Northwest Pacific Anticyclone Research Articles

Diversity of Northwest Pacific atmospheric circulation anomalies during post-ENSO summer

In present study, the diversity of summertime Northwest Pacific (NWP) atmospheric circulation anomalies following El Niño-Southern Oscillation is investigated by performing the inter-case empirical orthogonal function (EOF) analysis among 33 El Niño cases and 36 La Niña cases, respectively. Although an anomalous anticyclone is observed over the NWP for all El Niño cases’ composite, the circulation anomalies there vary from one case to another for each case. The EOF1 mode of NWP circulation anomalies explains 39.8% of inter-case variance, and its positive phase features an anomalous anticyclone and cyclone at south and north of 25°N, respectively. Therefore, the positive first principal component (PC1) corresponds to the anomalous NWP anticyclone, while the anticyclone shifts more northeastward and a cyclone appears at its south side for negative PC1. The PC1-related NWP circulation anomalies are largely controlled by the pronounced central and eastern Pacific sea surface temperature cooling, which indicates the diverse El Niño decay rate. Furthermore, four categories are obtained according to the El Niño decay rate by the nonlinear k-means cluster analysis, and the results further confirm that the close relationship between NWP circulation anomalies and El Niño decay rate. The PC1-regressed land rainfall anomalies highly resembles the composite results in the key areas of Asian monsoon region: the central China and South Asia, indicating variable rainfall anomalies in these areas during post-El Niño summer. The conclusion obtained from La Niña cases are generally same to El Niño.

East Asian Summer Monsoon Rainfall Anomalies in 2020 and the Role of Northwest Pacific Anticyclone on the Intraseasonal‐to‐Interannual Timescales

AbstractIn summer 2020, extreme rainfall occurred over the middle and lower reaches of the Yangtze River Basin (YRB). The accumulated precipitation amount of this rainfall event broke the record since 1961. On the interannual timescale, seasonal mean rainfall anomalies in the YRB during summer 2020 are related to the tropical North Indian Ocean (NIO) sea surface temperature (SST) warming and a La Niña event in the equatorial eastern Pacific. Both the NIO SST warming and La Niña help to generate the anomalous tropospheric low‐level Northwest Pacific anticyclone (NWP‐AAC) via the Matsuno‐Gill type response. On the subseasonal timescale, rainfall anomalies in the YRB exhibit two significant periods during summer 2020, intraseasonal (40–60 days) and quasi‐biweekly (∼10 days). The former intraseasonal component is the focus of this study. The intraseasonal NWP‐AAC, which is a part of tropical intraseasonal oscillation, plays a dominant role in connecting the East Asian summer monsoon (EASM) rainfall with tropical convection. Using the pattern regression, a real‐time index (RTI) is proposed to represent the intraseasonal‐to‐interannual variability of the NWP‐AAC. The RTI captures the quasi‐biennial tendency of NWP‐AAC as well as its intraseasonal variability. These suggest that the NWP‐AAC is a common mode across timescales. Also, the RTI well depicts the EASM rainfall variability in 2020. Our results provide an alternative view of cross‐scale EASM variability, with an emphasis on the conduit effect of NWP‐AAC from intraseasonal to interannual timescale. Hence, the RTI could be utilized for subseasonal to seasonal climate monitoring in the Asian summer monsoon region.

The Interdecadal Change of Relationship Between Summer Water Vapor Content Over Tibetan Plateau and Spring Sea Surface Temperature in Indian Ocean

