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Taylor, PC, Cai M, Hu AX, Meehl J, Washington W, Zhang GJ.  2013.  A Decomposition of Feedback Contributions to Polar Warming Amplification. Journal of Climate. 26:7023-7043.   10.1175/jcli-d-12-00696.1   AbstractWebsite

Polar surface temperatures are expected to warm 2-3 times faster than the global-mean surface temperature: a phenomenon referred to as polar warming amplification. Therefore, understanding the individual process contributions to the polar warming is critical to understanding global climate sensitivity. The Coupled Feedback Response Analysis Method (CFRAM) is applied to decompose the annual- and zonal-mean vertical temperature response within a transient 1% yr(-1) CO2 increase simulation of the NCAR Community Climate System Model, version 4 (CCSM4), into individual radiative and nonradiative climate feedback process contributions. The total transient annual-mean polar warming amplification (amplification factor) at the time of CO2 doubling is +2.12 (2.3) and +0.94 K (1.6) in the Northern and Southern Hemisphere, respectively. Surface albedo feedback is the largest contributor to the annual-mean polar warming amplification accounting for +1.82 and +1.04 K in the Northern and Southern Hemisphere, respectively. Net cloud feedback is found to be the second largest contributor to polar warming amplification (about +0.38 K in both hemispheres) and is driven by the enhanced downward longwave radiation to the surface resulting from increases in low polar water cloud. The external forcing and atmospheric dynamic transport also contribute positively to polar warming amplification: +0.29 and +0.32 K, respectively. Water vapor feedback contributes negatively to polar warming amplification because its induced surface warming is stronger in low latitudes. Ocean heat transport storage and surface turbulent flux feedbacks also contribute negatively to polar warming amplification. Ocean heat transport and storage terms play an important role in reducing the warming over the Southern Ocean and Northern Atlantic Ocean.

Tian, BJ, Zhang GJ, Ramanathan V.  2001.  Heat balance in the Pacific warm pool atmosphere during TOGA COARE and CEPEX. Journal of Climate. 14:1881-1893.   10.1175/1520-0442(2001)014<1881:hbitpw>;2   AbstractWebsite

The atmosphere above the western equatorial Pacific warm pool (WP) is an important source for the dynamic and thermodynamic forcing of the atmospheric general circulation. This study uses a high-resolution reanalysis and several observational datasets including Global Precipitation Climatology Project precipitation, Tropical Ocean Global Atmosphere (TOGA) Tropical Atmosphere Ocean moored buoys, and Earth Radiation Budget Experiment, TOGA Coupled Ocean-Atmosphere Response Experiment (COARE), and Central Equatorial Pacific Experiment (CEPEX) radiation data to examine the details of the dynamical processes that lead to this net positive forcing. The period chosen is the period of two field experiments: TOGA COARE and CEPEX during December 1992-March 1993. The four months used in the study were sufficient to establish that the warm pool atmosphere (WPA) was close to a state of radiative-convective-dynamic equilibrium. The analysis suggests that the large-scale circulation imports about 200 W m(-2) of sensible heat and about 140 W m(-2) of latent energy into the WPA mainly through the low-level mass convergence and exports about 420 W m(-2) potential energy mainly through the upper-level mass divergence. Thus the net effect of the large-scale dynamics is to export about 80 W m(-2) energy out of the WPA and cool the WPA by about 0.8 K day(-1). The dynamic cooling in addition to the radiative cooling of about 0.4 K day(-1) or 40 W m(-2) leads to a net radiative-dynamic cooling of about 1.2 K day(-1) or 120 W m(-2), which should be balanced by convective heating of the same magnitude. The WPA radiative cooling is only about 0.4 K day(-1), which is considerably smaller than previously cited values in the Tropics. This difference is largely due to the cloud radiative forcing (CRF), about 70 W m(-2), associated with the deep convective cirrus clouds in the WPA, which compensates the larger clear sky radiative cooling. Thus moist convection heats the WPA, not only through the direct convective heating, that is, the vertical eddy sensible heat and latent energy transport, but also through the indirect convective heating, that is, the CRF of deep convective clouds. The CRF of the deep convective clouds has a dipole structure, in other words, strong heating of the atmosphere through convergence of longwave radiation and a comparable cooling of the surface through the reduction of shortwave radiation at the surface. As a result, the deep convective clouds enhance the required atmospheric heat transport and reduce the required oceanic heat transport significantly in the WP. A more detailed understanding of these convective processes is required to improve our understanding of the heat transport by the large-scale circulation in the Tropics.

