Weak Southwest Monsoon Research Articles

Changes in C<sub>3</sub>/C<sub>4</sub> vegetation in the continental interior of the Central Himalayas associated with monsoonal paleoclimatic changes during the last 600 kyr

Abstract. A continuous lacustrine sediment core obtained from the Kathmandu Valley in the Central Himalayas revealed that cyclical changes in C3/C4 vegetation corresponded to global glacial-interglacial cycles from marine isotope stages (MIS) 15 to MIS 4. The C3/C4 vegetation shifts were reconstructed from significant changes in the δ13C values of bulk organic carbon. Glacial ages were characterized by significant 13C enrichment, due to the expansion of C4 plants, attributed to an intensification of aridity. Thus, the southwest (SW) summer monsoon, which brings the majority of rainfall to the Central Himalayan southern slopes, would have been weaker. Marine sediment cores from the Indian Ocean and Arabian Sea have demonstrated a weaker SW monsoon during glacial periods, and our results confirm that arid conditions and a weak SW monsoon prevailed in the continental interior of the Central Himalayas during glacial ages. This study provides the first continuous record for the continental interior of paleoenvironmental changes directly influenced by the Indian monsoon.

Early deglacial (∼19–17 ka) strengthening of the northeast monsoon

Earlier studies that mostly relied on bulk 14C dating indicated a weaker Southwest monsoon (SWM) and a stronger than present Northeast monsoon (NEM) during the Last Glacial Maximum (LGM, ∼18000 14C years or ∼21000 calendar years ago). We have analyzed a sediment core from the equatorial Indian Ocean, where planktonic foraminifera were dated by Accelerator Mass Spectrometry (AMS 14C). δ18O analysis of three species of foraminifera viz. Globigerinoides ruber, Globigerinoides sacculifer, and Globorotalia menardii reveals that the NEM strengthened during the early deglacial period (∼15000 14C years ago or ∼18000 calendar years ago) relative to the present and not during the LGM as surmised earlier.

A 200-ka pollen and oxygen-isotopic record from two sediment cores from the eastern Arabian Sea

We present here a pollen record from the northern Indian Ocean that goes back to 200 ka, the boundary between marine isotopic stages (MIS) 7 and 6. Pollen, oxygen-isotopic composition and organic carbon have been examined for two sediment cores from the eastern Arabian Sea (15°02′N and 71°41′E, 13°16′N and 71°00′E), to reconstruct the long-term palaeoclimate and palaeovegetation of the Indian subcontinent. Oxygen-isotope data suggest that glacial periods (MIS 2, 4 and 6) are characterised by low precipitation because of a weak southwest monsoon and a strong northeast monsoon. In contrast, interglacial periods (MIS 1, 3 and 5) are marked by high fresh water input resulting from a strong southwest monsoon. During the last glacial–interglacial cycle, sea-surface temperature and surface salinity changed as a result of variations in the evaporation–precipitation (E–P) balance. Throughout the core, the dominant pollen types are Poaceae and Chenopodiaceae and/or Amaranthaceae (>50%). Their dominance during glacial periods (MIS 2, 4 and 6) suggests that the climate was cold and dry. This dominance is also suggestive of salinity-tolerant vegetation colonizing large areas near seashore due to lower sea level or high E–P conditions. During interglacial periods (MIS 1, 3 and 5) when there was high precipitation, the arid taxa were sparse, whereas Poaceae and Piperaceae were abundant. Hence, Chenopodiaceae and/or Amaranthaceae and Artemisia are suggestive of cold and arid/semi-arid climate, and Poaceae and Piperaceae of warm and wet conditions. Mangrove pollen is not well represented in the cores.

A 200-ka pollen and oxygen-isotopic record from two sediment cores from the eastern Arabian Sea

Abstract We present here a pollen record from the northern Indian Ocean that goes back to 200 ka, the boundary between marine isotopic stages (MIS) 7 and 6. Pollen, oxygen-isotopic composition and organic carbon have been examined for two sediment cores from the eastern Arabian Sea (15°02′N and 71°41′E, 13°16′N and 71°00′E), to reconstruct the long-term palaeoclimate and palaeovegetation of the Indian subcontinent. Oxygen-isotope data suggest that glacial periods (MIS 2, 4 and 6) are characterised by low precipitation because of a weak southwest monsoon and a strong northeast monsoon. In contrast, interglacial periods (MIS 1, 3 and 5) are marked by high fresh water input resulting from a strong southwest monsoon. During the last glacial–interglacial cycle, sea-surface temperature and surface salinity changed as a result of variations in the evaporation–precipitation (E–P) balance. Throughout the core, the dominant pollen types are Poaceae and Chenopodiaceae and/or Amaranthaceae (>50%). Their dominance during glacial periods (MIS 2, 4 and 6) suggests that the climate was cold and dry. This dominance is also suggestive of salinity-tolerant vegetation colonizing large areas near seashore due to lower sea level or high E–P conditions. During interglacial periods (MIS 1, 3 and 5) when there was high precipitation, the arid taxa were sparse, whereas Poaceae and Piperaceae were abundant. Hence, Chenopodiaceae and/or Amaranthaceae and Artemisia are suggestive of cold and arid/semi-arid climate, and Poaceae and Piperaceae of warm and wet conditions. Mangrove pollen is not well represented in the cores.

