Browsing by Author "Heikkilä, UE"
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- Item10Be in last deglacial climate simulated by ECHAM5-HAM – Part I: climatological influences on 10Be deposition(Copernicus Publications, 2013-11-25) Heikkilä, UE; Phipps, SJ; Smith, AMReconstruction of solar irradiance has only been possible for the Holocene so far. During the last deglaciation, two solar proxies (10Be and 14C) deviate strongly, both of them being influenced by climatic changes in a different way. This work addresses the climate influence on 10Be deposition by means of ECHAM5-HAM atmospheric aerosol–climate model simulations, forced by sea surface temperatures and sea ice extent created by the CSIRO Mk3L coupled climate system model. Three time slice simulations were performed during the last deglaciation: 10 000 BP ("10k"), 11 000 BP ("11k") and 12 000 BP ("12k"), each 30 yr long. The same, theoretical, 10Be production rate was used in each simulation to isolate the impact of climate on 10Be deposition. The changes are found to follow roughly the reduction in the greenhouse gas concentrations within the simulations. The 10k and 11k simulations produce a surface cooling which is symmetrically amplified in the 12k simulation. The precipitation rate is only slightly reduced at high latitudes, but there is a northward shift in the polar jet in the Northern Hemisphere, and the stratospheric westerly winds are significantly weakened. These changes occur where the sea ice change is largest in the deglaciation simulations. This leads to a longer residence time of 10Be in the stratosphere by 30 (10k and 11k) to 80 (12k) days, increasing the atmospheric concentrations (25–30% in 10k and 11k and 100% in 12k). Furthermore the shift of westerlies in the troposphere leads to an increase of tropospheric 10Be concentrations, especially at high latitudes. The contribution of dry deposition generally increases, but decreases where sea ice changes are largest. In total, the 10Be deposition rate changes by no more than 20% at mid- to high latitudes, but by up to 50% in the tropics. We conclude that on "long" time scales (a year to a few years), climatic influences on 10Be deposition remain small (less than 50%) even though atmospheric concentrations can vary significantly. Averaged over a longer period, all 10Be produced has to be deposited by mass conservation. This dominates over any climatic influences on 10Be deposition. Snow concentrations, however, do not follow mass conservation and can potentially be impacted more by climate due to precipitation changes. Quantifying the impact of deglacial climate modulation on 10Be in terms of preserving the solar signal locally is analysed in an accompanying paper (Heikkilä et al., 10Be in late deglacial climate simulated by ECHAM5-HAM – Part 2: Isolating the solar signal from 10Be deposition). © Author(s) 2013.
- Item10Be in late deglacial climate simulated by ECHAM5-HAM – Part 2: Isolating the solar signal from 10Be deposition(Copernicus Publications, 2014-04-01) Heikkilä, UE; Shi, X; Phipps, SJ; Smith, AMThis study investigates the effect of deglacial climate on the deposition of the solar proxy 10Be globally, and at two specific locations, the GRIP site at Summit, Central Greenland, and the Law Dome site in coastal Antarctica. The deglacial climate is represented by three 30 year time slice simulations of 10 000 BP (years before present = 1950 CE), 11 000 and 12 000 BP, compared with a preindustrial control simulation. The model used is the ECHAM5-HAM atmospheric aerosol–climate model, driven with sea-surface temperatures and sea ice cover simulated using the CSIRO Mk3L coupled climate system model. The focus is on isolating the 10Be production signal, driven by solar variability, from the weather- or climate-driven noise in the 10Be deposition flux during different stages of climate. The production signal varies at lower frequencies, dominated by the 11 year solar cycle within the 30 year timescale of these experiments. The climatic noise is of higher frequencies than 11 years during the 30 year period studied. We first apply empirical orthogonal function (EOF) analysis to global 10Be deposition on the annual scale and find that the first principal component, consisting of the spatial pattern of mean 10Be deposition and the temporally varying solar signal, explains 64% of the variability. The following principal components are closely related to those of precipitation. Then, we apply ensemble empirical decomposition (EEMD) analysis to the time series of 10Be deposition at GRIP and at Law Dome, which is an effective method for adaptively decomposing the time series into different frequency components. The low-frequency components and the long-term trend represent production and have reduced noise compared to the entire frequency spectrum of the deposition. The high-frequency components represent climate-driven noise related to the seasonal cycle of e.g. precipitation and are closely connected to high frequencies of precipitation. These results firstly show that the 10Be atmospheric production signal is preserved in the deposition flux to surface even during climates very different from today's both in global data and at two specific locations. Secondly, noise can be effectively reduced from 10Be deposition data by simply applying the EOF analysis in the case of a reasonably large number of available data sets, or by decomposing the individual data sets to filter out high-frequency fluctuations. © Author(s) 2014.
