A Review of Mechanisms for Formation of an Anomalous Anticyclone in Western North Pacific During El Niño

Abstract The phase relationships of the western North Pacific (WNP) summer monsoon (WNPM) with the Australian monsoon (AM) and Indian monsoon (IM) are investigated using observational rainfall, SST, and NCEP reanalysis data for the period of 1979–2005. It is found that a strong WNPM often follows a strong AM but leads a weak AM, and a significant simultaneous negative correlation appears between WNPM and IM. The in-phase relationship from AM to the succeeding WNPM occurs often during the El Niño decaying phase when the warm eastern Pacific SST anomaly (SSTA) weakens AM through anomalous Walker circulation and the persistence of an anomalous WNP anticyclone from the boreal winter to summer leads to a weak WNPM. The out-of-phase relation from WNPM to the succeeding AM occurs either during the El Niño early onset year when the warm SSTA in June–August (JJA) is strong enough to force a low-level cyclonic flow anomaly in WNP and in December–February (DJF) the same warm SSTA forces a weak AM, or during the El Niño decaying phase when the persistence of the WNP anomalous anticyclone causes a weak WNPM and the transition of a warm to a cold episode causes a strong AM in DJF. The simultaneous negative correlation between WNPM and IM often appears either during the El Niño early onset years when the warm eastern Pacific SSTA induces the cyclonic wind shear that strengthens WNPM but suppresses convection over India, or during the El Niño decaying summer when a weak WNPM results from the persistence of the local anomalous anticyclone and a strong IM results from the El Niño-to-La Niña transition or a basin-wide Indian Ocean warming.

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Atmospheric Dynamic and Thermodynamic Processes Driving the Western North Pacific Anomalous Anticyclone during El Niño. Part I: Maintenance Mechanisms

Journal of Climate ◽

10.1175/jcli-d-16-0489.1 ◽

2017 ◽

Vol 30 (23) ◽

pp. 9621-9635 ◽

Cited By ~ 45

Author(s):

Bo Wu ◽

Tianjun Zhou ◽

Tim Li

Keyword(s):

El Niño ◽

North Pacific ◽

Western North Pacific ◽

El Nino ◽

Global Climate Model ◽

Eastern Pacific ◽

Circulation System ◽

Remote Forcing ◽

Anomalous Anticyclone ◽

Central Eastern

The western North Pacific anomalous anticyclone (WNPAC) is an important low-level circulation system that connects El Niño and the East Asian monsoon. In this study, the mechanisms responsible for the formation and maintenance of the WNPAC are explored. Part I of this study focuses on the WNPAC maintenance mechanisms during El Niño mature winter and the following spring. Moisture and moist static energy analyses indicated that the WNPAC is maintained by both the remote forcing from the equatorial central-eastern Pacific via the atmospheric bridge and the local air–sea interactions. Three pacemaker experiments by a coupled global climate model FGOALS-s2, with upper-700-m ocean temperature in the equatorial central-eastern Pacific restored to the observational anomalies plus model climatology, suggest that about 60% (70%) intensity of the WNPAC during the winter (spring) is contributed by the remote forcing from the equatorial central-eastern Pacific. The key remote forcing mechanism responsible for the maintenance of the WNPAC is revealed. In response to El Niño–related positive precipitation anomalies over the equatorial central-eastern Pacific, twin Rossby wave cyclonic anomalies are induced to the west. The northern branch of the twin cyclonic anomalies advects dry and low moist enthalpy air into the tropical western North Pacific, which suppresses local convection. The suppressed convection further drives the WNPAC.

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Response of western North Pacific anomalous anticyclone in the summer of decaying El Niño to global warming: Diverse projections based on CMIP6 and CMIP5 models

Journal of Climate ◽

10.1175/jcli-d-21-0352.1 ◽

2021 ◽

pp. 1-41

Author(s):

Chao He ◽

Zhenyuan Cui ◽

Chunzai Wang

Keyword(s):

Global Warming ◽

El Niño ◽

North Pacific ◽

Western North Pacific ◽

El Nino ◽

Southern Oscillation ◽

Tropical Indian Ocean ◽

Cmip5 Models ◽

Kelvin Wave ◽

Anomalous Anticyclone

AbstractThe anomalous anticyclone over the western North Pacific (WNPAC) is a key atmospheric bridge through which El Niño-Southern Oscillation (ENSO) affects East Asian climate. In this study, the response of the anomalous WNPAC to global warming under the high-emission scenario is investigated based on 40 models from CMIP6 and 30 models from CMIP5. Despite low inter-model consensus, the multi-model median (MMM) of CMIP6 models projects an enhanced anomalous WNPAC but the MMM of CMIP5 models projects a weakened anomalous WNPAC, both of which reach about 0.5 standard deviation of the decadal internal variability derived from the pre-industrial control experiment. As consistently projected by CMIP6 and CMIP5 models, a same magnitude of sea surface temperature anomaly (SSTA) over the tropical Indian Ocean (TIO) stimulates a weaker anomalous WNPAC under a warmer climate, and this mechanism is responsible for the weakened anomalous WNPAC based on the CMIP5-MMM. However, the above mechanism is overwhelmed by another mechanism related to the changes in tropical SSTA based on the CMIP6-MMM. As a result of the enhanced warm SSTA over the TIO and the eastward shift of the warm SSTA over the equatorial Pacific during the decaying El Niño, the warm Kelvin wave emanating from the TIO is enhanced along with the stronger zonal SSTA gradient based on the CMIP6-MMM, enhancing the anomalous WNPAC. The diverse changes in the zonal SSTA gradient between the TIO and the equatorial western Pacific also explain the inter-model diversity of the changes in anomalous WNPAC.

