Southern Annular Mode-like changes in southwestern Patagonia at centennial timescales over the last three millennia (original) (raw)
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Quaternary Science Reviews, 2012
Much of the ongoing discussion regarding synchrony or bipolar asynchrony of paleoclimate events has centered on the timing and structure of the last glacial termination in the southern mid-latitudes, in particular the southwestern Patagonian region (50 e55 S). Its location adjacent to the Drake Passage and near the southern margin of the southern westerly winds (SWW) allows examining the postulated links between the Southern OceaneSWW coupled system and atmospheric CO 2 variations through the last glacial termination. Results from two sites located in the Última Esperanza area (52 S) allow us to infer SWW-driven changes in hydrologic balance during this critical time interval. These findings indicate peatland development under temperate/wet conditions between 14,600 and 14,900 cal yr BP, followed by cooling and a lake transgressive phase that led to a shallow lake during the early part of the Antarctic Cold Reversal (ACR, 13,600e14,600 cal yr BP), followed in turn by a deeper lake and modest warming during Younger Dryas time (YD, w11,800e13,000 cal yr BP), superseded by terrestrialization and forest expansion at the beginning of the Holocene. We propose that the SWW (i) strengthened and shifted northward during ACR time causing a precipitation rise in northwestern and southwestern Patagonia coeval with mid-and high-latitude cooling and a halt in the deglacial atmospheric CO 2 rise; (ii) shifted southward during YD time causing a precipitation decline/increase in NW/SW Patagonia, respectively, high-latitude warming, and invigorated CO 2 release from the Southern Ocean; (iii) became weaker between w10,000 and 11,500 cal yr BP causing a precipitation decline throughout Patagonia, concurrent with peak mid-and high-latitude temperatures and atmospheric CO 2 concentrations.
Here we report results from a high-resolution palynological record and stratigraphic/geochronologic data related to a Neoglacial event in Torres del Paine National Park, southern Chile (51_S, 71_W), to investigate climatic variations in Southwest Patagonia during the last 5000 years. The record reveals a stepwise expansion of Nothofagus-dominated woodlands and forests with discrete pulses at 4400, 2900, 1300, and 570 cal yr BP. Superimposed upon this trend we identify a relative opening of the woodlands between 4100–2900 and 2300–1300 cal yr BP. Closed-canopy forests dominated the landscape between 570–60 cal yr BP, followed by a rapid decline at the end of the 19th century that coincided with intense fire activity and the appearance of Rumex cf. acetocella, an exotic species introduced by European settlers. We interpret these changes as variations in the intensity and/or position of the southern margin of the westerly winds, which culminated with a net eastward shift of the forest–steppe ecotone during the Little Ice Age. We propose that millennial-scale changes in either the latitudinal position and/or the overall strength of the southern westerlies may be responsible for vegetation changes, fire occurrence, and the dynamic behavior of Patagonian glaciers during the last 5000 years. Because the modern maximum in near-surface wind velocities and precipitation is located between 48_ and 50_S, we suggest that the core of the southern westerlies may have achieved this modern position w570 years ago.
Onset and Evolution of Southern Annular Mode-Like Changes at Centennial Timescale
Scientific Reports, 2018
The Southern Westerly Winds (SWW) are the surface expression of geostrophic winds that encircle the southern mid-latitudes. In conjunction with the Southern Ocean, they establish a coupled system that not only controls climate in the southern third of the world, but is also closely connected to the position of the Intertropical Convergence Zone and CO2 degassing from the deep ocean. Paradoxically, little is known about their behavior since the last ice age and relationships with mid-latitude glacier history and tropical climate variability. Here we present a lake sediment record from Chilean Patagonia (51°S) that reveals fluctuations of the low-level SWW at mid-latitudes, including strong westerlies during the Antarctic Cold Reversal, anomalously low intensity during the early Holocene, which was unfavorable for glacier growth, and strong SWW since ~7.5 ka. We detect nine positive Southern Annular Mode-like events at centennial timescale since ~5.8 ka that alternate with cold/wet in...