The interannual relationship between the spring sea surface temperature over the western tropical Indian Ocean (WTIO SST) and summer water vapor content over Tibetan Plateau (TPWVC) enhances significantly after 1992/1993. The regressed atmospheric circulation against WTIO SST index (WTIO SSTI) for two periods is explored to explain the interdecadal variation. During ID1 (1979–1991), the center of the anomalous anticyclone is generally located eastward and the weak easterly anomalies on its southern flank transport moisture from the western Pacific to Southeast China with no effects on TPWVC. In ID2 (1994–2017), the Northwest Pacific anticyclone, the anomalous easterlies, and the subtropical high at 500 hPa all move westward and enhance significantly; thus, it forms a westward moisture transport pathway delivering the water vapor from the western Pacific into Tibetan Plateau. A possible mechanism is raised. On the one hand, the SST anomalies (SSTA) related to WTIO SSTI extend eastward from spring to summer in ID2. With the increased mean SST in the Indo-western Pacific Ocean under the global warming and the stronger mean summer SST in the eastern Indian Ocean, the positive SSTA induce the enhanced Kelvin waves and Northwest Pacific anticyclone with strong easterly anomalies during ID2. But in ID1, the SSTA related to WTIO SST confined in the western-central Indian Ocean from spring to summer excite the decreased Kelvin waves with less significant easterly anomalies due to the weaker mean SST. On the other hand, the eastward shift of tropical summer SSTA generates the increased convection and rising motion over the Southeast Indian Ocean in ID2. They enhance the easterly anomalies on the southern flank of the Northwest Pacific anticyclone and induce anticyclonic shear through the meridional circulation. As a result, the easterly anomalies shift westward to transport more moisture into Tibetan Plateau. However, in ID1, the easterly anomalies of the anticyclone cannot be strengthened with no westward shift. Therefore, the above reasons lead to the interdecadal enhancement of relationship between the spring WTIO SST and summer TPWVC.

Different Mechanisms for Interannual Variability of the Northwest Pacific Subtropical Anticyclone in June and July

ABSTRACTThe interannual variability of the summertime Northwest Pacific (NWP) subtropical anticyclone (NWPSA) has been widely investigated for its important role in the variability and predictability of the East Asian summer monsoon. A preceding El Niño–Southern Oscillation (ENSO) event influences the NWPSA by anchoring ocean memory in the North Indian Ocean, NWP region, etc. This study reveals a discontinuity of the impact of ENSO on the interannual variability of the summertime NWPSA, which is small in June but significant in July. The different mechanisms for the interannual variability of the NWPSA in June and July are further examined using observations, reanalysis data, and an atmospheric general circulation model. The interannual variability of the NWPSA in June and July arises from different physical processes. The latter is closely related to a preceding ENSO, while the former is attributed to the sea surface temperature (SST) anomalies over the southern tropical Atlantic (STA) in the preceding spring that can persist into June, independent of a preceding El Niño event. The STA SST warming could cause an anomalous descent in the central Pacific via modulating the Walker circulation, leading to low-level anomalous divergence and easterlies in the western equatorial Pacific and thereby an anomalous NWPSA. The different mechanisms underlying the NWPSA variability in June and July may be rooted in the annual cycle of the SST and Intertropical Convergence Zone in the Atlantic region. This study has important implications for the potential predictability of the NWPSA.

Dominant modes of CMIP3/5 models simulating northwest Pacific circulation anomalies during post-ENSO summer and their SST dependence

Based on an intermodel empirical orthogonal function (EOF) analysis, this study has investigated the dominant modes of northwest Pacific (NWP) circulation anomalies during post-ENSO summer and their SST dependence involved in 47 Coupled Model Intercomparison Project phase 3 and phase 5 models. The first EOF mode, explaining 33.3% of total intermodel variance, features an anomalous cyclone over the tropical NWP and is controlled by the positive SST anomalies over the equatorial western Pacific (WP). The equatorial WP warming enhances local convection with lower- (upper-) level convergence (divergence), and the anomalous cyclone is a direct Rossby wave response to positive rainfall anomalies there. The second EOF mode, explaining 24.6% of total intermodel variance, is characterized by an anomalous NWP anticyclone (NWPAC). The related SST anomalies show warming in the tropical Indian Ocean (TIO) and equatorial central and eastern Pacific (CEP) and cooling in the NWP. The TIO (CEP) warming induces local wet anomalies, which trigger eastward (westward) Kelvin (Rossby) wave, resulting the adjustment of large-scale circulation. The resultant lower- (upper-) level divergence (convergence) suppresses convection over the NWP, inducing the anomalous NWPAC as a Rossby wave response. The NWP cooling influences NWPAC via positive thermodynamic feedback between local SST and circulation anomalies. Model results further confirm the role of leading mode-related SST anomalies affecting the simulation of NWP circulation.

