ALBERT

Description: Large changes in solar ultraviolet radiation can indirectly affect climate by inducing atmospheric changes. Specifically, it has been suggested that centennial-scale climate variability during the Holocene epoch was controlled by the Sun 2,3. However, the amplitude of solar forcing is small when compared with the climatic effects and, without reliable data sets, it is unclear which feedback mechanisms could have amplified the forcing. Here we analyse annually laminated sediments of Lake Meerfelder Maar, Germany, to derive variations in wind strength and the rate of 10 Be accumulation, a proxy for solar activity, from 3,300 to 2,000 years before present. We find a sharp increase in windiness and cosmogenic 10Be deposition 2,759 ± 39 varve years before present and a reduction in both entities 199 ± 9 annual layers later. We infer that the atmospheric circulation reacted abruptly and in phase with the solar minimum. A shift in atmospheric circulation in response to changes in solar activity is broadly consistent with atmospheric circulation patterns in long-term climate model simulations, and in reanalysis data that assimilate observations from recent solar minima into a climate model. We conclude that changes in atmospheric circulation amplified the solar signal and caused abrupt climate change about 2,800 years ago, coincident with a grand solar minimum. © 2012 Macmillan Publishers Limited. All rights reserved.

Description: On the Earth, the atmosphere, ocean, and land interact with each other. For example, an atmospheric pressure system directly influences the Sea Surface Heights (SSHs) in a barometric sense; the associated wind transfers momentum from the atmosphere into the ocean, which alters the ocean currents affecting again the SSHs. The integrated effects of all motion components directly influence the angular momentum of the Earth, while the integrated effect of all mass variations alters the Earth’s inertia. Both can excite the Earth Orientation Parameters (EOPs). In this study, we use the Community Earth System Model (CESM) to simulate mass and motion variations within a coupled climate system. The modeled mass and motion variations of all subcomponents are used to compute the total excitation functions, which then are compared to very precise global EOP observations, provided by the International Earth Rotation and Reference Systems Service (IERS). For further reference, the modeled excitation functions of the subcomponents are compared to operational excitations, provided by the German Research Centre for Geosciences (GFZ). This allows an evaluation of the global model behavior and of the subcomponents. Further, regions of particularly high influence on the excitations as well as regions of especially strong dynamical coupling are identified. Four CESM experiments were performed, one reference experiment featuring solely natural variations, while the others separate the influence of (I) a coupled ocean component; (II) the quasi-biennial oscillation (QBO) and (III) anthropogenic forcings, e.g. greenhouse gas (GHG) emissions and ozone depleting substances (ODS). The modeled EOPs are in good agreement with the reference data sets, but reveal an slight overestimation of the modeled atmospheric mass component in the North Pacific for annual to interannual timescales, leading to deviations in the X1 component. Analyzing variations among the CESM experiments reveal (I) the complete absence of interannual subtropical tropospheric jet variability when using a climatological ocean; (II) a significantly increased atmospheric mass variation in the arctic region in the absence of a QBO; and (III) hardly any modeled effect of the global dynamics with respect to anthropogenic forcings. Finally, the North Pacific - a region with particularly strong atmosphere-ocean coupling - is investigated, highlighting wind driven ocean mass variations within the model and GRACE observations. The identified significant wind patterns explain the modeled ocean mass variations and can be directly projected onto ERA-Interim data in order to estimate the independent GRACE observations. The here presented relation between the ERA-Interim winds and the GRACE gravity field observations supporting the following two conclusions: (I) ERA-Interim winds can be used to further refine GRACE observations; (II) GRACE observations contain assimilation worthy information for atmospheric models.

