The steady-state extratropical atmospheric response to thermal forcing is investigated in a simple atmospheric general circulation model. The thermal forcings qualitatively mimic three key aspects of ...anthropogenic climate change: warming in the tropical troposphere, cooling in the polar stratosphere, and warming at the polar surface. The principal novel findings are the following:
1) Warming in the tropical troposphere drives two robust responses in the model extratropical circulation: poleward shifts in the extratropical tropospheric storm tracks and a weakened stratospheric Brewer–Dobson circulation. The former result suggests heating in the tropical troposphere plays a fundamental role in the poleward contraction of the storm tracks found in Intergovernmental Panel on Climate Change (IPCC)-class climate change simulations; the latter result is in the opposite sense of the trends in the Brewer–Dobson circulation found in most previous climate change experiments.
2) Cooling in the polar stratosphere also drives a poleward shift in the extratropical storm tracks. The tropospheric response is largely consistent with that found in previous studies, but it is shown to be very sensitive to the level and depth of the forcing. In the stratosphere, the Brewer–Dobson circulation weakens at midlatitudes, but it strengthens at high latitudes because of anomalously poleward heat fluxes on the flank of the polar vortex.
3) Warming at the polar surface drives an equatorward shift of the storm tracks. The storm-track response to polar warming is in the opposite sense of the response to tropical tropospheric heating; hence large warming over the Arctic may act to attenuate the response of the Northern Hemisphere storm track to tropical heating.
4) The signs of the tropospheric and stratospheric responses to all thermal forcings considered here are robust to seasonal changes in the basic state, but the amplitude and details of the responses exhibit noticeable differences between equinoctial and wintertime conditions. Additionally, the responses exhibit marked nonlinearity in the sense that the response to multiple thermal forcings applied simultaneously is quantitatively different from the sum of the responses to the same forcings applied independently. Thus the response of the model to a given thermal forcing is demonstrably dependent on the other thermal forcings applied to the model.
Internal variability in the climate system gives rise to large uncertainty in projections of future climate. The uncertainty in future climate due to internal climate variability can be estimated ...fromlarge ensembles of climate change simulations in which the experiment setup is the same from one ensemble member to the next but for small perturbations in the initial atmospheric state. However, large ensembles are invariably computationally expensive and susceptible to model bias.
Here the authors outline an alternative approach for assessing the role of internal variability in future climate based on a simple analytic model and the statistics of the unforced climate variability. The analytic model is derived from the standard error of the regression and assumes that the statistics of the internal variability are roughly Gaussian and stationary in time. When applied to the statistics of an unforced control simulation, the analytic model provides a remarkably robust estimate of the uncertainty in future climate indicated by a large ensemble of climate change simulations. To the extent that observations can be used to estimate the amplitude of internal climate variability, it is argued that the uncertainty in future climate trends due to internal variability can be robustly estimated from the statistics of the observed climate.
Climate change has been and will be accompanied by widespread changes in surface temperature. It is clear that these changes include global-wide increases in mean surface temperature and changes in ...temperature variance that are more regionally-dependent
. It is less clear whether they also include changes in the persistence of surface temperature. This is important as the effects of weather events on ecosystems and society depend critically on the length of the event. Here we provide an extensive survey of the response of surface temperature persistence to climate change over the twenty-first century from the output of 150 simulations run on four different Earth system models, and from simulations run on simplified models with varying representations of radiative processes and large-scale dynamics. Together, the results indicate that climate change simulations are marked by widespread changes in surface temperature persistence that are generally most robust over ocean areas and arise due to a seemingly broad range of physical processes. The findings point to both the robustness of widespread changes in persistence under climate change, and the critical need to better understand, simulate and constrain such changes.
Climate variability in the high-latitude Southern Hemisphere (SH) is dominated by the SH annular mode, a large-scale pattern of variability characterized by fluctuations in the strength of the ...circumpolar vortex. We present evidence that recent trends in the SH tropospheric circulation can be interpreted as a bias toward the high-index polarity of this pattern, with stronger westerly flow encircling the polar cap. It is argued that the largest and most significant tropospheric trends can be traced to recent trends in the lower stratospheric polar vortex, which are due largely to photochemical ozone losses. During the summer-fall season, the trend toward stronger circumpolar flow has contributed substantially to the observed warming over the Antarctic Peninsula and Patagonia and to the cooling over eastern Antarctica and the Antarctic plateau.
A new data set of middle- and upper-stratospheric temperatures based on reprocessing of satellite radiances provides a view of stratospheric climate change during the period 1979-2005 that is ...strikingly different from that provided by earlier data sets. The new data call into question our understanding of observed stratospheric temperature trends and our ability to test simulations of the stratospheric response to emissions of greenhouse gases and ozone-depleting substances. Here we highlight the important issues raised by the new data and suggest how the climate science community can resolve them.
