Zonal Advection Research Articles

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.

The Surface-Layer Heat Balance in the Equatorial Pacific Ocean. Part II: Interannual Variability*

The surface-layer heat balance on interannual timescales in the equatorial Pacific has been examined in order to determine the processes responsible for sea surface temperature (SST) variability associated with warm and cold phases of the ENSO cycle (El Niño and La Niña). Principal datasets include multiyear time series of surface winds, upper-ocean temperature, and velocity obtained from the Tropical Atmosphere Ocean buoy array at four locations along the equator in the western (165°E), central (170°W), and eastern (140°W and 110°W) Pacific. A blended satellite/in situ SST product and surface heat fluxes based on the Comprehensive Ocean–Atmosphere Data Set are also used. Changes in heat storage, horizontal heat advection, and heat fluxes at the surface are estimated directly from data; vertical fluxes of heat out of the base of the mixed layer are calculated as a residual. Results indicate that all terms in the heat balance contribute to SST change on interannual timescales, depending on location and time period. Zonal advection is important everywhere, although relative to other processes, it is most significant in the central Pacific. The inferred vertical heat flux out of the base of the mixed layer is likewise important everywhere, especially so in the eastern equatorial Pacific where the mean thermocline is shallow. Meridional advection (primarily due to instability waves in this analysis) is a negative feedback term on SST change in the eastern equatorial Pacific, tending to counteract the development of warm and cold anomalies. Likewise, the net surface heat flux generally represents a negative feedback, tending to damp SST anomalies created by ocean dynamical processes. The implications of these results for ENSO modeling and theory are discussed.

Rectification of the Madden–Julian Oscillation into the ENSO Cycle

An ocean general circulation model, forced with idealized, purely oscillating wind stresses over the western equatorial Pacific similar to those observed during the Madden‐Julian oscillation (MJO), developed rectified low-frequency anomalies in SST and zonal currents, compared to a run in which the forcing was climatological. The rectification in SST resulted from increased evaporation under stronger than normal winds of either sign, from correlated intraseasonal oscillations in both vertical temperature gradient and upwelling speed forced by the winds, and from zonal advection due to nonlinearly generated equatorial currents. The net rectified signature produced by the MJO-like wind stresses was SST cooling (about 0.48C) in the west Pacific, and warming (about 0.18C) in the central Pacific, tending to flatten the background zonal SST gradient. It is hypothesized that, in a coupled system, such a pattern of SST anomalies would spawn additional westerly wind anomalies as a result of SST-induced changes in the low-level zonal pressure gradient. This was tested in an intermediate coupled

An eigen analysis of the interdecadal changes in the structure and frequency of ENSO mode

The mechanisms of interdecadal changes of El Niño‐Southern Oscillation (ENSO) modes are examined through an eigen analysis of a simple coupled ocean‐atmosphere model. It is shown that the observed interdecadal climate shift can effectively modify the strength of two major coupled feedbacks for the ENSO mode, namely, the zonal advection and thermocline feedbacks. These modifications lead to quantitative changes of the leading coupled mode in its frequency, growth rate, and spatial pattern, which are consistent with the observation.

