Atmospheric feedback explains disparate climate response to regional Arctic sea-ice loss

Arctic sea-ice loss is a consequence of anthropogenic global warming and can itself be a driver of climate change in the Arctic and at lower latitudes, with sea-ice minima likely favoring extreme events over Europe and North America. Yet the role that the sea-ice plays in ongoing climate change remains uncertain, partly due to a limited understanding of whether and how the exact geographical distribution of sea-ice loss impacts climate. Here we demonstrate that the climate response to sea-ice loss can vary widely depending on the pattern of sea-ice change, and show that this is due to the presence of an atmospheric feedback mechanism that amplifies the local and remote signals when broader scale sea-ice loss occurs. Our study thus highlights the need to better constrain the spatial pattern of future sea-ice when assessing its impacts on the climate in the Arctic and beyond.

Impact of Large-Scale Afforestation on Surface Temperature: A Case Study in the Kubuqi Desert, Inner Mongolia Based on the WRF Model

Afforestation activities in the Kubuqi Desert, Inner Mongolia, China, have substantially increased tree and shrub coverage in this region. In this study, the response of the surface temperature to afforestation is simulated with the Weather Research and Forecasting model. The surface temperature changes are decomposed into contributions from the intrinsic surface biophysical effect and atmospheric feedback, using the theory of intrinsic biophysical mechanism. The effect of afforestation on the surface temperature is 1.34 K, −0.48 K, 2.09 K and 0.22 K for the summer daytime, the summer nighttime, the winter daytime and the winter nighttime, respectively, for the grid cells that have experienced conversion from bare soil to shrub. The corresponding domain mean values are 0.15 K, −0.2 K, 0.67 K, and 0.06 K. The seasonal variation of surface temperature change is mainly caused by changes in roughness and Bowen ratio. In the daytime, the surface temperature changes are dominated by the biophysical effect, with albedo change being the main biophysical factor. In the nighttime, the biophysical effect (mainly associated with roughness change) and the atmospheric feedback (mainly associated with change in the background air temperature) contribute similar amounts to the surface temperature changes. We conclude that the atmospheric feedback can amplify the influence of the surface biophysical effect, especially in the nighttime.

Idealized Study of Ocean Impacts on Tropical Cyclone Intensity Forecasts

Idealized coupled tropical cyclone (TC) simulations are conducted to isolate ocean impacts on intensity forecasts. A one-dimensional ocean model is embedded into the Hurricane Weather Research and Forecasting (HWRF) mesoscale atmospheric forecast model. By inserting an initial vortex into a horizontally uniform atmosphere above a horizontally uniform ocean, the SST cooling rate becomes the dominant large-scale process controlling intensity evolution. Westward storm translation is introduced by bodily advecting ocean fields toward the east. The ocean model produces a realistic cold wake structure allowing the sensitivity of quasi-equilibrium intensity to storm (translation speed, size) and ocean (heat potential) parameters to be quantified. The atmosphere provides feedback through adjustments in 10-m temperature and humidity that reduce SST cooling impact on quasi-equilibrium intensity by up to 40%. When storms encounter an oceanic region with different heat potential, enthalpy flux adjustment is governed primarily by changes in air–sea temperature and humidity differences that respond within 2–4 h in the inner-core region, and secondarily by wind speed changes occurring over a time interval up to 18 h after the transition. Atmospheric feedback always acts to limit the change in enthalpy flux and intensity through adjustments in 10-m temperature and humidity. Intensity change is asymmetric, with a substantially smaller increase for storms encountering larger heat potential compared to the decrease for storms encountering smaller potential. The smaller increase results initially from the smaller wind speed present at the transition time plus stronger limiting atmospheric feedback. The smaller wind speed increase resulting from these two factors further enhances the asymmetry.