Northwest Pacific Anticyclonic Anomalies during Post–El Niño Summers Determined by the Pace of El Niño Decay

Abstract This study investigates the characteristics and maintaining mechanisms of the anomalous northwest Pacific anticyclone (NWPAC) following different El Niño decaying paces. In fast decaying El Niño summers, the positive SST anomalies in the tropical central-eastern Pacific (TCEP) have transformed to negative, and positive SST anomalies appear around the Maritime Continent (MC), whereas in slow decaying El Niño summers, positive SST anomalies are present in the TCEP and in the tropical Indian Ocean (TIO). During fast decaying El Niño summers, the cold Rossby wave in response to the negative TCEP SST anomalies has a primary contribution to maintaining the NWPAC anomalies. The warm Kelvin wave response and enhanced Hadley circulation anomalies forced by the positive MC SST anomalies also facilitate developing the NWPAC anomalies. During slow decaying El Niño summers, the warm Kelvin wave anchored over the TIO plays a crucial role in sustaining the NWPAC anomalies, while the warm Rossby wave triggered by the positive TCEP SST anomalies weakens the western part of the NWPAC anomalies. The southwesterly anomalies of the NWPAC anomalies during fast decaying El Niño summers can reach to higher latitudes than those during slow decaying El Niño summers. Correspondingly, positive rainfall anomalies appear in northern China and the Yangtze River basin in fast decaying El Niño summers but are only distributed in the Yangtze River basin in slow decaying El Niño summers. This study implies that the El Niño decaying pace is a key factor in East Asian summer climate.

Effect of excessive equatorial Pacific cold tongue bias on the El Niño-Northwest Pacific summer monsoon relationship in CMIP5 multi-model ensemble

El Nino induces an anomalous lower-tropospheric anticyclone over the tropical Northwest Pacific (NWP), accompanied by suppressed local convection and rainfall. The tropical NWP anomalies persist until the following summer, with major effects on the Asian summer monsoons. Based on the phase 5 of the Coupled Model Intercomparison Project (CMIP5) multi-model ensemble, this study finds that climate models commonly underestimate this El Nino-NWP teleconnection with too weak tropical NWP anticyclone and rainfall anomalies in post-El Nino summers, potentially limiting the models’ skill in seasonal prediction of the Asian summer monsoons. The analyses show that such underestimated NWP anomalies in post-El Nino summers in CMIP5 models can be traced back to the well-known excessive equatorial Pacific cold tongue error in the mean. Models with an excessive westward extension of Pacific cold tongue tend to displace westward the simulated pattern of El Nino-related warm SST anomalies along the equator. The warm SST biases over the western Pacific in CMIP5 models would enhance the local atmospheric convection/rainfall and induce low-level cyclonic circulation anomalies over the tropical NWP with a Gill-type Rossby wave response, resulting in the commonly underestimated NWP anticyclone and rainfall anomalies during post-El Nino summers. The present results, along with our previous finding that the equatorial cold tongue bias would distort the projections of tropical Pacific warming pattern under increased greenhouse gas scenario, imply that reducing equatorial cold tongue bias in models can substantially improve climate simulation and prediction/projection for the tropical Pacific and Asian monsoons.