A Theory for the Tropical Tropospheric Biennial Oscillation

The key questions of how the tropospheric biennial oscillation (TBO) maintains the same phase from northern summer in South Asia to southern summer in Australia, and how the reversed phase can last through three locally inactive seasons to the next monsoon, are studied by a simple tropical atmosphere–ocean–land model. The model has five boxes representing the South Asian and Australian monsoon regions and the equatorial Indian and western and eastern Pacific Oceans. The five regions interact with each other through the SST–monsoon, evaporation–wind, monsoon–Walker circulation, and wind stress–ocean thermocline feedbacks. A biennial oscillation emerges in a reasonable parameter regime, with model SST and wind variations resembling many aspects of the observed TBO. Warm SST anomalies (SSTA) in July in the equatorial Indian Ocean cause an increase of surface moisture convergence into South Asia, leading to a stronger monsoon. The monsoon heating on one hand induces a westerly wind anomaly in the Indian Ocean, and on the other hand intensifies a planetary-scale east–west circulation leading to anomalous easterlies over the western and central Pacific. The westerly anomaly over the Indian Ocean decreases the local SST, primarily by evaporation–wind feedback. The easterly anomaly in the central Pacific causes a deepening of the ocean thermocline in the western Pacific therefore increasing the subsurface and surface temperatures. In addition, a modest easterly anomaly in the western Pacific opposes the seasonal mean westerlies so evaporation is reduced. These effects overwhelm those of the cold zonal advection and anomalous upwelling. The net result is warm SSTA persisting in the western Pacific through northern fall, leading to a stronger Australian monsoon. Meanwhile, the warming in the western Pacific also induces a stronger local Walker cell and thus a surface westerly anomaly over the Indian Ocean. This westerly anomaly helps the cold SSTA to persist through the succeeding seasons, leading to a weaker Asian monsoon in the following summer. During northern winter the westerly anomaly associated with the stronger Australian monsoon, through anomalous ocean downwelling and reduction of evaporation (when the seasonal mean wind is easterly), reinvigorates the warm SSTA in the western Pacific, which has been weakened by the slow cold advection from the eastern Pacific. This further intensifies the eastern Walker cell and helps to keep the eastern Pacific cold. The authors’ theory indicates that the TBO is an inherent result of the interactions between northern summer and winter monsoon and the tropical Indian and Pacific Oceans. Thus, it is an important component of the tropical ocean–atmosphere interaction system, separate from the El Niño–Southern Oscillation. While the eastern Pacific plays only a passive role in this mechanism, the western Pacific–Maritime Continent region is crucially important. It serves as a bridge in space and time, both in connecting the convection anomaly from the northern summer to the northern winter monsoon and in channeling the feedback of the northern winter monsoon to the Indian Ocean.

Upper-Ocean Heat and Salt Balances in the Western Equatorial Pacific in Response to the Intraseasonal Oscillation during TOGA COARE*

During the TOGA COARE Intensive Observing Period (IOP) from November 1992 through February 1993, temperature, salinity, and velocity profiles were repeatedly obtained within a 130 km × 130 km region near the center of the Intensive Flux Array (IFA) in the western equatorial Pacific warm pool. Together with high quality measurements of air–sea heat flux, rain rate, upper-ocean microstructure, and penetrating solar radiation, they make up a unique dataset for upper-ocean heat and freshwater budget studies. Three survey cruises sampled different phases of the Intraseasonal Oscillation (ISO) during the IOP. Temporal evolution and advective terms in the heat and salt balance equations, on timescales of 3 days and longer, are estimated using the survey data. The upper-ocean (0–50 m) heat and salt budgets at the center of the IFA were estimated and are closed to within 10 W m−2 of observed air–sea heat fluxes and to within approximately 20% of observed rain rates during each of the three cruises. Generally, advection in the upper ocean cannot be neglected during the IOP. Zonal advection alternates sign but had a net warming and freshening tendency. Meridional advection decreased temperature and increased salinity in the surface layer, while vertical advection warmed and freshened the surface layer because of the general downwelling trend. Heat advection is as important as the net air–sea flux during the westerly wind burst time periods. The sub-ISO timescale upper-ocean dynamics, such as the strong meridional advection caused by inertial motions, are found to have important contributions to the upper-ocean heat and freshwater balances.

