Pacific Meridional Mode
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.
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.
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.
[7] 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.
[21] The PMM appears to be mainly a consequence of stochastic (random) climate forcing in the extratropics[22] albeit with influence from the atmospheric background state.
[24] The mid-latitude jet stream[25] and, according to Tseng et al. 2020, the East Asian winter monsoon can modulate the NPO-PMM connection.
[19] This occurs via the Atlantic and East Pacific Oceans, being cooled or warmed by the positive North American Dipole.
[32] The cooling Tropical Atlantic induces anticyclonic airflow anomalies over the East Pacific, which in turn oppose the trade winds and trigger a positive PMM.
Anomalies in their strength alter surface heat fluxes over the sea, causing SST changes that peak in spring[2] and spread southwestward.
[85] A circumglobal teleconnection influenced by the PMM and changes in atmospheric pressure systems[86] alters precipitation in the Yellow River valley,[87] and Rossby waves alter the precipitation in the Yangtze River valley of China[59][88] as they emanate from the PMM region westward and interact with the jet stream.
[18] Zhong, Liu and Notaro (2011) found that a positive PMM causes dry winters from the Great Plains into the Northeastern United States through a North Atlantic Oscillation-type teleconnection.
[93] Gibson et al. (2020) found a correlation between PMM and the occurrence of a ridge off the West Coast of the United States, a pattern associated with droughts there.
This is mainly due to the excitation of ENSO variability by the PMM,[100] which in turn induces anomalies in moisture transport,[101] and has been proposed as a predictor of Australian droughts.
Mechanistically, PMM influences ENSO state through several pathways:[8] Positive PMM events result in wind[100] and SST anomalies that resemble these preceding optimal El Niño conditions and westerly wind bursts, and also modulate sub-surface ocean heat content associated with El Niño development.
[113][114] Zonal advection of SST anomalies from the central to the eastern Pacific may allow the PMM to induce canonical El Niño.
[115] You and Furtado (2018) proposed that mismatches between the northern and southern PMM prevent the development of canonical El Niño events while congruence favours it.
[118] Cai, Wang and Santoso (2017) proposed that the unusually west-shifted warm SST anomalies during the 2014–16 El Niño event may have been a consequence of the positive PMM that year,[119] and Paek, Yu and Qian (2017) explained the sustained SST anomalies in the central Pacific during that year with the prolonged positive PMM conditions.
[124] He et al. (2020) identified the persistence of a positive PMM-like SST pattern as a mechanism that impedes the genesis of La Niña after a Central Pacific El Niño event.
[126] According to Shi et al. 2023, the extension of negative PMM associated SST anomalies helped maintain the 2020-23 La Niña.
[127] Not all PMM events trigger subsequent ENSO events,[4] a phenomenon that appears to be caused by varying SST patterns according to Zhao et al. (2020)[128] In the so-called "East PMM" the SST anomalies stay off the equatorial Pacific and are flanked by cold SST anomalies in the tropical East Pacific and impede El Niño development, while in the "West PMM", they extend into the Western Pacific and trigger winds favourable to El Niño development.
[129] The source of this variance is unclear but may relate to forcings from the Atlantic Ocean and diversity in the North Pacific Oscillation.
[137] Zhang et al. (2016) identified a positive correlation between West Pacific accumulated cyclone energy (ACE) and the PMM.
[138] Zuo et al. (2018) proposed that positive PMM events can facilitate an early onset of typhoon seasons through increased genesis in the eastern West Pacific.
[156] The "South Pacific Meridional Mode" (SPMM) is an analogous climate mode in the south Pacific;[22] Zhang, Clement and Di Nezio proposed its existence in 2014[157] and it operates in a nearly identical manner to the northern hemisphere PMM[158] albeit according to You and Furtado (2018) with SST anomalies peaking during (austral) summer and wind anomalies during (austral) winter.
Both the mean climate state—in particular the strength of the ocean surface heat flux variations caused by wind changes and the latitude of the ITCZ—and the storminess in the extratropics influence its variability.
[175] Dima, Lohann and Rimbu (2015) proposed that the Great Salinity Anomaly in the North Atlantic after 1970 modified the Pacific climate through a positive PMM state and suggested that Heinrich events during the late Pleistocene may have caused a similar teleconnection.
[188] Tomas, Deser and Sun (2016) identified a positive PMM and SPMM pattern in models as a response to Arctic sea ice loss.
[191] England et al. (2020) described the development of positive PMM and SPMM-like SST anomalies in response to a loss of Arctic and Antarctic sea ice in the late 21st century.
[192] Orihuela-Pinto et al. (2022) noted a weakening of PMM variability after a shutdown of the Atlantic meridional overturning circulation.