Trammell, JH, Jiang X, Li LM, Kao A, Zhang GJ, Chang EKM, Yung Y.  2016.  Temporal and spatial variability of precipitation from observations and models*. Journal of Climate. 29:2543-2555.   10.1175/jcli-d-15-0325.1   AbstractWebsite

Principal component analysis (PCA) is utilized to explore the temporal and spatial variability of precipitation from GPCP and a CAM5 simulation from 1979 to 2010. In the tropical region, the interannual variability of tropical precipitation is characterized by two dominant modes (El Nino and El Nino Modoki). The first and second modes of tropical GPCP precipitation capture 31.9% and 15.6% of the total variance, respectively. The first mode has positive precipitation anomalies over the western Pacific and negative precipitation anomalies over the central and eastern Pacific. The second mode has positive precipitation anomalies over the central Pacific and negative precipitation anomalies over the western and eastern Pacific. Similar variations are seen in the first two modes of tropical precipitation from a CAM5 simulation, although the magnitudes are slightly weaker than in the observations. Over the Northern Hemisphere (NH) high latitudes, the first mode, capturing 8.3% of the total variance of NH GPCP precipitation, is related to the northern annular mode (NAM). During the positive phase of NAM, there are negative precipitation anomalies over the Arctic and positive precipitation anomalies over the midlatitudes. Over the Southern Hemisphere (SH) high latitudes, the first mode, capturing 13.2% of the total variance of SH GPCP precipitation, is related to the southern annular mode (SAM). During the positive phase of the SAM, there are negative precipitation anomalies over the Antarctic and positive precipitation anomalies over the midlatitudes. The CAM5 precipitation simulation demonstrates similar results to those of the observations. However, they do not capture both the high precipitation anomalies over the northern Pacific Ocean or the position of the positive precipitation anomalies in the SH.

Tsintikidis, D, Zhang GJ.  1998.  A numerical study on the coupling between sea surface temperature and surface evaporation. Journal of Geophysical Research-Atmospheres. 103:31763-31774.   10.1029/1998jd200027   AbstractWebsite

The feedback between sea surface temperature (SST) and surface evaporation is an important issue in the study of climate change. To understand this feedback and its interaction with surface wind in the tropical Pacific Ocean (30 degrees N - 30 degrees S), and in particular over the warm pool region, a dynamic tropical atmospheric circulation model is used. The model consists of a two-layer free troposphere and a well-mixed boundary layer. It involves active interactions between the boundary layer flow, forced by an SST gradient, and the free atmospheric flow, forced by SST. Various SST fields (representing climatology, El Nino, and La Nina conditions) are used to drive the model. It is found that the binned averages of evaporation and wind speed increase with SST for up to about 300-301 K. From that point on they decrease with SST. In addition, negative SST anomalies correspond to excess latent heat flux and wind speed. These results are in agreement with relevant observations. To understand the thermodynamic versus dynamic effects of SST on surface evaporation, in one of the experiments we impose a sudden positive SST perturbation on the climatological SST field during the model integration. It is shown that while surface evaporation is initially enhanced in response to the SST change, as the atmospheric circulation gradually "feels" the SST perturbation, its dynamic effect through the circulation change becomes more apparent over the SST perturbation region. Overall, the results of our study show that in the low SST regime the behavior of evaporation is dictated by thermodynamics, whereas in the high SST regime it is dictated by atmospheric dynamic considerations.