Strong westerly winds that circle Antarctica are responsible for the upwelling of cold water from the Southern Ocean's depths. Since the middle of the twentieth century, these winds have increased in ...strength and shifted towards the South Pole5. ...the ocean - the Southern Ocean, in particular - is extremely difficult to observe systematically, even under the best conditions. ...technical and logistical hurdles prevent any single method from providing a coherent picture that will determine the specifics of climate change as a new global equilibrium emerges. Ice modellers and other climate scientists are working hard to integrate land ice into global coupled climate models, but, unfortunately, none of the current generation of such models (nor those in previous generations) includes the melting of ice sheets on Antarctica or Greenland.
Meltwater from the Antarctic Ice Sheet is projected to cause up to one metre of sea-level rise by 2100 under the highest greenhouse gas concentration trajectory (RCP8.5) considered by the ...Intergovernmental Panel on Climate Change (IPCC). However, the effects of meltwater from the ice sheets and ice shelves of Antarctica are not included in the widely used CMIP5 climate models, which introduces bias into IPCC climate projections. Here we assess a large ensemble simulation of the CMIP5 model 'GFDL ESM2M' that accounts for RCP8.5-projected Antarctic Ice Sheet meltwater. We find that, relative to the standard RCP8.5 scenario, accounting for meltwater delays the exceedance of the maximum global-mean atmospheric warming targets of 1.5 and 2 degrees Celsius by more than a decade, enhances drying of the Southern Hemisphere and reduces drying of the Northern Hemisphere, increases the formation of Antarctic sea ice (consistent with recent observations of increasing Antarctic sea-ice area) and warms the subsurface ocean around the Antarctic coast. Moreover, the meltwater-induced subsurface ocean warming could lead to further ice-sheet and ice-shelf melting through a positive feedback mechanism, highlighting the importance of including meltwater effects in simulations of future climate.
•We present simulations of Titan’s middle and lower atmosphere with the Titan Atmospheric Model (TAM) GCM.•Vertical and latitudinal temperature profiles from the surface through the stratopause are ...reproduced.•Superrotation develops naturally in the model.•Comparison to observations indicates the prevalence of dry conditions at low latitudes.•Polar and equatorial precipitation are consistent with observed clouds, but mid-latitude cloudiness remains a puzzle.
Simulation results are presented from a new general circulation model (GCM) of Titan, the Titan Atmospheric Model (TAM), which couples the Flexible Modeling System (FMS) spectral dynamical core to a suite of external/sub-grid-scale physics. These include a new non-gray radiative transfer module that takes advantage of recent data from Cassini–Huygens, large-scale condensation and quasi-equilibrium moist convection schemes, a surface model with “bucket” hydrology, and boundary layer turbulent diffusion. The model produces a realistic temperature structure from the surface to the lower mesosphere, including a stratopause, as well as satisfactory superrotation. The latter is shown to depend on the dynamical core’s ability to build up angular momentum from surface torques. Simulated latitudinal temperature contrasts are adequate, compared to observations, and polar temperature anomalies agree with observations. In the lower atmosphere, the insolation distribution is shown to strongly impact turbulent fluxes, and surface heating is maximum at mid-latitudes. Surface liquids are unstable at mid- and low-latitudes, and quickly migrate poleward. The simulated humidity profile and distribution of surface temperatures, compared to observations, corroborate the prevalence of dry conditions at low latitudes. Polar cloud activity is well represented, though the observed mid-latitude clouds remain somewhat puzzling, and some formation alternatives are suggested.
Although the Southern Ocean is thought to account for a significant portion of the contemporary oceanic uptake of carbon dioxide (CO2), flux estimates in this region are based on sparse observations ...that are strongly biased toward summer. Here we present new estimates of Southern Ocean air‐sea CO2 fluxes calculated with measurements from biogeochemical profiling floats deployed by the Southern Ocean Carbon and Climate Observations and Modeling project during 2014–2017. Compared to ship‐based CO2 flux estimates, the float‐based fluxes find significantly stronger outgassing in the zone around Antarctica where carbon‐rich deep waters upwell to the surface ocean. Although interannual variability contributes, this difference principally stems from the lack of autumn and winter ship‐based observations in this high‐latitude region. These results suggest that our current understanding of the distribution of oceanic CO2 sources and sinks may need revision and underscore the need for sustained year‐round biogeochemical observations in the Southern Ocean.
Plain Language Summary
The Southern Ocean absorbs a great deal of carbon dioxide from the atmosphere and helps to shape the climate of Earth. However, we do not have many observations from this part of the world, especially in winter, because it is remote and inhospitable. Here we present new observations from robotic drifting buoys that take measurements of temperature, salinity, and other water properties year‐round. We use these data to estimate the amount of carbon dioxide being absorbed by the Southern Ocean. In the open water region close to Antarctica, the new estimates are remarkably different from the previous estimates, which were based on data collected from ships. We discuss some possible reasons that the float‐based estimate is different and how this changes our understanding of how the ocean absorbs carbon dioxide.
