An overview of Holocene monsoon variability of Sri Lanka and its association with Indian subcontinent climate records

: Located in the core of the Indian monsoon domain, Sri Lanka has a classic tropical monsoonal climate, thus making it an ideal platform for paleoclimatic investigations. During past decades, extensive studies have been carried out to explore the Indian Ocean monsoon variability across different time scales including in Sri Lanka. This review synthesized a comprehensive picture of the paleo-monsoon variability of Sri Lanka during the Holocene and compared it with the regional records from the Indian monsoon regime. Available paleoclimate information indicated that the early Holocene is characterized by a strong monsoon phase in Sri Lanka. This enhanced monsoon activity phase was followed by a weak monsoon phases around 8.0 kyr BP. Mid-Holocene was identified as a semi-arid to arid or a period with decreased monsoon activity. The late Holocene showed a highly variable climate with a large spatial variability across Sri Lanka. These observations are almost consistent with the climatic records of the Indian monsoon regime. Dissimilarities in the timing of onset and withdrawal of monsoons were also noted in different geographical localities in Sri Lanka that are possibly due to localized responses of the climate system. Paleoclimate records from Sri Lanka revealed that summer and winter monsoons behaved mostly in a similar way, implying similar forcing mechanisms. Despite the contrasting differences among regional records across shorter time scales, the monsoon climate of Sri Lanka during the Holocene has varied coherently with the climate of other regions of the Indian monsoon domain including the Indian subcontinent, the Bay of Bengal, Northern Andaman Sea, Eastern, and Western Arabian Sea, Northern and Southern Oman, Yemen and as far as some locations from China .


INTRODUCTION
The term 'tropical monsoon' is technically used to describe a major wind system that shows a profound seasonal contrast of the predominant wind regime between the summer and winter seasons. The monsoons occur in equatorial regions of South Asia, Africa, Central America, and Australia. Monsoons are considered to be a unique atmospheric circulation phenomenon in the tropics that has distinctive local, regional and global climate impacts (Wang et al., 2014). It is one of the most important climatic systems that affect the livelihood of over 60% of the world population. From among monsoon systems in the world, the Asian Monsoon System (AMS) is the strongest monsoon circulation compared to all other regional monsoons (Gadgil, 2003;Cane, 2010;Rashid et al., 2011). The fluctuations of monsoons over South Asia have drastic socio-economic constraints since weak monsoon activities affect agriculture through extreme droughts while strong monsoons cause natural disasters such as floods and landslides (Ratnayake and Herath, 2005;Huang et al., 2012). The seasonal reversal of monsoon circulation affects the climate of Asian landmass as well as the Indian Ocean and the South China Sea. Such variations cause cool and dry winters, rainy summers, variations in sea surface temperature, and ocean salinity patterns. In addition, seasonal changes in the monsoon wind regime could lead to variation in the upwelling process in oceans (Morrison et al., 1998;Jayaram and Kumar, 2018;Patel et al., 2019).
In Sri Lankan and South India, monsoon rainfalls serve as an important factor for agricultural activities. Thus, lifestyles of people in the region have developed on the variabilities in monsoonal features. For instance, the timing of the monsoon burst and termination, amount of precipitation, rainfall intensities have immense socioeconomic impacts on communities that are engaged in agriculture. Even the earliest human civilization in the Indian subcontinent was shifted following the spatiotemporal variation of the South Asian Monsoon (SAM) (Kathayat et al., 2017).
During the last few decades, monsoons have sparked the interest of the climate research community (Wang et al., 2014). Since then extensive scientific investigations have been carried out to explore the unique atmospheric circulation phenomenon. Among them, the Indian monsoon is one of the most explored weather phenomena of the Asian subcontinent , since the Asian monsoon has known to be stronger during warm interglacial or interstadial climate and weaker during cooler glacial or stadial periods (Tiwari et al., 2010). Sri Lanka, a small island in the Indian Ocean, located 800 km north of the equator, is largely affected by the monsoon climates. Located in the core of the Indian monsoon domain, the climate of Sri Lanka is described as tropical monsoonal that has a distinct rainfall rhythm throughout the year. Thus the rain-fed agro-economy in Sri Lanka is highly vulnerable to seasonal changes in the monsoon climate. Seasonal crop calendars are also prepared following the timing of the monsoon rainfall. Early-onset or late withdrawal of monsoons causes devastating impacts such as floods and landslides or droughts. Therefore, a long-term observation of the variability of the monsoon system is extremely important to predict future scenarios and to reduce the impacts of extreme events.
