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

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.

scholarly journals Weakened Anomalous Western North Pacific Anticyclone during an El Niño–Decaying Summer under a Warmer Climate: Dominant Role of the Weakened Impact of the Tropical Indian Ocean on the Atmosphere

2018 ◽  
Vol 32 (1) ◽  
pp. 213-230 ◽  
Author(s):  
Chao He ◽  
Tianjun Zhou ◽  
Tim Li
Keyword(s):  
Indian Ocean ◽  
El Niño ◽  
North Pacific ◽  
Western North Pacific ◽  
El Nino ◽  
Tropical Indian Ocean ◽  
Tropospheric Temperature ◽  
Coupled Models ◽  
Subtropical Anticyclone ◽  
Mean State

Abstract The western North Pacific subtropical anticyclone (WNPAC) is the most prominent atmospheric circulation anomaly over the subtropical Northern Hemisphere during the decaying summer of an El Niño event. Based on a comparison between the RCP8.5 and the historical experiments of 30 coupled models from the CMIP5, we show evidence that the anomalous WNPAC during the El Niño–decaying summer is weaker in a warmer climate although the amplitude of the El Niño remains generally unchanged. The weakened impact of the sea surface temperature anomaly (SSTA) over the tropical Indian Ocean (TIO) on the atmosphere is essential for the weakened anomalous WNPAC. In a warmer climate, the warm tropospheric temperature (TT) anomaly in the tropical free troposphere stimulated by the El Niño–related SSTA is enhanced through stronger moist adiabatic adjustment in a warmer mean state, even if the SSTA of El Niño is unchanged. But the amplitude of the warm SSTA over TIO remains generally unchanged in an El Niño–decaying summer, the static stability of the boundary layer over TIO is increased, and the positive rainfall anomaly over TIO is weakened. As a result, the warm Kelvin wave emanating from TIO is weakened because of a weaker latent heating anomaly over TIO, which is responsible for the weakened WNPAC anomaly. Numerical experiments support the weakened sensitivity of precipitation anomaly over TIO to local SSTA under an increase of mean-state SST and its essential role in the weakened anomalous WNPAC, independent of any change in the SSTA.

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

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.

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*

2010 ◽  
Vol 23 (11) ◽  
pp. 2974-2986 ◽  
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.

scholarly journals Interdecadal Relationship between the Mean State and El Niño Types*

2013 ◽  
Vol 26 (2) ◽  
pp. 361-379 ◽  
Author(s):  
Pei-Hsuan Chung ◽  
Tim Li
Keyword(s):  
El Niño ◽  
El Nino ◽  
Atmospheric Precipitation ◽  
Frequent Occurrence ◽  
Central Pacific ◽  
Cp El Niño ◽  
Budget Analysis ◽  
Ep El Niño ◽  
The Mean ◽  
Mean State

Abstract The interdecadal change of the mean state and two types of El Niño was investigated based on the analysis of observational data from 1980 to 2010. It was found that easterly trades and sea surface temperature (SST) gradients across the equatorial Pacific undergo a regime change in 1998/99, with enhanced trades and a significant cooling (warming) over tropical eastern (western) Pacific in the later period. Accompanying this mean state change is more frequent occurrence of central Pacific (CP) El Niño during 1999–2010. The diagnosis of air–sea feedback strength showed that atmospheric precipitation and wind responses to CP El Niño are greater than those to the eastern Pacific (EP) El Niño for given a unit SST anomaly (SSTA) forcing. The oceanic response to the same wind forcing, however, is greater in the EP El Niño than in the CP El Niño. A mixed layer heat budget analysis reveals that zonal advection (thermocline change induced vertical advection) primarily contributes to the CP (EP) El Niño growth. The role of the mean SST zonal gradient in El Niño selection was investigated through idealized numerical experiments. With the increase of the background zonal SST gradient, the anomalous wind and convection response to a specified EP or CP SSTA shift to the west. Such a difference results in a bifurcation of maximum SSTA tendency, as shown from a simple ocean model. The numerical results support the notion that a shift to the La Niño–like interdecadal mean state is responsible for more frequent occurrence of CP-type El Niño.

scholarly journals Tropical Indian Ocean Influence on Northwest Pacific Tropical Cyclones in Summer following Strong El Niño*

2011 ◽  
Vol 24 (1) ◽  
pp. 315-322 ◽  
Author(s):  
Yan Du ◽  
Lei Yang ◽  
Shang-Ping Xie
Keyword(s):  
Indian Ocean ◽  
El Niño ◽  
El Nino ◽  
Tropical Indian Ocean ◽  
Vertical Shear ◽  
Northwest Pacific ◽  
Kelvin Wave ◽  
Anomalous Anticyclone ◽  
The Western Pacific ◽  
Inducing Surface