Late glacial and Holocene climate variability, southernmost Patagonia
Quaternary Science Reviews, 2020
Late glacial-Holocene palaeoecological record, constrained by a robust chronology, from a peat bog near Punta Burslem (54°54'S, 67°57'W) on Isla Navarino, southernmost Patagonia documents the shifts in intensity and focus of the Southern Westerly Winds (SWWs) at these high latitudes. Such long-term records are required to reconstruct and better understand the likely regional impacts of a poleward shift and intensification of the SWWs predicted under global warming scenarios. Deglaciation at Punta Burslem occurs sometime before c. 17,000 cal a BP, and the post glacial landscape is dominated by cold tolerant pioneer species. Nothofagus woodland is established by c. 12,250 cal a BP, this moisture sensitive vegetation type retreats in the early to mid-Holocene from c. 9700 to 7050 cal a BP reflecting an intense and sustained drier phase associated with a prolonged poleward contraction of the SWWs. After c. 6000 cal a BP there is a regional trend to cooler and wetter climate. However, we identify at least five periods of rapid climate change (RCC) leading to drier conditions at this southern extreme of Patagonia: c. 5350-4750 cal a BP, c.4300-3300 cal a BP, c. 2600-1850 cal a BP, c. 1350-1100 cal a BP and c. 550-350 cal a BP. From a synthesis of our Isla Navarino records and a latitudinal spread (34°-64°S) of multiproxy records it is proposed that these periods of RCC and relatively drier conditions indicate latitudinal shifts in the location and intensity of the SWWs in response to climatic warming leading to reduced precipitation at the southern margins of Patagonia.
Precipitation linked to Atlantic moisture transport: clues to interpret Patagonian palaeoclimate
Climate Research, 2015
Westerlies are the main climatic feature in the mid-latitudes of the Southern Hemisphere (SH), driving the amount and distribution of precipitation. Patagonia is a vast region in South America's mid-latitudes, which encompasses 2 sub regions with highly distinct precipitation features. These two regions include wet Western Patagonia extending from the Pacific coast to the Andean highs (i.e. maximum elevations), and dry Eastern Patagonia situated leeward of the Andes in the Argentine steppe plains. Patagonia is influenced by strong mid-latitude westerlies throughout the year. Westerlies have been considered the unique driver of climate both in Western and Eastern Pata gonia. This research is focused on the Lago Cardiel catchment area in central Eastern Patagonia. A significant link between precipitation in that region and local zonal moisture transport from the Atlantic was established. A fraction of intense precipitation was related to strong local westward moisture transport, partly as a consequence of slow-moving weather systems crossing over Patagonia. As long as a dipolar pattern of long-term precipitation anomaly was observed between dry central Western/Southern Patagonia and wet central Eastern Patagonia, it could be interpreted as due to enhanced synoptic easterly moisture flux from the Atlantic. Thus, the westerlies rule was broken at least under blocking-like flows, which induced moist easterlies. The relatively wet 1940s exemplified this phenomenon. Such a conceptual framework can be applied to palaeoclimatic proxy record reconstructions as well as to general circulation model (GCM) outcomes for the late and mid-Holocene.
Patagonian and southern South Atlantic view of Holocene climate
Quaternary Science Reviews, 2016
We present a comprehensive 10 Be chronology for Holocene moraines in the Lago Argentino basin, on the east side of the South Patagonian Icefield. We focus on three different areas, where prior studies show ample glacier moraine records exist because they were formed by outlet glaciers sensitive to climate change. The 10 Be dated records are from the Lago Pearson, Herminita Península-Brazo Upsala, and Lago Frías areas, which span a distance of almost 100 km adjacent to the modern Icefield. New 10 Be ages show that expanded glaciers and moraine building events occurred at least at 6120 ± 390 (n ¼ 13), 4450 ± 220 (n ¼ 7), 1450 or 1410 ± 110 (n ¼ 18), 360 ± 30 (n ¼ 5), and 240 ± 20 (n ¼ 8) years ago. Furthermore, other less well-dated glacier expansions of the Upsala Glacier occurred between~1400 and~1000 and~2300 and~2000 years ago. The most extensive glaciers occurred over the interval from~6100 to~4500 years ago, and their margins over the last~600 years were well within and lower than those in the middle Holocene. The 10 Be ages agree with 14 C-limiting data for the glacier histories in this area. We then link southern South American, adjacent South Atlantic, and other Southern Hemisphere records to elucidate broader regional patterns of climate and their possible causes. In the early Holocene, a far southward position of the westerly winds fostered warmth, small Patagonian glaciers, and reduced sea ice coverage over the South Atlantic. Although we infer a pronounced southward displacement of the westerlies during the early Holocene, these conditions did not occur throughout the southern mid-high latitudes, an important exception being over the southwest Pacific sector. Subsequently, a northward locus and/or expansion of the winds over the Patagonia-South Atlantic sector promoted the largest glaciers between~6100 and~4500 years ago and greatest sea ice coverage. Over the last few millennia, the South Patagonian Icefield has experienced successive century-scale advances superimposed on a long-term net decrease in size. Our findings indicate that glaciers and sea ice in the Patagonian-South Atlantic sector of the Southern Hemisphere did not achieve their largest Holocene extents over the last millennium. We conclude that a pattern of more extensive Holocene ice prior to the last millennium is characteristic of the Southern Hemisphere middle latitudes, which differs from the glacier history traditionally thought for the Northern Hemisphere.
