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The Kuroshio Extension (KE) is known to undergo large decadal variations, especially after the 1976/77 climate regime shift. Due to strong ocean‐atmosphere interactions over the KE, the region may be a key region for basin‐scale Pacific decadal climate variability. Although baroclinic Rossby waves are suggested to influence the KE decadal variability, it remains unclear where and how decadal atmospheric forcing anomalies that induce the Rossby waves are generated. Using a linear Rossby wave model forced by reanalysis surface wind stress, the relative contribution of wind stress at each longitude is quantified for the first time. It is shown that the contribution is largest at 160°W, and 40% of the total variance in the decadal KE variability is explained by the wind forcing around 170°–150°W. Further analyses suggest that the atmospheric forcing anomalies are generated by the El Niño/Southern Oscillation Modoki through an atmospheric teleconnection after the regime shift. Plain Language Summary: The Kuroshio Extension (KE) is an intense eastward ocean flow in the western North Pacific formed after the Kuroshio separates from the Japanese coast. Satellite altimeter records and reanalysis products constrained by observational data have revealed that the KE strongly fluctuates with the decadal timescale between a stable and an unstable state, characterized by positive and negative sea surface height (SSH) anomalies to the south of the KE axis, respectively. A westward propagating oceanic wave with an SSH anomaly is considered as a primary cause of the phase reversal of the KE decadal variability. However, it has not been quantified to what extent wind forcing at each longitude contributes to the KE decadal variability through the ocean wave. In our study, we find that the largest contribution of wind forcing to the KE decadal variability is at 160°W, and 40% of the total variance in the KE variability is owing to wind forcing around 170°–150°W by simulation of westward propagating oceanic waves forced by reanalysis surface wind stress. Furthermore, it is suggested that the surface wind anomalies at 170°–150°W are remotely forced by a mode of climate variability in the tropical Pacific via an atmospheric bridge. Key Points: Wind forcing at 150°–170°W explains 40% of the total variance in Kuroshio Extension (KE) decadal variability via Rossby wavesThe dominant forcing at 150°–170°W is due to an atmospheric teleconnection of El Niño/Southern Oscillation (ENSO) ModokiThe statistically significant relation between KE decadal variability and ENSO Modoki is found only after the 1976/77 climate regime shift [ABSTRACT FROM AUTHOR]

Sea surface temperature (SST) anomalies in the Kuroshio Extension (KE) have been suggested to play a crucial role in decadal climate variability of the North Pacific affecting global climate and marine ecosystem variability. By analyzing the mixed layer heat budget, taking into account seasonality and mixed layer depth (MLD) variations, we here show for the first time that the KE SST front undergoes large decadal variations mainly owing to decadal modulations of the effective heat capacity affecting the SST sensitivity to surface heat fluxes during 1982–2015. More specifically, when the mixed layer becomes anomalously thick (shallow) to the south (north) of the front, it becomes less (more) sensitive to wintertime surface cooling. As a result, the SST front is strengthened, with positive (negative) SST anomalies to the south (north). A heat conservation model suggests that MLD anomalies are mainly due to thermocline depth anomalies. Plain Language Summary: Decadal climate variability of the North Pacific is known to affect global climate and marine ecosystem variability. The sea surface temperature (SST) front associated with the Kuroshio Extension (KE) in the northwestern Pacific is considered to play a key role in the turnabout of the North Pacific decadal climate variability. The fundamental question of how the strength of the SST front is modulated on the decadal timescale, however, has yet to be understood. Through quantitative analyses of oceanic and atmospheric data sets during 1982–2015, here we present for the first time that the KE SST front undergoes large decadal variations mainly owing to decadal modulation in thickness of the surface mixed layer that controls sensitivity of SST to surface heat exchange between the ocean and the atmosphere. Furthermore, a highly idealized model suggests that the anomalous mixed layer thickness is mainly due to anomalous oceanic stratification. Key Points: Generation mechanisms of decadal sea surface temperature (SST) anomalies associated with the Kuroshio Extension SST front is examinedA seasonally stratified mixed layer heat budget analysis with variable mixed layer depth (MLD) is conductedDecadal modulation in the MLD that controls sensitivity of SST to surface heat fluxes plays a key role during 1982–2015 [ABSTRACT FROM AUTHOR]

