While internal climate variability is known to affect climate projections, its influence is often underappreciated and confused with model error. Why? In general, modeling centers contribute a small number of realizations to international climate model assessments [e.g., phase 5 of the Coupled Model Intercomparison Project (CMIP5)]. As a result, model error and internal climate variability are difficult, and at times impossible, to disentangle. In response, the Community Earth System Model (CESM) community designed the CESM Large Ensemble (CESM-LE) with the explicit goal of enabling assessment of climate change in the presence of internal climate variability. All CESM-LE simulations use a single CMIP5 model (CESM with the Community Atmosphere Model, version 5). The core simulations replay the twenty to twenty-first century (1920–2100) 30 times under historical and representative concentration pathway 8.5 external forcing with small initial condition differences. Two companion 1000+-yr-long preindustrial control simulations (fully coupled, prognostic atmosphere and land only) allow assessment of internal climate variability in the absence of climate change. Comprehensive outputs, including many daily fields, are available as single-variable time series on the Earth System Grid for anyone to use. Early results demonstrate the substantial influence of internal climate variability on twentieth- to twenty-first-century climate trajectories. Global warming hiatus decades occur, similar to those recently observed. Internal climate variability alone can produce projection spread comparable to that in CMIP5. Scientists and stakeholders can use CESM-LE outputs to help interpret the observational record, to understand projection spread and to plan for a range of possible futures influenced by both internal climate variability and forced climate change.
High-resolution global climate modeling holds the promise of capturing planetary-scale climate modes and small-scale (regional and sometimes extreme) features simultaneously, including their mutual interaction. This paper discusses a new state-of-the-art high-resolution Community Earth System Model (CESM) simulation that was performed with these goals in mind. The atmospheric component was at 0.25 grid spacing, and ocean component at 0.1 . One hundred years of ''present-day'' simulation were completed. Major results were that annual mean sea surface temperature (SST) in the equatorial Pacific and ElNiño Southern Oscillation variability were well simulated compared to standard resolution models. Tropical and southern Atlantic SST also had much reduced bias compared to previous versions of the model. In addition, the high resolution of the model enabled small-scale features of the climate system to be represented, such as air-sea interaction over ocean frontal zones, mesoscale systems generated by the Rockies, and Tropical Cyclones. Associated single component runs and standard resolution coupled runs are used to help attribute the strengths and weaknesses of the fully coupled run. The high-resolution run employed 23,404 cores, costing 250 thousand processor-hours per simulated year and made about two simulated years per day on the NCAR-Wyoming supercomputer ''Yellowstone.''
The Caribbean region shows maxima in easterly winds greater than 12 m s Ϫ1 at 925 hPa in July and February, herein referred to as the summer and winter Caribbean low-level jet (LLJ), respectively. It is important to understand the controls and influences of the Caribbean LLJ because other LLJs have been observed to be related to precipitation variability. The purpose of this study is to identify the mechanisms of the Caribbean LLJ formation and variability and their association to the regional hydroclimate. Climatological fields are calculated from the North American Regional Reanalysis and the 40-yr ECMWF Re-Analysis from 1979 to 2001. It is observed that the low-level (925 hPa) zonal wind over the Caribbean basin has a semiannual cycle and an interannual variability, with greater standard deviation during boreal summer. The semiannual cycle has peaks in February and July, which are regional amplifications of the large-scale circulation. High mountains to the south of the Caribbean Sea influence the air temperature meridional gradient, providing a baroclinic structure that favors a stronger easterly wind. The boreal summer strengthening of the Caribbean LLJ is associated with subsidence over the subtropical North Atlantic from the May-to-July shift of the ITCZ and the evolution of the Central American monsoon. Additionally, the midsummer minimum of Caribbean precipitation is related to the Caribbean LLJ through greater moisture flux divergence. From May to September the moisture carried by the Caribbean LLJ into the Gulf of Mexico is strongest. The summer interannual variability of the Caribbean LLJ is due to the variability of the meridional pressure gradient across the Caribbean basin, influenced by tropical Pacific variability during summer.
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