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The water vapour feedback probably makes the largest contribution to climate sensitivity, and the second-largest contribution to its uncertainty, in the sense of disagreement between General Circulation Models (GCMs, the most physically detailed models of climate we have). Yet there has been no quantification of it which allows these differences to be attributed physically with the aim of constraining the true value. This paper develops a new breakdown of the non-cloud LW (longwave) response to climate change, which avoids the problems of the conventional breakdown, and applies it to a set of 4 GCMs. The basic physical differences are that temperature is used as the vertical coordinate, and relative humidity as the humidity variable. In this framework the different GCMs' feedbacks look more alike, consistent with our understanding that their water vapour responses are physically very similar. Also, in the global mean all the feedback components have the same sign, allowing us to conveniently attribute the overall response fractionally (e.g. about 60% from the 'partly-Simpsonian' component). The systematic cancellation between different feedback components in the conventional breakdown is lost, so now a difference in a feedback component actually contributes to a difference in climate sensitivity, and the differences between these GCMs in the non-cloud LW part of this can be traced to differences in formulation, mean climate and climate change response. Physical effects such as those due to variations in the formulation of LW radiative transfer become visible. Differences in the distribution of warming no longer dominate comparison of GCMs. The largest component depends locally only on the GCM's mean climate, so it can in principle be calculated for the real world and validated. However, components dependent on the climate change response probably account for most of the variation between GCMs. The effect of simply changing the humidity variable in the conventional breakdown is also examined. It gives some of this improvement-the loss of the cancellations that leave the conventional breakdown of no use to understand differences between GCMs' climate sensitivities-but not the link to mean climate. [ABSTRACT FROM AUTHOR]

Projected changes in near-surface relative humidity (RH) remain highly model-dependent over land and may have been underestimated by the former generation global climate models. Here the focus in on the recent CNRM-CM6-1 model, which shows an enhanced land surface drying in response to quadrupled atmospheric CO2 compared to its CNRM-CM5 predecessor. Atmosphere-only experiments with prescribed sea surface temperature (SST) are used to decompose the simulated RH changes into separate responses to uniform SST warming, pattern of SST anomalies, changes in sea-ice concentration, as well as direct radiative and physiological CO2 effects. Results show that the strong drying simulated by CNRM-CM6-1 is due to both fast CO2 effects and a SST-mediated response. The enhanced drying compared to CNRM-CM5 is partly due to the introduction of the physiological CO2 effect that was not accounted for in CNRM-CM5. The global ocean warming also contributes to the RH decline over land, in reasonable agreement with the moisture advection mechanism proposed by earlier studies which however does not fully capture the contrasted RH response between the two CNRM models. The SST anomaly pattern is a significant driver of changes in RH humidity at the regional scale, which are partly explained by changes in atmospheric circulation. The improved land surface model may also contribute to a stronger soil moisture feedback in CNRM-CM6-1, which can amplify the surface aridity induced by global warming and, thereby, lead to a non-linear response of RH. [ABSTRACT FROM AUTHOR]

Climate models simulate an increase in global precipitation at a rate of approximately 1–3% per Kelvin of global surface warming. This change is often interpreted through the lens of the atmospheric energy budget, in which the increase in global precipitation is mostly offset by an increase in net radiative cooling. Other studies have provided different interpretations from the perspective of the surface, where evaporation represents the turbulent transfer of latent heat to the atmosphere. Expanding on this surface perspective, here we derive a version of the Penman–Monteith equation that allows the change in ocean evaporation to be partitioned into a thermodynamic response to surface warming, and additional diagnostic contributions from changes in surface radiation, ocean heat uptake, and boundary-layer dynamics/relative humidity. In this framework, temperature is found to be the primary control on the rate of increase in global precipitation within model simulations of greenhouse gas warming, while the contributions from changes in surface radiation and ocean heat uptake are found to be secondary. The temperature contribution also dominates the spatial pattern of global evaporation change, leading to the largest fractional increases at high latitudes. In the surface energy budget, the thermodynamic increase in evaporation comes at the expense of the sensible heat flux, while radiative changes cause the sensible heat flux to increase. These tendencies on the sensible heat flux partly offset each other, resulting in a relatively small change in the global mean, and contributing to an impression that global precipitation is radiatively constrained. [ABSTRACT FROM AUTHOR]