Description: The Solar Cycle and the Quasi-Biennial Oscillation are two major components of natural climate variability. Their direct and indirect influences in the stratosphere and troposphere are subject of a number of studies. The so-called ``top-down' mechanism describes how solar UV changes can lead to a significant enhancement of the small initial signal and corresponding changes in stratospheric dynamics. How the signal then propagates to the surface is still under investigation. We continue the ``top-down' analysis further down to the ocean and show the dynamical ocean response with respect to the solar cycle and the QBO. For this we use two 110-year chemistry climate model experiments from NCAR's Whole Atmosphere Community Climate Model (WACCM), one with a time varying solar cycle only and one with an additionally nudged QBO, to force an ocean general circulation model, GFZ's Ocean Model for Circulation and Tides (OMCT). We find a significant ocean response to the solar cycle only in combination with a prescribed QBO. Especially in the Southern Hemisphere we find the tendency to positive Southern Annular Mode (SAM) like pattern in the surface pressure and associated wind anomalies during solar maximum conditions. These atmospheric anomalies propagate into the ocean and induce deviations in ocean currents down into deeper layers, inducing an integrated sea surface height signal. Finally, limitations of this study are discussed and it is concluded that comprehensive climate model studies require a middle atmosphere as well as a coupled ocean to investigate and understand natural climate variability. Key Points: - Modeled oceanic solar cycle response depends on realistically modeled stratosphe - A realistically modeled stratospheric solar cycle response requires a QBO

Description: Large changes in solar ultraviolet radiation can indirectly affect climate1 by inducing atmospheric changes. Specifically, it has been suggested that centennial-scale climate variability during the Holocene epoch was controlled by the Sun2, 3. However, the amplitude of solar forcing is small when compared with the climatic effects and, without reliable data sets, it is unclear which feedback mechanisms could have amplified the forcing. Here we analyse annually laminated sediments of Lake Meerfelder Maar, Germany, to derive variations in wind strength and the rate of 10Be accumulation, a proxy for solar activity, from 3,300 to 2,000 years before present. We find a sharp increase in windiness and cosmogenic 10Be deposition 2,759  ±  39 varve years before present and a reduction in both entities 199  ±  9 annual layers later. We infer that the atmospheric circulation reacted abruptly and in phase with the solar minimum. A shift in atmospheric circulation in response to changes in solar activity is broadly consistent with atmospheric circulation patterns in long-term climate model simulations, and in reanalysis data that assimilate observations from recent solar minima into a climate model. We conclude that changes in atmospheric circulation amplified the solar signal and caused abrupt climate change about 2,800 years ago, coincident with a grand solar minimum.

Description: One decade of time-variable gravity field observations from the GRACE satellite mission reveals low-frequency ocean bottom pressure (OBP) variability of up to 2.5 hPa centered at the northern flank of the subtropical gyre in the North Pacific. From a 145 year-long simulation with a coupled chemistry climate model, OBP variability is found to be related to the prevailing atmospheric sea-level pressure and surface wind conditions in the larger North Pacific area. The dominating atmospheric pressure patterns obtained from the climate model run allow in combination with ERA-Interim sea-level pressure and surface winds a reconstruction of the OBP variability in the North Pacific from atmospheric model data only, which correlates favorably (r=0.7) with GRACE ocean bottom pressure observations. The regression results indicate that GRACE-based OBP observations are indeed sensitive to changes in the prevailing sea-level pressure and thus surface wind conditions in the North Pacific, thereby opening opportunities to constrain atmospheric models from satellite gravity observations over the oceans.

Description: Major stratospheric sudden warmings are prominent disturbances of the Northern Hemisphere polar winter stratosphere. Understanding the factors controlling major warmings is required, since the associated circulation changes can propagate down into the troposphere and affect the surface climate, suggesting enhanced prediction skill when these processes are accurately represented in models. In this study we investigate how different natural and anthropogenic factors, namely, the quasi-biennial oscillation (QBO), sea surface temperatures (SSTs), anthropogenic greenhouse gases, and ozone-depleting substances, influence the frequency, variability, and life cycle of major warmings. This is done using sensitivity experiments performed with the National Center for Atmospheric Research's Community Earth System Model (CESM). CESM is able to simulate the life cycle of major warmings realistically. The QBO strengthens the climatological stratospheric polar night jet (PNJ) and significantly reduces the frequency of major warmings through reduction of planetary wave propagation into the PNJ region. Variability in SSTs weakens the PNJ and significantly increases the major warming frequency due to enhanced wave forcing. Even extreme climate change conditions (RCP8.5 scenario) do not influence the total frequency but determine the prewarming phase of major warmings. The amplitude and duration of major warmings seem to be mainly determined by internal stratospheric variability. We also suggest that SST variability, two-way ocean/atmosphere coupling, and hence the memory of the ocean are needed to reproduce the observed tropospheric negative Northern Annular Mode pattern after major warmings.