We investigate the impact that the four principal large‐scale patterns of Southern Hemisphere (SH) atmospheric circulation variability have on Antarctic surface air temperature (SAT): (1) the ...southern baroclinic annular mode (BAM), which is associated with variations in extratropical storm amplitude; (2) the Southern Annular Mode (SAM), associated with latitudinal shifts in the midlatitude jet; and (3) the two Pacific‐South American patterns (PSA1 and PSA2), which are characterized by wave trains originating in the tropical Pacific that extend across the SH extratropics. A key aspect is the use of 35 years of daily observations and reanalysis data, which affords a sufficiently large sample size to assess the signatures of the circulation patterns in both the mean and variability of daily mean SAT anomalies. The BAM exerts the weakest influence on Antarctic SAT, albeit it is still important over select regions. Consistent with previous studies, the SAM is shown to influence SAT across most of the continent throughout the year. The PSA1 also affects SAT across almost all of Antarctica. Regionally, both PSA patterns can exert a greater impact on SAT than the SAM but also have a significantly weaker influence during summer, reflecting the seasonality of the SH response to El Niño–Southern Oscillation. The SAM and PSA patterns have distinct signatures in daily SAT variance that are physically consistent with their signatures in extratropical dynamic variability. The broad‐scale climate linkages identified here provide benchmarks for interpreting the Antarctic climate response to future changes in tropical sea surface temperatures, ozone recovery, and greenhouse gas increases.
Key Points
Large‐scale circulation patterns exert significant influence on Antarctic surface temperatures
The impact of the Pacific‐South American patterns is as large as the Southern Annular Mode
The circulation patterns also affect daily temperature variability across much of Antarctica
Data sets used to monitor the Earth's climate indicate that the surface of the Earth warmed from ∼1910 to 1940, cooled slightly from ∼1940 to 1970, and then warmed markedly from ∼1970 onward. The ...weak cooling apparent in the middle part of the century has been interpreted in the context of a variety of physical factors, such as atmosphere-ocean interactions and anthropogenic emissions of sulphate aerosols. Here we call attention to a previously overlooked discontinuity in the record at 1945, which is a prominent feature of the cooling trend in the mid-twentieth century. The discontinuity is evident in published versions of the global-mean temperature time series, but stands out more clearly after the data are filtered for the effects of internal climate variability. We argue that the abrupt temperature drop of ∼0.3 °C in 1945 is the apparent result of uncorrected instrumental biases in the sea surface temperature record. Corrections for the discontinuity are expected to alter the character of mid-twentieth century temperature variability but not estimates of the century-long trend in global-mean temperatures.
We use an empirical statistical model to demonstrate significant skill in making extended-range forecasts of the monthly-mean Arctic Oscillation (AO). Forecast skill derives from persistent ...circulation anomalies in the lowermost stratosphere and is greatest during boreal winter. A comparison to the Southern Hemisphere provides evidence that both the time scale and predictability of the AO depend on the presence of persistent circulation anomalies just above the tropopause. These circulation anomalies most likely affect the troposphere through changes to waves in the upper troposphere, which induce surface pressure changes that correspond to the AO.
Arctic clouds exhibit a robust annual cycle with maximum
cloudiness in fall and minimum cloudiness in winter. These variations affect energy
flows in the Arctic with a large influence on the surface ...radiative fluxes.
Contemporary climate models struggle to reproduce the observed Arctic cloud
amount annual cycle and significantly disagree with each other. The goal of
this analysis is to quantify the cloud-influencing factors that contribute
to winter–summer cloud amount differences, as these seasons are primarily
responsible for the model discrepancies with observations. We find that
differences in the total cloud amount annual cycle are primarily caused by
differences in low, rather than high, clouds; the largest differences occur between
the surface and 950 hPa. Grouping models based on their seasonal cycles of
cloud amount and stratifying cloud amount by cloud-influencing factors, we
find that model groups disagree most under strong lower tropospheric
stability, weak to moderate mid-tropospheric subsidence, and cold lower
tropospheric air temperatures. Intergroup differences in low cloud amount
are found to be a function of lower tropospheric thermodynamic
characteristics. Further, we find that models with a larger low cloud amount
in winter have a larger ice condensate fraction, whereas models with a
larger low cloud amount in summer have a smaller ice condensate fraction.
Stratifying model output by the specifics of the cloud microphysical scheme
reveals that models treating cloud ice and liquid condensate as separate
prognostic variables simulate a larger ice condensate fraction than those
that treat total cloud condensate as a prognostic variable and use a
temperature-dependent phase partitioning. Thus, the cloud microphysical
parameterization is the primary cause of inter-model differences in the
Arctic cloud annual cycle, providing further evidence of the important role
that cloud ice microphysical processes play in the evolution and modeling of
the Arctic climate system.
Long-range prediction and the stratosphere Scaife, Adam A; Baldwin, Mark P; Butler, Amy H ...
Atmospheric chemistry and physics,
02/2022, Letnik:
22, Številka:
4
Journal Article
Recenzirano
Odprti dostop
Over recent years there have been concomitant advances in the
development of stratosphere-resolving numerical models, our understanding of
stratosphere–troposphere interaction, and the extension of ...long-range
forecasts to explicitly include the stratosphere. These advances are now
allowing for new and improved capability in long-range prediction. We present an
overview of this development and show how the inclusion of the stratosphere
in forecast systems aids monthly, seasonal, and annual-to-decadal climate
predictions and multidecadal projections. We end with an outlook towards the
future and identify areas of improvement that could further benefit these
rapidly evolving predictions.
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