Abstract In the summer following a strong El Niño, tropical cyclone (TC) number decreases over the Northwest (NW) Pacific despite little change in local sea surface temperature. The authors’ analysis suggests El Niño–induced tropical Indian Ocean (TIO) warming as the cause. The TIO warming forces a warm tropospheric Kelvin wave that propagates into the western Pacific. Inducing surface divergence off the equator, the tropospheric Kelvin wave suppresses convection and induces an anomalous anticyclone over the NW Pacific, both anomalies unfavorable for TCs. The westerly vertical shear associated with the warm Kelvin wave reduces the magnitude of vertical shear in the South China Sea and strengthens it in the NW Pacific, an east–west variation that causes TC activity to increase and decrease in respective regions. These results help improve seasonal TC prediction.

scholarly journals Modulation of Boreal Summer Intraseasonal Oscillations over the Western North Pacific by ENSO

2016 ◽  
Vol 29 (20) ◽  
pp. 7189-7201 ◽  
Author(s):  
Fei Liu ◽  
Tim Li ◽  
Hui Wang ◽  
Li Deng ◽  
Yuanwen Zhang
Keyword(s):  
Indian Ocean ◽  
El Niño ◽  
North Pacific ◽  
Western North Pacific ◽  
El Nino ◽  
Southern Oscillation ◽  
La Niña ◽  
Boreal Summer ◽  
Maritime Continent ◽  
La Nina

Abstract The authors investigate the effects of El Niño and La Niña on the intraseasonal oscillation (ISO) in the boreal summer (May–October) over the western North Pacific (WNP). It is found that during El Niño summers, the ISO is dominated by a higher-frequency oscillation with a period of around 20–40 days, whereas during La Niña summers the ISO is dominated by a lower-frequency period of around 40–70 days. The former is characterized by northwestward-propagating convection anomalies in the WNP, and the latter is characterized by northward- and eastward-propagating convective signals over the tropical Indian Ocean/Maritime Continent. The possible mechanisms through which El Niño–Southern Oscillation (ENSO)-induced background mean state changes influence the ISO behavior are examined through idealized numerical experiments. It is found that enhanced (weakened) mean moisture and easterly (westerly) vertical wind shear in the WNP during El Niño (La Niña) are the main causes of the strengthened (weakened) 20–40-day northwestward-propagating ISO mode, whereas the 40–70-day ISO initiated from the Indian Ocean can only affect the WNP during La Niña years because the dry (moist) background moisture near the Maritime Continent during El Niño (La Niña) suppresses (enhances) the ISO over the Maritime Continent, and the ISO propagates less over the Maritime Continent during El Niño years than in La Niña years.

On the Phase Relations between the Western North Pacific, Indian, and Australian Monsoons*

2010 ◽  
Vol 23 (21) ◽  
pp. 5572-5589 ◽  
Author(s):  
Dejun Gu ◽  
Tim Li ◽  
Zhongping Ji ◽  
Bin Zheng
Keyword(s):  
Negative Correlation ◽  
El Niño ◽  
North Pacific ◽  
Early Onset ◽  
Western North Pacific ◽  
El Nino ◽  
Phase Relationship ◽  
Eastern Pacific ◽  
Boreal Winter ◽  
Anomalous Anticyclone

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.

scholarly journals An Assessment of Multimodel Simulations for the Variability of Western North Pacific Tropical Cyclones and Its Association with ENSO

2016 ◽  
Vol 29 (18) ◽  
pp. 6401-6423 ◽  
Author(s):  
Rongqing Han ◽  
Hui Wang ◽  
Zeng-Zhen Hu ◽  
Arun Kumar ◽  
Weijing Li ◽  
...  
Keyword(s):  
Interannual Variability ◽  
Tropical Cyclones ◽  
El Niño ◽  
North Pacific ◽  
Western North Pacific ◽  
El Nino ◽  
Southern Oscillation ◽  
Annual Number ◽  
Central Pacific ◽  
Cp El Niño

Abstract An assessment of simulations of the interannual variability of tropical cyclones (TCs) over the western North Pacific (WNP) and its association with El Niño–Southern Oscillation (ENSO), as well as a subsequent diagnosis for possible causes of model biases generated from simulated large-scale climate conditions, are documented in the paper. The model experiments are carried out by the Hurricane Work Group under the U.S. Climate Variability and Predictability Research Program (CLIVAR) using five global climate models (GCMs) with a total of 16 ensemble members forced by the observed sea surface temperature and spanning the 28-yr period from 1982 to 2009. The results show GISS and GFDL model ensemble means best simulate the interannual variability of TCs, and the multimodel ensemble mean (MME) follows. Also, the MME has the closest climate mean annual number of WNP TCs and the smallest root-mean-square error to the observation. Most GCMs can simulate the interannual variability of WNP TCs well, with stronger TC activities during two types of El Niño—namely, eastern Pacific (EP) and central Pacific (CP) El Niño—and weaker activity during La Niña. However, none of the models capture the differences in TC activity between EP and CP El Niño as are shown in observations. The inability of models to distinguish the differences in TC activities between the two types of El Niño events may be due to the bias of the models in response to the shift of tropical heating associated with CP El Niño.

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