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Polar Amplification in CCSM4: Contributions from the Lapse Rate

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Polar Amplification in CCSM4: Contributions from the Lapse Rate 15 JUNE 2014 G R A V E R S E N E T A L . 4433 Polar Amplification in CCSM4: Contributions from the Lapse Rate and Surface Albedo Feedbacks RUNE G. GRAVERSEN Department of Meteorology, Stockholm University, Stockholm, Sweden PETER L. LANGEN Danish Meteorological Institute, Copenhagen, Denmark THORSTEN MAURITSEN Max Planck Institute, Hamburg, Germany (Manuscript received 10 September 2013, in final form 7 January 2014) ABSTRACT A vertically nonuniform warming of the troposphere yields a lapse rate feedback by altering the infrared irradiance to space relative to that of a vertically uniform tropospheric warming. The lapse rate feedback is negative at low latitudes, as a result of moist convective processes, and positive at high latitudes, due to stable stratification conditions that effectively trap warming near the surface. It is shown that this feedback pattern leads to polar amplification of the temperature response induced by a radiative forcing. The results are obtained by suppressing the lapse rate feedback in the Community Climate System Model, version 4 (CCSM4). The lapse rate feedback accounts for 15% of the Arctic amplification and 20% of the amplification in the Antarctic region. The fraction of the amplification that can be attributed to the surface albedo feedback, associated with melting of snow and ice, is 40% in the Arctic and 65% in Antarctica. It is further found that the surface albedo and lapse rate feedbacks interact considerably at high latitudes to the extent that they cannot be considered independent feedback mechanisms at the global scale. 1. Introduction (Serreze and Francis 2006; Graversen et al. 2008; Serreze and Barry 2011). The amplification may be due to the A forcing of the climate system due to a radiative forcing and feedback processes being stronger at high imbalance at the top of the atmosphere (TOA) results in latitudes. But it may also be a consequence of other a change of Earth’s surface temperatures. This temper- processes that redistribute energy within the climate ature response may activate feedback processes within system, and that are modified due to the forcing. For the climate system that either further enhance or dampen example, changes of the ocean and atmospheric energy the TOA imbalance, and hereby further increase or de- transport in response to a forcing induce warming at crease the temperature response. some latitudes and cooling at others (Graversen 2006). The temperature response will not be constant over Although such changes can have large local effects, they Earth’s surface but tends to be larger at higher than at constitute only weak global radiative feedbacks as they lower latitudes, which is referred to as polar temperature contribute little to the global-mean TOA imbalance. amplification (Manabe and Wetherald 1975; Hansen Many studies find that the meridional atmospheric en- et al. 2005; Holland and Bitz 2003). Observations reveal ergy transport does not contribute to polar amplification that the ongoing global warming is amplified in the Arctic but rather damps it (Hwang et al. 2011; Koenigk et al. 2013; Skific and Francis 2013), whereas other studies point in the opposite direction (Alexeev et al. 2005; Corresponding author address: Rune Grand Graversen, Department of Meteorology, Stockholm University, S - 106 91 Langen and Alexeev 2007; Graversen et al. 2008). In fact, Stockholm, Sweden. the sign of the energy transport is model dependent; E-mail: [email protected] models with weak Arctic warming tend to exhibit an DOI: 10.1175/JCLI-D-13-00551.1 Ó 2014 American Meteorological Society Unauthenticated | Downloaded 09/28/21 06:24 AM UTC 4434 JOURNAL OF CLIMATE VOLUME 27 increased transport, and vice versa (Pithan and Mauritsen In the PRP approach, the radiation code is run in a 2014). standalone mode with fields associated with a given Forcings associated with a change of the atmospheric feedback changed to a perturbation level and all other content of CO2 or a change of the solar constant do not fields kept to a control level. For instance, the albedo directly contribute to polar temperature amplification, feedback can be estimated by running the radiation code since the induced radiative imbalance at TOA is larger with surface albedo from a 23CO2 climate and all other at lower than at high latitudes (Hansen et al. 2005). In fields from a 13CO2 climate. The kernel method is sim- contrast, some feedback processes are believed to con- ilar to the PRP approach, but here a unit perturbation of tribute to the amplification—for instance, the surface the fields associated with a given feedback is considered albedo feedback (Arrhenius 1896) and the lapse rate in order to obtain what is referred to as radiative kernels. feedback (Manabe and Wetherald 1975), both of which