Land–Sea Thermal Contrast and Intensity of the North American Monsoon under Climate Change Conditions

Abstract

The hypothesis that global warming during the twenty-first century will increase the land–sea thermal contrast (LSTC) and therefore the intensity of early season precipitation of the North American monsoon (NAM) is examined. To test this hypothesis, future changes (2075–99 minus 1979–2004 means) in LSTC, moisture flux convergence (MFC), vertical velocity, and precipitation in the region are analyzed using six global climate models (GCMs) from phase 5 of the Coupled Model Intercomparison Project (CMIP5) under the representative concentration pathway 8.5 (RCP8.5) emission scenario. A surface LSTC index shows that the continent becomes warmer than the ocean in May in the North American Regional Reanalysis (NARR) and ECMWF Interim Re-Analysis (ERA-Interim) and in June in the mean ensemble of the GCMs (ens_GCMs), and the magnitude of the positive LSTC is greater in the reanalyses than in the ens_GCMs during the historic period. However, the reanalyses underestimate July–August precipitation in the NAM region, while the ens_GCMs reproduces the peak season surprisingly well but overestimates it the rest of the year. The future ens_GCMs projects a doubling of the magnitude of the positive surface LSTC and an earlier start of the continental summer warming in mid-May. Contrary to the stated hypothesis, however, the mean projection suggests a slight decrease of monsoon coastal precipitation during June–August (JJA), which is attributed to increased midtropospheric subsidence, a reduced midtropospheric LSTC, and reduced MFC in the NAM coastal region. In contrast, the future ens_GCMs produces increased MFC and precipitation over the adjacent mountains during JJA and significantly more rainfall over the entire NAM region during September–October, weakening the monsoon retreat.

Inter-annual variability of precipitation over Southern Mexico and Central America and its relationship to sea surface temperature from a set of future projections from CMIP5 GCMs and RegCM4 CORDEX simulations

“…9c-h, see also Coppola et al 2014) and consistent with previous CMIP3 (Giorgi and Bi 2005;Christensen et al 2007) and CMIP5 (Torres-Alavez et al 2014) GCM experiments. All the models also show an increase in precipitation over the EP due to enhanced convection caused by increased El Niño-like conditions (also found in other results for GCMs, see for example Torres-Alavez et al 2014). In line with the theory of Karnauskas and Busalacchi (2009), the warmer the EP, the less the ITCZ is displaced northward and therefore the drier the conditions over Central America and Southern Mexico.…”

“…In contrast to the TNA, the TNP is mostly projected to undergo a relatively larger warming (Leloup and Clement 2009). In turn, coupled model intercomparison project phase 5 (CMIP5) models also show decreases of rainfall in SMECAM especially during spring and summer (Maloney et al 2013;TorresAlavez et al 2014) associated with stronger easterlies and a southward displacement of the ITCZ which generate subsidence over the region (Torres-Alavez et al 2014). Moreover, future CMIP5 scenarios suggest an intensification of the CLLJ by the end of the 21 century (Maloney et al 2013).…”

Climate change projections from Coupled Model Intercomparison Project phase 5 multi‐model weighted ensembles for Mexico, the North American monsoon, and the mid‐summer drought region

“…() found that CMIP3 GCMs captured the observed MSD rainfall pattern in Meso‐America, but with reduced summer rainfall that could be partially associated to underestimation of sea surface temperatures (SSTs) in the Gulf of Mexico and the Caribbean Sea. These problems are also observed in CMIP5 models (e.g., Maloney et al ., ; Fuentes‐Franco et al ., ), which, as CMIP3 GCMs (Rauscher et al ., ), show increased future dryness in Mesoamerica by the end of the 21st century (Cook and Seager, ; Maloney et al ., ; Fuentes‐Franco et al ., ; ; Torres‐Alavez et al ., ). In the case of the North American monsoon (NAM) region, some GCMs have shown an improvement from CMIP3 to CMIP5; however, GCMs still tend to overestimate winter and spring precipitation and show a late retreat of the monsoon rainfall during autumn (e.g., Cavazos and Arriaga‐Ramirez, ; Geil et al ., ; Seth et al ., ; Torres‐Alavez et al ., ).…”

“…These problems are also observed in CMIP5 models (e.g., Maloney et al ., ; Fuentes‐Franco et al ., ), which, as CMIP3 GCMs (Rauscher et al ., ), show increased future dryness in Mesoamerica by the end of the 21st century (Cook and Seager, ; Maloney et al ., ; Fuentes‐Franco et al ., ; ; Torres‐Alavez et al ., ). In the case of the North American monsoon (NAM) region, some GCMs have shown an improvement from CMIP3 to CMIP5; however, GCMs still tend to overestimate winter and spring precipitation and show a late retreat of the monsoon rainfall during autumn (e.g., Cavazos and Arriaga‐Ramirez, ; Geil et al ., ; Seth et al ., ; Torres‐Alavez et al ., ). A weak start of the monsoon and a late retreat has been projected for the end of the century (e.g., Cook and Seager, ; Maloney et al ., ; Seth et al ., ; Torres‐Alavez et al ., ), but with large uncertainties according to dynamically downscaled GCMs from CMIP5 (Bukovsky et al ., ).…”

“…Socio‐economic sectors and interdisciplinary groups require fine‐scale projections (finer than the GCM outputs) for studies of vulnerability, impacts, and adaptation (VIA). There are few studies in Mexico that have evaluated empirical–statistical ensembles of GCMs at regional scale (e.g., Montero‐Martínez and Pérez‐López, ; Cavazos and Arriaga‐Ramirez, ; Magaña et al ., ; Cook and Seager, ; Torres‐Alavez et al ., ). Few others have used dynamical downscaling (Diro et al ., ; Bukovsky et al ., ; ; Fuentes‐Franco et al ., ; ; Chang et al ., ; Cerezo‐Mota et al ., ) to better understand present climate and its future changes.…”