Simulation of the 2014 Anomalous Warming in the Northeast Pacific

Abstract

ABSTRACTThe Nucleus for European Modelling of the Ocean (NEMO) numerical model was used to simulate the North Pacific Ocean beginning in January 1960. The model had a horizontal resolution of 0.25o, 46 vertical levels, and employed a spectral nudging assimilation scheme that, unlike standard nudging, nudges only specific frequency and wavenumber bands. This simulation was nudged to the mean and monthly Levitus climatology of potential temperature and salinity. The model was forced with mean monthly winds, net heat flux, and precipitation from the National Centers for Environmental Prediction (NCEP). The simulation was used to study a recent intrusion of much warmer and less saline water than normal from the west into the Gulf of Alaska, beginning in December 2013 and lasting until at least the early summer of 2014. The observed surface temperature anomalies were more than 4 standard deviations above normal. The model reproduced these anomalies in both a qualitative and quantitative manner, reproducing the same scale of anomalies over the region. An anomalous increase in the North Pacific Current (NPC) was found in the model in 2012 and the beginning of 2013, in agreement with observations. This increase in the NPC is associated with the positive phase of the North Pacific Gyre Oscillation. The causes of the temperature anomalies in the Gulf of Alaska could be due to three key factors: (i) an anomalously high, positive, surface heat flux in 2013 over the greater North Pacific; (ii) a significant decrease in the eastward flow of the NPC starting in late 2013 (with an accompanying decrease in cold water advection) after a period of historically strong eastward flow; and (iii) weaker winds throughout most of 2013 accompanied, however, by a shift to stronger northward winds (with an accompanying increase in warm water advection) in late 2013.