ALBERT

Description: We implement the effects of gravitational self-attraction and loading (SAL) into a global baroclinic ocean circulation model and investigate effects on sea level patterns, ocean circulation, and density distributions. We compute SAL modifications as an additional force on the water masses at every time step by decomposing the field of ocean bottom pressure anomalies into spherical harmonic functions and then applying Love numbers to account for the elastic properties of the solid Earth. Considering SAL in the postprocessing turns out to be insufficient, especially in coastal waters and on subweekly time scales, where SAL modifies local sea level by around 0.6–0.8 cm on average; in the open ocean, changes mostly remain around 0.3 cm. Modifications of water velocities as well as of heat and salt distributions are modeled, yet they are small. Simple parameterizations of SAL effects currently used in a number of ocean circulation models suffer from the process's inhomogeneity in space and time. These parameterizations improve the modeled sea level patterns but fail to reproduce SAL impacts on circulation and density distributions. We therefore suggest to explicitly consider the full SAL effect in ocean circulation models, especially when investigating sea level variations faster than around 4 days.

Description: Ocean bottom pressure gradients deduced from the satellite gravity mission Gravity Recovery and Climate Experiment (GRACE) were previously shown to provide barotropic transport variations of the Antarctic Circumpolar Current (ACC) with up to monthly resolution. Here, bottom pressure distributions from GRACE with monthly (GFZ RL04) and higher temporal resolution (CNES/GRGS with 10 days, ITG-GRACE2010 with daily resolution) are evaluated over the ACC area. Even on time scales shorter than 10 days, correlations with in situ bottom pressure records frequently exceed 0.6 with positive explained variances, giving evidence that high-frequency nontidal ocean mass variability is captured by the daily ITG-GRACE2010 solutions not already included in the applied background models. Bottom pressure is subsequently taken to calculate the barotropic component of the ACC transport variability across Drake Passage. For periods longer than 30 days, transport shows high correlations between 0.4 and 0.5 with several tide gauge records along the coast of Antarctica. Still significant correlations around 0.25 are obtained even for variability with periods shorter than 10 days. Since transport variations are predominantly affected by time-variable surface winds, GRACE-based transports are contrasted against an atmospheric index of the Southern Annular Mode (SAM), which represents the Southern Hemispheric wind variability. Correlations between the SAM and GRACE-based transports are consistently higher than correlations between any of the available sea level records in all frequency bands considered, indicating that GRACE is indeed able to accurately observe a hemispherically consistent pattern of bottom pressure (and hence ACC transport) variability that is otherwise at least partially masked in tide gauge records due to local weather effects, sea ice presence and steric signals.

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.

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 favourably (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: [1]  High-resolution load-induced crustal deformations calculated from numerical models are tested for their ability to predict hydrologically induced station height variability, as they are known to be large enough to affect epoch-wise parameters obtained from the analysis of global geodetic networks. Loading contributions due to terrestrial water storage as given by global hydrological models are calculated on a 0.5° global regular grid with daily temporal resolution. Apart from the dominant seasonal variations, the hydrological loading signal discloses also rapid changes exceeding 1 mm in several regions that can be associated with major precipitation events and river floods. Locally strong loading signals with exceptionally high amplitudes, in many cases even with non-seasonal nature, occur along the major river channels. Only high-resolution loading calculations considering also the water mass anomalies stored in the model riverflow can resolve the correct amplitudes in the surrounded carea up to 100 kilometers distance. The comparison of the modeled hydrological surface deformation with GPS station time series shows that high-resolution hydrological loading estimates based on global-scale models are able to explain a considerable fraction (up to 54%) of the observed vertical station movements caused by continental water storage variations.

Description: An improved version of the OMCT ocean model with 1° spatial resolution provides bottom pressure anomalies for the new release 05 of the GRACE Atmosphere and Ocean De-aliasing Level 1B (AOD1B) product. For high-frequency signals with periods below 30 days, this model explains up to 10 cm 2 of the residual sea level variance seen by ENVISAT in large parts of the Southern Ocean, corresponding to about 40% of the observed sea level residuals in many open ocean regions away from the tropics. Comparable amounts of variance are also explained by AOD1B RL05 for co-located in situ ocean bottom pressure recorders. Although secular trends contained in AOD1B RL05 cause GRACE KBRR residuals to increase in shallow water regions, we find a reduction of those residuals over all open ocean areas, indicating that AOD1B RL05 is much better suited to remove non-tidal high-frequency mass variability from satellite gravity observations than previous versions of AOD1B.

Description: Abstract Coupled climate models participating in the CMIP5 (Coupled Model Intercomparison Project Phase 5) exhibit a large inter‐model spread in the representation of long‐term trends in soil moisture and snow in response to anthropogenic climate change. We evaluate long‐term (1861/01‐2099/12) water storage trends from 21 CMIP5 models against observed trends in terrestrial water storage (TWS) obtained from 14 years (2002/04‐2016/08) of the GRACE (Gravity Recovery And Climate Experiment) satellite mission. This is complicated due to the incomplete representation of TWS in CMIP5 models and interannual climate variability masking long‐term trends in observations. We thus evaluate first the spread in projected trends among CMIP5 models and identify regions of broad model consensus. Second, we assess the extent to which these projected trends are already present during the historical period (1861/01‐2016/08) and thus potentially detectable in observational records available today. Third, we quantify the degree to which 14‐year tendencies can be expected to represent long‐term trends, finding that regional long‐term trends start to emerge from interannual variations after just 14 years while stable global trend patterns are detectable after 30 years. We classify regions of strong model consensus into areas where 1) climate‐related TWS changes are supported by the direction of GRACE trends, 2) mismatch of trends hints at possible model deficits, 3) the short observation time span and/or anthropogenic influences prevent reliable conclusions about long‐term wetting or drying. We thereby demonstrate the value of satellite observations of water storage to further constrain the response of the terrestrial water cycle to climate change.