Off-equatorial Western Pacific Research Articles

The role of interannual ENSO events in decadal timescale transitions of the Interdecadal Pacific Oscillation

The build-up of decadal timescale upper ocean heat content in the off-equatorial western tropical Pacific can provide necessary conditions for interannual El Niño/Southern Oscillation (ENSO) events to contribute to decadal timescale transitions of tropical Pacific SSTs to the opposite phase of the Interdecadal Pacific Oscillation (IPO). This can be viewed as a corollary to subseasonal westerly wind burst events contributing to El Niño interannual timescale transitions. A long pre-industrial control run with CESM1 is analyzed to show that there is a greater chance of ENSO activity to contribute to an IPO transition when off-equatorial western Pacific Ocean heat content reaches either a maximum (for El Niño to contribute to a transition to positive IPO) or minimum (for La Niña to contribute to a transition to negative IPO) as seen in observations. If above a necessary ocean heat content threshold, the convergence associated with westerly anomaly near-equatorial surface winds associated with El Niño activity can draw that heat content equatorward to sustain anomalously warm western and central Pacific SSTs. These are associated with positive precipitation and convective heating anomalies, a Gill-type response and wind stress curl anomalies that continue to feed warm water into the near-equatorial western Pacific. These conditions then sustain the decadal-timescale transition to positive IPO (with the opposite sign for transition to negative IPO). Associated central equatorial Pacific convective heating anomalies produce SLP and wind stress anomalies in the North and South Pacific that can excite westward-propagating off-equatorial oceanic Rossby waves to contribute to the western Pacific thermocline depth and consequent heat content anomalies.

Cases for the sole effect of the Indian Ocean Dipole in the rapid phase transition of the El Ni\xf1o\u2013Southern Oscillation

The role of the Indian Ocean sea surface temperature (SST) in the rapid phase transition of the El Niño–Southern Oscillation (ENSO) has received much attention over the last few decades. However, the distinctive role of the Indian Ocean Dipole (IOD) and its underlying dynamics in controlling the rapid transition remains debatable, since it is difficult to isolate the sole effect of the IOD using observation. By conducting model experiments, this study demonstrated that the positive IOD cases could contribute to the rapid phase transition from El Niño to La Niña without the assistance of the Indian Ocean Basin–wide Mode. The westerly wind anomalies over the tropical western Pacific induced not only the El Niño phase via descending Kelvin waves in the equatorial eastern Pacific but also the SST cooling in the off-equatorial western Pacific via ascending Rossby waves. The positive feedback between the upwelling Rossby waves and cool SST maintained the Philippine Sea anticyclone anomalies. The accompanying easterly wind anomalies over the tropical western Pacific triggered upwelling oceanic Kelvin waves, leading to the occurrence of La Niña. This study shows the possible role of the IOD in the rapid phase transition of ENSO.

Projected increase in El Niño-driven tropical cyclone frequency in the Pacific

The El Nino/Southern Oscillation influences tropical cyclone variability. Under climate change, cyclones around Pacific island nations are projected to increase in frequency during El Nino events and decrease during La Nina events. The El Nino/Southern Oscillation (ENSO) drives substantial variability in tropical cyclone (TC) activity around the world1,2,3. However, it remains uncertain how the projected future changes in ENSO under greenhouse warming4,5,6,7,8 will affect TC activity, apart from an expectation that the overall frequency of TCs is likely to decrease for most ocean basins9,10,11. Here we show robust changes in ENSO-driven variability in TC occurrence by the late twenty-first century. In particular, we show that TCs become more frequent (∼20–40%) during future-climate El Nino events compared with present-climate El Nino events—and less frequent during future-climate La Nina events—around a group of small island nations (for example, Fiji, Vanuatu, Marshall Islands and Hawaii) in the Pacific. We examine TCs across 20 models from the Coupled Model Intercomparison Project phase 5 database12, forced under historical and greenhouse warming conditions. The 12 most realistic models identified show a strong consensus on El Nino-driven changes in future-climate large-scale environmental conditions that modulate development of TCs over the off-equatorial western Pacific and the central North Pacific regions. These results have important implications for climate change and adaptation pathways for the vulnerable Pacific island nations.