簡単な大気海洋結合モデルのスローモードについて

An eigen analysis of the simple air-sea coupled model is carried out to understand the coupled mechanism with the slow time scale (so called ‘slow mode’) related to the zonal phase difference between the SST and atmospheric heating anomalies. Both frequency and instability of the slow mode are very sensitive to the zonal phase difference between the SST and atmospheric heating anomalies. When the positive atmospheric heating is located between one-quarter wavelength west and one-quarter wavelength east of the positive SST anomaly, the slow mode becomes the unstable eastward-propagating mode. The further eastward shifting of the atmospheric heating results in damping of the eastward-propagating mode resembling the forced Kelvin mode. On the other hand, the slow mode becomes the unstable westward-propagating mode when atmospheric heating lies farther west than one-quarter wavelength west from the SST anomaly. This unstable westward-propagating mode resembles the forced Rossby mode. SST changes in the eastward- and westward-propagating slow modes are mainly induced by the changes of the oceanic height and the zonal advection by the current, respectively. The slow mode in the fast-wave limit and the non-rotating system is also discussed.

Thermocline and Zonal Advective Feedbacks Within the Equatorial Ocean Recharge Oscillator Model for ENSO

Through the dynamical coupling between ocean and atmosphere, the vertical advection of anomalous subsurface temperature by the mean upwelling and the zonal advection of mean sea surface temperature (SST) by anomalous current constitute the so‐called the thermocline and zonal advective feedbacks that are essential for El Niño/Southern Oscillation (ENSO) coupled dynamics. In this paper, we demonstrate that these two feedbacks are dynamically linked because of the geostrophic balance between the upper ocean zonal current and the meridional gradient of the thermocline. Both feedbacks thus play important and similar roles in the growth and phase transition of ENSO. We further propose a new version of the conceptual recharge oscillator model for ENSO by including these two feedbacks. The new model retains the simplest possible form of a harmonic oscillator, yet presents a more complete description of slow physics for ENSO. Moreover, it also provides the reconciliation between the biased emphases in the ENSO theories oh the thermocline and zonal advective feedbacks.

Simple Coupled Midlatitude Climate Models

A set of simple analytical models is presented and evaluated for interannual to decadal coupled ocean–atmosphere modes at midlatitudes. The atmosphere and ocean are each in Sverdrup balance at these long timescales. The atmosphere’s temperature response to heating determines the spatial phase relation between SST and sea level pressure (SLP) anomalies. Vertical advection balancing heating produces high (low) SLP lying east of warm (cold) SST anomalies, as observed in the Antarctic circumpolar wave (ACW), the decadal North Pacific mode, and the interannual North Atlantic mode. Zonal advection in an atmosphere with a rigid lid produces low SLP east of warm SST. However, if an ad hoc equivalent barotropic atmospheric response is assumed, high SLP lies east of warm SST. Relaxation to heating produces behavior like the observed North Atlantic decadal pattern, with low SLP over warm SST. Meridional advection in the atmosphere cannot produce the observed SST/SLP patterns. The dominant balance in the ocean’s temperature equation determines the phase speed of the modes. The coupled mode is nondispersive in all models examined here, indicating the need for additional processes. For modes with an SST–SLP offset as observed in the ACW and North Pacific, Ekman convergence acting as a heat source causes eastward propagation relative to the mean ocean flow. Sverdrup response to Ekman convergence, acting on the mean meridional temperature gradient, causes westward propagation relative to the mean ocean flow. When the ocean temperature adjusts through surface heat flux alone, the mode is advected by the mean ocean flow and is damped. Relaxation to heating in the atmosphere, when operating with Sverdrup response in the ocean, produces the only complete solution presented here that exhibits growth, with an e-folding timescale of order (100 days). This solution appears appropriate for the North Atlantic decadal mode. In Northern Hemisphere basins, with meridional boundaries, the same sets of dynamics create the observed SST–SLP phase relation. An additional factor is the creation of SST anomalies through variations in the western boundary current strengths, which are related to the zonally integrated wind stress curl over the whole basin. If barotropic and hence fast adjustment is assumed, the resulting positive feedback can maintain or strengthen the coupled anomalies in the North Pacific and interannual North Atlantic modes.

