27,294 research outputs found
Why Simpler Computer Simulation Models Can Be Epistemically Better for Informing Decisions
For computer simulation models to usefully inform climate risk management, uncertainties in model projections must be explored and characterized. Because doing so requires running the model many ti..
Attenuation of acoustic waves in glacial ice and salt domes
Two classes of natural solid media (glacial ice and salt domes) are under
consideration as media in which to deploy instruments for detection of
neutrinos with energy >1e18 eV. Though insensitive to 1e11 to 1e16 eV neutrinos
for which observatories (e.g., AMANDA and IceCube) that utilize optical
Cherenkov radiation detectors are designed, radio and acoustic methods are
suited for searches for the very low fluxes of neutrinos with energies >1017
eV. This is because, due to the very long attenuation lengths of radio and
acoustic waves in ice and salt, detection modules can be spaced very far apart.
In this paper, I calculate the absorption and scattering coefficients as a
function of frequency and grain size for acoustic waves in glacial ice and salt
domes and show that experimental measurements on laboratory samples and in
glacial ice and salt domes are consistent with theory. For South Pole ice with
grain size 0.2 cm at -51 degrees C, scattering lengths are calculated to be
2000 km and 25 km at 10 kHz and 30 kHz, respectively, and the absorption length
is calculated to be 9 km at frequencies above 100 Hz. For NaCl (rock salt) with
grain size 0.75 cm, scattering lengths are calculated to be 120 km and 1.4 km
at 10 kHz and 30 kHz, and absorption lengths are calculated to be 30,000 km and
3300 km at 10 kHz and 30 kHz. Existing measurements are consistent with theory.
For ice, absorption is the limiting factor; for salt, scattering is the
limiting factor.Comment: 16 pages, 7 figures, submitted to Journal of Geophysical Research -
Solid Eart
ENSO suppression due to weakening of the North Atlantic thermohaline circulation
Changes of the North Atlantic thermohaline circulation (THC) excite wave patterns that readjust the thermocline globally. This paper examines the impact of a freshwater-induced THC shutdown on the depth of the Pacific thermocline and its subsequent modification of the El Niño–Southern Oscillation (ENSO) variability using an intermediate-complexity global coupled atmosphere–ocean–sea ice model and an intermediate ENSO model, respectively. It is shown by performing a numerical eigenanalysis and transient simulations that a THC shutdown in the North Atlantic goes along with reduced ENSO variability because of a deepening of the zonal mean tropical Pacific thermocline. A transient simulation also exhibits abrupt changes of ENSO behavior, depending on the rate of THC change. The global oceanic wave adjustment mechanism is shown to play a key role also on multidecadal time scales. Simulated multidecadal global sea surface temperature (SST) patterns show a large degree of similarity with previous climate reconstructions, suggesting that the observed pan-oceanic variability on these time scales is brought about by oceanic waves and by atmospheric teleconnections
Atmospheric Circulation Response to Short-Term Arctic Warming in an Idealized Model
Recent Arctic sea ice loss in fall has been posited to drive midlatitude circulation changes into winter and even spring. Past work has shown that sea ice loss can indeed trigger a weakening of the stratospheric polar vortex, which can lead to delayed surface weather changes. But the mechanisms of such changes and their relevant time scales have remained unclear. This study uses large ensembles of idealized GCM simulations to identify how and over what time scales the atmospheric circulation responds to short-term surface heat flux changes in high latitudes. The ensemble-mean response of the atmospheric circulation is approximately linear in the amplitude of the surface forcing. It is also insensitive to whether the forcing is zonally asymmetric or symmetric, that is, whether stationary waves are generated or not. The circulation response can be decomposed into a rapid thermal response and a slower dynamic adjustment. The adjustment arises through weakening of vertical wave activity fluxes from the troposphere into the stratosphere in response to polar warming, a mechanism that differs from sudden stratospheric warmings yet still results in a weakened stratospheric circulation. The stratospheric response is delayed and persists for about 2 months because the thermal response of the stratosphere is slow compared with that of the troposphere. The delayed stratospheric response feeds back onto the troposphere, but the tropospheric effects are weak compared with natural variability. The general pathway for the delayed response appears to be relatively independent of the atmospheric background state at the time of the anomalous surface forcing
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How tropical Pacific surface cooling contributed to accelerated sea ice melt from 2007 to 2012 as ice is thinned by anthropogenic forcing
Over the past 40 years the Arctic sea ice minimum in September has declined. The period between 2007 and 2012 showed accelerated melt contributed to the record minima of 2007 and 2012. Here, observational and model evidence shows that the changes in summer sea ice since the 2000s reflects a continuous anthropogenically forced melting masked by interdecadal variability of Arctic atmospheric circulation. This variation is partially driven by teleconnections originating from sea surface temperature (SST) changes in the east-central tropical Pacific via a Rossby wave train propagating into the Arctic (hereafter referred to as the “Pacific-Arctic teleconnection (PARC)”), which represents the leading internal mode connecting the pole to lower latitudes. This mode has contributed to accelerated warming and Arctic sea ice loss from 2007 to 2012, followed by slower declines in recent years, resulting in the appearance of a slowdown over the past 11 years. A pacemaker model simulation, in which we specify observed SST in the tropical eastern Pacific, demonstrates a physically plausible mechanism for the PARC mode. However, the model-based PARC mechanism is considerably weaker and only partially accounts for the observed acceleration of sea ice loss from 2007 to 2012. We also explore features of large-scale circulation patterns associated with extreme melting periods in a long (1800-yr) CESM preindustrial simulation. These results further support the role of remote SST forcing originating from the tropical Pacific in exciting significant warm episodes in the Arctic. However, further research is needed to identify the reasons for model limitations in reproducing the observed PARC mode featuring a Cold Pacific - Warm Arctic connection
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