PP13D-01 INVITED
Holocene variability in the North American monsoon and links to other monsoon systems
Unlike the Asian and African monsoon systems, with relatively rich Holocene paleoclimate histories, the history of the North American monsoon (NAM) remains poorly documented. One reason for this lack of explicit paleoclimate resolution may be that the NAM influences a region that also receives significant non-monsoonal precipitation. Thus records of past aridity can be ambiguous in their seasonal attribution and related mechanisms. In the Southwestern US, monsoon moisture makes up half at most of the seasonal precipitation, with the balance coming in from westerly storms in the winter half-season. Holocene records from this region are divided on whether the mid-Holocene was wetter or drier; the balance of evidence is consistent with a scenario that involves an intensified monsoon and drier winters. However, continuous, well-dated records of the NAM spanning the Holocene are virtually nonexistent. Here we present a Holocene record of stable isotope variability from a cave in southeastern Arizona, Cave of the Bells, that appears to track at least partly the long-term variations in the NAM. Precipitation isotope data from the modern system indicates a monsoon influence that is subtle and only appears in the wettest of recent monsoon seasons. Speleothem oxygen isotopic data from the mid- Holocene parallel insolation forcing on Milankovich time scales and show substantial higher-frequency variance whose period changes over the 3300-6400 yr BP record. In particular, we note a shift from century-scale to multidecadal variance around 4500 yr BP that coincides with variance changes in other records from the tropical/subtropical Americas. We argue that as the ITCZ shifted southward in the mid-Holocene, in response to orbital forcing, subtropical dry zones expanded and climate in these marginal areas became more variable. This interval is an important time of change in other monsoon systems, and we will discuss how the NAM record from Arizona compares with well-documented shifts in the Asian and African monsoons.
PP13D-02
Diurnal Weather Cycles at the Tropical Treeline in the North American Monsoon Region
High elevation atmospheric processes in the North American Monsoon region were investigated using half-hour weather data collected from May 22nd, 2001 to March 21st, 2007 at Nevado de Colima, Mexico (19° 35' N, 103° 37' W, 3760 m a.s.l.). After briefly expanding on temporal changes, which were first described by Biondi et al. 2005, I investigate diurnal weather cycles using the entire period of record. During the monsoon, precipitation falls mostly in the afternoon (from 12:00 to 20:00), with a peak around 17:00. Barometric pressure follows a regular wave pattern in all months, with a high at noon in between two lows, one around 5:30 and one between 17:00 and 19:00. Since barometric pressure is higher during the wet season, and the lows are found in all months, the afternoon low is not related to precipitation; rather it may help convective processes during the monsoon season. The diurnal pressure waves correspond to changes in wind speed, but it is unclear if turbulence drives the changes in pressure, as suggested by other authors. In particular, the amplitude of the atmospheric wave is greater in the dry season than in the wet season, in contrast to what was observed at high elevations in the Alps. The diurnal cycle of air temperature shows maxima during the spring, as the increased cloudiness during the summer wet season reduces incoming short wave radiation and its direct outcome, maximum air temperature. The dry season is characterized by greater excursions in air temperature, since the interval between maxima and minima is larger at all hours of the day. Soil temperature (especially the minima) is higher during the wet season, and shows an afternoon peak, most likely related to precipitation. Both minimum and maximum soil temperatures are at their lowest level around noon. Atmospheric vapor and vapor pressure deficit follow opposite patterns, as expected according to the season and the diurnal cycle of precipitation. These data provide a unique baseline for future comparisons, provide a clear example of the value of permanent plots in the portion of the American Cordillera that is affected by the North American Monsoon system, and fill a data gap in tropical treeline environments. From the uncovered patterns and processes, it also follows that better understanding of weather and climate processes is needed for both forecasting and backcasting of regional and global changes in monsoon systems across multiple timescales. http://woods.geography.unr.edu/dendrosite/mexico.htm
PP13D-03
West African monsoon variability along the Guinea coast during the Holocene
Variations in major element concentrations of anoxic sediments from Lake Bosumtwi, Ghana, are used to reconstruct changes in the West African monsoon during the Holocene with subdecadal to annual resolution. Low elemental concentrations occur during the early- to mid-Holocene, when lake level was high and the exposed catchment area contributing sediments to the lake basin was small. Late Holocene increases in elemental concentrations are consistent with enriched oxygen isotope values of authigenic carbonates, indicating decreasing lake level and enhanced erosional transport of terrigenous material to the basin following the onset of dry conditions. The highest elemental concentrations and the most enriched isotopic values are synchonous with paleolake lowstands in seismic data, supporting our interpretation of the proxy data as indicators of past hydrologic changes. The mid- to late-Holocene decline in West African monsoon strength commenced abruptly between 3.5 and 2.6 kyr, after which it continued to decrease gradually until ca. 0.8 kyr. This contrasts with dust records from North Africa, which indicate a single abrupt shift from wet to dry conditions at ca. 5.5 kyr, and with other monsoon systems (e.g. Indian, Asian), which indicate a gradual decline in monsoon strength throughout this period. This suggests that during the late Holocene, the West African monsoon underwent at least 2 abrupt reorganizations in response to gradually changing summer insolation. Superimposed on these low-frequency hydrologic changes is significant centennial-scale variability. Cross spectral correlation with the record of atmospheric carbon-14 indicates that at least some of this variability may be linked to changes in solar irradiance, as also suggested in records from the Asian and Indian monsoon systems.
