# Climate Perspectives in the Intra-Americas Seas
## 3 Regional Climate Features
### Rainfall
#### 3.4.1 Rainfall Distribution and the MSD  
The annual rainfall cycle is strongly controlled by the migration of the ITCZ. The central and western Colombian annual precipitation cycle results from the passage of the ITCZ. This consists of distinct rainy seasons (April-May and October-November) and drier or less rainy conditions (December-February and June-August) [122; 123]. From the northern Caribbean coast of Colombia to southern Mexico, the rainfall annual cycle is dominated by a relatively dry season from November to April, and a wet season from May to October. Further north, the bulk of annual rainfall over much of Mexico occurs during a four-month period (June to September) [124]. Rainfall decreases during the boreal summer months in the Pacific coast of Central America. Therefore, much of the IAS region features a rainy season with periods that are above or below average precipitation. Rainfall seasonality is marked by two precipitation peaks: May-June and September-October. In general, this features a bimodal distribution with a relative minimum precipitation during July and/or August. This relative  
Figure 4: Average annual cycle of rainfall for contiguous \(10^{\circ}\times 10^{\circ}\) areas as shown in the boxes based on monthly Global Precipitation Measurement (GPM) for the 2001–2019 period.  
minimum is known as the MSD and corresponds to a minimum in convective activity. Terms such as _cancula_ (regardless of the name being defined for temperate climate) and _veranillo_ have been used locally to refer to this condition depending on the region of occurrence [16; 125; 126]. Figure 4 shows the long-term rainfall annual cycle (dotted lines) and the maximum and minimum values (defined by the shaded area) averaged over \(10^{\circ}\times 10^{\circ}\), upon which the bimodal distribution that features rainfall may be observed. The amplitude of the MSD varies across the domain with northern Central America and the Caribbean, exhibiting the largest rainfall decrease during the boreal summer.  
The MSD is the result of a rainfall-enhancing mechanism that occurs twice because of the biannual crossing of solar declination [127]. The northward displacement of the ITCZ following the seasonal march of the SST and the westward extension of the NASH have also been identified as forcing mechanisms of the MSD [55; 128]. The variability of the CLIJ and associated direct circulations are a key element in MSD development. Several studies have characterised the MSD over Central America, the Caribbean islands, and Mexico, finding high spatio-temporal variability [126; 129; 130; 131]. Large-scale dynamics, dominated by low-level atmospheric patterns and orographic forcing associated with steep topography, generate distinct MSD signatures in the region. The MSD is highly sensitive to the main variability modes that affect the region. At higher frequencies, the MJO [132] is a key modulator of the intra-seasonal variability of the MSD [133]. Changes in intra-seasonal variability are relevant as they modulate the onset and offset of the MSD, having a significant impact on the lengths and intensity of the rainy seasons. Information on the onset and offset of the MSD is critical for agriculture, hydropower electricity generation, and water resources management. At interannual scales, the ENSO modulates MSD variability through variations in the CLIJ (see Section 4). Although heavy rainfall is a feature of the region, drier periods are equally important as they play a major role in agriculture, energy, and regional economy in general. Understanding the dynamics of the transitioning of dry-wet seasons, its variability, and long-term trends is fundamental for planning.