The impact of ENSO in the Mexican Pacific Ocean and adjacent areas has been well documented. The focus has been on strong events that modulated the sea surface temperature and nutrient and Chla concentrations. In the southern Gulf of California, near its connection with the Pacific Ocean, El Niño in 1982/1983 caused an increase in SST (≈ 3–3.5 °C) and a considerable ecosystem modification that negatively affected the distribution of silicoflagellates and other phytoplankton populations living in the Pacific Ocean [20]. During the 1997–1998 event, a clear impact on the SST and salinity fields was documented off Baja California [8] that induced low levels of Chla (average concentration of 0.13 mg m−3) [16]. Recently, based on satellite observations during the period from 2002 to 2015, the impact of ENSO events on the southern Gulf of California, which induced high SST values (> 24 °C), low Chla concentrations (< 2.00 mg m−3), and moderate winds [11], was detected. This was particularly notable during the years 2002 and 2009, when moderate ENSOs took place. However, during 2015, an increase in SST was shown (up to ≈ 26.5 °C) and low levels of Chla were observed (< 2.00 mg m−3) [11].
It has been established that ENSO affects the Mexican Pacific Ocean in two principal ways: (1) it is associated with a reduction in the upwelling productivity and raises the ocean temperature by ≈ 5 °C [24]; and (2) the southward motion of warm equatorial waters combined with high solar radiation at the surface induces an increase in sea surface temperature, which in turn reduces the primary biological productivity. Zamudio et al. [34] pointed out that the ENSO event that occurs along the southwest coast of Mexico has three distinct stages: (1) the generation of a coastal jet characterized by strong vertical shear flow; (2) an increase in the horizontal component of the shear flow along with an increase in the amplitude of its oscillations; and (3) the development of an instability in the jet, in which it breaks into anticyclonic eddies that separate from the coast and drift southwestward.
The results of this study reveal a clear impact of the “Godzilla El Niño” on the Chla concentrations in the Tehuantepec upwelling system. Based on satellite observations, Aguirre-Gómez et al. [1] identified a slight increase in the sea surface temperature from 3 to 4 °C during the 1997/1998 ENSO in the Gulf of Tehuantepec; the authors emphasize an inhibition of the upwelling events in the gulf compared with previous and subsequent years; their wind and vertical water velocity values were shown to favor the development of the upwelling. The increase of SST observed by the authors is in agreement with the results of this study, where an increase in SST of 5.13 °C resulted in a dramatic decrease in the Chla concentration in January 2016 compared with its value in January 2015, resulting in a difference of 1.56 mg m−3.
Similar observations have been documented for other regions, such as for the Peruvian upwelling system (also known as the Northern Humboldt Current System), where a pronounced bottom-up control mechanism was observed during the 1982/1983 ENSO years that resulted in a decrease in primary production associated with Chla-poor waters (< 0.3 mg m−3) [9, 30]. Values varying from 0.5 to 1 mg m−3 were reported during the 1997/1998 ENSO in the same region [6]. Most recently, using a regional coupled physical-biogeochemical model, Espinoza-Morriberón et al. [9], studied the dynamical processes involved in the productivity changes during El Niño events in the Peruvian system, where the nutrient content decreased dramatically (especially nitrate and iron). This in turn affected the phytoplankton growth (particularly diatoms) and triggered habitat changes, such as a high mortality for several fish populations.
In studies of the effects of ENSO events on the California Current System, significant chemical and biological perturbations have been documented. During the 1997/1998 events, the nutrient levels were limited substantially, which reduced the area available for biological production, increased the survival rate at higher tropical levels, and increased the flux of carbon dioxide from the ocean to the atmosphere [4]. Based on satellite-derived ocean color, similar observations were made by Kahru and Mitchell [13], who documented a significant decrease in surface Chla (< 0.10 mg m−3) off the U.S. west coast during the 1997/1998 ENSO. The impact of this event on the northern Chile upwelling system was also addressed by Thomas et al. [31]. Based on satellite observations, they documented strong variations in both SST and Chla, which had a negative impact on the whole area.
To date, several consequences of the “Godzilla El Niño 2015/2016” event have been documented with respect to various domains of interest in the Pacific Ocean. For example, the warmest water and strongest convection was reported to be hundreds of kilometers farther west along the Equator during this event than during previous events [14]. Two strong anomalies were documented in the eastern North Pacific: anomalous winds from the south, which weakened nutrient transport and resulted in substantial decreases in phytoplankton biomass, and a 3.5 °C increase in water temperature by January [32]. An abnormal increase in temperature led to decreased densities and nutrient concentrations in the upper 350 m at the Equator [27].
Santoso et al. [25] examined several variables that are relevant to ENSO genesis that characterized the 2015/2016 event. They found that the 2015/2016 event was marked by a record-breaking warm anomaly in the central Pacific, revealing similarities between this event and the 1982/1983 and 1997/1998 events.
The monthly and inter-annual variability in SST and Chla concentration has been widely attributed to ENSO events. For instance, in the Gulf of California, an important association was observed between the values of these variables and the months with negative ENSO anomalies (La Niña), i.e., high Chla concentration values and low SST values. Months with positive ENSO anomalies (El Niño) were correlated with high SST values and low Chla concentrations [11] results which agree with our observations. Significant inter-annual differences in climate index values were observed, i.e., the years with high SST values, such as 2009, were correlated with large positive ENSO anomalies and were identified as strong El Niño years. Years with negative ENSO anomalies (2007–2008 and 2010–2011) had lower SST values and had moderate La Niña events [11].
Based on the observations presented in this study, there are three consequences of the effects of the “Godzilla El Niño 2015/2016” on the Tehuantepec upwelling: (1) wind velocities and vertical water velocity were higher during this event than those observed during the 2013/2014 and 2014/2015 winters; (2) SST values were higher (> 28 °C) off the coast during winter 2015/2016 than they were during the previous winters, with a rise of 5.13 °C between January 2015 and January 2016; and (3) the Chla concentration was lower (< 1 mg m−3), which suggests the advection of nutrient-poor waters.
Although there were no measurements of the primary productivity in the Gulf of Tehuantepec during this event, it is assumed that there was a marked effect on the phytoplankton community and thus on the biological productivity. Because the impacts of ENSO tend to be more dramatic during extreme events, the changes associated with the structure of the water column and Chla surface concentration can lead to significant fatalities, economic loss, and large-scale environmental degradation [9, 25]. Our interpretation is that the wind and vertical water velocity slightly increased in the Tehuantepec gulf during the “Godzilla El Niño” event (winter 2015/2016); however, the levels of Chla were lower compared to previous years, particularly in the winter season. This indicates that although the wind favored strong upwelling, the upwelled water was oligotrophic.
The results of this study show that, from a synoptic point of view, the use of remote sensing for monitoring large-scale processes leads to a better understanding of systems in which the high biological productivity influences the economy of the region. However, many more detailed studies are required, including in situ and satellite observations as well as numerical modeling, to understand the impact of these ENSO events on higher trophic levels, address globally relevant questions, and improve the predictability, preparedness, and response on seasonal time-scales to this abnormal climate disruption. Finally, the results presented here highlight the value of efforts to improve knowledge of the effects of the largest ENSO event on record on systems with high biological productivity.