The origins of the anomalous warming in the California coastal ocean and San Francisco Bay during 2014–2016

Abstract

During 2014 exceptionally warm water temperatures developed across a wide area off the California coast and within San Francisco Bay (SFB) and persisted into 2016. Observations and numerical model output are used to document this warming and determine its origins. The coastal warming was mostly confined to the upper 100 m of the ocean and was manifested strongly in the two leading modes of upper ocean (0–100 m) temperature variability in the extratropical eastern Pacific. Observations suggest that the coastal warming in 2014 propagated into nearshore regions from the west while later indicating a warming influence that propagated from south to north into the region associated with the 2015–2016 El Niño event. An analysis of the upper ocean (0–100 m) heat budget in a Regional Ocean Modeling System (ROMS) simulation confirmed this scenario. The results from a set of sensitivity runs with the model in which the lateral boundary conditions varied supported the conclusions drawn from the heat budget analysis. Concerning the warming in the SFB, an examination of the observations and the heat budget in an unstructured‐grid numerical model simulation suggested that the warming during the second half of 2014 and early 2016 originated in the adjacent California coastal ocean and propagated through the Golden Gate into the Bay. The finding that the coastal and Bay warming are due to the relatively slow propagation of signals from remote sources raises the possibility that such warming events may be predictable many months or even several seasons in advance.

Predicting the Evolution of the 2014–2016 California Current System Marine Heatwave From an Ensemble of Coupled Global Climate Forecasts

“…This period of warming was forecast by models with some skill, though slightly lagged and with reduced magnitude relative to observations. Based on a heat budget analysis of a regional ocean model, Chao et al (2017) attributed the early 2014 upper ocean temperature increase in the central CCS to a combination of anomalous surface heat fluxes and oceanic influence from the west, and Zaba and Rudnick (2016) found anomalous surface heat flux to be a FIGURE 7 | Spatial anomaly correlation coefficients for January-initialized forecasts. At each lead time, anomaly correlation coefficients were calculated between forecast and observed SSTa for the area plotted in Figure 5 dominant driver of anomalous warming in the southern CCS in the first half of 2014.…”

“…While the CCS was persistently anomalously warm throughout 2014-2016 (Figure 1), regional and broad-scale anomalies during that period evolved in response to a suite of forcing mechanisms (Amaya et al, 2016) including (i) the "Ridiculously Resilient Ridge, " a blocking high pressure system (Swain et al, 2014) that gave rise to the Blob by reducing winddriven mixing and wintertime cooling in the northeast Pacific (Bond et al, 2015), (ii) the subsequent impact of offshore warm anomalies on the CCS, likely through both lateral advection and anomalous atmospheric forcing (Zaba and Rudnick, 2016;Chao et al, 2017;Jacox et al, 2018), (iii) evolution from the Blob warming pattern characteristic of North Pacific Gyre Oscillation (NPGO) variability to an arc-pattern warming resembling Pacific Decadal Oscillation (PDO) variability, a transition that may have been facilitated by tropical-extratropical teleconnections through an El Niño event in 2014-2015 that was initially predicted to be very strong but ultimately was weak (McPhaden, 2015;Amaya et al, 2016;Di Lorenzo and Mantua, 2016), and (iv) the 2015-2016 El Niño event that was one of the strongest on record based on equatorial Pacific SSTa, but whose CCS expression was quite different from that expected based on past strong El Niños (Jacox et al, 2016;Frischknecht et al, 2017).…”

Regional Structure in the Marine Heat Wave of Summer 2015 Off the Western United States

“…The MHW persisted over multiple years due to atmospheric teleconnections from the tropics (Di Lorenzo and Mantua, 2016;Liang et al, 2017;Tseng et al, 2017). This prolonged MHW in the northeast Pacific eventually overlapped with the 2015-16 El Niño (Jacox et al, 2016;Zaba and Rudnick, 2016;Chao et al, 2017;Zaba et al, 2018), though the impacts of the El Niño were weaker than usual along western North America (Barnard et al, 2017;Frischknecht et al, 2017;Paek et al, 2017). This MHW caused major damage to economically important fisheries and other ecosystems from Alaska through California associated with species shifts (Whitney, 2015;Cavole et al, 2016;Auth et al, 2017;Daly et al, 2017;Peterson et al, 2017;Du and Peterson, 2018;Gomez-Ocampo et al, 2018;Kahru et al, 2018) and an unprecedentedly large bloom of toxic algae that spanned the entire coastline (McCabe et al, 2016).…”

“…We analyze a heat budget for the ocean surface mixed layer to understand the cause of SST changes during the development of the split MHW. Many studies of the causes, spatial pattern, and timing of the northeast Pacific MHW of 2014-2016 focus on monthly anomalies (e.g., Bond et al, 2015;Di Lorenzo and Mantua, 2016;Gentemann et al, 2016;Chao et al, 2017), but those anomalies can be due to the presence of a few strong anomalies, or even a single anomaly, on synoptic time scales. Our study relates the SST variability to a wind forcing pattern that normally has a time scale of days, but during summer 2015 lasted for 2 weeks.…”

Biogeochemical Anomalies at Two Southern California Current System Moorings During the 2014–2016 Warm Anomaly‐El Niño Sequence

“…Anomalous sea surface temperatures (SSTs) appeared off California in spring 2014 and reached +5 °C in the upper 50 m of the water column (Gentemann et al, ; Zaba & Rudnick, ). The Warm Anomaly was associated with an “aborted El Niño” and was thus not attributed to direct equatorial El Niño forcing (Hu & Fedorov, ; Li et al, ), although atmospheric teleconnections from the equatorial Pacific likely influenced temperature anomalies in winter 2014–2015 (Chao et al, ; Di Lorenzo & Mantua, ). The Warm Anomaly persisted through spring 2015, at which point normal upwelling conditions produced cooler temperatures and high nutrients nearshore.…”