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  1. Abstract

    In the tropics, the absorbed solar radiation is larger than the outgoing longwave radiation, while the opposite is true at high latitudes. This basic fact implies a poleward energy transport (PET) in both hemispheres, which is accomplished by the atmosphere and oceans. The magnitude of PET is determined by the top of atmosphere gradient in the net radiation flux, and small changes to this quantity must change the total PET in the absence of changes in heat uptake. We analyze a large ensemble of 50 historic climate simulations from the CESM LENS2 project and find a significant PET anomaly in the latter half of the twentieth century. The temporal evolution of this anomaly—with a rapid increase after 1950, a peak near 1975, and a rapid decrease in the 1990s—mirrors the historic trend of sulfur dioxide (SO2, a significant aerosol predecessor) emissions from Europe and North America. This anomaly also appears in an analysis of the PET calculated from ERA5 reanalyses and from the CESM2 Single Forcing Large Ensemble. Consistent with previous studies, we find that historic SO2emissions from Europe and North America brightened clouds, which reflected additional solar radiation back to space in the midlatitudes: this shortwave anomaly increased the meridional gradient in the net TOA radiation flux and induced an anomalous northward energy transport. Finally, our results suggest that cryosphere processes become an additional important factor in setting the PET anomaly during the first years of the twenty-first century by contributing to the difference in absorbed solar radiation between hemispheres alongside cloud radiative effects.

    significance statement

    In this study, we analyze a large group of climate model simulations from 1850 to 2014 and find that this historical pollution changed the way that heat was transported from the tropics to Earth’s poles. We also find this change in heat transport when we analyzed an atmospheric reanalysis, which is a historical dataset that combines many meteorological observations into a best estimate of the past climate state. This extra reflection of sunlight from polluted clouds cooled the Northern Hemisphere, and we hypothesize that this cooling caused more heat transport out of the tropics. Last, we find that similar pollution emitted from China and India in more recent decades has not led to a change in Earth’s heat transport because of counteracting changes in snow and ice in the Northern Hemisphere.

     
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  2. Abstract

    We examine the hypothesis that the observed connection between the stratospheric quasi-biennial oscillation (QBO) and the strength of the Madden–Julian oscillation (MJO) is modulated by the sea surface temperature (SST)—for example, by El Niño–Southern Oscillation (ENSO). A composite analysis shows that, globally, La Niña SSTs are remarkably similar to those that occur during the easterly phase of the QBO. A maximum covariance analysis suggests that MJO power and SST are strongly linked on both the ENSO time scale and the QBO time scale. We analyze simulations with a modified configuration of version 2 of the Community Earth System Model, with a high top and fine vertical resolution. The model is able to simulate ENSO, the QBO, and the MJO. The ocean-coupled version of the model simulates the QBO, ENSO, and MJO, but does not simulate the observed QBO–MJO connection. When driven with prescribed observed SST anomalies based on composites for QBO east and QBO west (QBOE and QBOW), however, the same atmospheric model produces a modest enhancement of MJO power during QBOE relative to QBOW, as observed. We explore the possibility that the SST anomalies are forced by the QBO itself. Indeed, composite Hovmöller diagrams based on observations show the propagation of QBO zonal wind anomalies all the way from the upper stratosphere to the surface. Also, subsurface ocean temperature composites reveal a similarity between the western Pacific and Indian Ocean subsurface signal between La Niña and QBOE.

     
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  3. Abstract. Teleconnections from the Madden–Julian Oscillation (MJO) are a key source of predictability of weather on the extended timescale of about 10–40 d. The MJO teleconnection is sensitive to a number of factors, including the mean dry static stability, the mean flow, and the propagation and intensity characteristics of the MJO, which are traditionally difficult to separate across models. Each of these factors may evolve in response to increasing greenhouse gas emissions, which will impact MJO teleconnections and potentially impact predictability on extended timescales. Current state-of-the-art climate models do not agree on how MJO teleconnections over central and eastern North America will change in a future climate. Here, we use results from the Coupled Model Intercomparison Project Phase 6 (CMIP6) historical and SSP585 experiments in concert with a linear baroclinic model (LBM) to separate and investigate alternate mechanisms explaining why and how boreal winter (January) MJO teleconnections over the North Pacific and North America may change in a future climate and to identify key sources of inter-model uncertainty. LBM simulations suggest that a weakening teleconnection due to increases in tropical dry static stability alone is robust across CMIP6 models and that uncertainty in mean state winds is a key driver of uncertainty in future MJO teleconnections. Uncertainty in future changes to the MJO's intensity, eastward propagation speed, zonal wavenumber, and eastward propagation extent are other important sources of uncertainty in future MJO teleconnections. We find no systematic relationship between future changes in the Rossby wave source and the MJO teleconnection or between changes to the zonal wind or stationary Rossby wave number and the MJO teleconnection over the North Pacific and North America. LBM simulations suggest a reduction of the boreal winter MJO teleconnection over the North Pacific and an uncertain change over North America, with large spread over both regions that lends to weak confidence in the overall outlook. While quantitatively determining the relative importance of MJO versus mean state uncertainties in determining future teleconnections remains a challenge, the LBM simulations suggest that uncertainty in the mean state winds is a larger contributor to the uncertainty in future projections of the MJO teleconnection than the MJO. 
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  4. Abstract

    The intertropical convergence zone (ITCZ) exports energy and imports moisture. This has been understood for decades. By analyzing a set of uniform, nonrotating aquaplanet simulations, we show that energy export and moisture convergence are general characteristics of warm humid regions, and not just of the ITCZ. Using an analysis method based on the column relative humidity, we find that the absorption of longwave radiation by clouds supplies the energy that is exported from humid regions. The longwave absorption also induces a thermally direct circulation that lifts water vapor and converges moisture into regions that are already quite humid. An additional set of simulations shows that strong atmospheric energy convergence is absent when radiation is homogenized across the domain.

