subcontinent, and south of the Western Ghats region. The CRU dataset shows that the negative trend over the central subcontinent extends west to east, from south of Pakistan to central-east India, with a prominent horseshoe pattern with one of its arms placed on the foothills of the Himalayas, and the other obliquely along the central-east coast of India. The reduction in summer rainfall over central-east India during the past century is about 10–20%. This could have socio-economic consequences as the agriculture in this region is still largely rain-fed. Significant positive trends in precipitation are consistently observed to the north of the Western Ghats region, in both the datasets. However, the positive rainfall trends are confined to a small domain along the west coast.

Figure 1: Summer monsoon precipitation trends for the years 1901–2012. Observed trend in precipitation (mm day−1 112 year−1) in (a) IMD and (b) CRU datasets, during June–September, for the years 1901–2012. Contours denote regions significant at the 95% confidence level. Full size image

Warming Indian Ocean

Previous studies using atmospheric general circulation models (AGCM) forced by observed SST variations suggest that a decrease in global land monsoon rainfall can be caused by warming trends over the tropical oceans, especially the central-eastern Pacific and the western Indian Ocean27. Among these tropical oceans, the Pacific SST anomalies do not show any long-term significant trend, while the western Indian Ocean exhibited rapid warming throughout the past century28 (Fig. 2). In comparison with the rest of the Indian Ocean, the western Indian Ocean generally has cooler mean SSTs in summer, owing to the strong monsoon winds and the resultant upwelling over the west. Meanwhile, the central-east Indian Ocean is characterized by a warm pool with SSTs greater than 28.0 °C. Though previous studies demonstrate a basin-wide warming in the Indian Ocean during the past half century21,22, our analysis using long-term SST datasets show that over an extended period of 112 years, the western Indian Ocean (WIO, 50–65°E, 5°S-10°N) has experienced anomalous warming of 1.2 °C (Fig. 2), which is 0.5 °C greater than the warming over the warm pool region28. Even though a warming trend over the Indian Ocean may result in an increased supply of moisture locally over the ocean, it is not necessary that the surplus moisture is carried on to the peninsula29. The sustained Indian Ocean warming in the west has led to a greater westward extension of this Indian Ocean warm pool in recent decades21,28. The expansion of the Indian Ocean warm pool can in turn modify the land-sea thermal gradient, which could modulate the strength and flow of the monsoon circulation and the moisture laden winds towards the South Asian subcontinent. Hence, deciphering the relationship between the Indian Ocean warming and the monsoon precipitation might give a clue to the cause and effect of the observed rainfall trends over the region.

Figure 2: Summer sea surface temperature trends for the years 1901–2012. Observed trend in mean summer (June–September) SST (°C 112 year−1) over the global tropics during 1901–2012. Full size image

To evaluate the association of the nearly monotonic warming over the western Indian Ocean on the observed monsoon trends over the subcontinent, we carried out a correlation analysis between SST and precipitation anomalies over the two regions. Figure 3a,b show the correlation between the western Indian Ocean SST anomalies and precipitation anomalies over the South Asian region, using multiple datasets. The spatial distribution of the correlation coefficients is consistent among the different data sets. The fascinating aspect is that, the correlation of precipitation anomalies with the Indian Ocean SST anomalies and the trend in these precipitation anomalies are strikingly similar. A pattern correlation computed between the trends in Fig. 1b and the correlation coefficients in Fig. 3b (for the region 60°–100°E, 5°–35°N at 0.5° horizontal grid resolution) indicates that they are strongly correlated (r=0.73, significant at 95% confidence level). The most robust common feature among the trend and correlation patterns is a horseshoe pattern over central India. Apart from this, a dipolar pattern in the rainfall trends, with positive in the north and negative in the south is also visible along the Western Ghats of the Indian subcontinent (Fig. 1). A recent study30 suggests that enhanced warming to the north of western Indian Ocean in recent decades has also led to a poleward shift of the core of low-level monsoon winds, increasing the precipitation to the north of the Western Ghats, while weakening it over the south. This dipolar rainfall pattern between the south and north of the Western Ghats is also found to be associated with the western Indian Ocean warming at both the interannual and low frequency (for example, trend) timescales (Fig. 3).

