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Chunzai Wang

Abstract

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 Niño–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.

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Chunzai Wang

Abstract

Phenomena important for Atlantic climate variability include the Atlantic zonal equatorial mode, the tropical Atlantic meridional gradient mode, and the North Atlantic Oscillation (NAO). These climate phenomena and their associated atmospheric circulation cells are described and discussed using the NCEP–NCAR reanalysis field and the NCEP sea surface temperature (SST) from January 1950 to December 1999. Atmospheric divergent wind and vertical motion are used for the identification of atmospheric circulation cells. During the peak phase of the Atlantic equatorial mode, the Atlantic Walker circulation weakens and extends eastward, which results in surface westerly wind anomalies in the equatorial western Atlantic. These westerly wind anomalies are partly responsible for warming in the equatorial eastern Atlantic that occurs in the second half of the year. The Atlantic equatorial mode involves a positive ocean–atmosphere feedback associated with the Atlantic Walker circulation, similar to the Pacific El Niño. The tropical Atlantic meridional gradient mode is characterized by a strong SST gradient between the tropical North Atlantic (TNA) and the tropical South Atlantic. Corresponding to the meridional gradient mode is an atmospheric meridional circulation cell in which the air rises over the warm SST anomaly region, flows toward the cold SST anomaly region aloft, sinks in the cold SST anomaly region, then crosses the equator toward the warm SST region in the lower troposphere. The analysis presented here suggests that the Pacific El Niño can affect the TNA through the Walker and Hadley circulations, favoring the TNA warming in the subsequent spring of the Pacific El Niño year. The NAO, characterized by strong westerly airflow between the Icelandic low and the Azores high, is also related to an atmospheric meridional circulation. During the high NAO index, the atmospheric Ferrel and Hadley cells are strengthened, consistent with surface westerly and easterly wind anomalies in the North Atlantic and in the mid-to-tropical Atlantic, respectively.

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Chunzai Wang

Abstract

Atmospheric circulation cells associated with the El Niño–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 subtropics, and returning back to the Tropics in the lower troposphere. The anomalous Hadley cell in the western Pacific is opposite to that in the eastern Pacific. The divergent wind and vertical velocity also show a midlatitude zonal cell (MZC) over the North Pacific. The mean MZC is characterized by the air rising in the central North Pacific, flowing westward and eastward in the upper troposphere, descending in the east coast of Asia and the west coast of North America, then returning back to the central North Pacific in the lower troposphere. The anomalous MZC during the mature phase of El Niño shows an opposite rotation to the mean MZC, indicating a weakening of the MZC.

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Chunzai Wang

Abstract

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.

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Chunzai Wang and Xin Wang

Abstract

Based on their opposite influences on rainfall in southern China during boreal fall, this paper classifies El Niño Modoki into two groups: El Niño Modoki I and II, which show different origins and patterns of SST anomalies. The warm SST anomalies originate in the equatorial central Pacific and subtropical northeastern Pacific for El Niño Modoki I and II, respectively. Thus, El Niño Modoki I shows a symmetric SST anomaly distribution about the equator with the maximum warming in the equatorial central Pacific, whereas El Niño Modoki II displays an asymmetric distribution with the warm SST anomalies extending from the northeastern Pacific to the equatorial central Pacific. Additionally, the warm SST anomalies in the equatorial central Pacific extend farther westward for El Niño Modoki II than for El Niño Modoki I. Similar to the canonical El Niño, El Niño Modoki I is associated with an anomalous anticyclone in the Philippine Sea that induces southwesterly wind anomalies along the south coast of China and carries the moisture for increasing rainfall in southern China. For El Niño Modoki II, an anomalous cyclone resides east of the Philippines, associated with northerly wind anomalies and a decrease in rainfall in southern China. The canonical El Niño and El Niño Modoki I are associated with a westward extension of the western North Pacific subtropical high (WNPSH), whereas El Niño Modoki II shifts the WNPSH eastward. Differing from canonical El Niño and El Niño Modoki I, El Niño Modoki II corresponds to northwesterly anomalies of the typhoon steering flow, which are unfavorable for typhoons to make landfall in China.

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Mingmei Xie and Chunzai Wang

Abstract

The western North Pacific anomalous anticyclone (WNPAC) significantly affects East Asian climate. Previous studies have elucidated interannual variability of the WNPAC associated with El Niño, but decadal variability of the WNPAC remains unknown. The present paper investigates the dominant modes of decadal variability of the WNPAC by using observational data. The first decadal mode, characterized by an anomalous anticyclone centered over the western North Pacific, is associated with the Pacific decadal oscillation (PDO). The relationship between the first mode and the PDO shifted from in phase to out of phase around 1966. From 1900 to 1966 when the PDO and the first mode are in phase, the anticyclone is maintained by the effects of both the strengthened Aleutian low through meridional atmospheric forcing and Indian Ocean warming through enhanced zonal Walker circulation. From 1967 to 2012, the anticyclone is induced by cold SST anomalies over the central equatorial Pacific when the PDO and the first mode are out of phase. The second decadal mode is characterized by an anomalous anticyclone extending from southeastern China to the Philippine Sea and is associated with the Maritime Continent (MC). This anticyclone resides in the sinking branch of the local Hadley circulation, triggered by enhanced convection associated with the MC warming from 1900 to 2012. The finding of the decadal WNPAC in this paper may provide a new way to explain East Asian climate on a decadal time scale.