These kernels can be multiplied by the actual perturba- are examined here. The water vapor feedback, which tion of the fields in question, normalized by the temper- strongly enhances the global temperature response ature change at the surface, in order to obtain the (Arrhenius 1896; Held and Soden 2000), seems most feedback parameter. An advantage of the kernel method active at low latitudes (Langen et al. 2012), whereas is that the radiative kernels are to some extent in- the feedback associated with cloud changes is more dependent of models (Soden et al.2008); when the ker- uncertain. nels are already determined, it is straightforward and Although the tropospheric temperature will change in computationally inexpensive to calculate the feedback response to a forcing, the change may not be constant parameters. with height. The difference in temperature change with The online method wherein feedbacks are suppressed height gives rise to the lapse rate feedback. Because of in climate models has been applied earlier in order to the Clausius–Clapeyron relationship between tempera- study the surface albedo feedback (Hall 2004; Graversen ture and water vapor saturation pressure, the saturated and Wang 2009; Mauritsen et al. 2013), the water vapor mixing ratio of water vapor increases more at lower feedback (Schneider et al. 1999; Hall and Manabe 1999; than at upper levels in the troposphere when Earth is Langen et al. 2012; Mauritsen et al. 2013), and the cloud warming. In regions where strong convection is present, feedback (Vavrus 2004; Langen et al. 2012; Mauritsen such as at tropical latitudes, this leads to an increase of et al. 2013). In addition to studying the effect of the latent heat release and warming of the upper tropo- feedbacks, Langen et al. (2012) and Mauritsen et al. sphere (Hansen et al. 1984), which results in enhanced (2013) also tested the notion that the total temperature radiation back to space, and in a more efficient cooling change can be split into parts that can be attributed to of Earth. This contributes to a negative lapse rate feed- each of the feedbacks. Both studies concluded that this back. At the high latitudes, stable stratification conditions was indeed the case for the feedbacks considered. in the lower troposphere result in a larger warming of the In the present study we lock the lapse rate and the near-surface air than of the upper troposphere (Manabe surface albedo feedback. As far as the authors know, it is and Wetherald 1975), which contributes to a regionally the first time the lapse rate feedback has been sup- positive lapse rate feedback. Hence the lapse rate feed- pressed in a climate model. back is believed to be negative at low and positive at high latitudes, which leads to Arctic amplification (Pithan and 2. Model description Mauritsen 2014). In the present study the lapse rate and the surface The Community Climate System Model, version 4 albedo feedback are suppressed by locking the lapse rate (CCSM4; Gent et al. 2011), from the National Center for and the surface albedo one by one and in combination in Atmospheric Research (NCAR) is used. The model system a state-of-the-art climate model. Hereby the full effect includes submodels for the atmosphere, land surface pro- on the climate system due to each of the feedbacks is cesses, sea ice, and ocean. The atmosphere model has a fi- investigated. The contribution from these feedbacks to nite-volume dynamical core with 26 vertical layers and ;28 the polar amplification and the interactions between the horizontal resolution. It is run both in a slab-ocean mode feedbacks are studied. Feedback parameters that are (SOM) and a data-ocean mode (DOM), where in the latter assumed to be unique for each feedback (Hansen et al. theseasurfacetemperaturesandtheseaicearefixedto 1984) are estimated directly, as an alternative approach a climatology. In the SOM version, the ocean part includes to the indirect offline methods, such as the more com- an isothermal mixed layer only, which has a fixed horizontal monly used partial radiative perturbation (PRP) method transport of energy. The horizontal ocean energy transports (Wetherald and Manabe 1988) and the radiative kernel in the slab-ocean model, often referred to as the q fluxes, are method (Soden et al. 2008). determined from the climatology of an equilibrium run Unauthenticated | Downloaded 09/28/21 06:24 AM UTC 15 JUNE 2014 G R A V E R S E N E T A L . 4435 FIG. 1. Globally averaged, annual-mean (a) SAT and (b) sea ice extent as a function of model year for all slab- ocean experiments, the ocean–atmosphere coupled control run, and the data ocean control run with fixed sea surface temperatures and sea ice (SST/SI). The slab-ocean experiments are shown both for a 13CO2 and 23CO2 climate for the free experiment, the SAT-based locked LR, the TrMT-based locked LR, the locked SA, and the experiment with both SA and SAT-based LR locked. including a full dynamical ocean model (Bitz et al.
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