Eastward propagation of mid-latitude near-surface thermohaline anomaly and link to marine heat waves in the eastern North Pacific

Abstract Eastward propagations of near-surface and isopycnal thermohaline disturbances at 25.5–26.3 σ θ with a zonal speed of in-situ geostrophic current were observed in the mid-latitude (40–50° N) North Pacific and were linked to the marine heat wave (MHW) ‘The Blob’ in the Gulf of Alaska (GOA: 150–130° W and 40–50° N). Isopycnal depth anomalies were formed corresponding to wind-stress curl anomalies and propagated eastward. A warm and low-salinity disturbance was formed in the central Pacific in 2011–2012 under the negative maximum of Pacific Decadal Oscillation (PDO). This warm, low-salinity and low-density disturbance propagated eastward and was warmed by heating anomaly of surface heat flux, reduced entrainment and northward heat advection in 2013 due to the weakened westerly wind. The disturbance propagated further eastward and reached the GOA in early 2014. During the 2014–2015 MHW, heating by reduced entrainment and horizontal heat advection maintained the MHW, even though surface heat flux worked as cooling anomaly. The reduced entrainment was caused by the enhanced density stratification of the warm and low-density surface water, together with the entrainment of warm-saline subsurface water which was formed in 2013 in the area west of the GOA and subducted and propagated eastward as subsurface isopycnal warm-saline anomaly. Pycnocline heaving due to wind-induced upwelling corresponding to the strengthened Aleutian Low Pressure under the 2014–2015 El-Niño contributed to the stratification enhancement. The heating of the horizontal heat advection was due to weakened westerly wind on the northern side of strengthened and southward-shifted Aleutian Low Pressure under the El-Niño and positive PDO.