Pacific Meridional Mode

Pacific Meridional Mode (PMM) is a climate mode in the North Pacific. In its positive state, it is characterized by the coupling of weaker trade winds in the northeast Pacific Ocean between Hawaii and Baja California with decreased evaporation over the ocean, thus increasing sea surface temperatures (SST); and the reverse during its negative state. This coupling develops during the winter months and spreads southwestward towards the equator and the central and western Pacific during spring, until it reaches the Intertropical Convergence Zone (ITCZ), which tends to shift north in response to a positive PMM.

The North Pacific Oscillation (NPO) and the "North American Dipole"—two climate oscillations over the North Pacific and North America—trigger PMM modes during winter. Temperature fluctuations in the North Atlantic and the West Pacific oceans and changes in Arctic sea ice have also been proposed as triggers for PMM events.

The PMM is not the same thing as the El Niño-Southern Oscillation (ENSO), but there is evidence that PMM events can trigger ENSO events, especially Central Pacific El Niño events. The PMM state can also modulate hurricane activity in the East Pacific and typhoon activity in the West Pacific oceans and alter precipitation on the continents surrounding the Pacific Ocean. The South Pacific Ocean has a PMM-like mode known as the "South Pacific Meridional Mode" (SPMM) that also influences the ENSO cycle.

In the early 21st century, the intensity of the 2014–16 El Niño event and the highly active 2018 Pacific hurricane and typhoon seasons have been attributed to positive PMM events. With anthropogenic global warming, PMM activity is likely to increase, and some scientists have proposed that a loss of Antarctic and especially Arctic sea ice will induce future positive PMM events.

The existence and properties of the Pacific Meridional Mode were proposed by Chiang and Vimont in 2004. The Pacific Meridional Mode is a form of coupled variability between the latitude of the Intertropical Convergence Zone (ITCZ) and north–south sea surface temperature (SST) gradients in the subtropical Pacific Ocean. Anomalies in the temperature gradient induce shifts in the ITCZ's position, which in turn alters wind-surface heat flux processes that modify the SST structure. Specifically, weaker trade winds are coupled to warm SST anomalies in the North Pacific—in particular along the California coast and between Hawaii and Western North America— focused on the subtropical Pacific, while cold SST anomalies lie in the East Tropical Pacific. The weaker trade winds correspond to southwesterly wind anomalies and mean reduced evaporative cooling, and the ITCZ is displaced northward. Mathematically, the PMM is often defined by maximum covariance analysis of three-month mean SST and wind anomalies in the central and eastern Pacific, with a focus on the northern hemisphere (20°S-30°N, 175°E-85°W) and by removing the ENSO index through linear regression.

The PMM is most intense during the months of January through May. Wind anomalies peak in February and SST anomalies in March. The PMM responses tend to persist into late summer and autumn through interactions with the ITCZ, which reaches its highest latitude and thus strongest interaction with the PMM during these seasons.

Generally, the PMM does not extend farther south than the ITCZ and thus tends not to reach the equator as the ITCZ is normally in the northern hemisphere. This is because the wind-SST feedback operates mostly when the wind anomaly is opposite to the climatological mean wind. This is not the case south of the ITCZ where mean winds come from the south. It is also a primarily ocean mixed layer process, with oceanic dynamics playing a minor role.

In the North Pacific Ocean, the "Victoria mode" is another SST pattern that extends across the entire North Pacific, unlike the more regionally limited PMM which has been described as the eastern part of the Victoria mode. The distinction is that the Victoria mode is an SST pattern while the PMM is an SST-wind coupling pattern, and the Victoria Mode may be a more reliable predictor of ENSO than PMM according to Ren et al. 2023. Another North Pacific climate oscillation, the "North Pacific Mode", resembles the PMM.