Several paleoclimatic investigations have been carried out in Sri Lanka to explore monsoon variability across different time scales. These studies can be used to enhance our understanding of monsoon predictability, improve the accuracy of predictions, and refine the projection's climate modeling. Therefore, this review aims to synthesize a comprehensive picture of the paleomonsoon variability in Sri Lanka during the Holocene. The information gathered is compared with regional records, particularly with the records from the Indian subcontinent.

INDIAN MONSOON REGIME
The Indian monsoon is a dynamic and widely explored feature of the tropical atmospheric circulation system. The Asian Monsoon System (AMS) consists of two subsystems, the South Asian Monsoon (SAM) which is also known as the Indian Monsoon, and the East Asian Monsoon (EAM). The SAM is geographically occupied mostly over the Indian subcontinent and the Indian Ocean while EAM covers the East Asian regions of China, the Korean peninsula, and Japan (Huang et al., 2012). The AMS brings remarkable imprints on the global climate, hydrological cycles, and energy flow through the ecosystems, thus having significant socio-economic consequences (Gadgil, 2003;Rashid et al., 2007;Cane, 2010;Achyuthan et al., 2014). Indian monsoon often originates over the equatorial Indian Ocean and spread northward over land and eastward over the tropical ocean ( Figure 1).
As explained by the modern air mass theory, the monsoons are caused by the seasonal movement of the Inter-tropical Convergence Zone (ITCZ) over the equatorial region (Schneider et al., 2014). During the northern hemisphere summer, ITCZ shifts northward and causes southwest (summer) monsoons. When it is shifted towards the south, northeast (winter) monsoon occurs. The ITCZ migrates between latitudes of 20° N and 8° S depending on the boreal summer and winter, promoting the rainfall over the south Asian region (Schneider et al., 2014). The Indian Summer Monsoon (ISM), or south-west monsoon is a major component of the Asian monsoon system which has significant impacts on a hydrological cycle during summer months (Wang, 2002;Gadgil, 2003;Cane, 2010;Rashid et al., 2011;Ota et al., 2017). During the northern hemispheric summer, pressure differences over warm Tibetan Plateau and cooler Indian Ocean generate southwesterly winds which could result in high rainfalls over southern and south-west Asia (Webster et al., 1998;Wang et al., 2001;Rashid et al., 2007). The intensity of southwest monsoon links with strengths of the low pressure over the Tibetan Plateau and high pressure over the southern Indian Ocean, the effect of Somali Jet (Findlater Jet) and Somali current (Findlater current), Indian Ocean branch of Walker Cell (Walker circulation) and effect of the Indian Ocean Dipole (Trott et al., 2017;Wang et al., 2018).
During September, air pressure over northern India increases because of the rapid cooling of the northern Indian subcontinent, following the retreating Sun into the southern hemisphere. However, air over the adjacent Indian ocean does not cool that fast and holds the heat, resulting in a low-pressure zone. Due to this pressure gradient, cold air masses move from Himalayan and Indo-Gangetic Plain headed towards the Indian Ocean through the Deccan plateau. This is referred to as the northeast monsoon or retreating monsoon (Gupta et al., 2006;Clift and Plumb, 2008;Singhvi et al., 2010;Rajmanickam et al., 2017). The formation of the northeast monsoon is affected by the strengthening of high-pressure cells over Tibetan and Siberian plateaus during the northern hemisphere winter. During this period, the ITCZ shifts towards the south of India, with westward migration and subsequent weakening of the high-pressure cell in the southern Indian Ocean. In shorter time scales, monsoon intensity fluctuates with the sea surface temperature, notably in the tropical Indian Ocean, and with the extension of Eurasian snow cover (Philander et al., 1996;Webster et al., 1998;Robock et al., 2003;Rashid et al., 2011).

