PP44B-01 INVITED
Role of the Westerlies in the Ocean's Overturning and Carbon Cycle
Thirteen years ago, Bonnie Samuels and I proposed that the formation and overturning of North Atlantic Deep Water (NADW), the ocean's quintessential "thermohaline" circulation, might in fact be a response to the westerly winds that overlie the Antarctic Circumpolar Current (ACC) in the Southern Hemisphere (Toggweiler, J. R. and B. Samuels, Deep-Sea Res. I, 42, 477-500, 1995). Our idea was based on an experiment carried out in an ocean model in which the wind stress on the ocean south of 30 deg. S was raised and lowered by 50 percent. Little did we know that Mother Nature was carrying out a similar experiment, as the strongest westerlies had been shifting poleward and are better aligned with the ACC now than they were 50 years ago. It may just be a matter of time before the effect that Samuels and I wrote about can be seen, or not seen, in observations. The upwelling forced by the southern westerlies also draws CO2-rich deep water up to the surface around Antarctica, such that a poleward shift of the westerlies also causes CO2 to be vented up to the atmosphere from the deep ocean. Because the poleward shift is thought to be caused in part by global warming and higher CO2, the CO2 coming out of the deep ocean could reinforce the upwelling via further changes in the westerlies. The amount of CO2 coming out of the ocean is small compared to anthropogenic emissions so this reinforcement is probably not important today. It could, however, have been hugely important as the climate warmed at the end of the last ice age, when the deep ocean was the main source of higher CO2 (Toggweiler. J. R., J. Russel, and S. Carson, Paleoceanography, 21, PA2005, doi:10.1029/2005PA001154, 2006). In this presentation I will discuss the latest thinking on the westerlies, ocean circulation, and atmospheric CO2 in relation to the following question: did higher CO2 feed back on the circulation and ventilation of the deep ocean via poleward-shifted westerlies at the end of the last ice age, or did the westerlies simply shift poleward in a warming climate?
PP44B-02
Long-term modulation of the glacial mode of the climate system
Part of the puzzle to understand the forcing and feedback of the climate system by orbital parameters and atmospheric carbon dioxide may be found in long marine cores which comprise multiple glacial cycles. Our 800 kyr record of the position of the Subtropical Front (STF) in the Indian Ocean shows that the STF migrates northwards during glacial periods but the extent of its migration is modulated by a long term 400 kyr cycle. We argue that the maximum equatorial insolation, a function of eccentricity, can dominate the equator- pole gradient and influence changes in the westerly winds and the associated Southern Annular Mode. This signal is communicated to the high latitudes via the movement of the westerlies and the STF which act as a feedback to meridional overturning in the Atlantic, extremely so when the STF reaches its most northerly reconstructed position to impinge on South Africa. At this position the westerlies are weakest and decoupled from the topographically constrained ACC across all three southern hemisphere continents hence there is a minimum in transfer of wind energy to the ACC, the ultimate driver of overturning. Furthermore, the coincidence of the STF on South Africa also shuts down the Agulhas leakage "warm" pathway for the return of salt and heat to the Atlantic. The latitude of the STF therefore acts as an amplifier of low latitude eccentricity forcing of tropical temperatures to high latitude SSTs and ice volume via the strength of meridional overturning.
PP44B-03 INVITED
Linking the Present to the Past: The Importance of the Southern Hemisphere Storm Track for Southern Ocean CO2 Uptake
The location and intensity of the Southern Hemisphere storm track plays a major role in regulating the amount of CO2 exchanged between the Southern Ocean and overlying atmosphere. Changes in the storm track correspond to changes in wind stress and precipitation over the Southern Ocean, dramatically altering the circulation and biogeochemical cycling in this region. Here, we use a simplified ocean biogeochemical model to investigate the response of Southern Ocean CO2 cycling to 100-year changes in the intensity of Southern Hemisphere wind and precipitation. We will demonstrate that Southern Ocean CO2 uptake, and therefore atmospheric CO2 concentration, is strongly influenced by century-scale changes in wind and precipitation. While this study is set in the present-day climate of the Southern Hemisphere, similar mechanisms of change could have operated in the past. Paleoclimatic analogs should be used to evaluate these mechanisms, providing a basis for evaluation of current modeling efforts.
