# Climate Perspectives in the Intra-Americas Seas
## 2 Large-Scale Climate Forcing
### Pressure Patterns  
During summer, circulation in North America possesses a unique pattern with a lower tropospheric cyclone over the continent. Figure 1 shows that it is accompanied by anticyclonic circulations over the oceans on its western and eastern sides, the Western Pacific High (WPH) and the North Atlantic Subtropical High (NASH) respectively, with an opposite pattern in the upper troposphere [49]. The low-level circulation in the Atlantic region is dominated by the NASH, with associated geostrophic easterlies and south easterlies in the Gulf of Mexico. This semi-permanent feature is commonly referred to as the Azores High or Bermuda High, in Europe and North America, respectively [50]. Focusing on the magnitude and location of the highest central pressure of the NASH [51] showed that the anticyclone migrates in a somewhat elliptical pattern from month to month within an area bounded by 258-408\({}^{\circ}\) N and 208-508\({}^{\circ}\) W. The upper panel in Figure 1 contrasts the high-pressure systemsbetween summer months June-July-August (JJA) and winter months December-January-February (DJF), in terms of intensity and extension. During summer, the NASH is enhanced, and its western ridge extends to the south-eastern US [52], inducing a strong easterly flow across the Caribbean. This flow supplies warm and humid air to feed large-scale precipitation over tropical and subtropical North America [53]. The strength and orientation of the western ridge of the NASH plays an important role in modulating the intra-seasonal variability of summer rainfall over the IAS, characterised by coupling with marine boundary layer clouds [54]. The circulation associated with the most westward extension of the NASH during July and August is linked with the MSD (described in Section 3.4.1), through the reinforcement of low-level anticyclonic flow over the Gulf of Mexico [55]. The western ridge position of the NASH is associated with subsidence and clear skies, which makes the movement of cyclones difficult. However, when the NASH western ridge is located to the southwest of its summer climatological mean position, precipitation over the south-eastern US increases with enhanced moisture transport from adjacent tropical oceans [56]. Variations in precipitation located at the periphery of subtropical high systems have marked impacts on local circulation and the evolution of nearby weather systems. However, there have been few observational studies on precipitation in areas dominated by subtropical high, mainly due to the limited availability of well-matched measurements. Limited impacts on local precipitation due to prevailing subtropical high conditions have been observed. According to Reference [57], precipitation was only 30% lower under the NASH conditions than under non-NASH conditions. Whilst the impact of the NASH conditions on rainfall intensity was larger, it was still 50% less than intensity under non-NASH conditions. Precipitation under the NASH was highly correlated with vertical motion as active up draughts, occurring even under subtropical high  
Figure 1: Mean Sea–Level Pressure for (**A**) DJF and (**B**) JJA. 10 m wind vectors and zonal wind shear (shaded) for (**C**) DJF and (**D**) JJA and average rainfall for (**C**) January and (**D**) July respectively, for the 1979–2019 period based on fifth generation of the European Centre for Medium–Range Weather Forecasts atmospheric reanalyses of the global climate (ERA5) data. Both (**A**,**B**) show the contrasting seasonal expansion of the North Atlantic Subtropical High (NASH) western ridge. Surface winds show the coherence of the large–scale circulation with the pressure patterns and the area that features the Caribbean Low–Level Jet (CLIJ) is observed as a maximum of zonal wind shear. Complimentary (**E**) and (**F**) show the long–term average daily regional rainfall distribution.