MONSOON CLIMATE OF SRI LANKA
The rainfall regime of Sri Lanka is characterized by two monsoon periods known as the southwest monsoon (May to September) and the northeast monsoon (December to February). However, the annual average rainfall varies considerably across the country; with much higher rainfalls are on the central highlands and the western slopes of the central highlands receives an annual average rainfall exceeding 5000 mm ( Figure 2). In contrast, the eastern and northern parts of the island receive only about 1300 mm average annual rainfall. During its northward movement, ITCZ crosses Sri Lanka and activates the summer monsoon. As a result, the southwestern part of the island receives steady rain (Suppiah, 1997). The orographic effect of the island causes heavy rainfalls on the western slopes ( Figure 3). During the southwest monsoon period, cyclonic wind circulations in the mid-troposphere cause spells of heavy rain from June to August Suppiah, 1996). Further, the troughs of low pressure which may spread out of Sri Lanka from the northern Bay of Bengal cause stormy weather and heavy rain. With the southward movement of ITCZ, the summer monsoon season ends in September. The northeast monsoon (winter monsoon) arrives in Sri Lanka in December when ITCZ migrates southwards. In December, spells of heavy rain occur in the eastern lowlands as with passing waves of   small amplitude in the easterly wind stream and cyclonic wind circulations. Nearly 50% of the annual rainfall from the northeast monsoon is received by the eastern half of the country whereas the northwestern and southeastern regions receive relatively lower precipitation ( Figure 3). The orography of the island also causes steady rain on the eastern slopes of the highlands. During the inter-monsoon periods of March-April and October-November, tropical cyclones and thunderstorms have also influenced the climate of Sri Lanka .
Despite the small size of the country, Sri Lanka has a remarkable spatio-temporal rainfall variation with differences in a hydrological and agro-ecological environment that are controlled by the monsoon pattern and the topography (Jayasena et al., 2008;Chandrajith, 2020). Subsequently, the island is divided into two main climatic zones known as dry zone and wet zone however, an intermediate zone occurs in between main climatic zones (Domrös, 1979). Two third of the island, particularly north, northeast, and south-east terrains belongs to the dry zone regime ( Figure 3). Due to its position in the core of the Indian monsoon domain, the rain-fed agro-economy in the island is highly vulnerable to the influence of both summer and winter monsoons. Early-onset or late withdrawal of monsoons causes devastating impacts such as floods and landslides or droughts and even spreading of some vector bone diseases (Zubair, 2002;Ratnayake and Herath, 2005;Briët et al., 2008;Sirisena and Noordeen, 2014).

PALEOCLIMATE RECORDS FROM SRI LANKA
The great chronicle of Sri Lanka, the Mahāvansa provides the first documentary evidence on a palaeoclimatic event that occurred in Sri Lanka. The Mahāvansa refers to an extreme drought period that lasted around 700 BP on the island (Geiger and Boad, 1964). However, systematic paleo-environment and paleoclimate studies have been initiated in Sri Lanka in the '90s. Deraniyagala, (1992) reconstructed the late Quarternery paleo-environment of Sri Lanka and identified wet pluvials that were in phase with glacial stages while dry interpluvials were associated with inter-glacial stages. Besides, paleo-proxy records were investigated in different sites in Sri Lanka to reconstruct the Holocene climatic history. From among these studies, a palynological study of sediments from Potana cave in Sigiriya (Premathilake and Caratini, 1994) and Horton Plains (Premathilake, 2003;Premathilake and Risberg, 2003;Routh et al., 2014), geochemical and sedimentological investigations of sediment cores from the southeastern coastal plain (Ranasinghe et al., 2013a(Ranasinghe et al., , 2013b, southern coastal area (Ranasinghe et al., 2013a) and western coastal region (Gayantha et al., 2017;Ratnayake et al., 2017;Gayantha et al., 2019a,b;Gayantha et al., 2020) used to elucidate paleo-climate conditions prevailed in Sri Lanka ( Figure 2). Table 1 summarizes the paleo-climatic records available in Sri Lanka and the proxies used for these investigations.