Figure 3: Correlation between western Indian Ocean sea surface temperatures and monsoon precipitation. Correlation between SST over the western Indian Ocean (WIO, 50-65°E, 5°S-10°N) and precipitation over the South Asian subcontinent, for (a) HadISST and IMD precipitation, and (b) ERSST and CRU precipitation, for June–September 1901–2012. Contours denote regions significant at the 95% confidence level. The inset box includes parts of the central Indian subcontinent where the weakening trend in precipitation is significant (c) Time series of SST anomalies (°C, red) over western Indian Ocean along with CRU (blue) precipitation (mm day−1) over central Indian subcontinent (80–90°E, 20–30°N, inset box in b), smoothed with a 10 year moving average. Note that the correlation coefficient (r=−0.34) between HadISST and CRU precipitation is estimated using non-smoothed time series. Kendall’s rank correlation test for the two variables provided a tau coefficient of −0.3 (P<0.01, two tailed). Mann–Kendall test for the trend in the time series provided a tau coefficient of 0.6 for SST and −0.2 for precipitation, both significant at 95% confidence level. Full size image

For a time series analysis, we consider the precipitation over the central Indian subcontinent (80–90°E, 20–30°N, inset box in Fig. 3b) along with the western Indian Ocean SST anomalies (Fig. 3c). The rainfall region under consideration is chosen because the decreasing trend in rainfall is significant over this region, and also consistent with other studies utilizing station-wise analysis of rain gauge data6. Indeed, the SST and precipitation time series in Fig. 3c are negatively correlated (r=−0.35 at df=110, for the unsmoothed time series), significant at 95% confidence level. Analysis using detrended time series data also shows significant negative correlation over the same region (Supplementary Fig. 1). This means that similar processes may operate at interannual and lower frequency timescales to generate a secular downward monsoon rainfall trend in response to the warming of the western Indian Ocean, or alternatively, that a part of the Indian Ocean warming is due to the weakening of the monsoon22. The negative correlation is not observed during the first half of the 20th century, but becomes statistically significant post-1950s, probably due to the accelerated warming observed during the latter half. This implies that the relationship between a warm ocean and a weak monsoon is not linear. Since the ocean warming is a slow process, the heat has to build-up over several decades to make a substantial difference and reach the warm pool SST values (28.0 °C)21,28. The sustained western Indian Ocean warming has led to a greater spatial extension of the Indian Ocean warm pool in the recent decades only21.

Changing land-sea thermal contrast

As pointed out earlier, changes in the land-sea thermal contrast impact the strength of the monsoon16,17,18,19. Studies show that over the northern hemisphere, the surface temperatures over land increase more rapidly than over sea, under increasing greenhouse gases19. Ideally, such a scenario should strengthen the South Asian monsoon16,17,18,19. Figure 4a,b shows the climatological mean and trends of surface temperatures over the Indian Ocean and the South Asian monsoon domain, during summer. Obviously, the monotonic warming trend over the Indian Ocean is prominent, with the strongest warming over the west. At the same time the Indian subcontinent exhibits suppressed warming during the past century. In fact, the surface temperatures in the northern Indian peninsula show a cooling trend. This implies a weakening meridional thermal gradient over the South Asian domain, in contrast with earlier studies that suggest a strengthening of the thermal gradient under a globally warming scenario. The weakening land-sea thermal gradient is reproduced in other datasets also, indicating the robustness of the trend (Supplementary Fig. 2).

Figure 4: Summer mean surface temperatures and land-sea temperature gradients for the years 1901–2012. Observed (a) mean (°C) and (b) trend (°C 112 year−1) of surface temperatures during June–September, 1901–2012. For the trend, the colour shades represent regions significant at the 95% confidence level. (c) Observed trend (°C 65 year−1) of upper tropospheric (200 hPa) temperatures during June–September, 1948–2012. (d) Time series of the trend in land-sea contrast in the surface (T surf ) and tropospheric (T surf , 850-200hPa average) temperatures (°C). The land-sea contrast is estimated as the difference in the values between the boxes over the South Asian subcontinent (70–85°E, 10–30°N) and western Indian Ocean (50–65°E, 5°S-10°N), and the trend is estimated over 31 year sliding periods. The land surface temperature is from HadCRUT, SST from HadISST and tropospheric temperature from NCEP reanalysis. Full size image