A new framework of complex networks based on event synchronization that can be applied to the development of temperature anomalies associated with climate variation phenomena is developed to reveal propagation characteristics of ocean temperature anomalies and their asymmetry between positive and negative phases of the Indian Ocean Dipole (IOD). When a complex network based on sea surface temperature (SST) is constructed, it is revealed that SST anomalies in the eastern pole associated with both phases of the IOD tend to be preceded by those in the eastern equatorial region and from the south. When a complex network is constructed based on both surface and subsurface temperatures, it is found that the positive IOD tends to experience extreme subsurface temperature anomalies over more vast areas before extreme SST anomalies compared to the negative IOD. Plain Language Summary: The Indian Ocean Dipole (IOD) is an interannual climate phenomenon of the tropical Indian Ocean. During positive IOD events, anomalous cooling of sea surface temperature (SST) is observed in the southeastern tropical Indian Ocean, whereas anomalous warming occurs in the western tropical Indian Ocean. Anomalies with the opposite signs occur during negative IOD events. To investigate how such anomalous SST in the southeastern tropical Indian Ocean propagates during the IOD, a new framework of complex networks is proposed. When the complex network is constructed based only on SST, it is shown that both anomalously warm SST associated with the negative IOD and anomalously cool SST associated with the positive IOD tend to be preceded by those in the eastern equatorial region and to the south. When a complex network is constructed based on both surface and subsurface temperatures, it is found that the positive IOD tends to experience extreme subsurface temperature anomalies over more vast areas compared to the negative IOD. This suggests that the positive IOD may be more predictable than the negative IOD. Key Points: Complex network based on event synchronization is developed to examine temperature anomalies associated with the Indian Ocean Dipole (IOD)Sea surface temperature (SST) anomalies in the eastern pole tend to originate from the equator and to the south for both IOD phasesSST anomalies associated with the positive IOD tend to be preceded by subsurface temperature anomalies over more vast areas [ABSTRACT FROM AUTHOR]

Using observation/reanalysis data, influences of the Interdecadal Pacific Oscillation (IPO) on the frequency of the locally amplified Ningaloo Niño (i.e., events with local air‐sea interactions playing an important role) and their mechanisms are investigated in this study. It is found that the ratio of the locally amplified Ningaloo Niño increases during the negative phase of the IPO. This is due to positive interdecadal sea surface temperature anomalies to the northwest of Australia that promote deep atmospheric convection and anomalous cyclone off the west coast of Australia through a Matsuno‐Gill response. Based on a mixed layer heat budget analysis using outputs from a regional ocean model, it is shown that an enhanced warming of anomalously thin mixed layer by the shortwave radiation contributes dominantly to the interdecadal sea surface temperature anomalies. The shallow mixed layer depth is suggested to be generated by anomalously weak winds associated with the negative IPO. Plain Language Summary: Ningaloo Niño is an anomalously warm ocean condition along the west coast of Australia and causes heat stress to marine species (e.g., coral bleaching). There are two mechanisms that give rise to the Ningaloo Niño. One is atmospheric and oceanic remote forcing, which is associated with La Niña, and the other is associated with local air‐sea interactions. A Ningaloo Niño event in which the local air‐sea interactions play an important role is called "locally amplified Ningaloo Niño." In this study, it is confirmed that the Ningaloo Niño occurs more frequently during the negative Interdecadal Pacific Oscillation (IPO) than the positive IPO. Moreover, it is revealed for the first time that the ratio of the locally amplified events increases during the negative IPO due to intensified air‐sea interaction associated with warm ocean condition to the northwest of Australia. These results and mechanisms may contribute to improve decadal prediction of the Ningaloo Niño. Key Points: Ningaloo Niño occurs more frequently during the negative phase of the Interdecadal Pacific Oscillation (IPO)The ratio of the locally amplified Ningaloo Niño increases during the negative IPOPositive interdecadal SST anomalies to the northwest of Australia during the negative IPO enhance the local air‐sea interaction [ABSTRACT FROM AUTHOR]

During the boreal winter over the South China Sea (SCS), an elongated pool of cool sea surface temperatures (SSTs), known as the cold tongue region (CTR), exists. This paper examines the ocean thermodynamics behind the asymmetry in CTR interannual SST anomaly between strong and weak CTR events observed during November-December (ND) and January-February (JF) for 1982-2015. Generally, weak CTR events are more dominant than strong CTR events in terms of SST anomaly magnitude and spatial extent. Over the northern and southern halves of northern CTR, the key processes regulating the interannual ND CTR strength are the surface heat flux and horizontal advection, respectively. During JF, the surface heat flux mainly regulates the entire northern CTR. Over the southern CTR, the key processes controlling the ND strength are the surface heat flux and horizontal advection, while only the horizontal advection controls the JF strength. For the periods and regions where the surface heat flux is the dominant thermal process, considering the mixed layer depth effect, the latent heat flux in most cases is the most important component. [ABSTRACT FROM AUTHOR]

