Monterey Bay and contiguous waters of the California Current System have been observed repeatedly since 1929, most intensively since 1989 with ships, moorings, and autonomous vehicles. Here, seasonal, interannual, and multidecadal variations are linked to regional weather and large-scale climate ocean-atmosphere dynamics. In the springtime, the Northeast Pacific subtropical high-pressure system strengthens, intensifying northwesterly alongshore winds. These winds drive coastal upwelling that fertilizes nearshore surface water with phytoplankton nutrients, resulting in a dramatic increase in biological productivity. Upwelling weakens over summer into fall, allowing nutrient-depleted offshore water to move toward the coast. Southerly winter storm winds deepen the mixed layer and further enhance onshore flow. El Niño interrupts these seasonal cycles with varying intensity every three to eight years, but typically peaks during the low-productivity winter season, lessening its biological impact. Over the 1989­–2016 period of observation, a negative phase of the multidecadal Pacific Decadal Oscillation is observed as a 15-year cool period following the strong 1997–1998 El Niño. Two recently identified basin-scale phenomena, the central Pacific El Niño Modoki and the North Pacific Gyre Oscillation, increased in strength during this period. In Monterey Bay, primary productivity increased substantially during the cool period, at about 3% per year. This shift also marked the beginning of a monotonic decline in subsurface oxygen, which decreased by 3% annually in the 300–400 m depth horizon, above the oxygen minimum zone. Anthropogenically driven increases in surface pCO₂ and acidity (pH) are notable in Monterey Bay in spite of high near-surface variability. Recently, over 2014–2016, Monterey Bay has warmed, interrupting the 1988–2012 cooling trend. Even with the warm years included, however, there is no overall increasing trend in temperature at any depth from 1988 to the present. A recompilation of historical temperature data back to 1929 indicates that over this longer period, average and cool years have not been significantly different, but warm episodes have been hotter over the last few decades, leading to a trend of increasing temperature over the past 89 years. The 2014–2016 warm period included “the Blob” and an El Niño, and is reminiscent of similar conditions in the early 1940s. It is not known if the warm conditions will continue, or we will return to a cooler and drier than average period. Our observations highlight the value of long-term data. Such data collections will need to be automated to increase their value and sustainability.

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