PALEO-MONSOON VARIABILITY OF SRI LANKA DURING HOLOCENE
The early Holocene period of Sri Lanka was characterized by a strong monsoon phase as depicted by several studies (Premathilake and Risberg, 2003;Premathilake, 2006). Based on the pollen and spore records, information obtained from total organic carbon (TOC), total nitrogen (TN), δ 13 C and siliceous microfossils found in peat sequences from the Horton Plains indicated a per humid event that occurred from 10.2 to 9.9 kyr BP with a hyper-humid event around 8.7 kyr BP. Increased pollen abundance with higher TOC content and lower TN were observed during these events (Premathilake and Risberg, 2003). During a strong monsoon phase, the flourishing of vegetation could result in increased pollen and spore production. In addition, wet periods also favor high primary productivity that is reflected in increased TOC contents. Changes in the presence of dominant pollen types indicate vegetation changes due to shifts in warm/wet and dry/arid climate conditions (Misra and Bhattacharyya, 2014;Veena et al., 2014;Srivastava et al., 2017). Similarly, wet or dry monsoon climate can be identified based on the dominance of each taxon of siliceous microfossil types within a specific period (Premathilake, 2006;Misra and Bhattacharyya, 2014;Veena et al., 2014). Based on upwelling and terrestrial proxies, enhanced southwest monsoon activity during the early Holocene (10.0-8.0 kyr BP) was also detected in a marine record from the western continental shelf off Colombo (Ranasinghe et al., 2017). After the 12.5 kyr BP Younger Dryas cold event, wet and warm conditions were gradually increased and depicts an early Holocene summer monsoon strengthening as evident in LOI, TOC, TN, δ 13 C, δ 15 N, major and trace element ratios, and diffuse color reflectance proxies in sediment records from Gawaratenna Plateau (Ranasinghe et al., 2017). However, monsoon strength has weakened at around 8.2 kyr BP, showing a teleconnection with North Atlantic cold episodes.
In most paleoclimate records, the mid-Holocene of Sri Lanka was noted as a semi-arid to arid phase or a period of weakened monsoon activity (Figure 4). From 8.7 to 3.6 kyr BP, a semi-arid climate was characterized as indicated by decreasing pollen abundancy (Premathilake and Risberg, 2003). However, around 5.7 kyr BP, a slightly wetter climate was observed in the dry zone area where rainfall mainly receives from the northeast monsoon (Premathilake and Caratini, 1994). The Horton Plains, which receive excessively high rainfall at present, remained semi-arid during the mid-Holocene. Premathilake (2006) further revealed that the monsoon precipitation over the Horton Plains commenced to decrease from around 8.7 kyr BP and continued until 3.6 kyr BP. Therefore, this period can be considered as a semi-arid phase or a weakened phase of southwest monsoon with an extreme arid period in between 5.4 to 3.6 kyr BP (Premathilake, 2006). This was also supported by the marine sediment records from the western continental shelf where decreased flux of terrestrial Ti and Fe 2 O 3 were noted particularly during 8.2 to 2.0 kyr BP. A sedimentary record from the western continental shelf revealed a low terrestrial influx from 4.0  kyr BP to 2.0 kyr BP, indicating a decreased intensity of southwest monsoon precipitation. However, the monsoon winds were stronger during this period as depicted by upwelling proxies (Ranasinghe et al., 2016).
Mid-Holocene aridity was also detected in northeast monsoon records from the southeastern region in Sri Lanka. Geochemical proxies of lagoon sediments from the south-eastern coast indicated an arid phase from 7.3 to 6.75 kyr BP and a semi-arid interval extending from 6.25 to 4.60 kyr BP (Ranasinghe et al., 2013b). However, a short wet interval during mid-Holocene was observed during 6.50-6.25 kyr BP (Ranasinghe et al., 2013a), and a slightly more humid climate was noted around 5.7 kyr BP in the Potana Cave site (Premathilake and Caratini, 1994). Late Holocene (4.2 to 0.0 kyr BP) is characterized by fluctuating weak and enhanced monsoon conditions (Premathilake and Risberg, 2003;Premathilake, 2006;Ranasinghe et al., 2013a,b;Gayantha et al., 2017). The winter monsoon-dominated southeastern part of the island experienced an arid climate during 4.0-3.0 kyr BP but a wet climate during the 3.0-1.5 kyr BP period (Ranasinghe et al., 2013a). However, strengthened summer monsoon activity was observed during 3.6 to 2.0 kyr BP, but decreased around 2.2 kyr BP as indicated from archives from central highlands (Premathilake and Risberg, 2003). Multi-proxy paleo-climate data obtained by sedimentary achieves from the Bolgoda Lake on the west coast showed an intensification of south-west monsoon from 2.9 to 1.3 kyr BP (Gayantha et al., 2017). Signs of fading monsoons were also detected during the period of 2.4-2.0 kyr BP (Gayantha et al., 2019b) and 2.0 kyr BP to present (Premathilake and Risberg, 2003) in the south-west monsoon. A sediment core from the central highland also indicated a dry period from 4.0 to 1.0 kyr BP (Ranasinghe et al., 2017). Depicting the highly dynamic nature of the late Holocene summer monsoon, the periods extending from 1.78 to 1.29 kyr BP and 1.0 kyr BP to present were recorded as active monsoon phases (Ambillapitiya et al., 2017;Gayantha et al., 2017). Premathilake and Risberg (2003) also suggested an increasing trend in summer monsoon towards the late Holocene. Despite the wax and wane behavior of monsoon strength was recorded during the late Holocene period, an overall weakening of the southwest monsoon throughout the late Holocene was evident from lake sediments from the west coast (Gayantha et al., 2017;Ratnayake et al., 2017).