It has been pointed out that climatological-mean precipitation-evaporation difference ( P- E) should increase under global warming mainly through the increasing saturation level of moisture. This study focuses on evaporation changes under global warming and their dependency on the direct warming effect, on the basis of future projections from the Coupled Model Intercomparison Project Phase 5 (CMIP5). Over most of the tropical, subtropical and midlatitude regions, the direct contribution from surface temperature increase is found to dominate the projected increase in evaporation. This contribution is nevertheless offset partially, especially over the oceans, by contributions from weakening surface winds and increasing near-surface relative humidity. Greater warming of surface air than of the sea surface also acts to reduce surface evaporation, by reducing both the exchange coefficient and humidity contrast at the surface. Though generally of secondary importance, this contribution is the dominant factor over the subpolar oceans. Over the polar oceans, the effect of sea-ice retreat dominantly contributes to the evaporation increase in winter, whereas the reduced exchange coefficient and surface humidity contrast coupled with the sea-ice retreat account for most of the response during summertime. Over the continents, changes in the surface exchange coefficient, reflecting changes in soil moisture and vegetation among other factors, are important to modulate the direct effects of the warming and the generally reduced surface air relative humidity. [ABSTRACT FROM AUTHOR]

We analyzed the evolution of surface relative humidity (RH) and specific humidity (q) in Spain, based on complete records available from the State Meteorological Agency of Spain. The surface RH records used span the period 1920-2011, but because of spatial and temporal constraints in the dataset we used a subset of the data, covering the period 1961-2011. The subset contained 50 monthly series of RH, which were created through a process of quality control, reconstruction and homogenization. The data shows that there was a large decrease in RH over mainland Spain from 1961 to 2011, which was greatest in spring and summer. In contrast, there was no overall change in the specific humidity in this period, except in spring, when an increase was observed. The decrease in RH affected the entire country, but the changes in specific humidity were less homogeneous. For specific humidity there was a general increase in the northern and eastern parts of Spain, whereas negative trends dominated in the central and southern areas, mainly during the summer months. The results suggest that an increase in the water holding capacity of the atmosphere as a consequence of warming during recent decades has not been accompanied by an increase in the surface water vapor content, probably because the supply of water vapor from the main terrestrial and oceanic areas has been constrained. We discuss the implications of these findings for evapotranspiration processes, precipitation and water management in Spain. [ABSTRACT FROM AUTHOR]

A central paradox of the global monsoon record involves reported decreases in rainfall over land during an era in which the global hydrologic cycle is both expected and observed to intensify. It is within this context that this work develops a physical basis for both interpreting the observed record and anticipating changes in the monsoons in a warming climate while bolstering the concept of the global monsoon in the context of shared feedbacks. The global-land monsoon record across multiple reanalyses is first assessed. Trends that in other studies have been taken as real are shown to likely be spurious as a result of changes in the assimilated data streams both prior to and during the satellite era. Nonetheless, based on satellite estimates, robust increases in monsoon rainfall over ocean do exist and a physical basis for this land-ocean contrast remains lacking. To address the contrast's causes, simulated trends are therefore assessed. While projections of total rainfall are inconsistent across models, the robust land-ocean contrast identified in observations is confirmed. A feedback mechanism is proposed rooted in the facts that land areas warm disproportionately relative to ocean, and onshore flow is the chief source of monsoonal moisture. Reductions in lower tropospheric relative humidity over land domains are therefore inevitable and these have direct consequences for the monsoonal convective environment including an increase in the lifting condensation level and a shift in the distribution of convection generally towards less frequent and potentially more intense events. The mechanism is interpreted as an important modulating influence on the 'rich-get-richer' mechanism. Caveats for regional monsoons exist and are discussed. [ABSTRACT FROM AUTHOR]