Slow El Niño‐Southern Oscillation boundary waves

Abstract

Anomalies of sea surface temperature (SST) and heat storage in the upper 400 m of ocean (HS400) for 13 years from 1979 to 1991 are mapped onto a 2° latitude by 5° longitude grid each month over most of the global ocean from 30°S to 60°N. Time sequences at every grid location are band pass filtered to reveal El Niño‐Southern Oscillation (ENSO) signals on period scales ranging from 2 to 7 years. Time‐distance diagrams are constructed along the equator and eastern boundary of the North Pacific, North Atlantic, and South Indian Oceans. These time‐distance diagrams display slow poleward propagation of equatorial ENSO signals, consistently in HS400 and intermittently in SST. In the North Pacific Ocean, ENSO signals in HS400 take 9–18 months to propagate from the equator northward along the west coast of North America to the Gulf of Alaska. In the North Atlantic Ocean, ENSO signals in HS400 take 6–12 months to propagate from the equator along the west coast of Africa to the Strait of Gibraltar. In the South Indian Ocean, ENSO signals in HS400 take 9–18 months to propagate poleward from the equator along the west coasts of Indonesia and Australia to Perth. Speeds of poleward propagation of ENSO waves in both HS400 and SST along these eastern boundaries range from 5 to 30 cm s−1. These ENSO boundary waves propagate in the same direction as Kelvin waves, but with speeds an order of magnitude smaller. They provide a remote and delayed influence upon ENSO activity in the middle‐latitude eastern boundary in each ocean. Time‐distance diagrams are also constructed along the western boundary and equator of the North Pacific, North Atlantic, and South Pacific Oceans. These time‐distance diagrams display slow propagation of ENSO signals in SST and HS400 in both directions depending on the ocean. In the North Pacific Ocean, ENSO signals in both SST and HS400 propagate equatorward from Taiwan to New Guinea, taking 6–18 months to make this transit. Moreover, they arrive on the equator in phase with zonally propagating equatorial ENSO waves. This indicates that ENSO activity in the western subtropical North Pacific Ocean influences ENSO activity in the western equatorial Pacific Ocean through slow equatorward propagation. Speeds of equatorward propagation of ENSO waves in both HS400 and SST along this western boundary range from 5 to 20 cm s−1. In the North Atlantic and South Pacific Oceans this off‐equatorial influence is not possible, since slow ENSO waves in HS400 propagate poleward along these western boundaries.