23COASTAL Characteristics and Dynamics of the Santa Cruz Eddy

Mesoscale eddies with horizontal scales ranging from dozens to hundreds of kilometers are frequently observed along the U.S. West Coast, especially during late spring and summer months. In this study, the Santa Cruz Eddy (SCE), a mesoscale eddy that forms over Monterey Bay, has been examined using the Navy’s Coupled Ocean / Atmosphere Mesoscale Prediction System (COAMPS) model with an emphasis on its characteristics, evolution, and formation dynamics.

According to the four SCE events examined in this study, the SCE is a warm-core mesoscale cyclonic vortex in the marine boundary layer (MBL) capped by an inversion, with a diameter comparable to the dimensions of Monterey Bay (~40 km) when fully developed. The associated secondary circulation often results in spiral boundary layer clouds. The SCE occurs mostly during nighttime hours. It first appears on the leeside of the Santa Cruz Mountains (SCM) over the northern portion of Monterey Bay in the early evening, grows in size and total vorticity while drifting southward across the Bay, and eventually dissipates after midnight.

Dynamically, the SCE is associated with the interaction of the northwesterlies driven by the semi-permanent North Pacific High (NPH) pressure system, the coastal terrain, and the local circulations induced by land-sea differential heating. The nearly coastline-parallel northwesterlies coming from the north of the SCM are rich with positive vorticity filaments due to the coastal terrain effect. In the early evening hours, the northwesterlies start entering Monterey Bay in accordance with the development of the sea breeze. The total vorticity over Monterey Bay grows accordingly, leading to the development of the SCE. The SCE reaches its maximum strength and size around midnight, after which the vorticity-rich northwesterlies stop turning into Monterey Bay as the land breeze starts developing. The SCE spins down in approximately two hours, presumably due to friction, after the incoming vorticity flux is shut down.