Using observational and reanalysis data, the cloud‐shortwave radiation‐sea surface temperature (SST) feedback in the Ningaloo Niño/Niña region off the west coast of Australia is examined. It is found that this feedback operates as a negative feedback for positive SST anomalies and as a positive feedback for negative SST anomalies. This asymmetry is due to variation in the dominant cloud amount with SST. For positive SST anomalies, a decrease in the shortwave radiation resulting from an increase in high cloud amount associated with deep convection prevails, because SSTs in this region are relatively high compared to other eastern boundary regions and close to the convective threshold. Conversely, the region is under the influence of the subtropical high, and a decrease in the surface shortwave radiation associated with an increase in low cloud amount is dominant for negative SST anomalies. Plain Language Summary: Ningaloo Niño is an El Niño‐like phenomenon that occurs off the west coast of Australia and causes marine heatwave. This study investigates the feedback between shortwave radiation reaching the sea surface, cloud, and sea surface temperature (SST) that plays an important role in the development of this phenomenon. When the SST off the western Australia warms anomalously, the feedback acts as a negative feedback because increased high cloud amount reduces shortwave radiation reaching the sea surface. Conversely, when the SST cools anomalously, the feedback further cools the SST because the amount of low cloud increases and less shortwave radiation can reach the ocean surface. Dependence of the sign of the feedback to that of SST anomalies is a unique feature that has not been identified in other regions. This study enhances our understanding of local ocean‐atmosphere interaction associated with Ningaloo Niño and may help us to better predict this phenomenon in advance and mitigate its detrimental influences. Key Points: During Ningaloo Niño (Niña), surface shortwave radiation decreases owing to the dominance of an increase in high (low) cloud amountThe cloud‐shortwave radiation‐sea surface temperature feedback operates as a negative (positive) feedback for Ningaloo Niño (Niña)The unique asymmetric feedback is due to relatively high sea surface temperatures under the influence of the subtropical high [ABSTRACT FROM AUTHOR]

Understanding what controls horizontal variations in sea surface temperatures (SSTs) is one of the key science questions in climate research. Although various oceanic effects contribute to reinforcement/relaxation of horizontal variations in SSTs, the role of surface heat fluxes is surprisingly complex and can lead to significant biases in coupled models if improperly represented. In particular, the contribution of surface heat fluxes to surface frontogenesis/frontolysis depends not just on their gradients, but also on the distribution of mixed layer depth, which controls the effective heat capacity of the upper ocean. In this study, a new metric, referred to as the surface flux frontogenesis metric, is proposed that quantifies the relative importance of horizontal variations in surface heat fluxes and mixed layer depth. Global maps of this metric reveal that the role of surface heat fluxes in determining the horizontal SST gradient is highly variable geographically and by season. Furthermore, the metric can help explain characteristics of SST fronts in the northwestern Pacific, the Southern Ocean, the eastern equatorial Pacific, and the west coast of North America. Implications of this metric in coupled models will also be discussed. [ABSTRACT FROM AUTHOR]

Detailed mechanisms for frontogenesis/frontolysis of the sea surface temperature (SST) front in the Agulhas Return Current (ARC) region are investigated using outputs from a high-resolution coupled general circulation model. The SST front is maintained throughout the year through an approximate balance between frontolysis by surface heat flux and frontogenesis by horizontal advection. Although a southward (northward) cross-isotherm flow on the northern (southern) side of the front is weaker than a strong eastward along-isotherm current in the frontal region, this cross-isotherm confluent flow advects warmer (cooler) temperature toward the SST front north (south) of the front and acts as the dominant frontogenesis mechanism. In addition, stronger (weaker) frontogenesis in austral summer (winter) is attributed to the stronger (weaker) cross-isotherm confluence, which may be linked to seasonal variations of the Agulhas Current, ARC, and Antarctic Circumpolar Current. On the other hand, the contribution from entrainment is relatively small, because frontolysis by larger (smaller) entrainment velocity on the northern (southern) side opposes frontogenesis by less (more) effective cooling associated with a thicker (thinner) mixed layer and smaller (larger) temperature difference between the mixed layer and entrained water in the northern (southern) region. To gain further insight into the time-mean cross-isotherm confluent flow in the frontal region, the vorticity balance is examined. It is shown that anticyclonic (cyclonic) vorticity advection north (south) of the front by the mean cross-isotherm confluence is in balance with the sum of cyclonic (anticyclonic) vorticity advection by the mean along-isotherm flow and cross-isotherm eddy-mean interaction. [ABSTRACT FROM AUTHOR]

Both surface and subsurface salinity variability associated with positive Indian Ocean dipole (pIOD) events and its impacts on the sea surface temperature (SST) evolution are investigated through analysis of observational/reanalysis data and sensitivity experiments with a one-dimensional mixed layer model. During the pIOD, negative (positive) sea surface salinity (SSS) anomalies appear in the central-eastern equatorial Indian Ocean (southeastern tropical Indian Ocean). In addition to these SSS anomalies, positive (negative) salinity anomalies are found near the pycnocline in the eastern equatorial Indian Ocean (southern tropical Indian Ocean). A salinity balance analysis shows that these subsurface salinity anomalies are mainly generated by zonal and vertical salt advection anomalies induced by anomalous currents associated with the pIOD. These salinity anomalies stabilize (destabilize) the upper ocean stratification in the central-eastern equatorial (southeastern tropical) Indian Ocean. By decomposing observed densities into contribution from temperature and salinity anomalies, it is shown that the contribution from anomalous salinity stratification is comparable to that from anomalous thermal stratification. Furthermore, impacts of these salinity anomalies on the SST evolution are quantified for the first time using a one-dimensional mixed layer model. Since enhanced salinity stratification in the central-eastern equatorial Indian Ocean suppresses vertical mixing, significant warming of about 0.3°-0.5°C occurs. On the other hand, stronger vertical mixing associated with reduced salinity stratification results in significant SST cooling of about 0.2°-0.5°C in the southeastern tropical Indian Ocean. These results suggest that variations in salinity may potentially play a crucial role in the evolution of the pIOD. [ABSTRACT FROM AUTHOR]