RELATIONSHIP WITH REGIONAL RECORDS
Considering the regional coherence of the Indian monsoon domain, paleoclimate records of the Indian subcontinent, Bay of Bengal, Andaman Sea, Arabian Sea, and continental records around the Arabian Peninsular, provide useful insight into the highly dynamic nature of the Holocene monsoon climate ( Figure 5). Although only a few studies have been carried out in Sri Lanka, extensive investigations on the Holocene climatic variation were carried out in India and surrounding regions for the last few decades using different physical, geochemical and biological proxies. In particular, marine sediments from the Bay of Bengal, and cave stalagmites showed marked changes in biological, chemical, and physical proxies corresponding to active and weak monsoon phases as documented in Sri Lanka.

Early Holocene
Progressive strengthening of the Indian summer monsoon was observed from ~11.0 to 9.4 kyr BP in northwest India, based on lake sediment records from Kottla Dahar (Dixit et al., 2014). Enhanced monsoon activity phases were also identified from 10.0 to 7.0 kyr BP (Nair et al., 2010) and from 10.7 to 8.6 kyr BP (Sandeep et al., 2017) in lake records of Shantisagara lake in southern India. Comparable paleoclimate records were also noted in the eastern Arabian Sea (Sarkar et al., 1990), Oman and Yemen (Fleitmann et al., 2003;Fleitmann et al., 2007), and Andaman Sea (Rashid et al., 2011) and supported the monsoon intensification observed in Sri Lanka during the early Holocene. A rapid northward displacement of the mean position of summer ITCZ and Indian summer monsoon rain belt during early Holocene were reflected by the rapid decrease of δ 18 O in a cave stalagmite from Oman and Yemen (Fleitmann et al., 2007), while a wetter and humid climate was observed in a marine record from Andaman sea (Rashid et al., 2011). Strengthening of monsoon from 11.0 to 7.0 kyr BP was observed in sediments from the Ganges-Brahmaputra river delta on the east coast of India (Goodbred Jr. and Kuehl, 2000). These records are consistent with the observed strong monsoon activity recorded in Sri Lanka during the early Holocene. Paleoclimate archives from south India and surrounding regions showed the single-step development of the Indian monsoon during the early Holocene ( Figure  6).
Despite the observed single-step monsoon strengthening, an apparent two-step progression of Indian summer monsoon during early Holocene was noted in some paleoclimate records (Overpeck et al., 1996;Thamban et al., 2007;He et al., 2018). Abrupt strengthening of the Indian monsoon was observed from 13.0 to 12.5 kyr BP and from 10.0 to 9.5 kyr BP, implying that monsoon progression did not follow astronomical forcing at the dawn of Holocene (Overpeck et al., 1996). Thamban et al. (2007) also identified abrupt strengthening of monsoon, one at 9.5 kyr BP and the other at 9.1 kyr BP, based on marine records from the eastern Arabian Sea. Similar events were also recorded in the Central Tibetan Plateau around 11.7 kyr BP and 10.0 kyr BP inferred by decreased δ 18 O in Ostracods and a significant increase of sedimentary Ti concentration (He et al., 2018). During the second event, which extended from 10.0 to 7.0 kyr BP, leaf-wax hydrogen isotope composition (δDwax) values decreased progressively and reached the minimum indicating enhanced precipitation.
