The effect of anthropogenic aerosols on clouds is one of the most important and least understood aspects of human-induced climate change. Small changes in the amount of cloud coverage can produce a ...climate forcing equivalent in magnitude and opposite in sign to that caused by anthropogenic greenhouse gases, and changes in cloud height can shift the effect of clouds from cooling to warming. Focusing on the Amazon, we show a smooth transition between two opposing effects of aerosols on clouds: the microphysical and the radiative. We show how a feedback between the optical properties of aerosols and the cloud fraction can modify the aerosol forcing, changing the total radiative energy and redistributing it over the atmospheric column.
Midlatitude storm tracks are dominant features of the atmospheric circulation, transferring most of the heat, momentum and moisture in the extratropics. Despite their importance, the mechanisms ...controlling their seasonal variability are not fully understood. Over the North Pacific, transient eddy activity exhibits a distinct minimum during midwinter while reaching its maximum in late-fall and early-spring. This behavior occurs despite the enhanced jet and baroclinicity during midwinter, and is known as the Pacific midwinter minimum paradox. To explore the mechanisms that control the seasonal cycle of midlatitude storm tracks, we perform both observational analysis and numerical modeling. For the observational analysis, the seasonal cycle of storm tracks over the Pacific and Atlantic basins is explored using a 57-years reanalysis dataset. Particularly, the relationship between storm track intensity and the strength of the jet over the Atlantic is examined in years of a strong jet. It is found that in the Atlantic, similar to the well-known Pacific case, a midwinter minimum exists and is more pronounced when the jet is stronger. We find that in those years of a midwinter minimum the jet is more equatorward, resulting in less eddies, while for a more poleward eddy-driven jet, eddies tend to intensify. For the modeling study, we use an idealized moist atmospheric general circulation model (GCM) in which we implement an idealized seasonal cycle by varying the radiative parameters of the model. This allows to examine a wide range of climates, simulated by varying the top-ofatmosphere insolation. The model was used with several levels of complexity. First, the relationship between the strength of the subtropical jet (and the Hadley circulation) and storm track intensity in the zonally-symmetric model, without topography or land-sea contrasts, is investigated. We find that strengthening of the subtropical jet, which in the idealized GCM we explore by increasing the ocean heat flux, leads to baroclinic wave growth in the vicinity of the jet and to a weakening of nearsurface baroclinicity in midlatitudes. In the next level of complexity, the response of the storm track seasonal cycle to changes in the strength of the subtropical jet in a zonally-asymmetric configuration is examined. We find that a midwinter minimum of storm track intensity occurs in the model when a Gaussian-shaped mountain is implemented in the idealized model. The minimum becomes more pronounced as the strength of the subtropical jet is increased. By analyzing the energy budget, we find that reduced storm track intensity in midwinter can be explained by increased barotropic energy conversion in the subtropics, leading to loss of eddy energy, together with an equatorward shift of the baroclinic and the barotropic conversion peaks, resulting in an additional eddy energy loss in midlatitudes. The midwinter minimum in the model occurs when the Gaussian mountain resembles the Tibetan Plateau (in terms of height, width and latitudinal position) and thus generating a “Pacific”- like storm track. When an “Atlantic”-like configuration is applied, the midwinter minimum is less pronounced, indicating the role of stationary features in controlling this phenomenon
