Clouds and the atmospheric circulation response to warming

We study the effect of clouds on the atmospheric circulation response to CO2 quadrupling in an aquaplanet model with a slab-ocean lower boundary. The cloud effect is isolated by locking the clouds to either the control or 4xCO2 state in the shortwave (SW) or longwave (LW) radiation schemes. In our model, cloud-radiative changes explain more than half of the total poleward expansion of the Hadley cells, midlatitude jets, and storm tracks under CO2 quadrupling, even though they cause only one-fourth of the total global-mean surface warming. The effect of clouds on circulation results mainly from the SW cloud-radiative changes, which strongly enhance the Equator-to-pole temperature gradient at all levels in the troposphere, favoring stronger and poleward-shifted midlatitude eddies. By contrast, quadrupling CO2 while holding the clouds fixed causes strong polar amplification and weakened midlatitude baroclinicity at lower levels, yielding only a small poleward expansion of the circulation. Our results show that (a) the atmospheric circulation responds sensitively to cloud-driven changes in meridional and vertical temperature distribution, and (b) the spatial structure of cloud feedbacks likely plays a dominant role in the circulation response to greenhouse gas forcing. While the magnitude and spatial structure of the cloud feedback are expected to be highly model-dependent, an analysis of 4xCO2 simulations of CMIP5 models shows that the SW cloud feedback likely forces a poleward expansion of the tropospheric circulation in most climate models

The role of the stratospheric polar vortex for the austral jet response to greenhouse gas forcing

Future shifts of the austral midlatitude jet are subject to large uncertainties in climate model projections. Here we show that, in addition to other previously identified sources of inter-model uncertainty, changes in the timing of the stratospheric polar vortex breakdown modulate the austral jet response to greenhouse gas forcing during summertime (December-February). The relationship is such that a larger delay in vortex breakdown favors a more poleward jet shift, with an estimated 0.7-0.8-degree increase in jet shift per 10 days delay in vortex breakdown. The causality of the link between the timing of the vortex breakdown and the tropospheric jet response is demonstrated through climate modeling experiments with imposed changes in the seasonality of the stratospheric polar vortex. The vortex response is estimated to account for about 30% of the inter-model variance in the shift of the summertime austral jet, and about 45% of the mean jet shift

Controls of the transient climate response to emissions by physical feedbacks, heat uptake and carbon cycling

The surface warming response to carbon emissions is diagnosed using a suite of Earth system models, 9 CMIP6 and 7 CMIP5, following an annual 1% rise in atmospheric CO2 over 140 years. This surface warming response defines a climate metric, the Transient Climate Response to cumulative carbon Emissions (TCRE), which is important in estimating how much carbon may be emitted to avoid dangerous climate. The processes controlling these intermodel differences in the TCRE are revealed by defining the TCRE in terms of a product of three dependences: the surface warming dependence on radiative forcing (including the effects of physical climate feedbacks and planetary heat uptake), the radiative forcing dependence on changes in atmospheric carbon and the airborne fraction. Intermodel differences in the TCRE are mainly controlled by the thermal response involving the surface warming dependence on radiative forcing, which arise through large differences in physical climate feedbacks that are only partly compensated by smaller differences in ocean heat uptake. The other contributions to the TCRE from the radiative forcing and carbon responses are of comparable importance to the contribution from the thermal response on timescales of 50 years and longer for our subset of CMIP5 models and 100 years and longer for our subset of CMIP6 models. Hence, providing tighter constraints on how much carbon may be emitted based on the TCRE requires providing tighter bounds for estimates of the physical climate feedbacks, particularly from clouds, as well as to a lesser extent for the other contributions from the rate of ocean heat uptake, and the terrestrial and ocean cycling of carbon

Contributions of climate feedbacks to changes in atmospheric circulation

The projected response of the atmospheric circulation to the radiative changes induced by CO2 forcing and climate feedbacks is currently uncertain. In this modelling study, the impact of CO2-induced climate feedbacks on changes in jet latitude and speed is assessed by imposing surface albedo, cloud, and water vapor feedbacks as if they were forcings in two climate models, CAM4 and ECHAM6. The jet response to radiative feedbacks can be broadly interpreted through changes in midlatitude baroclinicity. Clouds enhance baroclinicity, favoring a strengthened, poleward shifted jet; this is mitigated by surface albedo changes which have the opposite effect on baroclinicity and the jet, while water vapor has opposing effects on upper- and lower-level baroclinicity with little net impact on the jet. Large differences between the CAM4 and ECHAM6 responses illustrate how model uncertainty in radiative feedbacks causes a large spread in the baroclinicity response to CO2 forcing. Across the CMIP5 models, differences in shortwave feedbacks by clouds and albedo are a dominant contribution to this spread. Forcing CAM4 with shortwave cloud and albedo feedbacks from a representative set of CMIP5 models yields a wide range of jet responses that strongly correlate with the meridional gradient of the anomalous shortwave heating and the associated baroclinicity response. Differences in shortwave feedbacks statistically explain about 50% of the inter-model spread in CMIP5 jet shifts for our set of models, demonstrating the importance of constraining radiative feedbacks for accurate projections of circulation changes

Time-evolving sea surface warming patterns modulate the climate change response of subtropical precipitation over land

