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Recent observations and model studies of the earth’s radiative energy balance have focused attention on the earth’s top of atmosphere (TOA) energy balance. This is the balance between the shortwave energy absorbed by the earth, which is represented by a spatially and temporally averaged absorbed flux , and the emitted longwave energy, which is represented by the corresponding averaged emitted flux . The TOA average net flux FN is defined as the difference between the two over the averaged area and time, which may be a local, regional, or global average. A global nonzero net flux represents a measure of imbalance between the energy being absorbed and emitted by the earth for the time interval in question. It is of interest to ask what the natural variability of the net flux might be and whether, during times of climate change, signals of important climate change processes might be detected against this natural background variation; examples of these signals include evidence of ocean heat storage, the effects of El Niño, and the radiative effects of volcanic eruptions. In this paper, the authors review the significance of the net flux, survey the observational evidence from a range of satellite instruments over several decades, and analyze some of the most recent observations from the Clouds and the Earth’s Radiant Energy System (CERES) program to determine what signals and what natural variability might be expected in the TOA net flux. Based on this analysis, the use of broadband radiation measurements for global climate change studies can be assessed. [ABSTRACT FROM AUTHOR]

This paper examines variations of the surface air temperature (SAT) over China and the globe in the twentieth century simulated by 19 coupled climate models driven by historical natural and anthropogenic forcings. Most models perform well in simulating both the global and the Northern Hemispheric mean SAT evolutions of the twentieth century. The inclusion of natural forcings improves the simulation, in particular for the first half of the century. The reproducibility of the SAT averaged over China is lower than that of the global and hemispheric averages, but it is still acceptable. The contribution of natural forcings to the SAT over China in the first half of the century is not as robust as that to the global and hemispheric averages. No model could successfully produce the reconstructed warming over China in the 1920s. The prescribed natural and anthropogenic forcings in the coupled climate models mainly produce the warming trends and the decadal- to interdecadal-scale SAT variations with poor performances at shorter time scales. The prominent warming trend in the last half of the century over China and its acceleration in recent decades are weakly simulated. There are discrepancies between the simulated and observed regional features of the SAT trend over China. Few models could produce the summertime cooling over the middle part of eastern China (27°–36°N), while two models acceptably produce the meridional gradients of the wintertime warming trends, with north China experiencing larger warming. Limitations of the current state-of-the-art coupled climate models in simulating spatial patterns of the twentieth-century SAT over China cast a shadow upon their capability toward projecting credible geographical distributions of future climate change through Intergovernmental Panel on Climate Change (IPCC) scenario simulations. [ABSTRACT FROM AUTHOR]

The characteristics of tropical mesoscale convective systems (MCSs) simulated with a finer-resolution (~50 km) version of the Geophysical Fluid Dynamics Laboratory (GFDL) AM4 model are evaluated by comparing with a comprehensive long-term observational dataset. It is shown that the model can capture the various aspects of MCSs reasonably well. The simulated spatial distribution of MCSs is broadly in agreement with the observations. This is also true for seasonality and interannual variability over different land and oceanic regions. The simulated MCSs are generally longer-lived, weaker, and larger than observed. Despite these biases, an event-scale analysis suggests that their duration, intensity, and size are strongly correlated. Specifically, longer-lived and stronger events tend to be bigger, which is consistent with the observations. The same model is used to investigate the response of tropical MCSs to global warming using time-slice simulations forced by prescribed sea surface temperatures and sea ice. There is an overall decrease in occurrence frequency, and the reduction over land is more prominent than over ocean. [ABSTRACT FROM AUTHOR]

It is unclear how the width and strength of the Hadley circulation are controlled and how they respond to climate changes. Simulations of global warming scenarios with comprehensive climate models suggest the Hadley circulation may widen and weaken as the climate warms. But these changes are not quantitatively consistent among models, and how they come about is not understood. Here, a wide range of climates is simulated with an idealized moist general circulation model (GCM) coupled to a simple representation of ocean heat transport, in order to place past and possible future changes in the Hadley circulation into a broader context and to investigate the mechanisms responsible for them. By comparison of simulations with and without ocean heat transport, it is shown that it is essential to take low-latitude ocean heat transport and its coupling to wind stress into account to obtain Hadley circulations in a dynamical regime resembling Earth''s, particularly in climates resembling present-day Earth''s and colder. As the optical thickness of an idealized longwave absorber in the simulations is increased and the climate warms, the Hadley circulation strengthens in colder climates and weakens in warmer climates; it has maximum strength in a climate close to present-day Earth''s. In climates resembling present-day Earth''s and colder, the Hadley circulation strength is largely controlled by the divergence of angular momentum fluxes associated with eddies of midlatitude origin; the latter scale with the mean available potential energy in midlatitudes. The importance of these eddy momentum fluxes for the Hadley circulation strength gradually diminishes as the climate warms. The Hadley circulation generally widens as the climate warms, but at a modest rate that depends sensitively on how it is determined. [ABSTRACT FROM AUTHOR]