Origins of Biases in CMIP5 Models Simulating Northwest Pacific Summertime Atmospheric Circulation Anomalies during the Decaying Phase of ENSO

Abstract The present study documents the biases of summertime northwest Pacific (NWP) atmospheric circulation anomalies during the decaying phase of ENSO and investigates their plausible reasons in 32 models from phase 5 of the Coupled Model Intercomparison Project. Based on an intermodel empirical orthogonal function (EOF) analysis of El Niño–Southern Oscillation (ENSO)-related 850-hPa wind anomalies, the dominant modes of biases are extracted. The first EOF mode, explaining 21.3% of total intermodel variance, is characterized by a cyclone over the NWP, indicating a weaker NWP anticyclone. The cyclone appears to be a Rossby wave response to unrealistic equatorial western Pacific (WP) sea surface temperature (SST) anomalies related to excessive equatorial Pacific cold tongue in the models. On one hand, the cold SST biases increase the mean zonal SST gradient, which further intensifies warm zonal advection, favoring the development and persistence of equatorial WP SST anomalies. On the other hand, they reduce the anomalous convection caused by ENSO-related warming, and the resultant increase in downward shortwave radiation contributes to the SST anomalies there. The second EOF mode, explaining 18.6% of total intermodel variance, features an anticyclone over the NWP with location shifted northward. The related SST anomalies in the Indo-Pacific sector show a tripole structure, with warming in the tropical Indian Ocean and equatorial central and eastern Pacific and cooling in the NWP. The Indo-Pacific SST anomalies are highly controlled by ENSO amplitude, which is determined by the intensity of subtropical cells via the adjustment of meridional and vertical advection in the models.

Weakening of Northwest Pacific Anticyclone Anomalies during Post–El Niño Summers under Global Warming

The northwest Pacific anticyclone (NWPAC) anomalies during post–El Niño summers are a key predictor of the summer climate in East Asia and the northwestern Pacific (NWP). Understanding how this will change under global warming is crucial to project the changes in the variability of the northwest Pacific summer monsoon. Outputs from 18 selected coupled models from phase 5 of the Coupled Model Intercomparison Project show that the anomalous NWPAC response to El Niño will likely be weakened under global warming, which is attributed to the decreased zonal contrast between the tropical Indian Ocean (TIO) warming and the NWP cooling during post–El Niño summers. Under global warming, the NWPAC anomalies during the El Niño mature winter are weakened because of decreased atmospheric circulation in response to El Niño–Southern Oscillation (ENSO), which leads to the weakening of local air–sea interaction and then decreases the cold NWP SST anomalies. Furthermore, the decreased surface heat flux anomalies, the weakened anticyclone anomalies over the southeastern Indian Ocean, and the slackened anomalous easterlies over the north Indian Ocean weaken the warm TIO SST anomalies. However, the strengthened tropospheric temperature anomalies could enhance the anomalous TIO warming. Although the changes in TIO SST anomalies are indistinctive, the weakening of the SST anomaly gradient between the TIO and the NWP is robust to weaken the NWPAC anomalies during post–El Niño summers. Moreover, the positive feedback between the TIO–NWP SST anomalies and the NWPAC anomalies will enhance the weakening of NWPAC under global warming.

Asymmetry in summertime atmospheric circulation anomalies over the northwest Pacific during decaying phase of El Niño and La Niña

The present study has revealed the asymmetric characteristics of summertime atmospheric circulation anomalies over the Indo-Pacific for El Nino and La Nina, as well as the possible mechanism. During the summer when El Nino has dissipated, there are basin-wide warming over the tropical Indian Ocean (TIO) and cooling over the equatorial central and eastern Pacific (CEP). The northwest Pacific (NWP) anticyclone is maintained by a combined effect of the TIO warming and equatorial CEP cooling. The impact of TIO warming is via the Kelvin wave-induced divergence mechanism, and the effect of CEP cooling is by stimulating a Rossby wave response to its northwest. Correspondingly, there are lower (upper) level convergence (divergence) over the TIO and divergence (convergence) over the NWP. The tropical atmospheric waves and large-scale circulation anomalies could couple together and maintain the anticyclone. However, for La Nina, there are easterly wind anomalies over the equatorial western Pacific, and the anomalous NWP cyclone (NWPC) is weak and shifts northwestward, which are largely different from El Nino. The equatorial CEP cooling plays a dominant role in the maintenance of NWPC. The negative rainfall anomalies induced by CEP cooling force the anticyclonic wind anomalies over the WP as a Rossby wave response, which further cause the lower (upper) level convergence (divergence) over the MC and NWP. The resultant convergence leads to the enhanced convection there, inducing the NWPC as a Rossby wave response. Besides, the equatorial easterly wind anomalies trigger the upwelling oceanic Kelvin wave and maintain the CEP cooling as the Bjerknes feedback.