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Relative Contributions of the Indian Ocean and Local SST Anomalies to the Maintenance of the Western North Pacific Anomalous Anticyclone during the El Niño Decaying Summer*

Journal of Climate ◽

10.1175/2010jcli3300.1 ◽

2010 ◽

Vol 23 (11) ◽

pp. 2974-2986 ◽

Cited By ~ 234

Author(s):

Bo Wu ◽

Tim Li ◽

Tianjun Zhou

Keyword(s):

Indian Ocean ◽

El Niño ◽

North Pacific ◽

Western North Pacific ◽

El Nino ◽

Late Summer ◽

Kelvin Waves ◽

Relative Role ◽

Local Forcing ◽

Anomalous Anticyclone

Abstract To investigate the relative role of the cold SST anomaly (SSTA) in the western North Pacific (WNP) or Indian Ocean basin mode (IOBM) in maintaining an anomalous anticyclone over the western North Pacific (WNPAC) during the El Niño decaying summer, a suite of numerical experiments is performed using an atmospheric general circulation model, ECHAM4. In sensitive experiments, the El Niño composite SSTA is specified in either the WNP or the tropical Indian Ocean, while the climatological SST is specified elsewhere. The results indicate that the WNPAC is maintained by the combined effects of the local forcing of the negative SSTA in the WNP and the remote forcing from the IOBM. The former (latter) contribution gradually weakens (enhances) from June to August. The negative SSTA in the WNP is crucial for the maintenance of the WNPAC in early summer. However, because of a negative air–sea feedback, the negative SSTA gradually decays, as does the local forcing effect. Enhanced local convection associated with the IOBM stimulates atmospheric Kelvin waves over the equatorial western Pacific. The impact of the Kelvin waves on the WNP circulation depends on the formation of the climatological WNP monsoon trough, which does not fully establish until late summer. Therefore, the IOBM plays a crucial role in late summer via the Kelvin wave induced anticyclonic shear and boundary layer divergence.

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Atmospheric Dynamic and Thermodynamic Processes Driving the Western North Pacific Anomalous Anticyclone during El Niño. Part II: Formation Processes

Journal of Climate ◽

10.1175/jcli-d-16-0495.1 ◽

2017 ◽

Vol 30 (23) ◽

pp. 9637-9650 ◽

Cited By ~ 38

Author(s):

Bo Wu ◽

Tianjun Zhou ◽

Tim Li

Keyword(s):

Rossby Wave ◽

El Niño ◽

North Pacific ◽

Western North Pacific ◽

El Nino ◽

Specific Humidity ◽

Central Pacific ◽

Meridional Gradient ◽

Sign Change ◽

Anomalous Anticyclone

In Part I, the authors showed that northerly anomalies associated with the Rossby wave response to El Niño heating anomalies in the equatorial central Pacific lead to the southward advection of low moist enthalpy air forming the western North Pacific anomalous anticyclone (WNPAC). Why does such a remote forcing not cause the formation of the anomalous anticyclone in El Niño–developing summer? The physical mechanism responsible for the timing of the WNPAC formation is investigated in Part II. Through both an observational analysis and idealized numerical model experiments, the authors find that the onset timing of the WNPAC relies on the following three factors. The first is a sign change (from positive to negative) of the meridional gradient of background low-level specific humidity over the key tropical western North Pacific (WNP) region in November. The second is a sign change (from positive to negative) of the meridional gradient of background relative vorticity, which efficiently reduces the westward stretch of the Rossby wave gyre anomalies west of the equatorial heating through equivalent beta effect. As a result, the northern branch of the twin cyclonic anomalies induced by El Niño heating withdraws eastward, leaving space for the onset of the WNPAC. The third factor is attributed to local sea surface temperature anomaly (SSTA) forcing. Pacemaker experiments with a coupled global model indicate that cold SSTAs in the tropical WNP play an important role in starting the anomalous anticyclone over the WNP in late fall. In the absence of the local cold SSTA forcing, the formation of the WNPAC would be delayed to El Niño mature winter.

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Impact of the mean state on El Niño induced western North Pacific anomalous anticyclone during its decaying summer in AMIP models

Journal of Climate ◽

10.1175/jcli-d-20-0747.1 ◽

2021 ◽

pp. 1-49

Author(s):

Xieyuan Wang ◽

Tim Li ◽

Chao He

Keyword(s):

Indian Ocean ◽

El Niño ◽

North Pacific ◽

Western North Pacific ◽

El Nino ◽

Tropical Indian Ocean ◽

Cp El Niño ◽

Anomalous Anticyclone ◽

The Mean ◽

Mean State

AbstractThrough the diagnosis of 29 Atmospheric Model Inter-comparison Project (AMIP) experiments from the CMIP5 inter-comparison project, we investigate the impact of the mean state on simulated western North Pacific anomalous anticyclone (WNPAC) during El Niño decaying summer. The result indicates that the inter-model difference of the JJA mean precipitation in the Indo-western Pacific warm pool is responsible for the difference of the WNPAC. During the decaying summer of an Eastern Pacific (EP) type El Niño, a model that simulates excessive mean rainfall over the western North Pacific (WNP) reproduces a stronger WNPAC response, through an enhanced local convection-circulation-moisture feedback. The intensity of the simulated WNPAC during the decay summer of a Central Pacific (CP) type El Niño, on the other hand, depends on the mean precipitation over the tropical Indian Ocean. The distinctive WNPAC-mean precipitation relationships between the EP and CP El Niño result from different anomalous SST patterns in the WNP. While the local SST anomaly plays an active role in maintaining the WNPAC during the EP El Niño, it plays a passive role during the CP El Niño. As a result, only the mean-state precipitation/moisture field in the tropical Indian Ocean modulates the circulation anomaly in the WNP in the latter case.

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