Long-lived cold blobs in the Northeast Pacific linked with the tropical La Niña

Abstract

This study explores the counterpart of marine heatwave, known as the cold blob, in the Northeast Pacific (NEP) by selecting the long-lived events with the normalized sea surface temperature (SST) anomalies averaged over the blob area (160°–135° W, 40°–50° N) smaller than − 0.75 for at least 5 months. There are 10 summer-peak events with a stronger intensity and 7 winter-peak events. The summer-peak cold blob events are mostly attributed to the vertical entrainment when the mixed layer depth is shoaling. For the winter-peak group, the greater heat loss at surface is the principal factor to stimulate the cold blob, associated with the intense westerly anomalies around the blob area. However, the vertical entrainment generally hinders the amplification of winter-peak cold blobs. Moreover, the advection of colder-than-normal water to the blob area contributes much less for both two groups. We conjecture that the NEP cold blob and tropical La Niña-like state are potentially linked. The summer-peak cold blob event is often followed by a La Niña event in the ensuing winter. On the other hand, the equatorial La Niña-like SST cooling in late summer and autumn in turn enhances the winter-peak cold blobs. We find the subtropical region, particularly near the Baja California, may act as the oceanic bridge to connect the NEP cold blob and equatorial La Niña.