Slope and Shelf Flow Anomalies Off Oregon Influenced by the El Niño Remote Oceanic Mechanism in 2014–2016

Abstract

Outputs of the regional ocean circulation model are analyzed to demonstrate the measurable impact of the El Niño remote oceanic forcing mechanism along the US West Coast during the major heat wave period of 2014–2016. The 2‐km horizontal resolution model, based on the Regional Ocean Modeling System (ROMS), was run for the period of 2009–2018. Though the model does not assimilate observations, it performs well by comparison with time series data explaining observed variability on temporal scales from several days to seasonal and interannual. The El Niño‐related oceanic anomalies provided by a global state estimate are introduced in the regional model at the southern boundary at 24N. These propagate alongshore with coastally trapped waves (CTWs) and influence the variability off Oregon (41°–46°N). In particular, CTWs are evident in the subsurface along‐slope current, vs, and in the depth of the 26.5 kg m−3 isopycnal surface over the slope, z26.5. In summer 2014 and 2015, vs anomalies are positive (northward) and z26.5 anomalies are negative (deeper) along the US West Coast. In addition to the CTW patterns, z26.5 anomalies also exhibit slow‐moving features associated with undercurrent widening, separation, and subsurface eddy variability. Over the Oregon shelf, El Niño conditions contributed to the sharp weakening of the southward alongshore current throughout the water column in July 2014 and 2015, despite the near‐average southward, upwelling‐favorable winds.

El Niño‐Related Stratification Anomalies Over the Continental Slope Off Oregon in Summer 2014 and 2015: The Potential Vorticity Advection Mechanism

“…To see where along the slope the q B anomalies reveal themselves and how they may compare to v s , the anomalies in v s (y, t) and q B (y, t) are shown as Hovmöller diagrams. Anomalies in v s (Figure 4a) exhibit fast propagating CTW patterns as discussed in Kurapov et al (2022). In spring-summer 2014 and summer 2015, episodes Our hypothesis is that the advection of the along-slope gradient of q by the anomalously strong v s drives the summer 2014 and 2015 q B anomalies.…”

“…For example, the negative anomaly in the coastal sea level was stronger in observations than the model and the observed surface shelf temperature was colder than modeled; at the same time the alongshore current over the shelf was very well predicted (see Figure 3a, 4a, and 6 in Kurapov et al. [2022]). Analyses of the oceanic conditions off Oregon in winter 2013–2014 may present new opportunities for learning about the regional ocean dynamics and will start with careful comparisons of the observed and modeled full temperature and salinity profiles during winter 2013–2014.…”

“…All the model implementation details can be found in Kurapov et al. (2022) and only a short summary is provided here. The model is based on the Regional Ocean Modeling System, ROMS (http://www.myroms.org), a three‐dimensional model describing the nonlinear evolution of the stratified ocean.…”

“…Kurapov et al. (2022) show that the poleward undercurrent is the salient feature of the v s ( y , t ) annual cycle, peaking in Oregon at the end of July with the speed of 0.07 m s −1 .…”

“…In a series of plots in Figures 3a–3h, it may be noticed that the z 26.5 anomaly near the slope leads the anomaly at the offshore extent of the cross section shown. This effect can be associated with the offshore planetary wave propagation (Kurapov et al., 2022).…”