The land-sea contrast in surface temperatures, though important, may not provide a comprehensive picture of the factors modulating the strength of the monsoon circulation. Once the monsoon rains set in on the South Asian subcontinent, the land surface cools down considerably. However, the troposphere above the land remains warm due to the latent heat release from the convective activity, keeping the thermal contrast functional31. This means that the tropospheric temperature is an ideal parameter for examining the thermodynamic forcing related to the monsoon31. However, unlike the surface temperature, robust records of air temperature at different levels of the troposphere are not available before the 1950s, which makes it difficult to examine the role of long-term tropospheric temperature trends on the monsoon. Nevertheless, to confirm that our results are valid for the recent decades also, we analyse the tropospheric temperatures over the monsoon domain. The trend in the thermal contrast is well reflected in the upper tropospheric (200 hPa) temperature anomalies also, as demonstrated in Fig. 4c. Along with a cooling trend over the larger Indo-Tibetan landmass, increasing positive temperature anomalies are observed over the western Indian Ocean, indicating its potential role in weakening the monsoon flow towards the Indian peninsula. The warming of the upper troposphere over the Indian Ocean (Fig. 4c) is co-located with that over the sea surface (Fig. 4b). This may be attributed to the fact that the tropical Indian Ocean is a highly convective region during the northern summer, as the mean SSTs are above the minimum convective threshold (26 °C) and the southwesterly monsoon winds are conducive for enhanced moisture convergence32. Further warming of the Indian Ocean enhances the convection23, transferring surplus heat to the upper troposphere33,34,35.

To evaluate the trends observed in land-sea thermal contrast in Fig. 4b,c, we analyse the difference between the summer tropospheric temperatures over western Indian Ocean and the Indian subcontinent, at 850 and 200 hPa levels (Fig. 4d). The surface temperature gradient (T surf ) shows a sharp decline throughout the past century, largely contributed by an increased warming over the western Indian Ocean. The tropospheric temperature gradient (T trop ) post-1950s also indicates a trend similar to that in the surface temperatures. This is suggestive of a warm Indian Ocean having a strong hold on the whole troposphere, from the surface to the top, possibly due to enhanced convective mixing over the ocean33,34,35. Indeed, a separate analysis of the trend at each of these levels gave consistent results of a warming troposphere over the Indian Ocean, and a weakening thermal contrast between the subcontinent and the ocean. All these results indicate a reduced land-sea thermal contrast during the past century, partly due to increased warming over the Indian Ocean, especially over the western region, and partly due to a relatively weaker warming over the Indian peninsula.

Weakening of the summer monsoon Hadley circulation

Many studies have shown that warm SST anomalies are accompanied by large variations in the lower and upper troposphere due to enhanced latent heating aloft from convection over the ocean33,34,35. Furthermore, these changes are highly correlated with the strength of the monsoon circulation31,34. A more recent study using a coupled model framework also points out that a spatial extension of Indian Ocean warm pool could enhance the convection over the ocean while introducing a dry bias over land, by modulating the meridional Hadley circulation36. It is therefore likely that the impact of the Indian Ocean warming on the monsoon has become more prominent in recent decades. Consistently, an examination of the vertical wind velocity over the South Asian domain (50–100°E) during the years 1948–2012 indicates largescale upward motion over the equatorial ocean (10°S-10°N), extending up to the upper troposphere and favouring intense local convection (Fig. 5a). Furthermore, this enhanced upward motion over the ocean is compensated by subsidence of air over the subcontinent (10–20°N), inhibiting convection over the landmass and drying the region, through a modulation of the local Hadley cell (Fig. 5a). This suggests that though the warming ocean engenders enhanced local rainfall due to increased moisture availability, it weakens the monsoon Hadley circulation and reduces the rainfall over the land, ultimately building up a competition among the land and ocean rainfall in the South Asian monsoon domain.