Sea surface temperature (SST) changes constitute a major indicator and driver of climate changes induced by greenhouse gas increases. The objective of the present study is to investigate the role played by the detailed structure of the SST change on the large-scale atmospheric circulation and the distribution of precipitation. For that purpose, simulations from the Institut Pierre-Simon Laplace Coupled Model, version 4 (IPSL-CM4) are used where the carbon dioxide (CO2) concentration is doubled. The response of IPSL-CM4 is characterized by the same robust mechanisms affecting the other coupled models in global warming simulations, that is, an increase of the hydrological cycle accompanied by a global weakening of the large-scale circulation. First, purely atmospheric simulations are performed to mimic the results of the coupled model. Then, specific simulations are set up to further study the underlying atmospheric mechanisms. These simulations use different prescribed SST anomalies, which correspond to a linear decomposition of the IPSL-CM4 SST changes in global, longitudinal, and latitudinal components. The simulation using a globally uniform increase of the SST is able to reproduce the modifications in the intensity of the hydrological cycle or in the mean upward mass flux, which also characterize the double CO2 simulation with the coupled model. But it is necessary (and largely sufficient) to also take into account the zonal-mean meridional structure of the SST changes to represent correctly the changes in the Hadley circulation strength or the zonal-mean precipitation changes simulated by the coupled model, even if these meridional changes by themselves do not change the mean thermodynamical state of the tropical atmosphere. The longitudinal SST anomalies of IPSL-CM4 also have an impact on the precipitation and large-scale tropical circulation and tend to introduce different changes over the Pacific and Atlantic Oceans. The longitudinal SST changes are demonstrated to have a smaller but opposite effect from that of the meridional anomalies on the Hadley cell circulations. Results indicate that the uncertainties in the simulated meridional patterns of the SST warming may have major consequences on the assessment of the changes of the Hadley circulation and zonal-mean precipitation in future climate projections. [ABSTRACT FROM AUTHOR]

The extratropical annular-mode-like atmospheric responses to ENSO and global warming and the internal variability of annular modes are associated with similar, yet distinct, dynamical characteristics. In particular, La Niña, global warming, and the positive phase of annular modes are all associated with a poleward shift of midlatitude jet streams and surface westerlies. To improve understanding of these phenomena, the authors identify and compare patterns of interannual variability and global warming trends in the midlatitude surface westerlies and the space–time spectra of associated eddy momentum fluxes by analyzing simulations of the present climate in an atmosphere-only climate model, in which the ENSO-induced extratropical response is validated with that in reanalysis data, and by projection of future climate changes using a coupled atmosphere–ocean model. While the response to ENSO is consistent with the refraction of midlatitude eddies due to subtropical wind anomalies, the interannual internal variability of the annular modes marks a change in the eastward propagation speed of midlatitude eddies. In response to global warming, the dominant eddies exhibit a trend toward faster eddy phase speeds in both hemispheres, in a manner similar to the positive phase of interannual internal variability. These diagnoses suggest that the annular mode trend due to greenhouse gas increases may be more related to extratropical processes, especially in the upper troposphere/lower stratosphere, rather than being forced from the deep tropics. [ABSTRACT FROM AUTHOR]

This study examines the local memory of atmospheric and oceanic changes associated with a tropical cyclone (TC). The memory is quantified through anomalous maximum potential intensity (MPI) evolution for 20 days prior to the arrival of a TC through 60 days after the TC passage. The local MPI weakens and is not restored to the evolving climatology until well after the TC has departed. Stabilization occurs through warming of the atmosphere and cooling of the ocean surface on different time scales. The time scale of MPI stabilization following TC passage is approximately 30–35 days for a tropical storm to 50–60 days for a category 3–5 hurricane, with significant storm-specific and basin-specific variability. The atmospheric stabilization (warming with respect to SST) begins with TC arrival and continues for approximately 7–10 days after passage, when the troposphere cools below normal. The rewarming of SST and the subsequent rewarming of the atmosphere occurs within approximately 35 days for all intensities, despite a positive (weakened) MPI anomaly through two months. This suggests that the atmosphere retains anomalous warmth beyond what can be attributable to sensible heating from the rewarmed SST. The maintenance of a positive MPI anomaly beyond 35 days is thus attributed to a feedback on larger scales that requires considerable further research. A TC’s passage through a region does not always lead to a weakening of the MPI. In regions poleward of the sharp SST gradient, the MPI one month after TC passage is often several millibars stronger than climatology. There are also mesoscale regions of destabilization one month after TC passage that may result partially from salinity changes driven by oceanic mixing as well as changes in precipitation and evaporation. [ABSTRACT FROM AUTHOR]