Significant weakening of the Indian summer monsoon was recorded from 11.1 to 10.7 kyr BP as identified in the Shantiasgara lake record (Sandeep et al., 2017)   South India (Resmi and Achyuthan, 2018). Apart from that, significantly weak monsoon phases from 10.8 to 10.2 kyr BP,9.8 to 8.8 kyr BP,and 8.4 to 8.0 kyr BP were also recorded in marine sediments from the Arabian Sea. These events have coincided with 10.3, 9.4, and 8.2 Bond events, respectively (Gupta et al., 2005). However, only the 8.2 Bond events were recorded in Sri Lanka. This cold episode was recorded as a period of low terrestrial influx which depicts a weakening of rainfall. An abrupt weakening of the Indian summer monsoon which was reported during the 8.3 to 7.9 kyr BP period, is coincided with the 8.2 kyr BP cold event as evident by abrupt desiccation marked by Nir' Pa Co lake record in southeastern Tibet (Dixit et al., 2014).

Mid Holocene
Semi-arid climate and increased arid conditions were recorded in the central highlands of Sri Lanka from 8.7-3.6 kyr BP and 8.0 to 3.6 kyr BP (Figure 4). This observation is consistent with the records from Kukkal and Shantisagara lakes (Rajmanickam et al., 2017;Sandeep et al., 2017). A gradual decrease in monsoon precipitation was also initiated in ~8.0 kyr BP as evident from cave stalagmite records from southern Oman (Fleitmann et al., 2003). Sediments from the Arabian sea and Riwasa lake in northwest India also recorded a similar event (Thamban et al., 2007;Dixit et al., 2014). A cave stalagmite record from Oman and Yemen showed decreasing monsoon precipitation in the mid-Holocene due to the southward migration of ITCZ (Fleitmann et al., 2007). These events are also evident in mid-Holocene paleoclimate records from both Sri Lanka and the Indian subcontinent. This was followed by weak to moderate precipitation conditions mainly from 5.0 to 4.2 kyr BP (Fleitmann et al., 2007). However, the eastern Arabian sea recorded a more arid episode from 6.0 to 3.5 kyr BP and a significantly weak monsoon period from 6.0 to 5.5 kyr BP (Sarkar et al., 2000;Thamban et al., 2007). Although the mid-Holocene is identified as a weak monsoon phase in most paleoclimate records in Sri Lanka, deviations to this generalized figure can be observed in some records. Such events include a short wet climate recorded during 6.50-6.25 kyr BP (Ranasinghe et al., 2013a) and a slightly more humid climate around 5.7 kyr BP (Premathilake and Caratini, 1994). The reported short wet period during 6.5-6.25 kyr BP in the winter monsoon is consistent with the warmer and wetter conditions that appeared during 6. 5-5.4 kyr BP (Srivastava et al., 2017).

Late Holocene
During the late Holocene period, intense monsoon activity phases were recorded in many parts of Sri Lanka as revealed by different paleoclimate proxies. Similar climate conditions were also observed in paleoclimate records from the Indian subcontinent and surrounding regions. Rapid intensification of the summer monsoon with wet and strong climate phases was observed between 3.2-1.8 kyr BP in sediment recodes from Kukkal Lake, South India (Rajmanickam et al., 2017). A strengthening of monsoon activity from ~3.6 to 1.5 kyr BP, recorded in Sri Lanka is comparable to the records from both the eastern Arabian Sea and northern Andaman Sea Sarkar et al., 2000). However, the above observations are not consistent with some terrestrial records that showed the onset of aridity occurred ~4.0 kyr BP (Sarkar et al., 2000). The dissimilarities in the timing of onset and retreating monsoons are obvious in both summer and winter monsoons in different geographical localities in Sri Lanka implying the localized response of the climate system to a unique tropical atmospheric circulation phenomenon.