Figure 5: Observed trends and model simulated changes in summer monsoon Hadley circulation in response to Indian Ocean warming. (a) Trend from NCEP re-analysis (Pa s−1 year−1) and (b) model simulated response (Pa s−1) to western Indian Ocean warming in vertical velocity (omega) during northern summer (June–September) along the South Asian domain (50–100°E). Positive (red) values indicate upward motion. Full size image

Climate model response to Indian Ocean warming

Attribution of the decreasing monsoon rainfall to the Indian Ocean warming from observations alone is very difficult and probably impossible. Thus, to delineate and examine the role of the Indian Ocean warming on the monsoon circulation and rainfall, model sensitivity experiments using an ocean-atmosphere coupled climate model were carried out. In the sensitivity experiment, positive SST anomalies similar to those in the observations were added to the western Indian Ocean region (Supplementary Fig. 3). Figure 5b shows the model response in the summer monsoon Hadley circulation, and features a weakening of the local Hadley cell, much similar to the observed trends. The fact that the warming has resulted in enhanced convection over the ocean and subsidence over the landmass serves as the evidence that the observed warming trend in the Indian Ocean has the potential to weaken the monsoon circulation.

Figure 6 compares the observed trends in precipitation and surface winds with the model-simulated anomalies in response to warming over the western Indian Ocean. The interesting, robust element in the model simulations (Fig. 6b) is the horseshoe pattern in the negative precipitation anomalies, with one arm placed over the Himalayan foothills and the other over the central Indian landmass, similar to the one in the precipitation trends (Figs 1 and 6a) and the correlation pattern with western Indian Ocean SST anomalies (Fig. 3a,b). Consistent with the reduced precipitation, the surface winds also suggest weakened mean southwesterly winds. The trend analysis (Fig. 6a) shows a Rossby wave-like response37 to the Indian Ocean warming, with northeasterly wind anomalies over most of the Arabian Sea, converging towards the central Indian Ocean. The model simulation (Fig. 6b) also shows similar results, with respect to the Rossby wave response near the subcontinent and over the Arabian Sea. However, the zonal direction of the wind anomalies near the equator appears different in the observations and the model. This is not an unexpected result as the periods under consideration and the model simulations are different, since the observed winds are available only for 1948–2012, a period during which the Indian Ocean experienced a basin-wide warming21,22. In fact previous studies using AGCM experiments driven by recent basin-wide warming in the tropical Indian Ocean reproduced the observed zonal wind changes in the recent decades38. In the model sensitivity experiment, however, the positive SST offset is prescribed only over the western Indian Ocean region, where the long-term warming trend is observed in the SST records during 1901–2012. This suggests that while western Indian Ocean warming plays a significant role in the long-term change in winds, basin-wide warming over the tropical Indian Ocean has emerged as a co-conspirator in the change in land-sea thermal contrast and winds in recent decades.

Figure 6: Observed trends and model simulated changes in summer monsoon precipitation in response to Indian Ocean warming. (a) Observed trend in precipitation (CRU data, 1901–2012, mm day−1) and near-surface winds (NCEP, 1948–2012, m s−1), for June-September. (b) Model-simulated mean precipitation (mm day−1) and wind (10 m) anomalies (m s−1) in response to warming over the western Indian Ocean. The model-simulated anomalies are estimated from the sensitivity run where SST anomalies of the order of 1.5 °C is introduced over the western Indian Ocean (CFSv2 WIO ), with respect to a model control run (CFSv2 CTL ). Full size image

A further comparison of the model response to the warming is provided in Supplementary Fig. 4. The thermal response (surface and upper troposphere) is similar to those in the observed trends over the ocean, but exhibit some discrepancies over land. More specifically, for the model response in temperature at 200 hPa, the results reproduce the upper tropospheric warming over the ocean, but do not replicate the cooling over the Indo-Tibetan landmass. This is possibly because of the role of other processes on the upper tropospheric temperatures, such as aerosol forcing39,40 and stratosphere–troposphere interactions41, which are not included in the model experiment. It may also be noted that previous studies41 have suggested that the NCEP upper tropospheric data overestimate the cooling trend over the Indo-Tibetan landmass. Nevertheless, the similarity between the rainfall and local Hadley responses between the model experiments and observed fields clearly demonstrates the importance of western Indian Ocean warming in determining the observed monsoon response.