PP44B-04
Wind-Driven Upwelling in the Southern Ocean and the Deglacial Rise of Atmospheric CO2
Wind-driven upwelling in the ocean around Antarctica regulates the exchange of CO2 and other gases between the deep sea and the atmosphere. Diatom productivity and burial of biogenic opal in marine sediments south of the Antarctic Polar Front are linked to the rate of upwelling, which supplies dissolved Si to the euphotic zone of the Southern Ocean. We find enhanced rates of opal burial during the termination of the last ice age in each sector of the Southern Ocean. In the record with the greatest temporal resolution, we find evidence for two intervals of enhanced upwelling, concurrent with the two intervals of rising atmospheric CO2 concentration during deglaciation. These results provide the first direct evidence linking increased ventilation of deep water masses in the Southern Ocean to the deglacial rise in atmospheric CO2 We suggest that the deglacial increase in upwelling of the Southern Ocean was a response to extreme cold conditions in the Northern Hemisphere during the time intervals surrounding Heinrich Event 1 and the Younger Dryas. Paleo proxy records and model simulations both point to a reorganization of atmospheric circulation at these times, including a southward shift in the Intertropical Convergence Zone as well as a southward shift in the Southern Hemisphere westerlies. The increased wind stress at the latitude of the Drake Passage associated with the southward displacement of the westerlies was instrumental in breaking down glacial ocean stratification while increasing the upwelling and ventilation of deep waters around Antarctica, as described by Toggweiler (Paleoceanography, 2006, doi10.1029/2005PA001154).
PP44B-05 INVITED
Glacial to Interglacial Changes in Southern Ocean Water Mass Geometry, the ACC, and the Southern Westerlies at Drake Passage
Resolving ocean and atmospheric variability in the Drake Passage region is crucial for advancing our understanding of the role of the Southern Ocean in affecting ocean and climate change. Modeling studies suggest that altering the position or strength of the Southern Westerly Winds (SWW) and the Antarctic Circumpolar Current (ACC) relative to the Drake Passage could play a central role in driving observed glacial-interglacial changes in the atmospheric concentration of carbon dioxide and the global ocean circulation. The records of past ocean-atmosphere changes contained in sediment archives provide a natural testing ground for these hypotheses. Here we present high-resolution benthic and planktonic foraminiferal δ13C and δ18O records from new sediment cores recovered along both meridional (IMAGES PACHIDERME cruise onboard the R/V Marion Dufresne of IPEV) and zonal (IPY PALEODRAKE cruise) transects. Together with existing Southern Ocean cores, the new records provide constraints on the vertical and spatial gradients in surface and bottom water properties necessary to portray changes in the position of water masses and frontal systems relative to Drake Passage and Southern Chile. Our initial planktonic δ18O results over the last deglaciation show a greater magnitude change in the northern Drake Passage (2.0‰) and along the Chilean margin (2.5‰) than is generally observed in records which are either south or far to the North of the Subantarctic Front (SAF) today. These results are consistent in both sign and magnitude with a northward shift in the Subantarctic Front and an increase in the flux of polar and supbolar water northward along the coast of Chile during the glaciation. In addition, the large glacial decrease (>1.5‰) in benthic foraminiferal (C. wuellerstorfi) δ13C values in core MD07-3128 (52S, 1032m) suggests that the boundary between intermediate water (relatively high δ13C) and circumpolar deep water (low δ13C) was shifted northward (or shoaled) during the glacial period. Our results suggest that the interbasinal transport of subpolar surface and thermocline water was reduced during the glacial maximum. We use the timing of these shifts in Southern Ocean dynamics to evaluate the hypothesis that they play key roles in altering glacial atmospheric CO2 concentrations and global ocean circulation.
PP44B-06
Modeling Glacial-Interglacial Changes in Dust and Sea Salt Concentrations in West Antarctic Deep Ice Cores: Implications for Southern Hemisphere Atmospheric Dynamics
Chemical concentrations and fluxes measured in ice cores provide a unique archive of information regarding atmospheric aerosol loading and transport processes, and can be used to evaluate climate and environmental processes on timescales ranging from seasonal to millennial. Traditionally, analysis of soluble calcium concentrations has been used as a proxy for terrestrial dust loading, while soluble sodium concentrations serve as a proxy for sea salt aerosol loading. While long time series of these ions have provided valuable information on glacial/interglacial changes in dust and sea salt aerosol, interpretations based on qualitative understanding of modern processes as well as GCM experiments continue to evolve as new records become available. Recently, deep ice core data from East Antarctica has expanded greatly with the EPICA Dome C and EDML records. Here we examine ion data from the Siple Dome, West Antarctica, deep ice core using a semi-empirical modeling approach, and compare results with those from the EPICA cores. We find that calcium (dust) records show coherence at all sites on millennial timescales, which may be related to source conditions, and lack of significant change in atmospheric transport that is consistent with GCM results. However, a lower correlation among Siple Dome and EPICA sites than between EPICA sites suggests there may be additional dust sources that affect West Antarctica. In addition, there is a higher dust flux on all timescales at the lower elevation Siple Dome site, implying a gradient of aerosol loading in the atmosphere. On the other hand, sea salt deposition at Siple Dome on millennial timescales is not related to EPICA sites, and shows no temperature dependence. Possible explanations are that lower elevation sites are more sensitive to shifts in storm tracks, analogous to modern ENSO dynamics) and/or that seasonal sea ice conditions have regional effects. We discuss the implications that both aerosol datasets have implications for reconstructing Southern Hemisphere westerlies and ocean/atmosphere process, particularly in the South Pacific sector where Siple Dome is located.
