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.
Concept
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.
Other modes
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.
The PMM is distinct from the El Niño–Southern Oscillation (ENSO), which is the principal climate variation in the Pacific Ocean. The two climate modes are not easily separated, however, and they both act to induce decadal climate variations in the Pacific. Separating the Pacific decadal oscillation/Interdecadal Pacific oscillation from PMM is also difficult.
Triggers
The PMM appears to be mainly a consequence of stochastic (random) climate forcing in the extratropics albeit with influence from the atmospheric background state. The North Pacific Oscillation (NPO)—the atmospheric counterpart of the North Pacific Gyre Mode— can trigger PMM events mainly via SST anomalies off the coast of Baja California. The mid-latitude jet stream and, according to Tseng et al. 2020, the East Asian winter monsoon can modulate the NPO-PMM connection.
Warming of the North Atlantic Ocean can favour the onset of negative PMM through Rossby waves generated above the Atlantic Warm Pool. These spread eastward into the Pacific, where they trigger northerly winds that subsequently influence the ocean state. Such warming occurs both as part of the positive state of the Atlantic Multidecadal Oscillation and of the negative state of the so-called "North Atlantic Tripole". The negative "North Atlantic Tripole" state features warm SST anomalies in the subpolar and tropical and cold SST anomalies in the subtropical North Atlantic. This last interaction has become important since the 1990.
The "North American Dipole" is an alternating pattern of atmospheric pressure anomalies over North America, with positive anomalies over the Caribbean and its surroundings and negative anomalies over the Labrador Sea, that is strongest during winter. A positive North American Dipole is often associated with the development of a positive PMM during the subsequent spring. This occurs via the Atlantic and East Pacific Oceans, being cooled or warmed by the positive North American Dipole. The cooling Tropical Atlantic induces anticyclonic airflow anomalies over the East Pacific, which in turn oppose the trade winds and trigger a positive PMM.
Other mechanisms have been described: According to Park et al. 2018, SST anomalies over the Western Hemisphere Warm Pool modulate PMM onset in late summer. An eastward extension of the Kuroshio Extension has been related to PMM development by Joh and Di Lorenzo in 2019 and may be part of a decade-long pattern of climate oscillation in the Pacific, in the form of atmospheric pressure anomalies that travel counterclockwise around the North Pacific. According to Chen, Yu and Wen (2014), the spring Arctic Oscillation can trigger SST anomalies that resemble those of PMM and the PMM may be the pathway through which the Arctic Oscillation influences ENSO events. Zhou, Yang and Zheng (2017) proposed that an increased latitude of the West Pacific Warm Pool can trigger negative PMM events through changes in trade winds and SST gradients. Pausata et al. (2020) found a development of positive PMM conditions in response to volcanic eruptions in the northern hemisphere tropics. Simon Wang, Jiang and Fosu (2015) proposed that ENSO triggering can occur through an Indian Ocean-West Pacific-PMM route, but the mechanism is unclear. Cao et al. (2021) proposed that increased snow cover over North America can trigger a negative PMM-like teleconnection. Lin et al. (2021) indicated that the solar cycle modifies the PMM, with an active Sun favouring a positive PMM state and an inactive one a negative PMM. Cai et al. (2022) proposed that increased snow cover over the Tibetan Plateau in winter can induce strengthening trade winds over the eastern subtropical Pacific, causing a negative PMM state. This process is mediated through tropospheric temperature anomalies that are transported by the jet stream to the Pacific that influence the NPO. This relationship has become established in the 2000s, presumably due to Pacific decadal oscillation and Atlantic multidecadal oscillation state changes. Zhang et al. (2022) stated that the NPO is too limited to the extratropics to drive a PMM-like variability, which would instead be mainly controlled by variability of the Aleutian Low.
There is little study on whether ENSO induces PMM changes with research in 2011, 2018 and 2023 suggesting that positive ENSO events could trigger negative PMM events and less commonly, negative ENSO events positive PMM events, while Capotondi et al. (2019) proposed that SST anomalies in the west-central Pacific can induce warming along the West Coast resembling that of the PMM.
Growth and demise
Variations in the strength of the North Pacific High due to extratropical climate variability (e.g. the North Pacific Oscillation [NPO] in winter) induce changes in the strength of the trade winds. Anomalies in their strength alter surface heat fluxes over the sea, causing SST changes that peak in spring and spread southwestward. This process, when triggered by NPO variations during winter, is known as the "seasonal footprinting mechanism", and it involves positive NPO anomalies weakening the trade winds. According to Wu et al. (2009), the decreased evaporation occurs southwest of the original SST anomaly because of the easterly trade winds, and thus the SST anomaly tends to spread southwestward while ocean transportation changes cause a northward movement. Particularly in the northwest Tropical Pacific, increased insolation during spring facilitates the growth of PMM events.
Cross-equatorial winds triggered by temperature gradients between the hemisphere facilitate the development of the PMM. As they cross the equator, the Coriolis force deflects them into a direction opposite to that of the trade winds, weakening them. In turn, the ~PMM facilitates the development of cross-equatorial winds, generating a positive feedback, especially since the cross-equatorial winds act to trigger a cooling response in the southern hemisphere and according to Wu et al. (2009) along the equator.
Cloud albedo feedbacks enhance the growth of the PMM, while ocean transport hinders it. According to Wu et al. (2009), turbulent heat fluxes act to dissipate the originating SST anomaly. After August, westerly winds south of the ITCZ act to dissipate SST anomalies.
Effects
The PMM is the major pathway through which the extratropics influence tropical climate in the Pacific Ocean. PMM variations influence tropical cyclone activity in the Pacific and Atlantic Oceans.
Among the phenomena associated with the PMM are: A PMM event in 2014 significantly influenced the 2013–2015 North Pacific marine heatwave, which had significant impacts on the ocean off the west coast of North America. Amaya et al. (2020) proposed that a positive PMM state in 2019 similarly enhanced the 2019 North Pacific marine heatwave through an ITCZ shift and resulting alterations in the atmospheric circulation. Vice versa, Chen, Shi and Lin (2021) proposed that certain "blob" events can trigger a positive PMM. The PMM-like coupling between SST and wind anomalies may control the mean latitude of the ITCZ. The PMM may act to dampen low-frequency climate variations in the tropics. Positive PMM weakens the West Pacific subtropical anticyclone. The PMM may alter the behaviours of the East Pacific oxygen minimum zones.
Other suggested correlations:
Muñoz, Wang and Enfield (2010) identified a teleconnection from the PMM to the spring SSTs in the Gulf of Mexico and the Caribbean.