Long equatorial wave reflection in the Pacific Ocean from TOPEX/POSEIDON data during the 1992-1998 period

The TOPEX/POSEIDON sea level data have been shown to observe the sea surface height variability accurately over the globe and, more particularly, in the tropical Pacific, core region of the El Nino/Southern Oscillation. Various studies have already provided descriptions of long equatorial wave propagation as well as estimates of their reflection at both the eastern and western boundaries using TOPEX/POSEIDON data over short time periods. In this present work, we are first interested in computing long equatorial wave amplitudes over a much longer period (October 1992–May 1998) focusing on the Kelvin and first three largest Rossby waves. Secondly, we are examining qualitatively potential theories of ENSO in the light of observations of the strong 1997–1998 El Nino event. At the eastern boundary, Kelvin waves are observed to reflect into first (third) mode Rossby waves with a reflection efficiency of 75% (65%) of that of an infinite meridional wall. In the western Pacific Ocean, the reflection of the first Rossby wave does explain more than 90% of the Kelvin wave variance. The second Rossby wave is found not to be correlated to the Kelvin wave amplitude, and the third Rossby wave is highly correlated to the Kelvin signal although its contribution is weak. The investigation during the 1997–1998 event of two theories potentially important for ENSO (the delayed action oscillator mechanism and the recent theory of Picaut et al., 1997) led us to the following conclusions. The westerly wind anomalies observed in the western Pacific in December 1996 and March 1997 forced downwelling Kelvin waves which advected the eastern edge of the warm pool eastward and deepened the thermocline in the east Pacific. Both zonal advection and vertical processes are suggested to act constructively during the onset of positive anomalies in the central and eastern Pacific. From May to September 1997, downwelling Kelvin wave wind-forced in the central Pacific and reflected downwelling Rossby waves (reinforced by local easterly wind anomalies) acted against each other, and the 28 °C isotherm was found to move slightly westward. In October 1997, strong westerly wind anomalies forced a strong downwelling Kelvin wave potentially responsible for the strong warming in the east Pacific. The 28 °C isotherm reached the eastern boundary. At that time, the equatorial sea surface temperatures are zonally homogeneous. The strong downwelling Rossby waves reflected at the boundary could not act to terminate the warm event through zonal advection as suggested by Picaut et al. However the reflected upwelling Kelvin waves coming from the western boundary weakened the downwelling Kelvin signal in the central and east Pacific leading to a decrease of the sea surface temperature anomalies. Following this decrease, westerly wind anomalies in the central Pacific weakened, and upwelling Kelvin waves reinforced by easterly wind anomalies in the western Pacific propagated toward the eastern Pacific. As a conclusion, both theories must be considered simultaneously to understand the variability observed during the 1997–1998 El Nino event. However observations strongly suggest the delayed action oscillator mechanism to be the major process at work during the weakening of the warm 1997–1998 ENSO.

Tropical Pacific Ocean Mixed Layer Heat Budget: The Pacific Cold Tongue

Abstract Data from satellite-tracked drifting buoys and VOS/XBT profiles for the years 1979–95 were used to evaluate the seasonal cycle of how major oceanic processes redistribute heat in the cold tongue region of the tropical Pacific. The most active processes for the annual cycle are local heat storage and heat export by entrainment of upwelling and by mean meridional advection. Heat export by zonal advection, however, is not negligible, and meridional eddy heat fluxes associated with tropical instability waves effect a negative feedback that offsets a considerable fraction of that produced by the mean meridional advection. All of these processes mimic the essentially one cycle per year of the surface wind stress, as do those of the depths of both the bottom of the surface mixed layer and the thermocline. Because it is associated with poleward Ekman transports, upwelling, and baroclinic adjustment near the equator, the zonal wind stress component appears to be the more important. The meridional wind str...