PP44B-07
A Dendrochronological Approach to Estimates of Past Changes in the Antarctic Oscillation
The Antarctic Oscillation (AAO) or Southern Annular Mode (SAM) is the dominant pattern of climate variability at mid- to high-latitudes in the Southern Hemisphere. The positive phase of the AAO is associated with decreased geopotential height over the polar cap, increased geopotential height over the midlatitudes, and a strengthening and poleward shift of the storm track over the Southern Ocean. Anomalies of the AAO are related to significant changes in climate not only over Antarctica, but also in the temperate regions of South America, Australia, South Africa and New Zealand. The positive phase of the AAO is associated with a significant cooling across eastern Antarctica and central Australia, and a marked warming over the Antarctic Peninsula, southern Patagonia, Tasmania and the south of New Zealand. This positive phase is also related to anomalously dry conditions over south-western South America, New Zealand and Tasmania and to anomalously wet conditions over much of Australia and South Africa. In a first attempt to reconstruct past variations in the AAO we take advantages of previously reconstructed temperature (Tasmania, southern New Zealand and southern Patagonia) and streamflow (Río Puelo) records from AAO-climatically-sensitive regions located in the middle latitudes of the Southern Hemisphere. Depending on the seasonal arrangements of the AAO, the reconstructions explain 34% (Spring: Sep-Dec), 47% (Summer-fall: Jan-May), and 51% (Annual: Oct-Aug) of the total variance in the instrumental AAO during the interval 1949-2001 (52 years). A conspicuous feature in most reconstructions is the positive trend in the AAO estimates during the past 50 years, suggesting that the recent atmospheric circulation in the Southern Hemisphere is unprecedented in the context of the past four centuries. The influences of AAO on the climate of South America may have implications for weather and seasonal forecasting, and, if the positive AAO trend continues, for future climate changes in the Southern Hemisphere.
PP44B-08
An Isotopic Record of Holocene Variations in the Southern Hemisphere Westerly Wind Field From SW Patagonia
The southern westerly winds are an important feature of atmospheric circulation in the mid- to high-latitudes of the Southern Hemisphere. Recent observations and model-based studies have shown that both the latitudinal position and the overall strength of the wind field can directly influence Southern Ocean circulation and have important implications for the global carbon cycle. Here we present a lacustrine paleoclimate record of past westerly wind variability from Lago Guanaco -- a small closed-basin alkaline lake in SW Patagonia (51°S, 72°W). The lake is located within the core of the modern westerly wind maximum at ~50°S at a location where precipitation at interannual timescales is positively correlated with zonal wind throughout a significant portion of the Southern Hemisphere. In order to reconstruct Holocene variations in climate related to the westerlies, we have developed a high-resolution bulk organic δ13C record to monitor changes in lake productivity, organic matter provenance, and zonal migrations in the precipitation-controlled forest-steppe ecotone. We interpret more negative δ13C values to reflect increased westerly flow caused by reduced lake productivity and thermal stratification, in addition to a potential contribution from terrestrial organic matter. The Lago Guanaco record exhibits a decline in bulk δ13C through the Holocene that we interpret as an overall increase in the strength of the westerlies at this latitude that culminates during the last two hundred years. The decline in bulk δ13C is not monotonic, but rather occurs as a series of steps centered at 7000, 5400, 4000, 2200, and 300 cal yr BP. Superimposed on the Holocene trend are millennial-scale variations with δ13C amplitudes greater than 1‰. The timing of the millennial-scale variations appear to be consistent with records of the El Niño-Southern Oscillation (ENSO) variability from the tropical Pacific: reduced westerly flow in the Guanaco record corresponds with increased El Niño activity. The modern link between ENSO and the southern westerlies is related to the frequent occurrence of blocking anticyclones east of the southern tip of South America during El Niño years that produce an equatorward migration of SE Pacific storm tracks, which reduce precipitation and wind over southern Patagonia. Although the modern link between ENSO and the westerlies in southern South America exhibits non-stationarity at decadal timescales, it appears that there was an important connection to the tropics during the Holocene that significantly modulated atmospheric circulation in the high southern latitudes over millennial timescales.