Using outputs from an ocean general circulation model, the relative importance of the processes contributing to the development of the Indian Ocean Dipole (IOD) is examined systematically based on two metrics. One metric quantifies the relative importance of the surface heat flux term against the oceanic terms, while the other metric compares the contribution from the horizontal advection and vertical diabatic terms. It is revealed that the anomalous cooling in the eastern tropical Indian Ocean associated with the positive IOD varies with events and four representative events are investigated in more detail. During the 1991 IOD, the horizontal advection term made the largest contribution to the eastern cooling, and the vertical diabatic term was equally important in the early stage of the development. In the 1994 event, negative SST anomalies were generated by the surface heat flux term at first, and then matured by the vertical diabatic term. Anomalous cooling during the 1997 IOD was mainly produced by the vertical diabatic term. In 2012, anomalous surface heat flux and horizontal advection played the crucial role in the development of the eastern pole, but the vertical diabatic term opposed to the anomalous cooling. Furthermore, the dependence of the seasonal prediction skill by a global ocean-atmosphere coupled general circulation model on the generation mechanisms was examined. It is demonstrated that events with the vertical diabatic term playing a more important role in the development of the eastern pole are better predicted than those with the vertical diabatic term making relatively small contribution or opposing the occurrence. [ABSTRACT FROM AUTHOR]

The impacts of tropical interbasin interaction (TBI) on the characteristics and predictability of sea surface temperature (SST) in the tropics are assessed with a linear inverse modelling (LIM) framework that uses SST and sea surface height anomalies in the tropical Pacific (PO), Atlantic (AO), and Indian Ocean (IO). The TBI pathways are shown to be successfully isolated in stochastically-forced simulations that modify off-diagonal elements of the linear operators. The removal of TBI leads to a substantial increase in the amplitude of El Niño-Southern Oscillation (ENSO) and related variability. Partial decoupling experiments that eliminate specific coupling components reveal that PO-IO interaction is the dominant contributor, whereas PO-AO and AO-IO interactions play a minor role. A series of retrospective forecast experiments with different operators shows that decoupling leads to a substantial decrease in ENSO prediction skill especially at longer lead times. The relative contributions of individual pathways to forecast skill are generally consistent with the results from the stochastically-forced experiments. Qualitatively similar results are obtained from an additional set of forecast experiments that partially apply initial conditions over specific basins, but several important differences were also found due to differences in the representations of each TBI pathway. Finally, the cause of contrasting SST anomalies over the AO after the extreme 1982/83 and 1997/98 El Niño events is explored using LIM forecast experiments to demonstrate the strength and flexibility of our LIM-based approach. [ABSTRACT FROM AUTHOR]

Detailed mechanisms for frontogenesis/frontolysis of the Agulhas Return Current (ARC) Front, defined as the maximum of the meridional sea surface temperature (SST) gradient at each longitude within the ARC region (40°-50°E, 55°-35°S), are investigated using observational datasets. Due to larger (smaller) latent heat release to the atmosphere on the northern (southern) side of the front, the meridional gradient of surface net heat flux (NHF) is found throughout the year. In austral summer, surface warming is weaker (stronger) on the northern (southern) side, and thus the NHF tends to relax the SST front. The weaker (stronger) surface warming, at the same time, leads to the deeper (shallower) mixed layer on the northern (southern) side. This enhances the frontolysis, because deeper (shallower) mixed layer is less (more) sensitive to surface warming. In austral winter, stronger (weaker) surface cooling on the northern (southern) side contributes to the frontolysis. However, deeper (shallower) mixed layer is induced by stronger (weaker) surface cooling on the northern (southern) side and suppresses the frontolysis, because the deeper (shallower) mixed layer is less (more) sensitive to surface cooling. Therefore, the frontolysis by the NHF becomes stronger (weaker) through the mixed layer processes in austral summer (winter). The cause of the meridional gradient of mixed layer depth is estimated using diagnostic entrainment velocity and the Monin-Obukhov depth. Furthermore, the above mechanisms obtained from the observation are confirmed using outputs from a high-resolution coupled general circulation model. Causes of model biases are also discussed. [ABSTRACT FROM AUTHOR]