Cane-ZebiakモデルにおけるENSOの位相遷移と成長に果たす東西移流フィードバックの役割

The turnabout and growth mechanisms of the ENSO are diagnostically studied by analyzing the SST budget of the Cane-Zebiak model. The SST change rates, which are directly linked to the phase transition and growth of the ENSO, are attributed to two processes: the anomalous vertical advection of subsurface temperature by the mean upwelling (thermocline feedback), and the zonal advection of the climatological mean SST by the anomalous zonal current (zonal advection feedback). The contributions to the phase transition, and growth of the ENSO, can be systematically separated by decomposing the equatorial thermocline depth anomaly, and zonal current anomaly into their zonal mean and zonal contrast fields. It is found that the thermocline feedback, associated with the zonal mean thermocline depth anomaly, and the zonal advection feedback by the equatorial zonal mean zonal current anomaly are responsible for the phase transition of the ENSO. Those associated with zonal contrast fields are responsible for the growth of the ENSO. The two processes in the SST change contribute to the phase transition and growth of the ENSO in an almost equally significant manner. They are closely related, as a result of the geostrophic balance between the meridional gradient of the thermocline depth and the zonal current. These findings suggest that the conceptual understanding of the ENSO in the Cane-Zebiak model, should include both of these two processes.

Upper ocean salinity balance in the western equatorial Pacific

The upper ocean salinity balance in the western equatorial Pacific warm pool was evaluated using up to 2.5 years of data (September 1991 through April 1994) from the Coupled Ocean‐Atmosphere Response Experiment's enhanced monitoring array of moorings. At the central mooring site (0°, 156°E), precipitation had a record‐length mean of 4.5 m yr−1, while evaporation had a mean of 1.4 m yr−1. This excess surface freshwater flux was balanced primarily by vertical mixing (estimated as the residual of a salt budget calculation) and by zonal advection. For timescales between a month and 2.5 years, surface salinity variability was dominated by zonal advection and only weakly correlated with precipitation, consistent with the concept of a zonally migrating “fresh pool.” The effects of precipitation on local surface salinity variations were more apparent for timescales shorter than a month. Shallow rain “puddles” tended to form in a matter of hours. However, owing to the combination of mixing and advection, these precipitation‐generated freshwater puddles were typically short‐lived.

An OGCM Study for the TOGA Decade. Part II: Barrier-Layer Formation and Variability

A set of OGCM experiments is used to investigate the processes responsible for barrier layer (BL) formation in the Pacific Ocean. As in existing datasets, BL appears in the present experiments both in the western Pacific (WP) and under the intertropical convergence zone (ITCZ). In the WP, the BL displays a strong interannual variability linked to ENSO variability, in qualitative agreement with the observations of Ando and McPhaden. In both the equatorial and 3°–8°S bands, a subduction process is responsible for BL formation. In the equatorial region, it results from a strong downwelling near the salinity front created by convergence between central Pacific salty water and WP freshwater. In the southern region, the subduction of the South Equatorial Current salty water involves mainly mixed layer thinning due to the freshening of the surface layer by rain and equatorial divergence of water from the eastward fresh equatorial jets. The formation of BL under the ITCZ is found to be mostly related to local precipitation. The impact of the BL presence is then investigated. The BL interannual variability modifies the surface layer heat budget by switching on and off the entrainment cooling. The haline stratification traps most of the wind stress in the surface layer of the fresh and warm pool and induces strong eastward currents in response to westerly wind bursts (WWBs). The overall effect of salinity stratification is to retain heat and momentum in the upper layer of the WP by restraining the exchanges with the cooler waters from below and from the central Pacific. The combined effect of zonal advection and mixing after a WWB results in an eastward shift of the thick BL regions along the equator. These properties of the BL structure might favor the growth of unstable air–sea interactions in the central Pacific after a WWB.