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Chunzai Wang and Liping Zhang

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The Atlantic multidecadal oscillation (AMO) is characterized by the sea surface warming (cooling) of the entire North Atlantic during its warm (cold) phase. Both observations and most of the phase 5 of the Coupled Model Intercomparison Project (CMIP5) models also show that the warm (cold) phase of the AMO is associated with a surface warming (cooling) and a subsurface cooling (warming) in the tropical North Atlantic (TNA). It is further shown that the warm phase of the AMO corresponds to a strengthening of the Atlantic meridional overturning circulation (AMOC) and a weakening of the Atlantic subtropical cell (STC), which both induce an anomalous northward current in the TNA subsurface ocean. Because the mean meridional temperature gradient of the subsurface ocean is positive because of the temperature dome around 9°N, the advection by the anomalous northward current cools the TNA subsurface ocean during the warm phase of the AMO. The opposite is true during the cold phase of the AMO. It is concluded that the anticorrelated ocean temperature variation in the TNA associated with the AMO is caused by the meridional current variation induced by variability of the AMOC and STC, but the AMOC plays a more important role than the STC. Observations do not seem to show an obvious anticorrelated salinity relation between the TNA surface and subsurface oceans, but most of CMIP5 models simulate an out-of-phase salinity variation. Similar to the temperature variation, the mechanism is the salinity advection by the meridional current variation induced by the AMOC and STC associated with the AMO.

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Jia-Zhen Wang and Chunzai Wang

Abstract

Super El Niño has been a research focus since the first event occurred. On the basis of observations and models, we propose that a super El Niño emerges if El Niño is an early-onset type coincident with the distribution of an Atlantic Niña (AN) in summer and a positive Indian Ocean dipole (IOD) in autumn, conditions referred to as the Indo-Atlantic Booster (IAB). The underlying physical mechanisms refer to three-ocean interactions with seasonality. Early onset endows super El Niño with adequate strength in summer to excite wind-driven responses over the Indian and Atlantic Oceans, which further facilitate IAB formation by coupling with the seasonal cycle. In return, IAB alternately produces additional zonal winds U over the Pacific Ocean, augmenting super El Niño via the Bjerknes feedback. Adding AN and IOD indices into the regression model of U leads to a better performance than the single Niño-3.4 model, with a rise in the total explained variances by 10%–20% and a reduction in the misestimations of super El Niños by 50%. Extended analyses using Coupled Model Intercomparison Project models further confirm the sufficiency and necessity of early onset and IAB on super El Niño formation. Approximately 70% of super El Niños are early-onset types accompanied by IAB and 60% of early-onset El Niños with IAB finally grow into extreme events. These results highlight the super El Niño as an outcome of pantropical interactions, so including both the Indian and Atlantic Oceans and their teleconnections with the Pacific Ocean will greatly improve super El Niño prediction.

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Chunzai Wang and Robert H. Weisberg

Abstract

A linear perturbation, coupled ocean–atmosphere model is revisited for further insights into the El Niño–Southern Oscillation phenomenon. The model oscillates as a slow, eastward propagating mode interpreted as a divergence mode, whose energetics are controlled by the ocean. Growth requires that the work performed by the wind stress minus the work required to effect the ocean divergence exceeds the loss terms. The intrinsic scale of the atmosphere relative to the basin width is important. For sustainable oscillations, the ocean basin must be large enough so that oppositely directed divergence can develop on opposite sides of the basin. The global aspect of the atmospheric pressure field suggests that continental heating may provide either a direct source affecting adjacent oceans, or a connection between oceans. The important model parameters are the coupling and warming coefficients and the ocean Kelvin wave speed. The importance of the Kelvin wave speed derives from its specification of the background buoyancy state for the ocean. Upon further simplification, an analytical solution gives similar parameter dependence as found numerically and shows that growth requires both large zonal wavelength and a zonal phase lag between the anomalies of wind stress and SST.

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Chunzai Wang and Robert H. Weisberg

Abstract

The stability, periodicity, and horizontal structure of equatorial modes in a coupled ocean-atmosphere model, simplified by the assumption that zonal wind stress anomalies are proportional to sea surface temperature anomalies lagged by a zonal phase difference, are examined analytically in an unbounded basin. The gravest coupled Rossby and Kelvin modes coexist with additional westward and eastward slow modes whose phase speeds are smaller than the former. Two of these four modes, one propagating westward and the other eastward, are destabilized in each case depending upon the model parameters. For some particular parameter choices. coupled Rossby and Kelvin modes merge with westward and eastward slow modes, respectively. For other parameters, however. they separate and remain distinct from the slow modes. For all of these modes the primary modifications by coupling relative to uncoupled oceanic equatorial waves are a decrease in phase speed and an increase in meridional scale.

Among the model parameter effects, those of the zonal phase lag between the wind stress and SST anomalies and the coefficients of thermal and mechanical damping are the most interesting. Positive and negative phase lags represent the wind stress anomalies located to the west and east of the SST anomalies, respectively. The frequency of all modes is symmetric about zero phase lag, whereas the growth rate is antisymmetric about zero phase lag relative to the uncoupled damping rate. Wind stress anomalies to the west of SST anomalies favor slow mode growth and coupled Rossby and Kelvin mode decay. Dissipation for the slow modes and the coupled Rossby and Kelvin modes is controlled differently. For the slow modes the dissipation is mainly thermal, whereas for coupled Rossby and Kelvin modes the dissipation is mainly mechanical.

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