Mesoscale eddies in the northeastern Pacific tropical‐subtropical transition zone (16°N–30°N; 130°W–102°W) are analyzed using nearly 18 years of satellite altimetry and an automated eddy‐identification algorithm. Eddies that lasted more than 10 weeks are described based on the analysis of 465 anticyclonic and 529 cyclonic eddy trajectories. We found three near‐coastal eddy‐prolific areas: (1) Punta Eugenia, (2) Cabo San Lucas, and (3) Cabo Corrientes. These three areas are located in places where the coastal morphology changes abruptly and strong surface current intensification occurs at some phase of the seasonal cycle. Although mesoscale eddies in these areas have been previously reported, this study provides their first statistically supported characterization. Punta Eugenia showed the highest eddy production (with more cyclones generated), followed by Cabo Corrientes (also with more cyclones) and Cabo San Lucas (with more anticyclones). Cabo Corrientes eddies showed the highest mean values in propagation speed, swirling speed and eddy kinetic energy, whereas Punta Eugenia eddies showed the lowest values. Cyclonic eddies increased their distance traveled and duration from south to north; in contrast anticyclonic eddies increased from north to south. In average, anticyclones tend to travel faster than cyclones in all the subregions. These long‐lived eddies were mainly nonlinear and therefore can redistribute coastal waters relatively far into the open ocean. The peaks in the seasonal signal of eddy generation can be associated with the peaks in the strength of the offshore currents and/or in the Coastal Upwelling Index. No clear relationship could be established between El Niño events and eddy generation.

The surface circulation around the entrance to the Gulf of California is described from satellite altimetry supported by 10 conductivity‐temperature‐depth (CTD) surveys. The sea surface height calculated from the 1/4° World Ocean Database 2001 climatology plus the Archiving, Validation, and Interpretation of Satellite Oceanographic altimeter data (October 1992 to January 2008) were RMS adjusted to the dynamic height calculated from CTD data; the 27.0 kg m⁻³ isopycnal provided the optimum reference. In the mean, the surface circulation shows a branch of the California Current heading landward toward the Gulf of California entrance, where it splits into two subbranches; these are separated by a cyclonic circulation attached to the coast south of Cabo Corrientes. This feature is produced by Sverdrup dynamics and is the first observational indication that the Mexican Coastal Current is generated locally by the wind stress curl, as previously suggested by numerical models. The global variance of the surface circulation can be separated into seasonal (explained variance 35%), interannual (explained variance 35%), and mesoscale (explained variance 30%) components. The seasonal signal, which shows the interplay of the poleward Mexican Coastal Current and the equatorward branch of the California Current, can be explained by a long Rossby wave model forced by the annual wind and by radiation from the coast. The interannual component is dominated by the El Niño‐Southern Oscillation, which induces in the gulf entrance an anticyclonic (cyclonic) circulation during El Niño (La Niña); this circulation includes a poleward‐flowing branch (during El Niño) parallel to the Pacific coast of the Baja California peninsula. The mesoscale variability is caused by intense eddy activity.

S: Central American cold surge outbreaks, known as Nortes, are standard features of the Gulf of Mexico (GoM) climate, having significant oceanic and atmospheric impacts over the region that are essential for the marine ecosystem. Fifteen years of analysis of weather reports showed that Nortes occur within a 9 months window (September to May), with a total number of 24 ± 5 episodes per season. On average, there were 2.5 ± 1 events per month, lasting between 1 and 3 days with a maximum duration of 13 consecutive days. A Principal Component Analysis of 15 years of in situ observations identified the air temperature (41.6% of the explained variance) and the wave height (36.3%) as the two main drivers of these events impacts. Cluster Analysis to these same data categorized the Nortes into three types: Moderate, Strong, and Severe, based on the environmental impact caused in the GoM. These events tend to transit between categories as they lose strength while travelling to southern latitudes. Severe events are more frequent in the northwestern coast, occurring from January to March, whereas moderate events are more common in the middle of the GoM during September–December. Our results showed that Nortes increase the wind stress over the sea surface, rising the height and period of the waves over the southwestern and central GoM. Nortes also disturb the sea‐level and the surface currents, provoking high amplitude oscillations along the coasts, and accelerating and shifting the currents towards the southwest. Nortes bring cold, dry air that cools the ocean surface and the lower atmosphere and diminishes its relative humidity, which, together with the wind velocity increments, promote a positive sensible and latent heat flux maximized on the northwestern and western coasts.

Upwelling events over the Yucatan Shelf are an important physical phenomenon to the region. They typically happen during spring and summer and had been studied for some time with a primary focus on the development on the eastern side of the shelf and later transport to the central part of the Peninsula. There has been very little effort looking at the impact of upwelling on the western shelf, on the Campeche side. Using a combination of observations and modeling from 2018, we show evidence for the first time, of the presence of upwelled water on the western side. Particle tracking, integrated back-in-time, was used to identify the origin of the upwelled water. Our results show that Caribbean Subtropical Underwater was brought from the northeast shelf, over 500 km away from the study area, by advection. This water took over a month (40 days) to arrive at the study region, traveling along-shelf with an average velocity of 14.5 cm/s. In the nearshore waters off the Campeche Coast, Caribbean Tropical Water was underlain by upwelling Caribbean Subtropical Underwater. Monthly averaged sea surface temperature (SST) anomalies from a 39-year time series suggest that upwelled water off Campeche is a regular phenomenon during summer, while the recurrence of westward advection is supported by climatological Lagrangian Coherent Structures. More studies are needed to explore the frequency of occurrence and impact of these events on the western shelf.