Assessment of CMIP5 Model Simulations of the North American Monsoon System

Abstract

Precipitation, geopotential height, and wind fields from 21 models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) are examined to determine how well this generation of general circulation models represents the North American monsoon system (NAMS). Results show no improvement since CMIP3 in the magnitude (root-mean-square error and bias) of the mean annual cycle of monthly precipitation over a core monsoon domain, but improvement in the phasing of the seasonal cycle in precipitation is notable. Monsoon onset is early for most models but is clearly visible in daily climatological precipitation, whereas monsoon retreat is highly variable and unclear in daily climatological precipitation. Models that best capture large-scale circulation patterns at a low level usually have realistic representations of the NAMS, but even the best models poorly represent monsoon retreat. Difficulty in reproducing monsoon retreat results from an inaccurate representation of gradients in low-level geopotential height across the larger region, which causes an unrealistic flux of low-level moisture from the tropics into the NAMS region that extends well into the postmonsoon season. Composites of the models with the best and worst representations of the NAMS indicate that adequate representation of the monsoon during the early to midseason can be achieved even with a large-scale circulation pattern bias, as long as the bias is spatially consistent over the larger region influencing monsoon development; in other words, as with monsoon retreat, it is the inaccuracy of the spatial gradients in geopotential height across the larger region that prevents some models from realistic representation of the early and midseason monsoon system.

The Impact of Horizontal Resolution on North American Monsoon Gulf of California Moisture Surges in a Suite of Coupled Global Climate Models

“…easterly low-level jet along the Mexican coast, thus creating an unrealistic northward tropical moisture transport that makes it difficult for models to realistically capture the timing of the monsoon retreat (Geil et al 2013). …”

“…State-of-the-art GCMs [e.g., those used to produce the archives of phases 3 and 5 of the Coupled Model Intercomparison Project (CMIP3 and CMIP5); Meehl et al 2007;Taylor et al 2013] still feature an atmospheric grid spacing that is too coarse (*100 km) to adequately resolve the GoC. This significant limitation has so far prevented the analysis of GoC surges in studies on the NAM in the CMIP3 and CMIP5 GCMs (e.g., Liang et al 2008;Geil et al 2013).…”

North American Climate in CMIP5 Experiments. Part I: Evaluation of Historical Simulations of Continental and Regional Climatology

“…The HadCM3, HadGEM2-ES, and CanESM2 also perform the best over the larger monsoon region (Table 12). Geil et al (2013) find that the models that best represent the seasonal shift of the monsoon ridge and subtropical highs over the North Pacific and Atlantic tend to have the least trouble ending the monsoon, suggesting there is room for improvement over the region through an improved representation of the seasonal cycle in these large-scale features.…”

“…On the other hand, there does seem to be improvement in the timing of seasonal precipitation shifts, with 13 out of 21 (62%) CMIP5 models having a phase lag of zero months as compared to 6 out of 17 (35%) CMIP3 models in Liang et al (2008). The top ranking models for phase, RMSE, and bias shown in Table 13 (HadCM3, HadGEM2-ES, CNRM-CM5, CanESM2, and HadGEM2-CC) are also the models with the highest spatial correlations of MayOctober 850-hPa geopotential heights and winds when compared with the Interim European Centre for MediumRange Weather Forecasts (ECMWF) Re-Analysis (ERAInterim; Geil et al 2013). The HadCM3, HadGEM2-ES, and CanESM2 also perform the best over the larger monsoon region (Table 12).…”

Comparing proxy and model estimates of hydroclimate variability and change over the Common Era

“…Nevertheless, models are generally successful at simulating the large-scale atmospheric circulations that drive areas of subsidence and uplift, and thus generally represent the zonal mean hydroclimate well, particularly in the multi-model ensemble mean (Flato et al, 2013). This is less true of regional hydroclimate dynamics, as many models struggle to reproduce the full characteristics of observed phenomena such as the global monsoon systems (e.g., Geil et al, 2013;Langford et al, 2014, for the North American monsoon; Sperber et al, 2013, for the Asian monsoon), some midlatitude storm tracks and the frequency of associated cyclonic systems (e.g., in the western and northern Atlantic; Colle et al, 2013;DunnSigouin and Son, 2013;Sheffield et al, 2013;Zappa et al, 2013), and atmospheric blocking (Masato et al, 2013). A full consideration of hydroclimate also includes the representation of land surface processes.…”