A palynological study coupled with magnetic susceptibility and stratigraphy of subsurface sediment from the South Kerala sedimentary basin (Kumaran et al., 2008) revealed that south-west monsoon became gradually reduced after ~5.0 kyr BP or 4.0 kyr BP thereby, having a late Holocene dry climate. A significant weakening of the Indian summer monsoon after ~5.0 kyr BP was noted in the Andaman Sea and southern India (Rashid et al., 2011;Sandeep et al., 2017). The declining trend of Indian summer monsoon precipitation was identified from cave stalagmite obtained from northeast India during the same period (Berkelhammer et al., 2012). In addition, abrupt weakening of south-west monsoon was observed in lake sediment record from Haryana, India around 4.1 kyr BP following 4.2 kyr BP Bond event (Dixit et al., 2014). The weak monsoon phases recorded in Sri Lanka during the late Holocene to the present are consistent with the weak monsoon phase recorded in the South Asian region including recodes from the Arabian Sea.
Climate variations were important during the late Holocene period since expansion of human settlements was expanded during this period. An extremely strong south-west monsoon phase was recorded in hydrological conditions observed during 1.8-1.0 kyr BP in higher central Himalayas (Bhushan et al., 2018) and increased winter monsoon precipitation was recorded from 1.88 to 1.44 kyr BP in South India (Resmi and Achyuthan, 2018). In contrast, a weakening of the southwest monsoon was recorded in central Sri Lanka from ca. 2.0 kyr BP to present (Premathilake and Risberg, 2003). This observation is supported by a weak monsoon phase recorded in continental margin sediments off Oman, during 1.9-1.4 kyr BP (Gupta et al., 2003). Furthermore, a prolonged and gradual intensification of the Indian summer monsoon was observed from 0.4 kyr BP to present in the northern Andaman Sea record (Ota et al., 2017). Despite the two discrete, short humid events with significantly high rainfall around 0.6 kyr BP and around 0.15 kyr BP in the southwest monsoon record (Premathilake and Risberg, 2003). Veena et al. (2014) also noted uninterrupted phases of enhanced monsoon activity during the periods of 1.4-0.76 kyr BP and 0.42-0.14 kyr BP in the Pookude Lake record. A colder phase with suppressed monsoon activities from 0.60-0.25 kyr BP (Srivastava et al., 2017), decreased winter monsoon precipitation from ca. 1.44 kyr BP to present (Resmi and Achyuthan, 2018) and wetter to drier transition around ~0.68 kyr BP which was continued till 0.34 kyr BP as identified in Kedarnath peat records, from Central Himalayas, river sediments of Palar River basin, South India and in a Cave Stalagmite record from Southern Oman, respectively (Fleitmann et al., 2004).

SUMMARY
Palaeoclimatic information available in Sri Lanka suggested that the early Holocene was characterized by a strong monsoon phase. This enhanced monsoon activity phase was followed by a weak monsoon period around 8.0 kyr BP. In most paleoclimate records, the mid-Holocene was identified as a semi-arid to arid phase or a period of decreased monsoon activity. Late Holocene is a highly variable climate and in millennial and centennial time scale. There is a large spatial variability that existed in paleomonsoon records from Sri Lanka during the late Holocene. The monsoon variability inferred from paleoclimate records in Sri Lanka is almost consistent with regional records from the Indian monsoon regime. Dissimilarities in the timing of onset and withdrawal of monsoons were obvious among different geographical localities in Sri Lanka for both summer and winter monsoons suggesting localized response of the climate system to these unique atmospheric circulation phenomena. The Indian summer monsoon showed a pronounced phase difference across the country. For example, the southwest monsoon affected the western part of the country experienced a strong monsoon phase while the central Highlands having a weak monsoon phase during the same time interval. Both phases are supported with regional paleo-monsoon records to a large extent. Paleoclimate records from Sri Lanka revealed that summer and winter monsoons behaved mostly in a similar way, implying similar forcing mechanisms. Despite the contrasting differences among regional records across shorter time scales, the paleo-monsoon climate of Sri Lanka during the Holocene has been varied coherently with other of the Indian monsoon domain including the Indian subcontinent, Bay of Bengal, Northern Andaman Sea, Eastern and Western Arabian Sea, Northern and Southern Oman, Yemen and as far as some locations in China where Indian Summer Monsoon has a greater impact than East Asian Summer Monsoon. Based on the monsoonal response at around 8.2 kyr BP, it could be concluded that Sri Lanka showed a teleconnection with the North Atlantic cooling events during Holocene.