# Climate Perspectives in the Intra-Americas Seas
## 4 Climate Variability
### Interannual Variability  
The ENSO is the result of coupled ocean-atmosphere dynamics in the tropical Pacific [169], and, is the leading mode of interannual variability. It is associated with SST shifts, circulation, and rainfall patterns across the globe [170]. After the El Nino in 1997-1998, interest in the impacts of El Nino over the IAS region had increased. During El Nino boreal winters, precipitation increases over north-western Mexico (orange shading in Figure 6A) due to a southward shift in the subtropical jet stream position [171]. Conversely, rainfall decreases around the isthmus of Tehuantepec and the Caribbean coast of Central America [13; 171] (blue shading in Figure 6A). During warm (cold) ENSO, the zonal easterlies increase (decrease) over the isthmus [172], decreasing (increasing) precipitation over the Pacific slope of Central America. Rainfall variations are a response to the modulation of moisture transport from the Caribbean [79] and changes in the ITCZ meridional position forced by the ENSO. During the boreal summer, strong El Nino events are associated with drier than normal conditions in southern and central Mexico, Central America, and parts of the Caribbean. Along the Caribbean coast in Central America, El Nino leads to above normal rainfall [173]. A negative correlation exists between the intensity of the CLIJ and precipitation (strong jet with weak precipitation and vice versa) in Central America and southern Mexico [11]. In these cases, the El Nino results in an anomalous descent over southern Mexico that inhibits convection [174; 175]. The ENSO affects the genesis, tracks, intensity, and lifetimes of TCs in the ENP and TNA, through changes in SST, vertical wind shear, and relative vorticity induced by wind anomalies in the lower troposphere and changes in upper-tropospheric temperatures [176; 177]. El Nino inhibits the formation of TCs through enhanced vertical wind shear, subsidence, and reduced relative humidity in the tropical Atlantic [178]. The contrast of ENSO phases for TC activity is shown in Figure 6B-D, with enhanced (decreased) activity over the ENP (TNA) during a warm ENSO. In contrast, during a cold ENSO, TCs are intensified when they reach the Caribbean coast of Central America by the reduction of vertical wind shear. The impact of the ENSO is often associated with rainfall extremes. Assessments of the impacts of El Nino associated with disasters between 1972 and 2010 indicate damages of approximately 2.904 and 1.11 billion USD in losses in the agricultural and forestry sectors [179]. Although the ENSO leads to interannual variability, other modes also affect regional climate. The Atlantic meridional mode (AMM), which describes the meridional gradient between northern and southern tropical Atlantic SST [180], also influences the activity of Atlantic TCs [181]. A negative AMM phase (cool northern and warm southern tropical Atlantic SST anomalies) inhibits Atlantic TCs. Strong phases of the ENSO and AMM may amplify or dampen the influence of one another on  
Figure 6: **(A)** Correlation between the Bivariate El Nino– Southern Oscillation Timeseries BEST index and ERA–5 rainfall anomalies (dotted areas indicate statistically significant values, blue shade for warm El Niño–Southern Oscillation (ENSO) lead rainfall reduction, and orange shade for rainfall increase during warm ENSO). Tropical cyclones (TCs) composites for **(B)** warm (1983, 1987, 1992, 1997, 2015), **(C)** cold (1988, 1998, 1999, 2007, 2010), and **(D)** neutral (1981, 1990, 2003, 2012, 2014) ENSO years based on the ENSO 3.4 region sea surface temperature (SST) anomalies for the Atlantic and eastern tropical Pacific based on the HURDAT2 archive. The results highlight differences in the amount and intensity of TCs depending on the ENSO phase, with warm (cold) ENSO featuring reduced (enhanced) cyclonic activity over the Caribbean with a contrasting pattern for the eastern tropical Pacific (ETPac). Atlantic TCs [182; 183].