The Annual Cycle of SST in the Eastern Tropical Pacific, Diagnosed in an Ocean GCM*

The annual onset of the east Pacific cold tongue is diagnosed in an ocean GCM simulation of the tropical Pacific. The model uses a mixed-layer scheme that explicitly simulates the processes of vertical exchange of heat and momentum with the deeper layers of the ocean; comparison with observations of temperature and currents shows that many important aspects of the model fields are realistic. As previous studies have found, the heat balance in the eastern tropical Pacific is notoriously complicated, and virtually every term in the balance plays a significant role at one time or another. However, despite many complications, the three-dimensional ocean advection terms in the cold tongue region tend to cancel each other in the annual cycle and, to first order, the variation of SST can be described as simply following the variation of net solar radiation at the sea surface (sun minus clouds). The cancellation is primarily between cooling due to equatorial upwelling and warming due to tropical instability waves, both of which are strongest in the second half of the year (when the winds are stronger). Even near the equator, where the ocean advection is relatively intense, the terms associated with cloudiness variations are among the largest contributions to the SST balance. The annual cycle of cloudiness transforms the semiannual solar cycle at the top of the atmosphere into a largely 1 cycle yr−1 variation of insolation at the sea surface. However, the annual cycle of cloudiness appears closely tied to SST in coupled feedbacks (positive for low stratus decks and negative for deep cumulus convection), so the annual cycle of SST cannot be fully diagnosed in an ocean-only modeling context as in the present study. Zonal advection was found to be a relatively small influence on annual equatorial cold tongue variations; in particular, there was little direct (oceanic) connection between the Peru coastal upwelling and equatorial annual cycles. Meridional advection driven by cross-equatorial winds has been conjectured as a key factor leading to the onset of the cold tongue. The results suggest that the SST changes due to this mechanism are modest, and if meridional advection is in fact a major influence, then it must be through interaction with another process (such as a coupled feedback with stratus cloudiness). At present, it is not possible to evaluate this feedback quantitatively.

Observations of sea surface salinity in the western Pacific fresh pool: Large‐scale changes in 1992–1995

This paper investigates the variability of sea surface salinity (SSS) in the western equatorial Pacific fresh pool. For this purpose, we processed data collected from thermosalinographs embarked on merchant ships. Two main cross‐equatorial shipping lines that are representative of the oceanic conditions in the western tropical Pacific were selected: the Japan‐Tarawa‐Fiji line that crosses the equator near 173°E (eastern track) and the New‐Caledonia‐Japan line that crosses the equator near 156°E (western track). We show that there is a strong SSS variability in the region at monthly as well as interannual timescales. This high variability is attributed to the successive passages of a zonal salinity front, trapped in the (5°N–5°S) equatorial band and migrating in phase with the southern oscillation index. We also found the eastern track to be more variable in SSS because it is more exposed to these SSS front incursions. We carried out a detailed study of the mechanisms responsible for this variability; it revealed that the rainfall input acts as a source of freshwater responsible for the existence of a contrasted distribution of SSS (mainly high‐salinity waters in the central Pacific and low‐salinity waters in the western Pacific). However, the main mechanism responsible for the SSS variability is zonal advection that makes the two distinct masses of water converge, resulting in a salinity front which shifts back and forth in the equatorial band.

The role of ocean-atmosphere interactions in tropical cooling during the last glacial maximum

A simulation with a coupled atmosphere-ocean general circulation model configured for the Last Glacial Maximum delivered a tropical climate that is much cooler than that produced by atmosphere-only models. The main reason is a decrease in tropical sea surface temperatures, up to 6 degrees C in the western tropical Pacific, which occurs because of two processes. The trade winds induce equatorial upwelling and zonal advection of cold water that further intensify the trade winds, and an exchange of water occurs between the tropical and extratropical Pacific in which the poleward surface flow is balanced by equatorward flow of cold water in the thermocline. Simulated tropical temperature depressions are of the same magnitude as those that have been proposed from recent proxy data.