PP13D-04
Monsoon Variability In The Western Arabian Sea During Last 10,000 Years BP: A Planktic Foraminiferal Abundances And It's Stable Isotope Records
Abstract: The western Arabian Sea responds to the southwest monsoon winds by upwelling colder and nutrient rich waters from the deeper layers, causing a reduction in the sea surface temperature and enhanced biological productivity. A number of paleoclimatic studies have been carried out in this region to elucidate past monsoon variability (Sirocco et al., 1993; Gupta et al, 2003; Tiwari, 2005; Saher et.al.; 2007). Globigerina bulloides, a planktic foraminiferal species normally inhabiting surface ocean waters in temperate latitudes ( Be and Tolderlund , 1977) also becomes abundant at tropical latitudes upwelling occurs, and in these cases its abundance can exceed considerably. The conspicuous fluctuation in the abundance of Gg.bulloides during upwelling and non upwelling intervals is established through several studies ( Thiede and Junger, 1980, Gupta et al, 2003) This robust relation has been used as a proxy for wind velocity at several different times in the past in the Arabian Sea (Anderson et.al., 2002). A significant result from some of these centennially resolved Holocene records is declining abundance of Globigerina bulloides which is paralleled by reduced insolation record and this has been inferred as declining strength of Asian Monsoon. We are presenting here the data from the core SS4018 from near the Gulf of Aden, Western Arabian Sea taken at a water depth of 2830 m, precisely dated by the radiocarbon method using Accelerator Mass Spectrometry on planktonic foraminiferal separates. We have carried out the planktic foraminiferal census counts for each sample to know the relative abundance of key species. In addition to this, we have also employed multi- proxy approach such as oxygen and carbon isotopes of planktic foraminiferal tests, TOC, CaCO3 (%) to strengthen our interpretation and also to understand the relationships amongst the proxies themselves. Abundance of the key planktic foraminiferal species and other proxy records reveal at least 3 major climatic events (M-1, M-2 and M-3) during the last 10,000yrs. The earliest one (M-1) is a major decline in the upwelling intensity during 7.9.-5.6 kyBP. We have observed the lowest abundance of upwelling indicator species (UIS), it goes down from 70(%) to 30(%), mixed dweller (MD) and thermocline dweller (TD) species were showing their maximum abundance and at that time Organic carbon (OC) value revealed the lowest amount. Oxygen stable isotope records were also suggesting the same results. The second event at 5.5-2.2 kyBP (M-2) corresponds to average upwelling. We have observed the 2-3 high peaks of (UIS), on average UIS were showing around 50-55(%) of their abundance during this period, thus we have concluded the moderate intensity of SWM winds during M-2. The third event at last 2ky (M-3), decreasing trend in the strength of the SWM has been observed on the basis of sharp decline in the abundance of UIS and OC amount. For this interval, we have observed the increasing trend in the MD and TD planktic foraminiferal species.
PP13D-05
15-ky Peat Paleo-Monsoon Reconstructions from Lipid Biomarker Contents of Three Bogs in China
Paleoclimate reconstructions based on lipid biomarkers from peat bogs are relatively uncommon, although this approach has been widely applied to lake sediments where it yields important histories of climate-related changes in types of vegetation and organic matter preservation. We describe stratigraphic patterns in peat lipid biomarker distributions extracted from three well-dated peat cores. One core is from the Dingnan bog on the border of Guangdong and Jiangxi Provinces of southeastern China. A second core is from the Zoige-Hongyuan bog, which is on the Qinghai-Xizang Plateau of west China on the northeast edge of the Tibetan Plateau. The third peat core is from a shallow lake that is the source of the Hani River in Jiling Province of northeastern China. The combination of the three locations provides a broad regional history of the evolution of monsoon-influenced climate in China from the late-glacial through the Holocene. The start of peat accumulation in these locations at about 15 ka indicates the onset of wetter post-glacial climate. A period of warmer climate corresponding to the Holocene Optimum between 9 ka to 5 ka is evident in greater alterations of n-alkanoic acids and n-alkanols and in the appearance of biomarkers diagnostic of greater microbial activity. Changes in local precipitation that raised the water level of the Zoige-Hongyuan bog are indicated by increases in the proportions of C23 and C25 n- alkanes that are mainly derived from submerged/floating plants and by conversion of the Dingnan bog to a lake. Alternations in submergent and emergent plant n-alkane proxies reveal millennial scale rises and falls of the water level in these bogs, thereby reflecting variations in East Asian and Indian Ocean monsoon precipitation during the Holocene. These events correlate well with cold-dry events recorded in same regions by other climatic proxies. Our results reflect Holocene instability in the Asian monsoon system and indicate that millennial-scale changes in precipitation are more sensitive than temperature in reflecting the paleo-monsoon history of China.
PP13D-06
Indian Ocean Dipole and Australian Monsoon variability in model simulations of the late Quaternary
A set of simulations with the Fast Ocean Atmosphere Model (FOAM) are used to investigate variability of the Indian Ocean Dipole (IOD) and interactions with the Australian summer monsoon over the late Quaternary. Time slices simulations were carried out for 55, 35, 21, 11, 6 and 0 ka, allowing the response of the IOD and monsoon to be investigated for different mean climates. The strength of the Asian summer monsoon and El Nino-Southern Oscillation (ENSO) was evaluated for each time slice in order to determine the influence of Asian monsoon and ENSO variability on the strength and timing of IOD events and impacts on the Australian monsoon. The model response was evaluated in comparison with previously published proxy records of Indian Ocean and Australian monsoon variability. Additional sensitivity experiments were used to investigate the relative importance of each component of palaeoclimate forcing: insolation, vegetation and land cover, sea level and atmospheric composition.