Discussion

The hydrographic features and the numerical simulations described here confirm the existence of upwelling waters along the western Yucatan shelf,  brought from the northeast by coastal current advection. In their works of upwelling events over the northeastern Yucatecan coast, \citet{1979}\citet{Merino_1997}, and \citet{Jouanno_2018} proposed that local wind acts indirectly in the upwelling intensity and variability by inducing high-frequency variations in the current magnitude over the platform. Once the upwelled water intrudes the shelf from the eastern notch, the wind-forced currents spread this water along the north coast,  particularly during spring and summer. Similar to their findings,  \citet*{Mart_inez_Lopez_1998} detected a wind stress-forced westward current with maximum transport in July–August (\citealp{Zavala_Hidalgo_2003}). Off Campeche, more than 1.5 years of  ADCP observations off  Lerma and Champoton (result not shown) registered a mean 5 cm/s southwestward surface flow with maximum velocities reaching 43 cm/s. On the other hand, considering the 500 km traveled by the upwelled waters in 40 days gives an averaged advected velocity of  14.5 cm/s, comparable to the westward current velocity simulations of 5-20 cm/s off Yucatan (\citealp*{Mart_inez_Lopez_1998}\citealp{r2016}).
An interesting feature promoted by these upwelling events is the two-layer water column distribution in such an open and shallow environment, as depicted in the T/s diagram (Fig. \ref{554331}). Other authors had evidenced the large spatial coverture of these events over the shelf (\citealp{r2016}) and its lasting duration from spring to autumn (\citealp{Merino_1997}), which had demonstrated its importance on the productivity of the bank, as Nitrate distributions of the upwelled water layer fertilize the shelf (\citealp*{FURNAS1987161}\citealp{Merino_1997}). Despite we did not present any Nitrate data over the western side we believe upwelling events enhance its productivity, although more hydrographic evidence is needed to enlarge our understanding of this phenomenon on this side of the shelf and the role it may play in the seasonal and interannual variation of the regional fishing resources.
Some works have studied the seasonality of the upwelling events over the Yucatan Shelf (\citealp{rl1980}\citealp{Merino_1997}) and attributed it to the Yucatan Current flow intensity, which strengthens during spring and summer and weakens in autumn-winter, implying bottom friction mechanisms. On the other hand, \citet{r2016} explored the relationship between the alongshore wind stress over the shelf and the seasonality of the upwelling events, using a Longitude-time (Hövmuller) diagram of the mean monthly upwelling index (his Fig. 9). They found upwelling events throughout the year with two strong periods: March and July and October and December, the former being stronger. To explore the regularity of the advected upwelled waters over the western coast, we analyzed the alongshore SST anomaly distribution from the Caribbean sea (87°W) until the Tabasco coast (>92°W, Figure \ref{880332}), where cooling events are related to negative anomalies (blue color values). From the eastern side of the shelf until the western side, the Campeche coast is shown between longitudes 92-90°W, cooling events are seen alongshore all year round. Although SST is related to different surface heat budget processes and could be noisy to attributed it solely to the upwelling events, it showed an interesting resemblance to the findings of  \citealp{Merino_1997} and \citet{r2016}. It exhibited colder SST anomalies between July to August (in summer) off Campeche, and between April to August (spring and summer) off the Yucatecan coast. During winter (from January to March) surface cooling events encompassed all the region, from the Caribbean until Tabasco, although their SST anomalies are low and are probably related to the northerly cold wind burst events (the "Nortes") that affect all the GoM.