- ItemBeryllium-10 transport to Antarctica: results from seasonally resolved observations and modeling(John Wiley & Sons, Inc, 2011-12-15) Pedro, JB; Heikkilä, UE; Klekociuk, AR; Smith, AM; van Ommen, TD; Curran, MAJCosmogenic 10Be measured in polar ice cores has important application in the reconstruction of past solar activity. However, the processes controlling its atmospheric transport and deposition to the ice sheets are not fully understood. Here we use the seasonal changes in 10Be concentrations in a 10 year monthly resolved ice core record from the Law Dome site (East Antarctica) in conjunction with ECHAM5-HAM general circulation model (GCM) simulations of 10Be and 7Be deposition as tools to examine this problem. Maximum 10Be concentrations are observed in the ice core during the austral late summer to early autumn (summer-autumn), while minimum concentrations are observed during the austral winter. The GCM simulations, corroborated by earlier observations of 10Be:7Be ratios in Antarctica from the Georg von Neumayer air sampling station, suggest that the 10Be concentration maximum is linked to direct input of stratospheric 10Be from the Antarctic stratosphere to the lower levels of the Antarctic troposphere during the austral summer-autumn. This result contrasts with the modeled transport of 10Be to Greenland, where the seasonal maximum in stratospheric input is seen in the late winter to spring, synchronous with the timing of the seasonal maximum in midlatitude stratosphere to troposphere exchange. Our results suggest that a different combination of processes is responsible for the transport of 10Be to the Antarctic and Greenland ice sheets. © 2011 American Geophysical Union
- ItemConstraints on long-term changes in solar activity from the range of variability of cosmogenic radionuclide records(Copernicus Publications, 2011-09-03) Muscheler, R; Heikkilä, UEThere is a variety of different cosmogenic radionuclide-based reconstructions of solar activity variations for the past. Especially the longer-term changes and the absolute levels of past solar activity are uncertain as it is illustrated by the differences between these reconstructions. On the one hand there are differences between 10Be and 14C records that are commonly used as proxies for the varying solar modulation of galactic cosmic rays. On the other hand estimates of past changes in the geomagnetic shielding also include relatively large uncertainties. Here, we concentrate on variations in cosmogenic radionuclide records on time scales of 50 to 500 yr. We show that these are to a large extent independent of the geomagnetic field intensity. The range of variability of cosmogenic radionuclide records allows us to set constraints about long-term changes in solar activity. These records indicate that present solar activity levels were reached or exceeded regularly in the past. © Author(s) 2011.
- ItemInfluence of model resolution of model resolution on the atmospheric transport of Be-10(Copernicus Gesellschaft MBH, 2012-01-01) Heikkilä, UE; Smith, AMUnderstanding the transport path of the solar activity proxy Be-10 from source to archive is crucial for the interpretation of its observed variability. The extent of mixing of the strong production signal has been quantified in a previous study (Heikkila et al., 2009). In this study we perform sensitivity studies to investigate the influence of model resolution on the degree of mixing and transport path of Be-10 in the atmosphere using the ECHAM5-HAM aerosol-climate model. This study permits us to choose an acceptable resolution, and so minimum CPU time, to produce reconstructions as physically accurate as possible. Five model resolutions are applied: T21L19: a coarse horizontal and vertical resolution with model top at ca. 30 km, T42L31: an average horizontal and fine vertical one, T42L39: similar vertical resolution than L19 but including the middle atmosphere up to ca. 80 km, T63L31: a fine horizontal and vertical resolution and T63L47: a fine resolution horizontally and vertically with middle atmosphere. Comparison with observations suggests that a finer horizontal and vertical resolution might be beneficial, producing a reduced meridional gradient, although the spread between observations was much larger than between the five model runs. In terms of atmospheric mixing the differences became more distinguishable. All resolutions agreed that the main driver of deposition variability, observed in natural archives, is the input of stratospheric Be-10 (total contribution 68 %) which is transported into the troposphere at latitudes 30-50 degrees. In the troposphere the model resolutions deviated largely in the dispersion of the stratospheric component over latitude. The finest resolution (T63L47) predicted the least dispersion towards low latitudes but the most towards the poles, whereas the coarsest resolution (T21L19) suggested the opposite. The tropospheric components of Be-10 differed less between the five model runs. The largest differences were found in the polar tropospheric components, which contribute the least to total production (approximate to 4 %). We conclude that the use of the T42 horizontal resolution seems to be sufficient in terms of atmospheric mixing of a stratospheric tracer because no substantial improvement was seen when the resolution was increased from T42 to T63. The use of the middle atmospheric configuration is a trade-off between correctly describing stratospheric dynamics and having to reduce vertical resolution. The use of a high vertical resolution seemed more beneficial than the middle atmospheric configuration in this study. The differences found between the T42L31 and T63L31 resolutions were so small that T42L31 is a good choice because of its computational efficiency. © Author(s) 2012.
