Its central core can be found between 10° N and 20° N at about 925 hPa and has an easterly zonal wind that can be larger than 13 m s −1.

The CLLJ is part of the trade winds that prevail all year long in the Caribbean basin. The CLLJ and its interactions with the NASH are another important feature in MAR. They detected the MSD on WTs that are characterized by an strengthening of the NASH and the Caribbean Low-Level Jet (CLLJ) with a reduction of low-level humidity and precipitation patterns consistent with the MSD. Díaz-Esteban and Raga analyzed the WTs that influence precipitation behavior in southeastern Mexico, focusing on the regional atmospheric quasi-permanent circulation centers and El-Nino Southern Oscillation. This dry period is commonly referred to as the Mid-Summer Drought (MSD) in current literature. A dry “spell” or local minimum is more evident in southeastern Mexico and northern Central America, featuring a decrease in precipitation during July and August. The NAMS precipitation variability during the wet period exhibits a clear double-peak structure in the climate record at low latitudes.

The onset of NAMS occurs between the end of May and early June and reaches a mature stage between July and August and a decay stage in late September–October. Economic activities in the region are highly dependent on precipitation amounts and onset timing of the monsoon season. The North American Monsoon System (NAMS) precipitation represents more than 50% of the total annual over Southwestern US and Northwestern Mexico. It was found that positive (negative) precipitation anomalies in winter months are related to a stronger (weaker) Aleutian Low and a weaker (stronger) North Pacific High (NPH). In more modern times using the SOM method, Cavazos detected a relation between large-scale circulation patterns on both daily precipitation and low-level humidity fields over the northeastern region of Mexico and Southeast Texas.

Mosiño and Jáuregui recognized the importance of WTs associated with the longitudinal excursions of the North American Subtropical High (NASH) into the southern United States of America (US), the Gulf of Mexico and southern Mexico during wet and dry periods. A review by Jáuregui revealed that, at least for Mexico, there are three distinct seasonal periods for precipitation: a wet period that lasts from May to October and a dry period (November to April) composed of two seasons-dry-cold (November to January) and dry-warm (February to April) for winter and spring, respectively. In this study we refer to this landmass and adjacent oceanic areas as the Middle Americas Region (MAR). While there have been studies over North America, e.g.,, using WT, here we choose to focus on the region that comprises Mexico, the Intra-Americas Seas, Central America, and northern South America. 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. Low-frequency transitions are related to intraseasonal and seasonal scales. 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. During the dry period, a strong trough wind advects cold air masses from mid-latitudes to the subtropics over the western Atlantic Ocean. The NASH influence and intensification of the Caribbean low-level jet and the North American monsoon system is 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 influence of quasi-permanent pressure centers over the region, such as North Atlantic Subtropical High (NASH) and North Pacific High (NPH) are well captured. This classification is performed using Self-Organizing Maps (SOMs) with mean sea-level pressure standardized anomalies from reanalysis. 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.

Adjacency matrix nodebox