ENSO and East Asian winter monsoon relationship modulation associated with the anomalous northwest Pacific anticyclone

Using observational datasets and numerical model experiments, the mechanism on the slowly varying change in the relationship between the El Nino-Southern Oscillation (ENSO) and the East Asian winter monsoon (EAWM) is investigated. The decadal-window (11-, 15-, and 21-year) moving correlations show a significant change in the boreal wintertime ENSO–EAWM relationship between two sub-periods of 1976‒1992 and 1997‒2013. Such recent change in ENSO–EAWM relationship is mainly attributed to the changes in the intensity and zonal location of the anomalous lower-tropospheric northwest Pacific anticyclone (NWP-AC). NWP-AC commonly develops near the region of the Philippine Sea during the ENSO’s peak phase and plays an important role of bridging the tropical convection and mid-latitude teleconnection. On one hand, the intensity of the NWP-AC is influenced by the interdecadal variation in a linkage between ENSO and the Indian Ocean sea surface temperature (SST) variability, referring that a strong connection between the Pacific and Indian Oceans results in the strengthening of NWP-AC response to ENSO. On the other hand, the zonal displacement of the NWP-AC is associated with the Pacific Decadal Oscillation (PDO) and the Atlantic Multidecadal Oscillation (AMO). That is, the tropical Pacific mean state (i.e., zonal SST gradient between climatologically warm western Pacific and cold eastern Pacific)—strengthened by either the negative PDO phase or the positive AMO phase—drives the anomalous ENSO-induced convection to be shifted to the west. With this westward shift, the zonal center of the NWP-AC also migrates westward over the Philippine Islands and exerts stronger connection between ENSO and EAWM. In contrast, the relaxed zonal SST contrast associated with either the positive PDO phase or the negative AMO phase tends to exhibit weaker ENSO–EAWM relationship via both of eastward shifted zonal centers of the anomalous ENSO-induced convection and the NWP-AC. Finally, a series of the numerical experiments conducted by an atmospheric general circulation model supports the observational findings.

Reply to “Comments on ‘Combination Mode Dynamics of the Anomalous Northwest Pacific Anticyclone’”*

Abstract In this reply, the authors clarify the points made in the original paper in 2015 and show that issues raised in the comment by Li et al. are unsubstantiated. The main conclusions can be summarized as follows: 1) The time evolution of the anomalous low-level northwest Pacific anticyclone (NWP-AC) is largely caused by combination mode (C-mode) dynamics. 2) The theoretical C-mode index accurately captures the rapid development of the anomalous NWP-AC. 3) Thermodynamic air–sea coupling does not play a major role for the rapid phase transition of the NWP-AC and the meridionally antisymmetric atmospheric circulation response during the peak phase of El Niño events in boreal winter.