The Indian Ocean Dipole (IOD) is known as a climate mode in the tropical Indian Ocean accompanied by negative (positive) sea surface temperature (SST) anomalies over the eastern (western) pole during its positive phase. However, the western pole of the IOD is not always covered totally by positive SST anomalies. For this reason, the IOD is further classified into two types in this study based on SST anomalies in the western pole. The first type (hereafter 'canonical IOD') is associated with negative (positive) SST anomalies in the eastern (central to western) tropical Indian Ocean. The second type (hereafter 'IOD Modoki'), on the other hand, is associated with negative SST anomalies in the eastern and western tropical Indian Ocean and positive SST anomalies in the central tropical Indian Ocean. Based on composite analyses, it is found that easterly wind anomalies cover the whole equatorial Indian Ocean in the canonical IOD, and as a result, positive rainfall anomalies are observed over East Africa. Also, due to the basin-wide easterly wind anomalies, the canonical IOD is accompanied by strong sea surface height (SSH) anomalies. In contrast, zonal wind anomalies converge in the central tropical Indian Ocean in the IOD Modoki, and no significant precipitation anomalies are found over East Africa. Also, only weak SSH anomalies are seen, because equatorial downwelling anomalies induced by westerly wind anomalies in the west are counteracted by equatorial upwelling anomalies caused by easterly wind anomalies in the east. [ABSTRACT FROM AUTHOR]

Using observational data and outputs from an ocean general circulation model, the interdecadal changes in the Indian Ocean subtropical dipole (IOSD) are investigated for the first time. It is found that the frequency of the IOSD has become higher because of a decreasing trend in the mixed layer depth (MLD) over the southwestern pole in January and February. Positive (negative) sea surface temperature (SST) anomalies associated with the IOSD are generated when the mixed layer becomes anomalously shallow (deep). The thinner mixed layer in the recent decade amplifies this effect and even weak atmospheric forcing may trigger the IOSD. From a diagnosis of the Monin-Obukhov depth, it is shown that an increasing trend of surface heat flux, which is due to the decrease of wind speed (increase of specific humidity near the sea surface) associated with the poleward shift of westerly jet in January (the strengthening of Mascarene high in February), causes the decreasing trend of the MLD. On the other hand, the smaller amplitude in the recent decades is because the IOSD starts to develop in December, but the deeper mixed layer in December in the recent decade provides an unfavorable condition for its development. In addition, the shallower mixed layer in January and February may also amplify the negative feedback processes that damp the SST anomalies. Since no interdecadal changes in interannual variability of atmospheric forcing corresponding to that in the IOSD are observed, the interdecadal trend in the MLD is essential for that of the IOSD. [ABSTRACT FROM AUTHOR]

Using daily rainfall data from 183 stations, influences of the Madden-Julian Oscillation (MJO) on precipitation over southern Iran are examined. When composites of daily rainfall anomalies are constructed by introducing eight phases in the time evolution, wet conditions are observed in MJO phases 1-2 and 7-8. During these phases, atmospheric circulation anomalies favor moisture advection from the western Indian Ocean to southern Iran. In contrast, dry conditions during MJO phases 3 to 6 are associated with advection of drier air from the north. [ABSTRACT FROM AUTHOR]

A recently identified climate mode called Ningaloo Niño (Niña) is associated with positive (negative) sea surface temperature (SST) anomalies off the west coast of Australia and negative (positive) sea level pressure (SLP) anomalies in the overlying atmosphere. By conducting a series of numerical experiments with an atmospheric general circulation model, generation mechanisms of the atmospheric circulation anomalies accompanied by Ningaloo Niño/Niña are examined. Even when SST is allowed to vary interannually only in the eastern South Indian Ocean, negative (positive) SLP anomalies are formed off the west coast of Australia in Ningaloo Niño (Niña) years, supporting the existence of local ocean-atmosphere interaction. When the model is forced by SST anomalies outside of the eastern South Indian Ocean, negative (positive) SLP anomalies are also generated in Ningaloo Niño (Niña) years owing to a Matsuno-Gill type response to atmospheric convection anomalies in the tropical Pacific. It is found that the latter impact is stronger in the current atmospheric general circulation model. Regarding climatic impacts, it is shown that Ningaloo Niño (Niña) induces wet (dry) anomalies over the northwestern part of Australia even when SST anomalies outside of the eastern South Indian Ocean are excluded from the SST forcing. [ABSTRACT FROM AUTHOR]