Zonal displacement of the western equatorial Pacific “fresh pool”

The distribution of salt in the tropical Pacific Ocean and its variability are potentially important in better understanding the El Niño Southern Oscillation (ENSO) ocean‐atmosphere coupled system. Complementary data sets (including sea surface salinity (SSS) derived from a ship‐of‐opportunity network, Geosat sea level and derived zonal surface currents, and outgoing longwave radiation‐derived precipitation, evaporation, and cruise measurements along the 165°E longitude) are used to describe and understand SSS changes in the western equatorial Pacific over several ENSO cycles. It is first shown that the 1974–1989 zonal displacements of the eastern edge of the western equatorial Pacific “fresh pool” (SSS < 35), marked by a salinity front and closely related to the eastern edge of the warm pool, were dominated by interannual variations that are highly correlated with the Southern Oscillation Index (SOI). Owing to the availability of basin‐wide Geosat surface current data, it is then shown that the eastward extension of the eastern edge of the fresh pool during the 1986–1987 El Niño, as well as the westward retreat during the 1988–1989 La Niña, were chiefly the result of zonal salt advection in the equatorial band. Although the equatorial upwelling extended toward the western Pacific during La Niña, its effectiveness in changing the near‐surface salinity (and temperature) is found questionable. Bearing in mind the uncertainty in determining open‐ocean precipitation, evaporation, and mixed‐layer depth, it appears that the net freshwater flux probably played a minor role in the zonal displacements of the eastern edge of the fresh pool, as compared to zonal salt advection. The roles of first baroclinic Kelvin and first meridional mode Rossby waves are discussed considering that they account in a large part for zonal advection.

An Advective-Reflective Conceptual Model for the Oscillatory Nature of the ENSO

Recent findings about zonal displacements of the Pacific warm pool required a notable modification of the delayed action oscillator theory, the current leading theory for the El Niño–Southern Oscillation (ENSO). Simulations with a linearized coupled ocean-atmosphere model resulted in 3- to 6-year ENSO-like oscillations, with many of the variable model parameters found to be very close to their observed values. This simple model suggests that ocean processes that are ignored or underestimated in the delayed action oscillator theory, such as zonal current convergence, zonal advection of sea surface temperature, and equatorial wave reflection from the eastern ocean boundary, are fundamental to the development of the ENSO, in particular to its manifestations in the central equatorial Pacific.

The singular vectors of a coupled ocean-atmosphere model of ENSO. I: Thermodynamics, energetics and error growth

The singular vectors of a dynamical system are the most rapidly growing perturbations that can exist in the system before nonlinearity becomes important. We have studied the singular vectors of an intermediate coupled ocean-atmosphere model of El Niño/Southern Oscillation (ENSO) in an effort to understand the dynamics responsible for the growth of small perturbations in the tropics. In particular, we have examined how the singular vectors of the observed seasonal cycle are influenced by various thermodynamic processes which operate in the upper-ocean mixed-layer, and which affect sea-surface temperature (SST). These processes include vertical movements of the main thermocline, zonal advection and vertical upwelling. the main findings are: the singular vector spectrum is dominated by the fastest growing member, regardless of which thermodynamic processes are active; the growth factors of the singular vectors exhibit a strong seasonal dependence; the western and central parts of the Pacific Ocean are the areas most often favoured for growth by the singular vectors, and the general criteria that must be met for singular-vector growth have been determined; at a given time of year, the growth factors of the singular vectors are sensitive to different combinations of thermodynamic processes, although the configuration of the dominant singular-vector wind field after optimal growth has been achieved is very similar in all cases. This suggests that in the tropics, the coupled system has a preferred response that the ocean thermodynamics controlling SST conspire to produce, regardless of which processes are operating in the mixed layer; the observed ENSO cycle is likely to influence singular-vector growth significantly. These results and ideas have important ramifications for the growth of errors and uncertainties in models during ENSO forecasts, and for the way this influences the predictability of ENSO. They also have important ramifications for the way perturbations grow in the real coupled ocean-atmosphere system.