Greenhouse gas (GHG) emissions affect precipitation worldwide. The response is commonly described by two timescales linked to different processes: a rapid adjustment to radiative forcing, followed by a slower response to surface warming. However, additional timescales exist in the surface warming response, tied to the time evolution of the sea surface temperature (SST) response. Here we show that in climate model projections the rapid adjustment and surface mean warming are insufficient to explain the time evolution of the hydro-climate response in three key Mediterranean-like areas, namely California, Chile and the Mediterranean. The time evolution of those responses critically depends on distinct shifts in the regional atmospheric circulation associated with the existence of distinct fast and slow SST warming patterns. As a result, Mediterranean and Chilean drying are in quasi-equilibrium with GHG concentrations, meaning that the drying will not continue after GHG concentrations are stabilised, whereas California wetting will largely emerge only after GHG concentrations are stabilised. The rapid adjustment contributes to a reduction in precipitation but has a limited impact on the balance between precipitation and evaporation. In these Mediterranean-like regions, future hydro-climate related impacts will be substantially modulated by the time evolution of the pattern of SST warming that is realised in the real world

Mechanisms of the negative shortwave cloud feedback in mid to high latitudes

Increases in cloud optical depth and liquid water path (LWP) are robust features of global warming model simulations in high latitudes, yielding a negative shortwave cloud feedback, but the mechanisms are still uncertain. We assess the importance of microphysical processes for the negative optical depth feedback by perturbing temperature in the microphysics schemes of two aquaplanet models, both of which have separate prognostic equations for liquid water and ice. We find that most of the LWP increase with warming is caused by a suppression of ice microphysical processes in mixed-phase clouds, resulting in reduced conversion efficiencies of liquid water to ice and precipitation. Perturbing the temperature-dependent phase partitioning of convective condensate also yields a small LWP increase. Together, the perturbations in large-scale microphysics and convective condensate partitioning explain more than two-thirds of the LWP response relative to a reference case with increased SSTs, and capture all of the vertical structure of the liquid water response. In support of these findings, we show the existence of a very robust positive relationship between monthly-mean LWP and temperature in CMIP5 models and observations in mixed-phase cloud regions only. In models, the historical LWP sensitivity to temperature is a good predictor of the forced global warming response poleward of about 45°, although models appear to overestimate the LWP response to warming compared to observations. We conclude that in climate models, the suppression of ice-phase microphysical processes that deplete cloud liquid water is a key driver of the LWP increase with warming and of the associated negative shortwave cloud feedback

Observational evidence for a negative shortwave cloud feedback in middle to high latitudes

Exploiting the observed robust relationships between temperature and optical depth in extratropical clouds, we calculate the shortwave cloud feedback from historical data, by regressing observed and modeled cloud property histograms onto local temperature in middle to high southern latitudes. In this region, all CMIP5 models and observational data sets predict a negative cloud feedback, mainly driven by optical thickening. Between 45° and 60°S, the mean observed shortwave feedback (−0.91 ± 0.82 W m−2 K−1, relative to local rather than global mean warming) is very close to the multimodel mean feedback in RCP8.5 (−0.98 W m−2 K−1), despite differences in the meridional structure. In models, historical temperature-cloud property relationships reliably predict the forced RCP8.5 response. Because simple theory predicts this optical thickening with warming, and cloud amount changes are relatively small, we conclude that the shortwave cloud feedback is very likely negative in the real world at middle to high latitudes

Classifying the tropospheric precursor patterns of sudden stratospheric warmings

Classifying the tropospheric precursor patterns of sudden stratospheric warmings (SSWs) may provide insight into the different physical mechanisms of SSWs. Based on 37 major SSWs during the 1958–2014 winters in the ERA reanalysis data sets, the self-organizing maps method is used to classify the tropospheric precursor patterns of SSWs. The cluster analysis indicates that one of the precursor patterns appears as a mixed pattern consisting of the negative-signed Western Hemisphere circulation pattern and the positive phase of the Pacific-North America pattern. The mixed pattern exhibits higher statistical significance as a precursor pattern of SSWs than other previously identified precursors such as the subpolar North Pacific low, Atlantic blocking, and the western Pacific pattern. Other clusters confirm northern European blocking and Gulf of Alaska blocking as precursors of SSWs. Linear interference with the climatological planetary waves provides a simple interpretation for the precursors. The relationship between the classified precursor patterns of SSWs and ENSO phases as well as the types of SSWs is discussed

The role of synoptic waves in the formation and maintenance of the Western Hemisphere circulation pattern

Previous studies have demonstrated that the NAO, the leading mode of atmospheric low-frequency variability over the North Atlantic, could be linked to Northeast Pacific climate variability via the downstream propagation of synoptic waves. In those studies, the NAO and the Northeast Pacific climate variability are considered as two separate modes that explain the variance over the North Atlantic sector and the East Pacific–North America sector, respectively. A newly identified low-frequency atmospheric regime — the Western Hemisphere (WH) circulation pattern—provides a unique example of a mode of variability that accounts for variance over the whole North Atlantic–North America– North Pacific sector. The role of synoptic waves in the formation and maintenance of the WH pattern is investigated using the ERA reanalysis datasets. Persistent WH events are characterized by the propagation of quasi-stationary Rossby waves across the North Pacific–North America–North Atlantic regions and by associated storm track anomalies. The eddy-induced low-frequency height anomalies maintain the anomalous low-frequency ridge over the Gulf of Alaska, which induces more equatorward propagation of synoptic waves on its downstream side. The eddy forcing favors the strengthening of the mid-latitude jet and the deepening of the mid-high latitude trough over the North Atlantic, whereas the deepening of the trough over eastern North America mostly arises from the quasi-stationary waves propagating from the North Pacific. A case study for the 2013/14 winter is examined to illustrate the downstream development of synoptic waves. The roles of synoptic waves in the formation and maintenance of the WH pattern and in linking the Northeast Pacific ridge anomaly with the NAO are discussed