Using both observational and reanalysis data, evolution processes of a regional climate phenomenon off Western Australia named recently 'Ningaloo Niño (Niña)' are studied in detail. It is also shown that the Ningaloo Niño (Niña) has significant impacts on the precipitation over Australia. The Ningaloo Niño (Niña), which is associated with positive (negative) sea surface temperature (SST) anomalies and atmospheric anomalies off the western coast of Australia, peaks during austral summer and is classified into two types based on the difference in the evolution process. The first type called a locally amplified mode develops through an intrinsic unstable air-sea interaction off the western coast of Australia; an anomalous cyclone (anticyclone) generated by positive (negative) SST anomalies forces northerly (southerly) alongshore wind anomalies, which induce coastal downwelling (upwelling) anomalies, and enhance the positive (negative) SST anomalies further. The second type called a non-locally amplified mode is associated with coastally trapped waves originating in either the western tropical Pacific, mostly related to El Niño/Southern Oscillation, or the northern coast of Australia. Positive (negative) SST anomalies in both modes are associated with an anomalous low (high) off the western coast of Australia. The sea level pressure (SLP) anomalies in the locally amplified mode are regionally confined with a cell-like pattern and produce a sharp offshore pressure gradient along the western coast of Australia, whereas those in the non-locally amplified mode tend to show a zonally elongated pattern. The difference is found to be related to conditions of the continental SLP modulated by the Australian summer monsoon and/or the Southern Annular Mode. [ABSTRACT FROM AUTHOR]

The Madden-Julian Oscillation (MJO) is the major mode of intraseasonal variability (30-60 days) in the tropics, having large rainfall impacts globally, and possibly on southern Africa. However, the latter impact is not well understood and needs to be further explored. The life cycle of the MJO, known to be asymmetric, has been nevertheless analyzed usually through methods constrained by both linearity and orthogonality, such as empirical orthogonal function analysis. Here we explore a non-linear classification method, the self-organizing map (SOM), a type of artificial neural network used to produce a low-dimensional representation of high-dimensional datasets, to capture more accurately the life cycle of the MJO and its global impacts. The classification is applied on intraseasonal anomalies of outgoing longwave radiation within the tropical region over the 1980-2009 period. Using the SOM to describe the MJO is a new approach, complimentary to the usual real-time multivariate MJO index. It efficiently captures this propagative phenomenon and its seasonality, and is shown to provide additional temporal and spatial information on MJO activity. For each node, the subtropical convection is analyzed, with a particular focus on the southern Africa region. Results show that the convection activity over the central tropical Indian Ocean is a key factor influencing the intraseasonal convective activity over the southern African region. Enhanced (suppressed) convection over the central Indian Ocean tends to suppress (enhance) convection over the southern African region with a 10-day lag by modulating the moisture transport. [ABSTRACT FROM AUTHOR]

Prediction skill for southern African (16°-33°E, 22°-35°S) summer precipitation in the Scale Interaction Experiment-Frontier coupled model is assessed for the period of 1982-2008. Using three different observation datasets, deterministic forecasts are evaluated by anomaly correlation coefficients, whereas scores of relative operating characteristic and relative operating level are used to evaluate probabilistic forecasts. We have found that these scores for December-February precipitation forecasts initialized on October 1st are significant at 95 % confidence level. On a local scale, the level of prediction skill in the northwestern and central parts of southern Africa is higher than that in northeastern South Africa. El Niño/Southern Oscillation (ENSO) provides the major source of predictability, but the relationship with ENSO is too strong in the model. The Benguela Niño, the basin mode in the tropical Indian Ocean, the subtropical dipole modes in the South Atlantic and the southern Indian Oceans and ENSO Modoki may provide additional sources of predictability. Within the wet season from October to the following April, the precipitation anomalies in December-February are the most predictable. This study presents promising results for seasonal prediction of precipitation anomaly in the extratropics, where seasonal prediction has been considered a difficult task. [ABSTRACT FROM AUTHOR]

The sea surface temperature (SST) in the western Arabian Sea upwelling region is known to influence the amount of precipitation associated with the Indian summer monsoon. Thus, understanding what determines the SST in this region is an important issue. Using outputs from an ocean general circulation model with and without strong damping in the eastern equatorial Indian Ocean, this study examines how the reflection of semiannual Kelvin waves at the eastern boundary of the Indian Ocean may influence the western Arabian Sea upwelling region. The downwelling Kelvin waves generated in boreal spring are reflected at the eastern boundary and reach the western equatorial Indian Ocean as reflected Rossby waves about 6 months later. The resulting westward current along the equator in the western equatorial Indian Ocean transports warmer water to the western Arabian Sea upwelling region. Thus, the SST in this region becomes colder especially in boreal fall without the reflected Rossby waves. These results are further supported by the analysis of the mixed layer temperature balance. Surprisingly, vertical processes do not contribute to the SST difference, even though the thermocline becomes shallower without the downwelling Rossby waves. This is because the mixed layer is shoaling rapidly from September to November, and there is basically no entrainment of water from below. In contrast, the reflected Rossby waves do not have large impacts on the SST in other seasons mainly because the zonal SST gradient is not as strong and/or the amplitude of Rossby waves is weaker. [ABSTRACT FROM AUTHOR]