- ItemPhase of solar activity affects response of solar proxy 10Be(Esevier, 2013-10-15) Heikkilä, UE; Muscheler, R; Smith, AMThe solar proxy 10Be, measured in natural archives, has previously been assumed to lag solar activity by 1–2 yr. This reflects its residence time in the stratosphere where its main source lies. This study, based on a 30-yr global model simulation of the atmospheric transport of 10Be, shows that the stratospheric fraction of production varies significantly with the phase of solar activity. This leads to a lag between production change and deposition response of ca. 1 yr during the decreasing phase of solar activity but nearly no lag when solar activity increases. No lag is found for 7Be. The fraction of 10Be production in the stratosphere varies from ca. 65% to 69% between solar minimum (ϕ=400 MV) and maximum (ϕ=1200 MV), being highest during low solar activity when production is highest. When solar activity starts to decrease and production rate increase, the change is largest in polar stratosphere where residence time is long. This leads to a delayed response in 10Be deposition. When production rate decreases, the relative production change is larger in the troposphere, leading to a quicker deposition response. The difference between phases is larger in southern hemisphere. The form of reconstructed 11-yr cycles based on high-resolution 10Be records could therefore be biased if no correction is applied. © 2013, Elsevier Ltd.
- ItemProduction rate and climate influences on the variability of 10Be deposition simulated by ECHAM5-HAM: Globally, in Greenland and in Antarctica(American Geophysical Union, 2013-03-27) Heikkilä, UE; Smith, AMIce core concentrations of Be-10 are used as a proxy for solar activity, but they might be affected by atmospheric transport and deposition and their changes. During the Holocene, the influence is likely to be small, but during glacials it has to be accounted for. First, the climate influence has to be understood during the present climate. This study uses an ECHAM5-HAM 30-year climatological simulation of Be-10 to investigate the production and climate-related influences on Be-10 deposition with focus on Greenland and Antarctica. We examine the climate modes driving snow accumulation and hence potentially Be-10 deposition over a climatologically relevant period. The North Atlantic Oscillation (NAO) is found to be the main driver of changes in precipitation and Be-10 deposition in Greenland, in agreement with previous studies. In Antarctica, the picture is more complex as precipitation and Be-10 deposition are only weakly correlated with the Southern Annular Mode (SAM), El Nino-Southern Oscillation (ENSO), or Zonal Wave 3 pattern (ZW3). The results suggest that on seasonal scale, Be-10 deposition is linked with both precipitation rate and tropopause height, mainly due to the similar seasonal cycle. However, the correlation with tropopause height persists on the annual time scale. All in all, Be-10 variability in Antarctica is an interplay of several processes whose contribution varies in time and space. When interpreting Be-10 ice core records for solar activity, the time scale is essentially important. On seasonal scale, the Be-10 signal is dominated by weather influences, but on multiannual scales, the production rate is the main driver. On multidecadal scale, large long-term trends in climatic factors have the potential to distort the signal again as is seen in Be-10 records during glacials. This study shows how climate modes connect to Be-10 variability and how this connection could be used to correct for the climate impact. The established connections during present climatic conditions can be used as a basis to investigate these connections during glacial climate in a glacial model simulation. © 2013, American Geophysical Union.
- ItemA quasi-monthly record of 10Be concentration at Law Dome, Antarctica, from 2000 to 2015(Antarctic Climate and Ecosystems Cooperative Research Centre, 2016-03-07) Smith, AM; Curran, MAJ; Etheridge, DM; Galton-Fenzi, BK; Heikkilä, UE; Klekociuk, AR; Moy, AD; Pedro, JB; Simon, KJ; van Ommen, TDThis paper presents an overview of work undertaken over a number of Australian Antarctic Science projects, beginning in season 2001/02 with a shallow snow pit. In season 2005/06 this was augmented with a 260 m thermally drilled ice core and a 4.5 m snow pit. A core taken in 2008/09 overlapped the 2005/06 core and pit samples. From 2009/10, short cores spanning a few year’s deposition, along with snow pit samples spanning about half a year, have been taken each season. This has continued through to the current 2015/16 season. The cores permit an overlap with earlier years to match the chronology and to yield samples for 10Be analysis at the Australian Nuclear Science and Technology Organisation (ANSTO) by the technique of accelerator mass spectrometry (AMS). Together, the data provide a unique, continuous, quasi-monthly record over 2000 to 2015 as we have moved from Solar Cycle 23 to 24. The snow pits yield larger samples for 7Be analysis, earlier by gamma spectroscopy but lately by AMS. Along with comparison with neutron monitor data and GCM modelling, this unique, high-precision record has enabled us to learn much about the production, transport and deposition of 10Be to Law Dome and to improve our use of 10Be as a proxy for past solar variability.