The tropical cyclone season in the North Pacific in 2015 was extremely active, primarily due to the ongoing strong El Niño. Numerous records were broken across the region and in the central North Pacific (an area bounded by the 140°W and 180°W lines of longitude) it was the most active season on record by all measures.

By December, tropical cyclone activity in the northern hemisphere usually comes to an end, but on New Year’s Eve an unusual tropical depression formed in the central North Pacific very close to the equator. The depression dissipated early in January, but was a precursor of what was to come a week later. Tropical Storm Pali formed in the far southwestern corner of the central North Pacific region at a location closer to the equator than any other storm on record in the western hemisphere (east of the International Dateline).

Pali has fluctuated in strength in the last few days as it has drifted northwards, but a recent burst of intensification has resulted in it becoming the earliest central North Pacific hurricane to form in a calendar year on record. Pali beats the previous record set by Hurricane Ekeka in late January 1992.

Hurricane Pali at 0330 UTC on 12 January 2016
Image courtesy of the US Naval Research Laboratory

Although Hurricane Pali is no threat to any major land masses it will be watched closely in the next few days due to an unusual track being forecast by several computer models. It is currently predicted to sink southwards towards the equator and some models even suggest it could reach or even cross the equator as a tropical storm.

Conventional understanding of the science behind storm formation tells us that cyclones rarely form close to the equator since the Coriolis Effect, which induces rotation, is so small. In recent history there have been a couple of notable storm formations close to the equator. In 2001 Tropical Storm Vamei developed at latitude 1.5°N close to Singapore and in 2004 Tropical Storm Agni was briefly observed to cross the equator into the southern hemisphere as a weak depression before developing into a tropical storm at latitude 0.7°N in the Indian Ocean. However, there is no observed precedent of a full blown hurricane such as Pali moving close to the equator. Thus Pali should provide a useful insight into the behaviour cyclones in what is usually a cyclone-free zone.

Official warnings for the tropical cyclones in the central North Pacific are produced by the Central Pacific Hurricane Center. The Met Office routinely supplies predictions of cyclone tracks from its global forecast model to regional meteorological centres worldwide, which are used along with guidance from other models in the production of forecasts and guidance. We also provide updates on current tropical storms via @metofficestorms on Twitter.