Tropical Atlantic-Korea teleconnection pattern during boreal summer season

The remote impact of tropical Atlantic sea surface temperature (SST) variability on Korean summer precipitation is examined based on observational data analysis along with the idealized and hindcast model experiments. Observations show a significant correlation (i.e. 0.64) between Korean precipitation anomalies (averaged over 120–130°E, 35–40°N) and the tropical Atlantic SST index (averaged over 60°W–20°E, 30°S–30°N) during the June–July–August (JJA) season for the 1979–2010 period. Our observational analysis and partial-data assimilation experiments using the coupled general circulation model demonstrate that tropical Atlantic SST warming induces the equatorial low-level easterly over the western Pacific through a reorganization of the global Walker Circulation, causing a decreased precipitation over the off-equatorial western Pacific. As a Gill-type response to this diabatic forcing, an anomalous low-level anticyclonic circulation appears over the Philippine Sea, which transports wet air from the tropics to East Asia through low-level southerly, resulting an enhanced precipitation in the Korean peninsula. Multi-model hindcast experiments also show that predictive skills of Korean summer precipitation are improved by utilizing predictions of tropical Atlantic SST anomalies as a predictor for Korean precipitation anomalies.

Role of sea surface temperature anomalies in the tropical Indo-Pacific region in the northeast Asia severe drought in summer 2014: month-to-month perspective

The severe drought over northeast Asia in summer 2014 and the contribution to it by sea surface temperature (SST) anomalies in the tropical Indo-Pacific region were investigated from the month-to-month perspective. The severe drought was accompanied by weak lower-level summer monsoon flow and featured an obvious northward movement during summer. The mid-latitude Asian summer (MAS) pattern and East Asia/Pacific teleconnection (EAP) pattern, induced by the Indian summer monsoon (ISM) and western North Pacific summer monsoon (WNPSM) rainfall anomalies respectively, were two main bridges between the SST anomalies in the tropical Indo-Pacific region and the severe drought. Warming in the Arabian Sea induced reduced rainfall over northeast India and then triggered a negative MAS pattern favoring the severe drought in June 2014. In July 2014, warming in the tropical western North Pacific led to a strong WNPSM and increased rainfall over the Philippine Sea, triggering a positive EAP pattern. The equatorial eastern Pacific and local warming resulted in increased rainfall over the off-equatorial western Pacific and triggered an EAP-like pattern. The EAP pattern and EAP-like pattern contributed to the severe drought in July 2014. A negative Indian Ocean dipole induced an anomalous meridional circulation, and warming in the equatorial eastern Pacific induced an anomalous zonal circulation, in August 2014. The two anomalous cells led to a weak ISM and WNPSM, triggering the negative MAS and EAP patterns responsible for the severe drought. Two possible reasons for the northward movement of the drought were also proposed.

The weakening of the ENSO–Indian Ocean Dipole (IOD) coupling strength in recent decades

This study examines a recent weakening of the coupling between the El Nino-Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) mode after the 2000s and 2010s compared to the previous two decades (1980s and 1990s). The correlation between the IOD during the September–November season and the Nino3.4 index during the December–February season is 0.21 for 1999–2014, while for the previous two decades (1979–1998) it is 0.64. It is found that this weakening of the ENSO–IOD coupling during the 2000s and 2010s is associated with different spatial patterns in ENSO evolution during the boreal spring and summer seasons. During the boreal spring season of the El Nino developing phase, positive precipitation anomalies over the northern off-equatorial western Pacific is systematically weakened during the 2000s and 2010s. This also weakens the low-level cross-equatorial southerly flow, which can cause local negative precipitation anomalies over the maritime continent through increased evaporation and cold and dry moist energy advection. The weakened negative precipitation anomalies over the maritime continent reduces the amplitude of the equatorial easterly over the IO, therefore, suppresses a ENSO-related IOD variability. An analysis using climate models that participated in the Coupled Model Intercomparison Project phase 5 (CMIP5) supports this observational findings that the amplitude of the cross-equatorial southerly flow and associated suppressed convective activities over the maritime continent during the El Nino developing season are critical for determining the ENSO–IOD coupling strength in climate models.

