Large Eddy Simulations of Upper-Ocean Response to a Midlatitude Storm and Comparison with Observations

Abstract

A large eddy simulation (LES) model is used to investigate an upper-ocean response to a fall storm in the open ocean of the North Pacific Ocean. The storm is characterized by rapid increases in wind speed and surface heat loss but a relatively steady wave field. The LES model shows that surface convergence zones or windrows organize into line patterns aligned with the wind direction, evolving from nearly parallel lines to irregular structures featuring Y junctions as the wind speed increases. The downwelling-to-upwelling velocity ratio ranges between 1.2 and 1.6, indicating a moderate level of asymmetry between the downwelling and upwelling plumes in Langmuir circulation. During the storm, the turbulent Langmuir number Lat increases from 0.2 to 0.5 while the vertical turbulence intensity σw2 decreases from 1.4 to 0.7 u*2, where u* is the friction velocity. The order of turbulence intensities in three directions switches from crosswind ≈ vertical > downwind directions to downwind > crosswind > vertical directions. This suggests a transition from Langmuir to shear turbulence as the storm progresses. The Hoennikker number (Ho) remains below 0.1 and the strong evaporative heat loss does not contribute much to the turbulence generation in the ocean mixed layer. The LES results are compared with in situ and acoustic measurements collected during the storm. Patterns of model-predicted near-surface downwelling zones are in good agreement with horizontal distributions of bubble clouds revealed in sidescan sonar images. Striking similarity is also found in the temperature anomalies between the LES model and high-resolution thermistor chain measurements.