Key Points
Measurements from biogeochemical profiling floats were used to estimate air‐sea fluxes of carbon dioxide
Significant annual net outgassing of carbon dioxide was observed in the high‐latitude Antarctic‐Southern Zone
In this region, a large difference with previous estimates was found in winter when ship‐based sampling is sparse
Mountain glaciers are highly sensitive to climate change. However, the extent to which glaciers capture regional to hemisphere‐scale atmospheric processes remains uncertain, hindering paleoclimatic ...interpretations derived from moraine‐based glacier reconstructions. Here, we evaluate how mid‐latitude glacier systems monitor climate by comparing climate reanalysis products with glacier annual equilibrium line altitude (ELA) elevations from the antipodal Southern Alps of New Zealand and European Alps. We find significant regional and hemispheric correlations between glacier annual ELA and summer tropospheric temperatures. Annual ELA also exhibit positive correlations with the latitude of the westerly jets in both hemispheres. These results indicate that westerly wind‐belt latitude modulates the proportion of cold versus warm air masses influencing these glacier systems. These results highlight the sensitivity of mid‐latitude glaciers to atmospheric temperatures and circulation, with implications for interpreting moraine‐based paleoclimate reconstructions. Combined impacts of ongoing tropospheric warming and poleward‐shifting westerlies will likely accelerate recession of mid‐latitude glaciers.
Plain Language Summary
Mountain glaciers respond to climate change by gaining mass when the climate cools and losing mass when the climate warms. However, the extent to which these glacial fluctuations are reflective of local, regional, and hemispheric climate variations is less clear, hindering climatic interpretation of paleo‐glacier reconstructions developed from glacial landforms. This study evaluates the climatic footprint monitored by antipodal mid‐latitude glacier populations by comparing gridded reconstructions of global temperature and wind changes with glacier annual snowline elevations in the Southern Alps of New Zealand and annual equilibrium line altitude elevations in the European Alps. Our results indicate that (a) these glacier systems co‐vary with atmospheric temperatures on regional and even hemispheric scales throughout all levels of the troposphere, and (b) the latitudes of the westerly wind belts are important for regulating the proportion of cold versus warm air masses influencing glacier mass‐balance. Altogether, our results indicate that mid‐latitude mountain glacier fluctuations reflect temperature changes integrated over large regions of the atmosphere. With ongoing climate change, the combination of global atmospheric warming and poleward‐shifting westerlies is likely to accelerate recession of mid‐latitude glaciers in both hemispheres.
Key Points
Mid‐latitude glacier annual equilibrium line altitude corresponds to broad regions of atmospheric temperature
Mid‐latitude glacier annual equilibrium line altitude is sensitive to latitudinal shifts of the mid‐latitude westerlies
The influence of the westerlies on glaciers has important implications for interpreting past and predicting future climate change
The NOAA Science Advisory Board appointed a task force to prepare a white paper on the use of observing system simulation experiments (OSSEs). Considering the importance and timeliness of this topic ...and based on this white paper, here we briefly review the use of OSSEs in the United States, discuss their values and limitations, and develop five recommendations for moving forward: national coordination of relevant research efforts, acceleration of OSSE development for Earth system models, consideration of the potential impact on OSSEs of deficiencies in the current data assimilation and prediction system, innovative and new applications of OSSEs, and extension of OSSEs to societal impacts. OSSEs can be complemented by calculations of forecast sensitivity to observations, which simultaneously evaluate the impact of different observation types in a forecast model system.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Previous studies found large biases between individual observational and model estimates of historical ocean anthropogenic carbon uptake. We show that the largest bias between the Coupled Model ...Intercomparison Project phase 5 (CMIP5) ensemble mean and between two observational estimates of ocean anthropogenic carbon is due to a difference in start date. After adjusting the CMIP5 and observational estimates to the 1791–1995 period, all three carbon uptake estimates agree to within 3 Pg of C, about 4% of the total. The CMIP5 ensemble mean spatial bias compared to the observations is generally smaller than the observational error, apart from a negative bias in the Southern Ocean and a positive bias in the Southern Indian and Pacific Oceans compensating each other in the global mean. This dipole pattern is likely due to an equatorward and weak bias in the position of Southern Hemisphere westerlies and lack of mode and intermediate water ventilation.
Key Points
Observations and model simulations of ocean anthropogenic carbon assume different start dates
Once referenced to the same period, 1971–1995, models and observations of ocean anthropogenic carbon agree to within 4%
A model bias in the mean position of Southern Hemisphere westerlies results in a bias in the pattern of Southern Hemisphere carbon uptake
A coupled climate model with poleward-intensified westerly winds simulates significantly higher storage of heat and anthropogenic carbon dioxide by the Southern Ocean in the future when compared with ...the storage in a model with initially weaker, equatorward-biased westerlies. This difference results from the larger outcrop area of the dense waters around Antarctica and more vigorous divergence, which remains robust even as rising atmospheric greenhouse gas levels induce warming that reduces the density of surface waters in the Southern Ocean. These results imply that the impact of warming on the stratification of the global ocean may be reduced by the poleward intensification of the westerlies, allowing the ocean to remove additional heat and anthropogenic carbon dioxide from the atmosphere.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
PDF
