Long-term changes in North American monsoon (NAM) precipitation intensity in the southwestern United States are evaluated through the use of convective-permitting model simulations of objectively identified severe weather events during “historical past” (1950–70) and “present day” (1991–2010) periods. Severe weather events are the days on which the highest atmospheric instability and moisture occur within a long-term regional climate simulation. Simulations of severe weather event days are performed with convective-permitting (2.5 km) grid spacing, and these simulations are compared with available observed precipitation data to evaluate the model performance and to verify any statistically significant model-simulated trends in precipitation. Statistical evaluation of precipitation extremes is performed using a peaks-over-threshold approach with a generalized Pareto distribution. A statistically significant long-term increase in atmospheric moisture and instability is associated with an increase in extreme monsoon precipitation in observations and simulations of severe weather events, corresponding to similar behavior in station-based precipitation observations in the Southwest. Precipitation is becoming more intense within the context of the diurnal cycle of convection. The largest modeled increases in extreme-event precipitation occur in central and southwestern Arizona, where mesoscale convective systems account for a majority of monsoon precipitation and where relatively large modeled increases in precipitable water occur. Therefore, it is concluded that a more favorable thermodynamic environment in the southwestern United States is facilitating stronger organized monsoon convection during at least the last 20 years.

In this study, twenty large-scale circulation patterns are identified to generate a synoptic classification of Weather Types (WT) over a region that comprises Mexico, the Intra-Americas Seas, Central America, and northern South America. This classification is performed using Self-Organizing Maps (SOMs) with mean sea-level pressure standardized anomalies from reanalysis. The influence of quasi-permanent pressure centers over the region, such as North Atlantic Subtropical High (NASH) and North Pacific High (NPH) are well captured. Seasonal variability of high-pressure centers for dry (November–April) and wet (May–October) periods over the entire region are also well represented in amplitude and pattern among the WTs. The NASH influence and intensification of the Caribbean low-level jet and the North American monsoon system is well captured. During the dry period, a strong trough wind advects cold air masses from mid-latitudes to the subtropics over the western Atlantic Ocean. High-frequency transitions among WTs tend to cluster around the nearest neighbors in SOM space, while low-frequency transitions occur along columns instead of rows in the SOM matrix. Low-frequency transitions are related to intraseasonal and seasonal scales. The constructed catalog can identify near-surface atmospheric circulation patterns from a unified perspective of synoptic climate variability, and it is in high agreement with previous studies for the region.

Abstract. This work examines the origin of atmospheric water vapor arriving to the western North American monsoon (WNAM) region over a 34-year period (1981–2014) using a Lagrangian approach. This methodology computes budgets of evaporation minus precipitation (E−P) by calculating changes in the specific humidity of thousands of air particles advected into the study area by the observed winds. The length of the period analyzed (34 years) allows the method to identify oceanic and terrestrial sources of moisture to the WNAM region from a climatological perspective. During the wet season, the WNAM region itself is on average the main evaporative source, followed by the Gulf of California. However, water vapor originating from the Caribbean Sea, the Gulf of Mexico, and terrestrial eastern Mexico is found to influence regional-scale rainfall generation. Enhanced (reduced) moisture transport from the Caribbean Sea and the Gulf of Mexico from 4 to 6 days before precipitation events seems to be responsible for increased (decreased) rainfall intensity on regional scales during the monsoon peak. Westward propagating mid- to upper-level inverted troughs (IVs) seem to favor these water vapor fluxes from the east. In particular, a 200 % increase in the moisture flux from the Caribbean Sea to the WNAM region is found to be followed by the occurrence of heavy precipitation in the WNAM area a few days later. Low-level troughs off the coast of northwestern Mexico and upper-level IVs over the Gulf of Mexico are also related to these extreme rainfall events.

Precipitation, low‐level flow and atmospheric moisture transport fields over the Intra‐Americas region from a Weather Research and Forecasting model (WRF) multi‐decadal simulation are presented. The period of analysis is 1982–2012, and the focus is on mean monthly states and their variability. WRF results are compared to global precipitation products and to the Interim Reanalysis from the ECWMF (ERA‐Interim). An analysis of the coupling between precipitation over northern South America and regional scale sea surface temperatures (SSTs) and winds within ERA‐Interim and WRF is also presented. Over land, precipitation biases in WRF are mostly positive, especially over Central America and northern South America. This bias is associated with an excess in atmospheric moisture convergence in WRF compared to ERA‐Interim, which in turn is related to stronger low‐level circulation structures and differences in the moisture transport field between WRF and ERA‐Interim at the regional scale. The largest differences in the flow are observed over the eastern Pacific, where the flow is correlated with precipitation over southern Mexico, Central America and northern South America during the boreal summer. During early winter, correlation patterns of WRF precipitation and SST anomalies over the tropical Pacific are substantially weaker over western Texas and northwestern South America compared to the coupling between the global precipitation products and observed SSTs. For northwestern South America, the low correlation seems to be associated to a lack of coupling between regional scale winds and SST anomalies. During the boreal summer, the coupling between Caribbean SSTs and precipitation over parts of Mexico and Central America is stronger in WRF compared to observation‐based products.

Mesoscale Convective Systems (MCS) may vary greatly with respect to their morphology, propagation mechanism, intensity, and under which synoptic-scale conditions as a function of topographic complexity. In this study, we develop a long-term climatology of MCS during the North American Monsoon focusing on MCS morphology, lifecycle, and intensity as well as possible propagation mechanisms. We employ an MCS tracking and classification technique based on 23 years (1995 to 2017) of GOES IR satellite data. MCS intensity is also gauged with 7 years (2011 to 2017) of Vaisala GLD360 lightning data and, finally, monthly and interannual variability in synoptic conditions are examined with ERA5 reanalysis data. Our results based on 1594 identified MCS reveal that 98% are morphologically classified as Persistent Elongated Convective Systems. During the 23 summers (June through September) observed, the number of MCS varied considerably, averaging 70 MCS with minimum of 41 and maximum of 94. MCS typically have an average duration of around 8 h ± with a 2 h standard deviation. Propagation speeds, estimated with Hovmöller diagrams in addition to MCS centroid initial and final position, vary slightly depending on the trajectory. A notable result suggests that MCS propagation speeds are more consistent density currents or cold pools and not gravity waves nor steering-level winds. The results of this study could also provide a dataset for examining larger-scale controls on MCS frequency in addition to assesing convective parameterization and convective-resolving models in regions of complex topography.