North Pacific Pressure Dipole Index (NPPD)

The North Pacific pressure dipole is a teleconnection irregular pressure pattern oscillation shifting between positive, neutral, and native phases. The index is made up of 3 locations where pressure patterns are monitored. The first and foremost is in Anchorage Alaska, the second is San Diego, and, the third in Hilo, Hawaii. The North Pacific pressure dipole is very important as it Alters the extension & amplitude of the North Pacific jet stream and shifts precipitation anomalies around the United States. Since The North Pacific pressure dipole index is affected by the 500mb geopotential height layer it could be arguably noted as a tripole and dipole pattern.

The Positive Phase:

The positive phase of the North Pacific pressure dipole is characterized by a weak unstable Aleutian low over the Bearing Sea with higher than normal atmospheric heights and surface pressures over Anchorage, Alaska, with above or near average sea level pressure and height anomalies over San Diego in California with near to above average heights and pressures over Hilo Hawaii. This usually leads to a more Amplified wave pattern with increased Ridging over the West Coast with below average precipitation anomalies and above average surface temperatures, This is usually coupled with a retraction of the Eastern Asian jet or known as the West Pacific jet stream. A great example of a strongly positive North Pacific pressure dipole happened in January of 2021, where the Aleutian low was pretty much non-existent, This led to one of the driest January’s in modern records for the west And huge Arctic outbreaks for the high plains of the US. Below-average anomalous precipitation is observed in the four corners and the Southern Tier of the United States but can be near to above average in the Northeast and the Eastern seaboard.

The Negative Phase:

The negative phase of the North Pacific pressure dipole is basically the opposite of the positive phase, a very strong enhanced Aleutian low with well below average sea level pressure anomalies and Atmospheric Heights observed over Anchorage, Alaska, With near to below average air pressure anomalies with below average height anomalies over San Diego with near to below average height anomalies observed over Hawaii, But the heights here can even be slightly above average and still for the index to remain negative. The negative phase is coupled with the extension and enhancement of a very strong Pacific jet stream that extends just Northeast of Hawaii at latitude 35 to 40, Therefore the resulting conditions would be near to below-average precipitation anomalies over Anchorage, Alaska, since the jet is more enhanced and further south from its positive position with well above average precipitation anomalies across California, the desert Southwest and, the Deep South. Precipitation anomalies can vary somewhat across the southeast and the Northeast since the extension of this jet becomes more variable with Eastward extent. Temperatures can be below average with non El Nino events across the West with above average temperatures with El Nino events in progress. The desert Southwest usually experiences below average sea level temperatures and the Lower 48 states such as the Deep South. Near to above average temperatures are usually observed in the Northeast and Eastern seaboard with at times persistent High latitude blocking over Hudson Bay that enhances above average temperature anomalies in the Northern Tier of the US, Especially during an El Nino episode.