Recent advances in the chronology and the paleoclimatic understanding of Antarctic ice-core records point towards a larger heterogeneity of latitudinal climate fluctuations than previously thought. Thus, realistic paleoclimate reconstructions rely in the development of a tight array of well-constrained records with a dense latitudinal coverage. Climatic records from southernmost South America are critical corner-stones to link these Antarctic paleoclimatic archives with their South American counterparts. At 55°S on the Island of Tierra del Fuego, Lago Fagnano is located in one of the most substantially and extensively glaciated regions of southernmost South America during the Late Pleistocene. This elongated lake is the largest (~110 km long) and non-ice covered lake at high southern latitudes. A multi-proxy study of selected cores allows the characterization of a Holocene sedimentary record. Detailed petrophysical, sedimentological and geochemical studies of a complete lacustrine laminated sequence reveal variations in major and trace elements, as well as organic-content, suggesting high variability in environmental conditions. The comparison of these results with other regional records allows the identification of major known Late Holocene climatic intervals and to propose a time for the onset of the Southern Westerlies in Tierra del Fuego. These results improve our understanding of the forcing mechanisms behind climate change in southernmost Patagonia.
Holocene variability of the Southern Hemisphere westerlies in Argentinean Patagonia (52 S)
High-resolution analyses of allochthonous pollen input into crater lake sediments of Laguna Potrok Aike in the semi-arid Patagonian steppe reflect the variability of zonal wind intensities during the Holocene at 521 southern latitude. These indicators for Southern Hemisphere westerlies (SHW) strength vary on centennial timescales in concert with carbon/nitrogen (C/N) ratios and titanium (Ti) contents, interpreted as differential organic matter sources and minerogenic input to the sediment, respectively. The correlations underline a linkage between hydrological variability and west wind variability in Extra-Andean Patagonia. A shift to generally more intense SHW suggests intensification towards modern wind conditions at that latitude since 9.2 ka cal BP. r
The last glacial termination in the Coyhaique sector of central Patagonia
Quaternary Science Reviews, 2019
Southern South America is the only continental landmass that intersects the core of the Southern Westerly Winds (SWW), and thus is important for studying their role as a driver/conduit for the initiation/propagation of climate signals since the last glaciation. Their interaction with the Southern Ocean (SO) affects global climate through its influence on high-latitude upwelling and biological productivity, deep-water convection sites and, consequently, ventilation of CO 2 from the deep ocean. Variations in the SWW-SO coupled system have been postulated as fundamental drivers of climate change during glacial terminations and the current interglacial. Hence, deciphering the evolution of the SWW from sensitive locations in the southern middle latitudes is essential for understanding important climatic transitions during and since the Last Glacial Termination (T1). Terrestrial records from the central Patagonian Andes (CPA) (44-49 S), however, show heterogeneities in the timing, rates, and direction of climate change during T1, impeding detailed assessment of its drivers at regional, hemispheric, and global scales. Here we present new data on glacier, vegetation, and fire-regime changes in the Coyhaique sector (45 34 0 S) of CPA to improve our understanding on the timing and structure of the T1, including the behavior of the SWW. Our results indicate glacial recession from the youngest Last Glacial Maximum moraines just before~17.9 ka and development of an ice-dammed proglacial lake during the early stages of T1. Drainage of the ice-dammed lake, triggered by renewed glacial recession, was near-synchronous with the onset of a gradual multi-millennial trend toward arboreal dominance that started at~16 ka east and west of the Andes at that latitude. We detect increased influence of the SWW at~45 S starting at~16.6 ka, relative to the first millennium of T1, that led to positive anomalies in precipitation between~16e14.4 and~12.8e11.5 ka, followed by negative anomalies between~11.5e9 ka. The synchronous spread of arboreal vegetation east and west of the CPA divide during T1, despite the trans-Andean precipitation contrasts, suggests an upward shift in the temperature-controlled Andean tree line, underscoring the role of deglacial warming as the critical driver for afforestation at regional scale.
Frontiers in Earth Science, 2021
The last glacial termination was a key event during Earth’s Quaternary history that was associated with rapid, high-magnitude environmental and climatic change. Identifying its trigger mechanisms is critical for understanding Earth’s modern climate system over millennial timescales. It has been proposed that latitudinal shifts of the Southern Hemisphere Westerly Wind belt and the coupled Subtropical Front are important components of the changes leading to global deglaciation, making them essential to investigate and reconstruct empirically. The Patagonian Andes are part of the only continental landmass that fully intersects the Southern Westerly Winds, and thus present an opportunity to study their former latitudinal migrations through time and to constrain southern mid-latitude palaeo-climates. Here we use a combination of geomorphological mapping, terrestrial cosmogenic nuclide exposure dating and glacial numerical modelling to reconstruct the late-Last Glacial Maximum (LGM) behav...