Comments on “Combination Mode Dynamics of the Anomalous Northwest Pacific Anticyclone”*

Abstract In a recent paper, Stuecker et al. applied a “combination mode” (C-mode) theory to explain the formation of the anomalous western North Pacific anticyclone (WNPAC) during El Niño events. The C-mode, arising from interaction between the annual cycle and ENSO, is an Indo-Pacific basin mode with two “near annual” time scales (roughly 10 and 15 months, respectively). This comment discusses to what extent the C-mode can explain the WNPAC dynamics. The major findings are the following: 1) spectral analysis of the Indo-Pacific circulation anomaly fields indicates that the 10-month mode is not observed and the 15-month mode is only seen in the western North Pacific (WNP), where its spectral peak is statistically insignificant; 2) the 15-month mode (with a period of 13–19 months) accounts for only a small portion (13%) of the observed sea level pressure anomaly in the WNP; and 3) the C-mode evolution does not capture the observed timing of the WNPAC onset in the northern fall of El Niño developing year. In addition it is shown, based on observational analyses and numerical experiments, that local atmosphere–ocean interaction plays an important role in formation of the anomalous anticyclonic center over the Philippine Sea.

Combined influence of remote and local SST forcing on Indian Summer Monsoon Rainfall variability

The combined influence of tropical Indian Ocean (TIO) and Pacific Ocean (TPO) sea surface temperature (SST) anomalies on Indian summer monsoon rainfall (ISMR) variability is studied in the context of mid-1970s regime shift. The rainfall pattern on the various stages of monsoon during the developing and decaying summer of El Nino is emphasized. Analysis reveals that ISMR anomalies during El Nino developing summer in epoch-1 (1950–1979) are mainly driven by El Nino forcing throughout the season, whereas TIO SST exhibits only a passive influence. On the other hand in epoch-2 (1980–2009) ISMR does not show any significant relation with Pacific during the onset phase of monsoon whereas withdrawal phase is strongly influenced by El Nino. Again the eastern Indian Ocean cooling and westward shift in northwest Pacific (NWP) cyclonic circulation during epoch-2 have strong positive influence on the rainfall over the central and eastern India during the matured phase of monsoon. ISMR in the El Nino decaying summer does not show any significant anomalies in epoch-1 as both Pacific and Indian Ocean warming dissipate by the summer. On the other hand in epoch-2 ISMR anomalies are significant and display strong variability throughout the season. In the onset phase of monsoon, central and east India experience strong negative precipitation anomalies due to westward extension of persistent NWP anticyclone (forced by persisting Indian Ocean warming). The persistent TIO warming induces positive precipitation anomalies in the withdrawal phase of monsoon by changing the atmospheric circulation and modulating the water vapour flux. Moisture budget analysis unravels the dominant processes responsible for the differences between the two epochs. The moisture convergence and moisture advection are very weak (strong) over Indian land mass during epoch-1 (epoch-2) in El Nino decaying summer. The changing moisture availability and convergence play important role in explaining the weakening of ENSO monsoon relation in the recent years. The local TIO SST forcing and NWP circulation are prominent forcing factors for the interannual variability of ISMR during epoch-2.

Tropical Indian Ocean response to the decay phase of El Niño in a coupled model and associated changes in south and east-Asian summer monsoon circulation and rainfall