     
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  5. Abstract

    Studies in recent decades have demonstrated a robust relationship between tropical precipitation and column relative humidity (CRH). The present study identifies a similar relationship between CRH and the atmospheric cloud radiative effect (ACRE) calculated from satellite observations. Like precipitation, the ACRE begins to increase rapidly when CRH exceeds a critical value near 70%. We show that the ACRE can be estimated from CRH, similar to the way that CRH has been used to estimate precipitation. Our method reproduces the annual mean spatial structure of the ACRE in the tropics, and skillfully estimates the mean ACRE on monthly and daily time scales in six regions of the tropics. We propose that the exponential dependence of precipitation on CRH may be partially explained by cloud‐longwave feedbacks, which facilitate a shift from convective to stratiform conditions.

     
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  6. Abstract

    A 3‐D cloud‐resolving model has been used to investigate the domain size dependence of simulations of convective self‐aggregation (CSA) in radiative‐convective equilibrium. We investigate how large a domain is needed to allow multiple convective clusters and also how the properties equilibrated CSA depend on domain size. We used doubly periodic square domains of widths 768, 1,536, 3,072, and 6,144 km, over 350 simulated days. In the 768‐, 1,536‐, and 3,072‐km domains, the simulations produced circular convective clusters surrounded by broader regions of dry, subsiding air. In the 6,144‐km domain, the convection ultimately forms two semiconnected bands. As the domain size increases, equilibrated CSA moistens in two ways. First, as the circulation widens, this leads to stronger boundary layer winds and a more humid boundary layer. Second, the stronger inflow into the convective region boundary layer is associated with a warmer convective region boundary layer, which leads to intensified deep convection, more melting and freezing near the freezing level, enhanced midlevel stability, increased congestus activity, and detrainment of moist air into the dry region. In the larger domains, the deep convection and congestus slowly oscillate out of phase with each other with a time period of about 25 to 30 days. We hypothesize that other important domain size sensitivities, including a decrease in net moist static energy export from the convective region, are fundamentally linked to the increasing relationship between domain size and boundary layer wind speed. Our results suggest that the statistics of CSA converge only for domains wider than about 3,000 km.

     
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  7. Abstract

    The response of the Madden‐Julian oscillation (MJO) to ocean feedbacks is studied with coupled and uncoupled simulations of four general circulation models (GCMs). Monthly mean sea surface temperature (SST) from each coupled model is prescribed to its respective uncoupled simulation, to ensure identical SST mean‐state and low‐frequency variability between simulation pairs. Consistent with previous studies, coupling improves each model's ability to propagate MJO convection beyond the Maritime Continent. Analysis of the MJO moist static energy budget reveals that improved MJO eastward propagation in all four coupled models arises from enhanced meridional advection of column water vapor (CWV). Despite the identical mean‐state SST in each coupled and uncoupled simulation pair, coupling increases mean‐state CWV near the equator, sharpening equatorward moisture gradients and enhancing meridional moisture advection and MJO propagation. CWV composites during MJO and non‐MJO periods demonstrate that the MJO itself does not cause enhanced moisture gradients. Instead, analysis of low‐level subgrid‐scale moistening conditioned by rainfall rate (R) and SST anomaly reveals that coupling enhances low‐level convective moistening forR> 5 mm day−1; this enhancement is most prominent near the equator. The low‐level moistening process varies among the four models, which we interpret in terms of their ocean model configurations, cumulus parameterizations, and sensitivities of convection to column relative humidity.

     
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  8. Abstract

    Today’s global Earth system models began as simple regional models of tropospheric weather systems. Over the past century, the physical realism of the models has steadily increased, while the scope of the models has broadened to include the global troposphere and stratosphere, the ocean, the vegetated land surface, and terrestrial ice sheets. This chapter gives an approximately chronological account of the many and profound conceptual and technological advances that made today’s models possible. For brevity, we omit any discussion of the roles of chemistry and biogeochemistry, and terrestrial ice sheets.

     
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  9. Abstract

    Tropical South America plays a central role in global climate. Bowen ratio teleconnects to circulation and precipitation processes far afield, and the global CO2growth rate is strongly influenced by carbon cycle processes in South America. However, quantification of basin‐wide seasonality of flux partitioning between latent and sensible heat, the response to anomalies around climatic norms, and understanding of the processes and mechanisms that control the carbon cycle remains elusive. Here, we investigate simulated surface‐atmosphere interaction at a single site in Brazil, using models with different representations of precipitation and cloud processes, as well as differences in scale of coupling between the surface and atmosphere. We find that the model with parameterized clouds/precipitation has a tendency toward unrealistic perpetual light precipitation, while models with explicit treatment of clouds produce more intense and less frequent rain. Models that couple the surface to the atmosphere on the scale of kilometers, as opposed to tens or hundreds of kilometers, produce even more realistic distributions of rainfall. Rainfall intensity has direct consequences for the “fate of water,” or the pathway that a hydrometeor follows once it interacts with the surface. We find that the model with explicit treatment of cloud processes, coupled to the surface at small scales, is the most realistic when compared to observations. These results have implications for simulations of global climate, as the use of models with explicit (as opposed to parameterized) cloud representations becomes more widespread.

     
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