Variations in productivity and eolian fluxes in the northeastern Arabian Sea during the past 110 ka

High-resolution (one to two samples/ka) radionuclide proxy records from core 93KL in the northeastern Arabian Sea provide evidence for millennial climate variability over the past 110 ka. We interpret 230Th-normalized 232Th fluxes as a proxy for eolian input, and authigenic uranium concentrations as a proxy for past productivity. We attribute orbital and suborbital variations in both proxies to changes in the intensity of the southwest Indian Ocean monsoon. The highest 230Th-normalized 232Th fluxes occur at times that are consistent with the timing of the Younger Dryas, Heinrich events 1–7 and cold Dansgaard–Oeschger stadial events recorded in the GISP2 ice core. Such high dust fluxes may be due to a weakened southwest monsoon in conjunction with strengthened northwesterlies from the Arabian Peninsula and Mesopotamia. Authigenic uranium concentrations, on the other hand, are highest during warm Dansgaard–Oeschger interstadials when the southwest monsoon is intensified relative to the northwesterly winds. Our results also indicate that on orbital timescales maximum average eolian fluxes coincide with the timing of marine isotopic stage (MIS) 2 and 4, while minimum fluxes occur during MIS 1, 3 and 5. Although the forcing mechanism(s) controlling suborbital variabilities in monsoonal intensity is still debated, our findings suggest an atmospheric teleconnection between the low-latitude southwest monsoon and North Atlantic climate.

Late Quaternary climate history of the Horton Plains, central Sri Lanka

A 6 m long core was retrieved from a mire at ca 2200 m a.s.l. in the Horton Plains National Park, central Sri Lanka. The material collected consists of a mixture of organic matter and clastic particles, which have been subject to bio-, litho- and chronostratigraphic analyses. The pollen spectra suggest semi-arid conditions and a relatively species-poor plant community from >24,000 until 18,500 cal yr BP associated with a weaker South West Monsoon (SWM). During the late Pleistocene, the climate was fluctuating between relatively dry and humid conditions as the result of changes in the monsoonal regime. The onset of the moonson caused a semi-humid climate resulting in an expansion of the Upper Montane Rain Forest (UMRF). The strengthening of the SWM was interrupted by two relatively dry climatic events, each lasting ca 2000 years. The early Holocene was characterised by a per-humid event followed by a hyper-humid event, both influenced by a further strengthening of the SWM due to the orbitally induced maximum increment of summer insolation. The middle Holocene was marked by a trend towards semi-arid climatic conditions. During the late Holocene, the SWM rains strengthened again.

A synoptic study of active and weak southwest monsoon over Andhra Pradesh

An attempt has been made in this study to identify the synoptic systems that cause active or vigorous monsoon conditions over the three meteorological sub-divisions of Andhra Pradesh using the data for the years 1986 to 2000. Despite significant decrease in the frequency of formation of monsoon depressions in recent years, monsoon performance is noticed to be very good. Monsoon lows and upper air cyclonic-circulations (cycirs) found to be the chief synoptic systems which activate the monsoon over the state. Typical cases of these synoptic situations are discussed.

Sections in the western Indian ocean—variability in the temperature structure

The seasonal cycle of the temperature structure in the western Indian Ocean is presented along two sections monitored with XBT's. Both sections cut the Great Whirl near 8°N, where variations of isotherm depths are concentrated between June and November. Between 5°N and 8°S the variability is predominantly semi-annual, whereas further south it is annual. The seasonal cycle of vertical displacements of isotherms for temperatures colder than 15°C is similar to the cycle obtained in the core of the thermocline, with a slight lag. Variability in other parameters, such as SST, heat content or mixed layer depth, has little in common with vertical isotherm displacements. The deviations from the mean seasonal cycle are generally coherent over 5° of latitude. They are smaller near the equator and are not well correlated vertically, nor on long time scales. However, north of the equator anomalies may be related to variations in the strength of the southwest monsoon. In September and in November 1982, a year with a weak southwest monsoon, the intensity of the Great Whirl was weaker than normal. In 1983, the reverse occurred.

Surface circulation of the Indian Ocean during the last glacial maximum, approximately 18,000 yr B.P.

A seasonal reconstruction of the Indian Ocean during the last glacial maximum (∼18,000 yr B.P.) reveals that its surface circulation and sea surface temperature patterns were significantly different from the modern Indian Ocean. This reconstruction is based on the planktonic foraminiferal biogeography and estimated sea surface temperatures in 42 Indian Ocean samples. Compared to modern conditions, the polar front was 5° to 10° latitude further north during the last glacial maximum; the Subtropical Convergence was 2° to 5° latitude further north. The West Australian Current was more intense as part of the West Wind Drift was deflected northward along the coast of Australia. The Agulhas Current was cooler and weaker during the summer and more saline and subtropical during the winter. In general, the low latitudes underwent little temperature change. The western Arabian Sea was warmer which implies less upwelling and a weaker Southwest Monsoon. On the average, the Indian Ocean was 1.9°C cooler in February and 1.7°C cooler in August during the last glacial maximum.

Zonal Temperature-Anomaly Maps of Indian Ocean Surface Waters: Modern and Ice-Age Patterns

Maps of sea surface temperature anomalies in the Indian Ocean in modern and ice-age times reveal striking changes in its surface circulation. During the last glacial maximum (18,000 years before the present), the Indian Ocean had colder average zonal surface temperatures, a cooler and less extensive Agulhas Current, a distinct eastern boundary current, and decreased upwelling and a weaker southwest monsoon in its northwestern region.

SATELLITE EVIDENCE OF SEA-AIR INTERACTIONS DURING THE INDIAN MONSOON

A cloud-free zone adjoining the Indian Peninsula, observed from Gemini XI on Sept. 14, 1966, is attributable to upwelling. A possible connection between a weak southwest monsoon, coastal upwelling, and below-normal rainfall is suggested.