The ongoing and projected warming in the northern high latitudes (NHL; poleward of 601N) may lead to dramatic changes in the terrestrial carbon cycle. On the one hand, warming and increasing atmospheric CO 2 concentration stimulate vegetation productivity, taking up CO 2 . On the other hand, warming accelerates the decomposition of soil organic matter (SOM), releasing carbon into the atmosphere. Here, the NHL terrestrial carbon storage is investigated based on 10 models from the Coupled Carbon Cycle Climate Model Intercomparison Project. Our analysis suggests that the NHL will be a carbon sink of 0.3 AE 0.3 Pg C yr À1 by 2100. The cumulative land organic carbon storage is modeled to increase by 38 AE 20 Pg C over 1901 levels, of which 17 AE 8 Pg C comes from vegetation (43%) and 21 AE 16 Pg C from the soil (8%). Both CO 2 fertilization and warming enhance vegetation growth in the NHL. Although the intense warming there enhances SOM decomposition, soil organic carbon (SOC) storage continues to increase in the 21st century. This is because higher vegetation productivity leads to more turnover (litterfall) into the soil, a process that has received relatively little attention. However, the projected growth rate of SOC begins to level off after 2060 when SOM decomposition accelerates at high temperature and then catches up with the increasing input from vegetation turnover. Such competing mechanisms may lead to a switch of the NHL SOC pool from a sink to a source after 2100 under more intense warming, but large uncertainty exists due to our incomplete understanding of processes such as the strength of the CO 2 fertilization effect, permafrost, and the role of soil moisture. Unlike the CO 2 fertilization effect that enhances vegetation productivity across the world, global warming increases the productivity at high latitudes but tends to reduce it in the tropics and mid-latitudes. These effects are further enhanced as a result of positive carbon cycle-climate feedbacks due to additional CO 2 and warming.

Major volcanic events with a high loading of stratospheric aerosol have long been known to cause cooling, but their impact on precipitation has only recently been emphasized, especially as an analog for potential geoengineering of climate. Here, the authors use a coupled atmosphere-ocean-land-vegetation model in conjunction with observations to study the effects of volcanic aerosol on the tropical and subtropical precipitation. The small internal variability in the model enables a clear identification of the volcanic impact, which is broadly supported by observations, especially for the large Pinatubo event. Area averaged rainfall over land between 408S and 408N decreases by about 0.15 mm day 21 , 4-5 months after the height of a major volcanic aerosol loading, such as from Pinatubo, with regional changes as large as 0.6 mm day 21 or higher, such as over the Amazon and equatorial Africa. These anomalies migrate seasonally, following the movement of monsoon rainfall. This is because the low heat capacity of the land leads to rapid response of rainfall there, owing to the energy imbalance caused by volcanic aerosol cooling. In contrast, precipitation response over the ocean is much slower and considerably damped because of the much larger heat capacity. In addition, the difference in heat capacities over land and over ocean leads to an anomalous land-sea thermal contrast, which could further contribute to the reduction of rainfall over land. The volcano-induced drought may have significant impact on the ecosystem, agriculture, and the carbon cycle, especially in the monsoon regions.

This study focuses on the assessment of the spatiotemporal structure of ENSO variability and its winter climate teleconnections to North America in the Intergovernmental Panel on Climate Change’s (IPCC) Fourth Assessment Report (AR4) simulations of twentieth-century climate. The 1950–99 period simulations of six IPCC models are analyzed in an effort to benchmark models in the simulation of this leading mode of interannual variability: the Geophysical Fluid Dynamics Laboratory (GFDL) Coupled Model version 2.1 (CM2.1), the coupled ocean–atmosphere model of the Goddard Institute for Space Studies (GISS-EH), the NCAR Community Climate System Model version 3 (CCSM3), the NCAR Parallel Coupled Model (PCM), the Hadley Centre Coupled Atmosphere–Ocean General Circulation Model version 3 (HadCM3), and version 3.2 of the Model for Interdisciplinary Research on Climate at high resolution [MIROC3.2 (hires)]. The standard deviation of monthly SST anomalies is maximum in the Niño-3 region in all six simulations, indicating progress in the modeling of ocean–atmosphere variability. The broad success in modeling ENSO’s SST footprint—quite realistic in CCSM3—is however tempered by the difficulties in modeling ENSO evolution: for example, the biennial oscillation in CCSM3 and the lack of regular warm-to-cold phase transition in the MIROC model. The spatiotemporal structure, including seasonal phase locking, is, on the whole, well modeled by HadCM3; but there is room for improvement, notably, in modeling the SST footprint in the western Pacific. ENSO precipitation anomalies over the tropical Pacific and links to North American winter precipitation are also realistic in the HadCM3 simulation and, to an extent, in PCM. Hydroclimate teleconnections that lean on a stationary component of the flow, such as surface air temperature links, are however not well modeled by HadCM3 since the midlatitude ridge in the ENSO response is incorrectly placed in the simulation; PCM fares better. The analysis reveals that climate models are improving but are still unable to simulate many features of ENSO variability and its circulation and hydroclimate teleconnections to North America. Predicting regional climate variability/change remains an onerous burden on models.

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The stationary wave response to global climate change in the Geophysical Fluid Dynamics Laboratory's R30 coupled ocean-atmosphere GCM is studied. An ensemble of climate change simulations that use a standard prescription for time-dependent increases of greenhouse gas and sulfate aerosol concentrations is compared to a multiple-century control simulation with these constituents fixed at preindustrial levels. The primary response to climate change is to zonalize the atmospheric circulation, that is, to reduce the amplitude of the stationary waves in all seasons. This zonalization is particularly strong in the boreal summer over the Tropics. In January, changes in the stationary waves resemble that of an El Niño, and all months exhibit an El Niño-like increase of precipitation in the central tropical Pacific. The dynamics of the stationary wave changes are studied with a linear stationary wave model, which is shown to simulate the stationary wave response to climate change remarkably well. The linear model is used to decompose the response into parts associated with changes to the zonal-mean basic state and with changes to the zonally asymmetric ''forcings'' such as diabatic heating and transient eddy fluxes. The decomposition reveals that at least as much of the climate change response is accounted for by the change to the zonal-mean basic state as by the change to the zonally asymmetric forcings. For the January response in the Pacific-North American sector, it is also found that the diabatic heating forcing contribution dominates the climate change response but is significantly cancelled and phase shifted by the transient eddy forcing. The importance of the zonal mean and of the diabatic heating forcing contrasts strongly with previous linear stationary wave models of the El Niño, despite the similarity of the January stationary wave response to El Niño. In particular, in El Niño, changes to the zonal-mean circulation contribute little to the stationary wave response, and the transient eddy forcing dominates. The conclusions from the linear stationary wave model apparently contradict previous findings on the stationary wave response to climate change response in a coarse-resolution version of this model.