Seasonal predictability of the Indian Summer Monsoon : What role do land surface conditions play?

Anomalous springtime snow amounts over Eurasia may provide long term memory to the climate system by affecting the land surface energy and moisture budgets. In turn the anomalous land surface conditions introduced by snow anomalies may influence monsoon variability. In this paper, results from a programme of seasonal forecast ensembles are used to address, specifically, the influence of western Eurasian land surface conditions on the variability and hence predictability of the Indian summer monsoon. The factors that are important for establishing spring time land surface conditions over western Eurasia, particularly snow amounts are also investigated. The results have shown that high snow amounts over western Eurasia are linked to La Nina, suggesting that the El-Nino/Southern Oscillation (ENSO) has an influence on the wintertime climate of Eurasia. The signature of these snow depth anomalies is carried through to the summer in terms of changes in soil wetness and surface temperature. An ensemble of summer integrations with climatological sea surface temperature (SST) has been used to investigate the impact of these anomalous land surface conditions on monsoon variability. The results have shown that the monsoon circulation is substantially weakened in association with above normal snow amounts over western Eurasia, whilst All India Rainfall is slightly increased. Results from a parallel ensemble with observed SSTs show an opposite response in All India Rainfall, suggesting that the forcing by SST anomalies is potentially dominating the monsoon's inter-annual variability. The results have demonstrated that land surface conditions can have a significant impact on the large scale monsoon circulation and to a lesser extent on Indian Summer Monsoon rainfall, although the mechanisms involved have yet to be identified. It is suggested that interactions between the mid-latitude circulation and the monsoon may hold the key to understanding the link between Eurasian land surface conditions and monsoon variability. If that is the case then predictability of this relationship is likely to be limited, due to the high level of internal variability of the mid-latitude circulation.

Monsoon variability in recent years from synoptic scale disturbances and semi-permanent systems

In the recent decade from 1987 to 1996, the Indian summer monsoon rainfall has shown less interannual variability in comparison with its earlier decade. Except 1987 and 1988, the area weighted average monsoon rainfall of all other years are within 10% (normal) of its long period average value over India. The paper discusses monsoon rainfall and several other associated circulations features with their variability in interannual scale during 1987-96. The results show that though the variability of monsoon rainfall is less during the decade, there is a significant interannual variation in the number of synoptic systems, their days, intensities and number of days of presence of monsoon trough and Tibetan anticyclone. The years with positive side (negative side) of normal seasonal rainfall are characterised by more (less) number of days of synoptic disturbances and more (less) number of days of presence of monsoon trough and Tibetan anticyclone in their favourable positions. However, overall activity of heat low, tropical easterly jet and sub-tropical westerly jet in the season have no direct relation with seasonal monsoon rainfall. In addition, the dates of onset and withdrawal of monsoon over India and the number of days monsoon took to over all India also have no relation with the monsoon rainfall.

Deciphering a timeline of demise at Medieval Angkor, Cambodia using remote sensing

The Greater Angkor Region was the center of the Khmer Empire from the 9th until the 13th to the 14th centuries CE, when it entered a period of decline. Many studies have suggested that the decline of Angkor was precipitated by several factors, including severe monsoons, geopolitical shifts, and invasions. In this paper, we use light detection and ranging and ground penetrating radar to investigate the possible intersection of two of these existential threats in one feature: the North Bank Wall. Our results indicate that this feature was designed with dual functionality of extending the urban area’s defenses to the east of Angkor Thom while maintaining the existing infrastructure for the distribution and disposal of water. These findings suggest that the North Bank Wall was built before the severe droughts in the mid-13th century. The timing of the construction indicates that the perceived need for additional security—whether from internal factional disputes or external adversaries—predated the final adaptations to the hydraulic network during the unprecedented monsoon variability of the 14th century. These results indicate that perceived political unrest may have played a more important role in the decline of the site than previously known.

Monsoon Variability Reconstructed from Sedimentological and Mineral Magnetic Studies from Vaghad Tank Deposits, Nashik District, Maharashtra

An attempt made to reconstruct the monsoon variability using sedimentological, geochemical and mineral magnetic studies from deposits in Vaghad Tank, Nashik district, Maharashtra (India). The ~140 years multi-proxy data of the 3.3 meter thick sedimentary section of the tank exhibits some minor changes in sediment characteristics up to the depth of ~150 cm. The grain-size analysis and mineral magnetic studies of 67 samples of sediment suggests that, the sediment dominated by clay. Overall, sedimentary profile does not exhibit any systematic trend in the sediment properties. Finally, the present study concludes no significant changes in the past monsoon conditions have been occurred during the last century but some minor changes in the hydrodynamic conditions have been noticed during the last few decades.

A Paleoclimate Prognosis of the Future Asian Summer Monsoon Variability

In recent years, more and more record-breaking extreme weather/climate events have been reported from the Asian monsoon region, which have caused tremendous loss of property and lives. In this paper, we analyzed the Asian summer monsoon (ASM) variability during the Holocene and evaluated future climate extremes in monsoonal China from a paleoclimatic view. We found a significant regime transition to more chaotic fluctuations, with enhanced decadal variability of the ASM since 6.6 ka BP. We suggested the gradual intensification of ENSO was responsible for enhancing the ASM variability since the late mid-Holocene. If the observed relationship of monsoon mean intensity, ENSO and decadal variability of the ASM in the past 11.2 ka continue to exist, enhanced decadal variability of ASM in the future warming world will be expected. As a result, the intensification of daily precipitation extremes, superimposed on enhanced decadal variability of ASM, might make the record-breaking extremes more frequent in the future, increasing the risk of climate-related disasters in China.