Comparison of the Stability of the Atlantic Thermohaline Circulation in Two Coupled Atmosphere–Ocean General Circulation Models

Two coupled atmosphere–ocean general circulation models developed at GFDL show differing stability properties of the Atlantic thermohaline circulation (THC) in the Coupled Model Intercomparison Project/Paleoclimate Modeling Intercomparison Project (CMIP/PMIP) coordinated “water-hosing” experiment. In contrast to the R30 model in which the “off” state of the THC is stable, it is unstable in the CM2.1. This discrepancy has also been found among other climate models. Here a comprehensive analysis is performed to investigate the causes for the differing behaviors of the THC. In agreement with previous work, it is found that the different stability of the THC is closely related to the simulation of a reversed thermohaline circulation (RTHC) and the atmospheric feedback. After the shutdown of the THC, the RTHC is well developed and stable in R30. It transports freshwater into the subtropical North Atlantic, preventing the recovery of the salinity and stabilizing the off mode of the THC. The flux adjustment is a large term in the water budget of the Atlantic Ocean. In contrast, the RTHC is weak and unstable in CM2.1. The atmospheric feedback associated with the southward shift of the Atlantic ITCZ is much more significant. The oceanic freshwater convergence into the subtropical North Atlantic cannot completely compensate for the evaporation, leading to the recovery of the THC in CM2.1. The rapid salinity recovery in the subtropical North Atlantic excites large-scale baroclinic eddies, which propagate northward into the Nordic seas and Irminger Sea. As the large-scale eddies reach the high latitudes of the North Atlantic, the oceanic deep convection restarts. The differences in the southward propagation of the salinity and temperature anomalies from the hosing perturbation region in R30 and CM2.1, and associated different development of a reversed meridional density gradient in the upper South Atlantic, are the cause of the differences in the behavior of the RTHC. The present study sheds light on important physical and dynamical processes in simulating the dynamical behavior of the THC.

What Maintains the SST Front North of the Eastern Pacific Equatorial Cold Tongue?*

A coupled ocean–atmosphere regional model suggests a mechanism for formation of a sharp sea surface temperature (SST) front north of the equator in the eastern Pacific Ocean in boreal summer and fall. Meridional convergence of Ekman transport at 5°N is forced by eastward turning of the southeasterly cross-equatorial wind, but the SST front forms considerably south of the maximum Ekman convergence. Geostrophic equatorward flow at 3°N in the lower half of the isothermally mixed layer enhances mixed layer convergence. Cold water is upwelled on or south of the equator and is advected poleward by mean mixed layer flow and by eddies. The mixed layer current convergence in the north confines the cold advection, so the SST front stays close to the equator. Warm advection from the north and cold advection from the south strengthen the front. In the Southern Hemisphere, a continuous southwestward current advects cold water far from the upwelling core. The cold tongue is warmed by the net surface flux, which is dominated by solar radiation. Evaporation and net surface cooling are at a maximum just north of the SST front where relatively cool dry air is advected northward over warm SST. The surface heat flux is decomposed into a response to SST alone, and an atmospheric feedback. The atmospheric feedback enhances cooling on the north side of the front by 178 W m−2, about half of which is due to enhanced evaporation from cold dry advection, while the other half is due to cloud radiative forcing.

On the Radiative and Dynamical Feedbacks over the Equatorial Pacific Cold Tongue

An analysis of the climatic feedbacks in the NCAR Community Climate Model, version 3 (CCM3) over the equatorial Pacific cold tongue is presented. Using interannual signals in the underlying SST, the radiative and dynamical feedbacks have been calculated using both observations and outputs from the NCAR CCM3. The results show that the positive feedback from the greenhouse effect of water vapor in the model largely agrees with that from observations. The dynamical feedback from the atmospheric transport in the model is also comparable to that from observations. However, the negative feedback from the solar forcing of clouds in the model is significantly weaker than the observed, while the positive feedback from the greenhouse effect of clouds is significantly larger. Consequently, the net atmospheric feedback in the CCM3 over the equatorial cold tongue region is strongly positive (5.1 W m−2 K−1), while the net atmospheric feedback in the real atmosphere is strongly negative (−6.4 W m−2 K−1). A further analysis with the aid of the International Satellite Cloud Climatology Project (ISCCP) data suggests that cloud cover response to changes in the SST may be a significant error source for the cloud feedbacks. It is also noted that the surface heating over the cold tongue in CCM3 is considerably weaker than in observations. In light of results from a linear feedback system, as well as those from a more sophisticated coupled model, it is suggested that the discrepancy in the net atmospheric feedback may have contributed significantly to the cold bias in the equatorial Pacific in the NCAR Climate System Model (CSM).