Temperature and dissolved oxygen concentration in the Pacific Ocean at the northern region of the oxygen minimum zone off Mexico between the last two PDO cool phases

The changes in temperature and dissolved oxygen concentration in the Pacific Ocean in the northern region of the shallow oxygen minimum zone (OMZ) off Mexico were analyzed on the basis of the Word Ocean Database and a series of oceanographic cruises (LEGOZ-Mex). In order to test the changes in both parameters between two similar oceanographic scenarios according to the Pacific Decadal Oscillation (PDO), a comparison was made between the last two cool PDO phases of 1962–1974 and 2002–2012 when conditions might be expected to be relatively similar. Results show that the surface layer (upper layer of 50 m depth) has been warming significantly in two of the three areas included in this study: 1) the southern end of the California Current (0.8 °C; p < 0.001), 2) the southern Gulf of California (0.49 °C; p < 0.001), and 3) the Tropical Transitional Pacific (0.4 °C). Simultaneously, the water column has been deoxygenating significantly (p < 0.001), not only in the subsurface shallow OMZ layer (~ 100 to 300 m depth) in the Tropical Transitional area (− 45%), as has been recorded previously, but from 300 to 1000 m depth (by between −38% and - 57%). The exception was the California Current area at the northern limit of the shallow OMZ, where the deoxygenation was only significant (p < 0.001) from 300 to 1000 m depth (~ − 20%). We conclude that the significant warming of the surface layer in the Pacific OMZ off Mexico has increased the stratification of the water column and contributed to the deoxygenation of the interior ocean, at least between the last two cool phases of the PDO. These changes may be significantly affecting the marine biota that inhabit the shallow OMZs.