This study investigates the response of tropical Indian Ocean (TIO) sea surface temperature (SST) to El Nino decay phase and its impacts on South and East Asian summer monsoon in the National Centers for Environmental Prediction Climate Forecast System version 2 free run. The TIO basin-wide warming induced by El Nino at its peak phase (winter; DJF) and next spring (MAM + 1) are reasonably well captured by the model but with weak magnitude. This TIO basin-wide SST warming persists until summer (JJA + 1) and exert strong impact on summer monsoon rainfall and circulation as revealed in the observations. However, TIO SST anomalies are very weak in the model during the El Nino decaying summers. Though El Nino decay is delayed by 2 months in the model, decay of TIO SST warming is faster than the observations. Anomalous latent heat loss from ocean and a feeble southern TIO Rossby waves associated with weak wind response to El Nino are mainly accountable for rapid decay of TIO SST warming by mid-summer in the model. This suggests that JJA + 1 TIO SST response to El Nino decay phase in the model is poorly represented. The model is able to capture the SST anomalies associated with the northwest Pacific anticyclone at the peak phase of El Nino but fail to maintain that during the decay phase in MAM + 1 and JJA + 1. It is found that precipitation and circulation anomalies associated with TIO SST warming over the South and East Asian regions are disorganized in the model during the decay phase of El Nino. Rainfall anomalies over the southwest TIO, west coast of India, northern flank of northwest Pacific anticyclone and over Japan in JJA + 1 are poorly represented by the model. Analysis of lower troposphere stream function and rotational wind component reveals that northwest Pacific anticyclone shifted far eastward to the date line in the model during JJA + 1 unlike in the observations. Anomalous divergence observed over the western TIO and convergence in the northwest Pacific are absent in the model during JJA + 1. Extension of anomalous tropospheric warming from TIO region to equatorial western Pacific is also very weak in the model due to poor representation of TIO SSTs and the subsequent absence of any Kelvin wave response. Anomalous Walker circulation persisted from DJF to JJA + 1 due to El Nino late decay in the model unlike in the observations. This is also found to be responsible for the redundant changes in SST, rainfall and circulation over the Indo-western Pacific in the model. This study demonstrates that it is essential to represent the decay phase of El Nino and the associated TIO response accurately to have realistic simulations of summer monsoon in the decaying year.

A study of biases in simulation of the Indian Ocean basin mode and its capacitor effect in CMIP3/CMIP5 models

Based on 15 Coupled Model Intercomparison Project (CMIP) phase 3 (CMIP3) and 32 CMIP phase 5 (CMIP5) models, a detailed diagnosis was carried out to understand what compose the biases in simulation of the Indian Ocean basin mode (IOBM) and its capacitor effect. Cloud-radiation-SST (CRS) feedback and wind-evaporation-SST (WES) feedback are the two major atmospheric processes for SST changes. Most CMIP models simulate a stronger CRS feedback and a weaker WES feedback. During boreal fall of the El Nino/Southern Oscillation developing year and the following spring, there are weak biases of suppressed rainfall anomalies over the Maritime Continent and anomalous anticyclone over South Indian Ocean. Most CMIP models simulate reasonable short wave radiation (SWR) and weaker latent heat flux (LHF) anomalies. This leads to a weak bias of atmospheric processes. During winter, however, the rainfall anomalies are stronger due to west bias, and the anomalous anticyclone is comparable to observations. As such, most models simulate stronger SWR and reasonable LHF anomalies, leading to a strong bias of atmospheric processes. The thermocline feedback is stronger in most models. Though there is a deep bias of climatology thermocline, most models capture reasonable sea surface height-induced SST anomalies. Therefore, the effect of oceanic processes offset the weak bias of atmospheric processes in spring, and the tropical Indian Ocean warming persists into summer. However, anomalous northwest Pacific (NWP) anticyclone is weaker due to weak and west bias of the capacitor effect. The unrealistic western Pacific SST anomalies in models favor the westward extension of Rossby wave from the Pacific, weakening the effect of Kelvin wave from the Indian Ocean. Moreover, the western Pacific warming forces the NWP anticyclone move farther north than observations, suggesting a major forcing from the Pacific. Compared to CMIP3, CMIP5 models simulate the feedbacks more realistically and display less diversity. Thus, the overall performance of CMIP5 models is better than that of CMIP3 models.