Predictability of the subtropical dipole modes is assessed using the SINTEX-F coupled model. Despite the known difficulty in predicting subtropical climate due to large internal variability of the atmosphere and weak ocean-atmosphere coupling, it is shown for the first time that the coupled model can successfully predict the South Atlantic Subtropical Dipole (SASD) 1 season ahead, and the prediction skill is better than the persistence in all the 1-12 month lead hindcast experiments. There is a prediction barrier in austral winter due to the seasonal phase locking of the SASD to austral summer. The prediction skill is lower for the Indian Ocean Subtropical Dipole (IOSD) than for the SASD, and only slightly better than the persistence till 6-month lead because of the low predictability of the sea surface temperature anomaly in its southwestern pole. However, for some strong IOSD events in the last three decades, the model can predict them 1 season ahead. The co-occurrence of the negative SASD and IOSD in 1997/1998 austral summer can be predicted from July 1st of 1997. This is because the negative sea level pressure anomalies over the South Atlantic and the southern Indian Ocean in September-October (November-December) that trigger the occurrence of the negative SASD and IOSD are related to the well predicted tropical Indian Ocean Dipole (El Niño/Southern Oscillation). Owing to the overall good performances of the SINTEX-F model in predicting the SASD, some strong IOSD, and El Niño/Southern Oscillation, the prediction skill of the southern African summer precipitation is high in the SINTEX-F model. [ABSTRACT FROM AUTHOR]

This study examines southern African summer rainfall and tropical temperate troughs (TTTs) simulated with three versions of an atmospheric general circulation model differing only in the convection scheme. All three versions provide realistic simulations of key aspects of the summer (November-February) rainfall, such as the spatial distribution of total rainfall and the percentage of rainfall associated with TTTs. However, one version has a large bias in the onset of the rainy season. Results from self-organizing map (SOM) analysis on simulated daily precipitation data reveals that this is because the occurrence of TTTs is underestimated in November. This model bias is not related to westerly wind shear that provides favorable conditions for the development of TTTs. Rather, it is related to excessive upper level convergence and associated subsidence over southern Africa. Furthermore, the model versions are shown to be successful in capturing the observed drier (wetter) conditions over the southern African region during El Niño (La Niña) years. The SOM analysis reveals that nodes associated with TTTs in the southern (northern) part of the domain are observed less (more) often during El Niño years, while nodes associated with TTTs occur more frequently during La Niña years. Also, nodes associated with dry conditions over southern Africa are more (less) frequently observed during El Niño (La Niña) years. The models tend to perform better for La Niña events, because they are more successful in representing the observed frequency of different synoptic patterns. [ABSTRACT FROM AUTHOR]

Based on experiments using a coupled general circulation model which resolves tropical ocean-atmosphere coupled phenomena such as El Niño/Southern Oscillation (ENSO) and the Indian Ocean Dipole, forcing mechanisms of the Indian Ocean subtropical dipole (IOSD) are investigated. In the control experiment, as in the observation, several types of the IOSD are generated by the variations in the Mascarene High during austral summer and characterized by a dipole pattern of sea surface temperature (SST) anomalies in the northeastern and southwestern parts of the southern Indian Ocean. In another experiment, where the SST outside the southern Indian Ocean is nudged toward the monthly climatology of the simulated SST, one type of the IOSD occurs, but it is less frequent and associated with the zonal wavenumber four pattern of equivalently barotropic geopotential height anomalies in high latitudes, suggesting an interesting link with the Antarctic Circumpolar Wave. This indicates that, even without the atmospheric teleconnection from tropical coupled climate modes, the IOSD may develop in association with the atmospheric variability in high latitudes of the Southern Hemisphere. In the other experiment, where only the southern Indian Ocean and the tropical Pacific are freely interactive with the atmosphere, two types of both positive and negative IOSD occur. Since the occurrence frequency of the IOSD significantly increases as compared to the second experiment, this result confirms that the atmospheric teleconnection from ocean-atmosphere coupled modes in the tropical Pacific such as ENSO may also induce the variations in the Mascarene High that generate the IOSD. The present research, even within the realm of model studies, shows clearly that the predictability of the IOSD in mid-latitudes is related to both low and high-latitudes climate variations. [ABSTRACT FROM AUTHOR]

The interannual variation of precipitation in the southern part of Iran and its link with the large-scale climate modes are examined using monthly data from 183 meteorological stations during 1974-2005. The majority of precipitation occurs during the rainy season from October to May. The interannual variation in fall and early winter during the first part of the rainy season shows apparently a significant positive correlation with the Indian Ocean Dipole (IOD) and El Niño-Southern Oscillation (ENSO). However, a partial correlation analysis used to extract the respective influence of IOD and ENSO shows a significant positive correlation only with the IOD and not with ENSO. The southeasterly moisture flux anomaly over the Arabian Sea turns anti-cyclonically and transport more moisture to the southern part of Iran from the Arabian Sea, the Red Sea, and the Persian Gulf during the positive IOD. On the other hand, the moisture flux has northerly anomaly over Iran during the negative IOD, which results in reduced moisture supply from the south. During the latter part of the rainy season in late winter and spring, the interannual variation of precipitation is more strongly influenced by modes of variability over the Mediterranean Sea. The induced large-scale atmospheric circulation anomaly controls moisture supply from the Red Sea and the Persian Gulf. [ABSTRACT FROM AUTHOR]