Impact of Two Distinct Teleconnection Patterns Induced by Western Central Pacific SST Anomalies on Korean Temperature Variability during the Early Boreal Summer

Abstract This study investigates the impact of western central Pacific sea surface temperatures (SSTs) on the temperature variability over the Korea Peninsula during the early boreal summer season. The authors found a significant positive correlation between June temperature anomalies across the Korea Peninsula and a horseshoe pattern of ENSO-related SST anomalies in the western central Pacific during May. While this SST pattern is relatively stationary throughout the boreal summer, the SST-driven atmospheric teleconnection exhibits dramatic subseasonal changes. During May, positive precipitation anomalies caused by SST warming in the northern central Pacific (NCP; 20°–30°N, 160°E–160°W) induce a wave train extending in a northwest–southeast direction. During June, while this wave train pattern is weakened, a Pacific–Japan (PJ) pattern is formed by positive precipitation anomalies over the off-equatorial western Pacific (off-WP). Even though dramatic subseasonal changes exist in the teleconnection patterns, both patterns contribute to temperature warming over Korea. It is shown that the subseasonal change in the atmospheric teleconnection is caused by changes in climatological precipitation. During May, significant climatological precipitation related to the pre-mei-yu and pre-baiu fronts occurs over the NCP. These wet background conditions provide a favorable environment for anomalous convection over the NCP but not over the off-equatorial western Pacific. On the other hand, during June, convective activities over the off-WP result in the formation of a PJ pattern, enhanced by an increase in climatological precipitation associated with the activation of the western North Pacific (WNP) monsoon. These results indicate that climatological conditions play an important role in the formation of atmospheric teleconnection linking Pacific and East Asian climates.

A Study of the Impact of Off-Equatorial Warm Pool SST Anomalies on ENSO Cycles

Abstract Concurrent with most large El Niño events, cold sea surface temperature (SST) anomalies are observed over the western Pacific warm pool region (WPWP). Observational evidence that SST anomalies that form in the off-equatorial western Pacific during El Niño–Southern Oscillation (ENSO) cycles are forced by subsurface ocean processes equatorward of 12°N and air–sea fluxes poleward of 12°N is presented. It is demonstrated that diurnal mixing in the ocean equatorward of 12°N plays a significant role in bringing subsurface temperature anomalies to the sea surface during an El Niño event. The role of SST anomalies equatorward of 12°N in ENSO cycles is tested in the Zebiak–Cane coupled model, modified to allow for the impact of subsurface temperatures on SSTs. This coupled model successfully simulates cold SST anomalies in the off-equatorial northwestern Pacific that are observed to occur during the warm phase of ENSO and the atmospheric response to these anomalies, which is composed of both westerlies in the central Pacific and easterlies in the far western equatorial Pacific. It is found that there is little net change in the zonal mean wind stress at the equator, suggesting that the westerlies cancel the impact of the easterlies on the basin-scale tilt of the equatorial zonal mean thermocline depth. The anomalous westerly winds in the central equatorial Pacific are found to increase the amplitude of an El Niño event directly by increasing anomalous warm zonal advection and reducing upwelling. Moreover, the off-equatorial anticyclonic wind stress associated with the cold SST anomalies during the warm phase of ENSO tends to reduce the discharge of the equatorial heat content. Thus, the coupled processes over the western Pacific warm pool can serve as a positive feedback to amplify ENSO cycles.

Atmospheric Circulation Cells Associated with the El Niño–Southern Oscillation

Abstract Atmospheric circulation cells associated with the El Nino–Southern Oscillation (ENSO) are described and examined using the NCEP–NCAR reanalysis field and the NCEP sea surface temperature (SST) from January 1950 to December 1999. The divergent wind and pressure vertical velocity are employed for the identification of atmospheric circulation cells. The warm phase of ENSO shows positive SST anomalies in the equatorial eastern Pacific and along the east coast of Asia and the west coast of North America, and negative SST anomalies in the off-equatorial western Pacific and in the central North Pacific. Associated with this SST anomaly distribution are variations of atmospheric zonal and meridional circulation cells over the Pacific. The equatorial zonal Walker circulation cell is weakened, consistent with previous schematic diagrams. The anomalous meridional Hadley circulation cell in the eastern Pacific shows the air rising in the Tropics, flowing poleward in the upper troposphere, sinking in the subt...

A Unified Oscillator Model for the El Niño–Southern Oscillation

The delayed oscillator, the western Pacific oscillator, the recharge-discharge oscillator, and the advective- reflective oscillator have been proposed to interpret the oscillatory nature of the El Nino-Southern Oscillation (ENSO). All of these oscillator models assume a positive ocean-atmosphere feedback in the equatorial eastern and central Pacific. The delayed oscillator assumes that the western Pacific is an inactive region and wave reflection at the western boundary provides a negative feedback for the coupled system to oscillate. The western Pacific oscillator emphasizes an active role of the western Pacific in ENSO. The recharge-discharge oscillator argues that discharge and recharge of equatorial heat content cause the coupled system to oscillate. The advective- reflective oscillator emphasizes the importance of zonal advection associated with wave reflection at both the western and eastern boundaries. Motivated by the existence of these different oscillator models, a unified oscillator model is formulated and derived from the dynamics and thermodynamics of the coupled ocean-atmosphere system. Consistent with ENSO anomaly patterns observed in the tropical Pacific, this oscillator model considers sea surface temperature anomalies in the equatorial eastern Pacific, zonal wind stress anomalies in both the equatorial central Pacific and the equatorial western Pacific, and thermocline depth anomalies in the off-equatorial western Pacific. If the western Pacific wind-forced response is neglected, thermocline and zonal wind stress anomalies in the western Pacific are decoupled from the coupled system, and the unified oscillator reduces to the delayed oscillator. If wave reflection at the western boundary is neglected, the unified oscillator reduces to the western Pacific oscillator. The mathematical form of the recharge-discharge oscillator can also be derived from this unified oscillator. Most of the physics of the advective-reflective oscillator are implicitly included in the unified oscillator, and the negative feedback of wave reflection at the eastern boundary is added to the unified oscillator. With appropriate model parameters chosen to be consistent with those of previous oscillator models, the unified oscillator model oscillates on interannual timescales.

On the Atmospheric Responses to Tropical Pacific Heating during the Mature Phase of El Niño

The atmospheric heating and sea surface temperature (SST) anomalies during the mature phase of El Niño are observed to show both eastern and western Pacific anomaly patterns, with positive anomalies in the equatorial eastern/central Pacific and negative anomalies in the off-equatorial western Pacific. The detailed spatial patterns of the heating anomalies differ from the SST anomalies. The heating anomalies are more equatorially confined than the SST anomalies, and maxima of positive and negative heating anomalies are located farther to the west than the SST anomalies. The Gill–Zebiak atmospheric model assumes that the atmospheric initial heating has the same spatial patterns as the SST anomalies. This assumption results in some unrealistic model simulations for El Niño. When the model heating anomaly forcing is modified to resemble the observed heating anomalies during the mature phase of El Niño, the model simulations have been improved to 1) successfully simulate equatorial easterly wind anomalies in the western Pacific, 2) correctly simulate the position of maximum westerly wind anomalies, and 3) reduce unrealistic easterly wind anomalies in the off-equatorial eastern Pacific. This paper shows that off-equatorial western Pacific negative atmospheric heating (or cold SST) anomalies are important in producing equatorial easterly wind anomalies in the western Pacific. These off-equatorial cold SST anomalies in the western Pacific also contribute to equatorial westerly wind anomalies observed in the central Pacific during the mature phase of El Niño. Although off-equatorial cold SST anomalies in the western Pacific are smaller than equatorial positive SST anomalies in the eastern Pacific, they are enough to produce atmospheric responses of comparable magnitude to the equatorial eastern Pacific. This is because the atmospheric mean state is convergent in the western Pacific and divergent in the equatorial eastern Pacific. By either removing the atmospheric mean convergence or removing off-equatorial cold SST anomalies in the western Pacific, the atmospheric responses show no equatorial easterly wind anomalies in the western Pacific. In the Gill–Zebiak model, the mean wind divergence field is an important background state, whereas the mean SST is secondary.

Western Pacific interannual variability associated with the El Niño‐Southern Oscillation

Observations of sea surface temperature (SST), sea level pressure (SLP), surface wind, and outgoing longwave radiation (OLR) show that the El Niño‐Southern Oscillation (ENSO) displays western Pacific anomaly patterns in addition to eastern Pacific anomaly patterns. During the warm phase of ENSO, warm SST and low SLP anomalies in the equatorial eastern Pacific and low OLR anomalies in the equatorial central Pacific are accompanied by cold SST and high SLP anomalies in the off‐equatorial western Pacific and high OLR anomalies in the off‐equatorial far western Pacific. Also, while the zonal wind anomalies over the equatorial central Pacific are westerly, those over the equatorial far western Pacific are easterly. The nearly out‐of‐phase behavior between the eastern and western tropical Pacific is also observed during the cold phase of ENSO, but with anomalies of opposite sign. These western Pacific interannual anomaly patterns are robust features of ENSO, independent of data sets. It is argued that equatorial easterly (westerly) wind anomalies over the far western Pacific during the warm (cold) phase of ENSO are initiated by off‐equatorial western Pacific cold (warm) SST anomalies, and that these winds are important for the evolution of ENSO. An atmosphere model is employed to demonstrate that small off‐equatorial western Pacific cold (warm) SST anomalies (compared to those in the east) are sufficient to produce equatorial easterly (westerly) wind anomalies as observed over the far western Pacific. The coupled ocean‐atmosphere model of Zebiak and Cane is then modified to investigate the evolution of the western Pacific interannual anomaly patterns in a coupled ocean‐atmosphere system, by including a meridional structure to the subsurface temperature parameterization in the western Pacific. The modified model produces both western and eastern Pacific interannual anomaly patterns.

Recent Global Warming and Interdecadal Ocean-Atmosphere Coupled Phenomena

Hydrographic observations in the western tropical Pacific show not only the heat content variability related to ENSO but also another decadal trend related to the recent global warming issue. Since the background of SST is high in the western tropical Pacific, even weak SST anomalies may strongly affect the location of the Aleutian Low as well as the Asian monsoon in the extratropics in winter. In the off-equatorial western Pacific the ocean behaves like a dynamic slave to the winter Asian monsoon as demonstrated by the oceanic model simulation. In particular, the winter monsoon variations are responsible for the maturity or immaturity of the cold Mindanao Dome off the Philippine coast. The active (inactive) summer monsoon followed by the increasingly anomalous easterlies (westerlies) over the western tropical Pacific from summer through winter appears to be responsible for the positive (negative) SST anomalies in the western tropical Pacific. This suggests that an interesting positive feedback mechanism may exist in the Asian monsoon system, i. e., a coupled ocean-atmosphere-land system. Since vast amounts of short term natural climate variabilities ranging from several years to decades seem to originate in those SST variations in the western tropical Pacific, it will be necessary to look more closely at the Asian monsoon system.