Combination Mode Dynamics of the Anomalous Northwest Pacific Anticyclone*

Abstract Nonlinear interactions between ENSO and the western Pacific warm pool annual cycle generate an atmospheric combination mode (C-mode) of wind variability. The authors demonstrate that C-mode dynamics are responsible for the development of an anomalous low-level northwest Pacific anticyclone (NWP-AC) during El Niño events. The NWP-AC is embedded in a large-scale meridionally antisymmetric Indo-Pacific atmospheric circulation response and has been shown to exhibit large impacts on precipitation in Asia. In contrast to previous studies, the authors find the role of air–sea coupling in the Indian Ocean and northwestern Pacific only of secondary importance for the NWP-AC genesis. Moreover, the NWP-AC is clearly marked in the frequency domain with near-annual combination tones, which have been overlooked in previous Indo-Pacific climate studies. Furthermore, the authors hypothesize a positive feedback loop involving the anomalous low-level NWP-AC through El Niño and C-mode interactions: the development of the NWP-AC as a result of the C-mode acts to rapidly terminate El Niño events. The subsequent phase shift from retreating El Niño conditions toward a developing La Niña phase terminates the low-level cyclonic circulation response in the central Pacific and thus indirectly enhances the NWP-AC and allows it to persist until boreal summer. Anomalous local circulation features in the Indo-Pacific (e.g., the NWP-AC) can be considered a superposition of the quasi-symmetric linear ENSO response and the meridionally antisymmetric annual cycle modulated ENSO response (C-mode). The authors emphasize that it is not adequate to assess ENSO impacts by considering only interannual time scales. C-mode dynamics are an essential (extended) part of ENSO and result in a wide range of deterministic high-frequency variability.

Remote forcing of Indian Ocean warming on Northwest Pacific during El Niño decaying years: a FOAM model approach

This paper attempts to analyze in detail the remote influence of the Indian Ocean Basin warming on the Northwest Pacific (NWP) during the year of decaying El Nino. Observation data and the Fast Ocean-Atmosphere coupled Model 1.5 were used to investigate the triggering conditions under which the remote influence is formed between the positive sea surface temperature (SST) anomaly in the North Indian Ocean and the Anomalous Northwest Pacific anticyclone (ANWPA). Our research show that it is only when there is a contributory background wind field over the Indian Ocean, i.e., when the Indian Summer Monsoon (ISM) reaches its peak, that the warmer SST anomaly in the North Indian Ocean incites significant easterly wind anomalies in the lower atmosphere of the Indo-West tropical Pacific. This then produces the remote influence on the ANWPA. Therefore, the SST anomaly in the North Indian Ocean might interfere with the prediction of the East Asia Summer Monsoon in the year of decaying El Nino. Both the sustaining effect of local negative SST anomalies in the NWP, and the remote effect of positive SST anomalies in the North Indian Ocean on the ANWPA, should be considered in further research.

The critical role of Indian summer monsoon on the remote forcing between Indian and Northwest Pacific during El Niño decaying year

Recent studies have found a connection between Indian Ocean Basin Warming and the anomalous Northwest Pacific Anticyclone (ANPWA) during El Nino decaying year. This study focuses on the necessary condition for this connection by using observation and numerical simulation. The seasonal transition of the Indian Ocean sea surface wind is critical to the climatic effect of Indian Ocean Basin Warming. When the South Asian Summer Monsoon reaches its peak, the background wind becomes desirable for basin warming, which then affects the climate in the Northwest Pacific. Via the Kelvin waves and Ekman divergence, the wind anomalies exist in the lower atmosphere east of the Indian Ocean warm Sea Surface Temperature (SST) anomalies, and intensify and sustain the ANWPA throughout the El Nino decaying summer. This impact plays an important role in the inter-annual variability of the East Asian Summer Monsoon.

Impact of Northwest Pacific anticyclone on the Indian summer monsoon region

Influence of northwest (NW) Pacific anticyclone on the Indian summer monsoon (ISM), particularly over the head Bay of Bengal and monsoon trough region, is investigated. Strong NW Pacific anticyclone during summer induces negative precipitation anomalies over the head Bay of Bengal and Gangetic Plain region. Westward extension of moisture divergence and dry moisture transport from NW Pacific associated with anticyclone (ridge) and local Hadley cell-induced subsidence are responsible for these negative precipitation anomalies. The impact is maximum when the anticyclone and Indian Ocean basin warming co-occur. This contributes significantly to year-to-year variability of ISM.