Using diagnostic analyses and an AGCM simulation, the detailed mechanism of Indian Ocean Dipole (IOD) influence on the early winter Tibetan Plateau snow cover (EWTPSC) is clarified. In early winter of pure positive IOD years with no co-occurrence of El Niño, the anomalous dipole diabatic heating over the tropical Indian Ocean excites the baroclinic response in the tropics. Since both baroclinic and barotropic components of the basic zonal wind over the Arabian Peninsula increase dramatically in early winter due to the equatorward retreat of the westerly jet, the baroclinic mode excites the barotropic Rossby wave that propagates northeastward and induces a barotropic cyclonic anomaly north of India. This enables the moisture transport cyclonically from the northern Indian Ocean toward the Tibetan Plateau. The convergence of moisture over the plateau explains the positive influence of IOD on the EWTPSC. In contrast, the basic zonal wind over the Arabian Peninsula is weak in autumn. This is not favorable for excitation of the barotropic Rossby wave and teleconnection, even though the IOD-related diabatic heating anomaly in autumn similar to that in early winter exists. This result explains the insignificant (significant positive) partial correlation between IOD and the autumn (early winter) Tibetan Plateau snow cover after excluding the influence of ENSO. The sensitivity experiment forced by the IOD-related SST anomaly within the tropical Indian Ocean well reproduces the baroclinic response in the tropics, the teleconnection from the Arabian Peninsula, and the increased moisture supply to the Tibetan Plateau. Also, the seasonality of the atmospheric response to the IOD is simulated. [ABSTRACT FROM AUTHOR]

Using observation data and outputs from the 'twentieth-century climate in coupled models' (20c3m) control runs of coupled general circulation models submitted to the Coupled Model Intercomparison Project, phase 3 (CMIP3), the ability of CMIP3 models to simulate the Indian Ocean subtropical dipole (IOSD) and its influence on the rainfall anomaly over the southern African region is investigated. Many models simulate the IOSD, but the location and shape of the sea surface temperature anomaly vary among models. This model bias is closely linked to the bias in simulating the anomalous strengthening and southward shift of the subtropical high. Almost all models fail to simulate the rainfall anomaly associated with the IOSD owing to the inaccurate simulation of the location of sea surface temperature and sea level pressure anomalies. [ABSTRACT FROM AUTHOR]

The growth and decay mechanisms of subtropical dipole modes in the southern Indian and South Atlantic Oceans and their impacts on southern African rainfall are investigated using results from a coupled general circulation model originally developed for predicting tropical climate variations. The second (most) dominant mode of interannual sea surface temperature (SST) variations in the southern Indian (South Atlantic) Ocean represents a northeast-southwest oriented dipole, now called subtropical dipole mode. The positive (negative) SST interannual anomaly pole starts to grow in austral spring and reaches its peak in February. In austral late spring, the suppressed (enhanced) latent heat flux loss associated with the variations in the subtropical high causes a thinner (thicker) than normal mixed layer thickness that, in turn, enhances (reduces) the warming of the mixed layer by the climatological shortwave radiation. The positive (negative) pole gradually decays in austral fall because the mixed layer cooling by the entrainment is enhanced (reduced), mostly owing to the larger (smaller) temperature difference between the mixed layer and the entrained water. The increased (decreased) latent heat loss due to the warmer (colder) SST also contributes to the decay of the positive (negative) pole. Although further verification using longer observational data is required, the present coupled model suggests that the South Atlantic subtropical dipole may play a more important role in rainfall variations over the southern African region than the Indian Ocean subtropical dipole. [ABSTRACT FROM AUTHOR]

The seasonal and interannual variations of the sea surface temperature (SST) above the Seychelles Dome (SD) are investigated using outputs from an OGCM. The SST warms from August to April and cools from May to July. The surface heat flux plays the most important role in the seasonal variation, and it is mostly due to shortwave radiation. The horizontal advection tends to warm the SST in austral winter owing to the southward Ekman heat transport associated with the Indian summer monsoon. The cooling by the vertical turbulent diffusion becomes most effective in austral summer owing to the thin mixed layer during that time. On the interannual time scale, the SST becomes anomalously warm (cool) when the SD is weak (strong). In contrast to the seasonal variation, the vertical diffusion plays the most important role and causes anomalous warming (cooling). This warming (cooling) is due to the anomalously warm (cold) water below the mixed layer as a result of the deeper (shallower) thermocline in response to ocean dynamics. Also, the cooling by the vertical diffusion becomes less (more) efficient, because the mixed layer is anomalously thick (thin). The horizontal advection contributes to the anomalous warming (cooling) due to the anomalous southward (northward) