The Atlantic Multidecadal Oscillation (AMO) is a major mode of climate variability with important societal impacts. Most previous explanations identify the driver of the AMO as the ocean circulation, specifically the Atlantic Meridional Overturning Circulation (AMOC). Here we show that the main features of the observed AMO are reproduced in models where the ocean heat transport is prescribed and thus cannot be the driver. Allowing the ocean circulation to interact with the atmosphere does not significantly alter the characteristics of the AMO in the current generation of climate models. These results suggest that the AMO is the response to stochastic forcing from the mid-latitude atmospheric circulation, with thermal coupling playing a role in the tropics. In this view, the AMOC and other ocean circulation changes would be largely a response to, not a cause of, the AMO.
ABSTRACT:A parametrization for ice supersaturation is introduced into the ECMWF Integrated Forecast System (IFS), compatible with the cloud scheme that allows partial cloud coverage. It is based on the simple, but often justifiable, diagnostic assumption that the ice nucleation and subsequent depositional growth time-scales are short compared to the model time step, thus supersaturation is only permitted in the clear-sky portion of the grid cell.Results from model integrations using the new scheme are presented, which is demonstrated to increase uppertropospheric humidity, decrease high-level cloud cover and, to a much lesser extent, cloud ice amounts, all as expected from simple arguments. Evaluation of the relative distribution of supersaturated humidity amounts shows good agreement with the observed climatology derived from in situ aircraft observations. With the new scheme, the global distribution of frequency of occurrence of supersaturated regions compares well with remotely sensed microwave limb sounder (MLS) data, with the most marked errors of underprediction occurring in regions where the model is known to underpredict deep convection. Finally, it is also demonstrated that the new scheme leads to improved predictions of permanent contrail cloud over southern England, which indirectly implies upper-tropospheric humidity fields are better represented for this region.
The year 2000 radiative forcing (RF) due to changes in O 3 and CH 4 (and the CH 4-induced stratospheric water vapour) as a result of emissions of short-lived gases (oxides of nitrogen (NO x), carbon monoxide and non-methane hydrocarbons) from three transport sectors (ROAD, maritime SHIPping and AIRcraft) are calculated using results from five global atmospheric chemistry models. Using results from these models plus other published data, we quantify the uncertainties. The RF due to short-term O 3 changes (i.e. as an immediate response to the emissions without allowing for the long-term CH 4 changes) is positive and highest for ROAD transport (31 mW m À2) compared to SHIP (24 mW m À2) and AIR (17 mW m À2) sectors in four of the models. All five models calculate negative RF from the CH 4 perturbations, with a larger impact from the SHIP sector than for ROAD and AIR. The net RF of O 3 and CH 4 combined (i.e. including the impact of CH 4 on ozone and stratospheric water vapour) is positive for ROAD (þ16(AE13) (one standard deviation) mW m À2) and AIR (þ6(AE5) mW m À2) traffic sectors and is negative for SHIP (À18(AE10) mW m À2) sector in all five models. Global Warming Potentials (GWP) and Global Temperature change Potentials (GTP) are presented for AIR NO x emissions; there is a wide spread in the results from the 5 chemistry models, and it is shown that differences in the methane response relative to the O 3 response drive much of the spread.
We report on the first observation of neutrino induced production of muon pairs in the electromagnetic field of a nucleus. The data has been obtained using the CHARM II detector exposed to the CERN wideband neutrino and antineutrino beams. A clear signal of 55 +/- 16 events is seen in a sample of dimuons of opposite charge without visible recoil at the vertex. The cross section is determined and found to be in agreement with the standard model prediction
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