Large‐scale long‐period analyses like NCEP reanalysis have become invaluable for generating robust statistics for model validation and carrying out comprehensive diagnostics. While three strong features of these analyses are their availability for a long period with daily temporal resolution and global coverage, they suffer from coarse spatial resolution and strong model inputs, especially for variables like rainfall. This work evaluates daily NCEP rainfall as a high‐resolution analysis through comparison with high‐resolution rainfall based on satellite and GTS gauge observations. We use daily composite rainfalls over the Indian summer monsoon domain for the period 2001–2005 from satellite observations available at 10‐km resolution and 10‐km interpolated NCEP rainfall (I‐NCEP). Comparison of observed and I‐NCEP rainfall, in terms of parameters like correlation coefficient and phase synchronization, shows that the I‐NCEP rainfall provides a good representation of high‐resolution observations even at mesoscales and provides a good long‐period, high‐resolution data set for model validation and evaluation.
This study presents an analysis of observed data sets from multiple sources, including observations from a network of Argo floats during (2002–2003), with the aim of investigating the role of the southwest monsoon circulation in affecting the interactions between the oceanic mixed layer and the underlying thermocline in the northern Indian Ocean. Examination of the seasonal cycle of the upper‐ocean thermal structure shows that the surface cooling of the Arabian Sea, during the southwest monsoon season, is accompanied by significant warming of the thermocline. It is seen that the thermocline is warmer by about 1.2°C in the south‐central Arabian Sea during the southwest monsoon season relative to other months. Offline computations of the profiles of vertical diffusivity of heat reveal stronger and deeper penetration of heat into the Arabian Sea during the southwest monsoon season. The results presented in the paper demonstrate that the combined effects of strong wind‐driven mixing by the monsoonal winds, weak density stratification in the upper‐ocean, and downwelling in south‐central Arabian Sea, along with strong vertical diffusivity, favor downward transfer of warm waters from the surface into the thermocline. Besides the climatological seasonal cycle, the present study also examines the impact of monsoon interannual variability on the upper‐ocean response, by analysis of long‐term observed records during (1955–2001) as well as the Argo observations for (2002–2003). It is found that the interannual variations in the ocean response reveal signatures of the influence of strong and weak southwest monsoons on the mixed layer and thermocline variabilities.
Abstract El Niño–Southern Oscillation (ENSO) is an aperiodic oscillation of sea surface temperature (SST)-induced interannual rainfall variability in south India (SI) that has a direct impact on rain-fed agricultural production and the economy of the region. The study analyzed the influence of ENSO-related rainfall variability on crop yield of south Indian tea-growing regions (SITR) for the period of 1971–2015. The relationship between SST anomalies from June to August over the Niño-3 sector of the tropical Pacific Ocean and tea production anomalies of SI shows a positive correlation. However, SST has a negative relationship with rainfall in the regions of the southwest monsoon but not with the northeast monsoon region of the Nilgiris. The correlation between rainfall and crop yield in SI ( r = 0.045) is positively weak and statistically insignificant ( p > 0.05). Tea production is influenced more by the cold phase than the warm phase of ENSO, whereas rainfall is greatly influenced by the warm phase. Tea production across the regions indicated that none of the ENSO phase categories based on Niño-3 has significantly greater production than any of the other ENSO phases. Therefore, the predictability of tea production on the basis of ENSO phases is limited. Our findings highlight that the crop production of SITR appeared to be less responsive to the ENSO phases. This may be due to improvements in production technology that mitigated the problems associated with rainfall variability.
Projections of climate change are emerging to play major roles in many applications. However, assessing reliability of climate change projections, especially at regional scales, remains a major challenge. An important question is the degree of progress made since the earlier IPCC simulations (CMIP3) to the latest, recently completed CMIP5. We consider the continental Indian monsoon as an example and apply a hierarchical approach for assessing reliability, using the accuracy in simulating the historical trend as the primary criterion. While the scope has increased in CMIP5, there is essentially no improvement in skill in projections since CMIP3 in terms of reliability (confidence). Thus, it may be necessary to consider acceptable models for specific assessment rather than simple ensemble. Analysis of climate indices shows that in both CMIP5 and CMIP3 certain common processes at large and regional scales as well as slow timescales are associated with successful simulation of trend and mean.
Monsoon droughts over the Indian subcontinent emanate from failures in the seasonal (June–September) monsoon rains. While prolonged dry‐spells (“monsoon‐breaks”) pervade on sub‐seasonal/intra‐seasonal time‐scales, the underlying causes for these long‐lasting anomalies remain elusive. Based on analyses of a suite of observed data sets, we report an ocean‐atmosphere dynamical coupling on intra‐seasonal time‐scales, in the tropical Indian Ocean, which is pivotal in forcing extended monsoon‐breaks and causing droughts over the subcontinent. This coupling involves a feedback between the monsoonal flow and thermocline depth in the Equatorial Eastern Indian Ocean (EEIO), in which an anomaly of the summer monsoon circulation induces downwelling and maintains a higher‐than‐normal heat‐content. The near‐equatorial anomalies induce strong and sustained suppression of monsoon rainfall over the subcontinent. It is concluded that the intra‐seasonal evolution of the ocean‐monsoon coupled system is a vital key to unlocking the dynamics of monsoon droughts.
In the backdrop of a changing climate, we investigate whether the Indian summer monsoon is changing either in terms of duration or spatial coverage. Such an analysis specifically for the continental Indian region has both conceptual and societal implications, and has been lacking. We show here, based on an analysis of daily gridded observed rainfall data for the period 1951–2003, that there are decreasing trends in both early and late monsoon rainfall and number of rainy days, implying a shorter monsoon over India. Similarly, there is a sharp decrease in the area that receives a certain amount of rainfall and number of rainy days during the season. These trends are consistent with other variables like OLR and rainfall from independent datasets; in particular, the land‐ocean temperature contrast has a decreasing trend, consistent with a weakening monsoon. The results emphasize need for careful regional analysis in drawing conclusions regarding agro‐ecological sustainability in a changing climate.
Extreme rainfall events today pose a serious threat to many populated and urbanized areas worldwide. An accurate estimate of frequency and distribution of these events can significantly aid in policy planning and observation system design. We report here a high-resolution (10 km) analysis of heavy rainfall episodes (defined as 24-h rainfall exceeding 250 mm) over the Indian region. The dataset, recently developed by NOAA, USA, provides daily composite rainfalls for the period 2001-06 at locations approximately 10 km apart. We first assess the reliability of the dataset by comparing it with daily gridded (1° x 1°) rainfall data from IMD and three-hourly gridded (0.25° x 0.25°) data from TRMM for the overlap years (2001-04). A category-wise analysis of the high-resolution data reveals a number of hotspots of vulnerability; in particular, the semiarid region in northwest India emerges as a high-vulnerability area in terms of extreme rainfall events. The high-resolution analysis also clearly reveals the corridor of the monsoon trough, lined by a flower-pot distribution of extreme rainfall events along the flanks. This can be a valuable input for precision design of field experiments on the continental trough or on localized extreme events like thunderstorms. Other important implications for areas like vulnerability assessment, planning and mesoscale forecasting are discussed.
Diagnostic analysis of observations and a series of ensemble simulations using an atmospheric general circulation model (GCM) have been carried out with a view to understanding the processes responsible for the widespread suppression of the seasonal summer monsoon rainfall over the Indian subcontinent in 2000. During this period, the equatorial and southern tropical Indian Ocean (EQSIO) was characterized by persistent warmer than normal sea surface temperature (SST), increased atmospheric moisture convergence, and enhanced precipitation. These abnormal conditions not only offered an ideal prototype of the regional convective anomalies over the subcontinent and Indian Ocean, but also provided a basis for investigating the causes for the intensification and maintenance of the seasonal anomaly patterns. The findings of this study reveal that the strengthening of the convective activity over the region of the southern equatorial trough played a key role in inducing anomalous subsidence over the subcontinent and thereby weakened the monsoon Hadley cell. The leading empirical orthogonal function (EOF) of the intraseasonal variability of observed rainfall was characterized by a north–south asymmetric pattern of negative anomaly over India and positive anomaly over the region of the EQSIO and accounted for about 21% of the total rainfall variance during 2000. The GCM-simulated response to forcing by SST anomalies during 2000 is found to be consistent with observations in reasonably capturing the seasonal monsoon anomalies and the intraseasonal variability. Further, it is shown from the GCM experiments that the warm Indian Ocean (IO) SST anomalies influenced the regional intraseasonal variability in a significant manner by favoring higher probability of occurrence of enhanced rainfall activity over the EQSIO region and, in turn, led to higher probability of occurrence of dry spells and prolonged break-monsoon conditions over the subcontinent. In particular, the simulated break-monsoon anomaly pattern of decreased rainfall over the subcontinent and increased rainfall over the EQSIO is shown to intensify and persist in response to the IO SST anomalies during 2000. These results clearly bring out the significance of the IO SST anomalies in altering the regional intraseasonal variability and thereby affecting the seasonal mean monsoon. Further studies will be required in order to investigate the detailed physical mechanisms that couple the variability of convection over the IO region with the local SST boundary forcing and the large-scale monsoon dynamics.
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