Extreme water mass anomaly observed in the Hawaii ocean time‐series

Extremely anomalous water mass properties, with deviations as high as 35σ, were observed in the thermocline during January 2001 at the Hawaii Ocean Time‐series site north of Oahu. The spatial distribution of the anomalous waters is consistent with a submesoscale vortex with radius ∼15 km, possibly a remnant of a mesoscale eddy. The most plausible source location of the anomalous waters is offshore of Mexico near Baja California. Given the southwestward subtropical gyre circulation, it is unlikely that these waters were transported directly westward to Hawaii. Unusual northward transport of Equatorial Waters along the North American coast during the 1997–98 El Niño event, and subsequent transport southwestward in the core of a California Undercurrent eddy is more plausible. El Niño modulation of eddy transport and diffusion of water mass properties may substantially impact the mean distribution of properties in the North Pacific subtropical gyre.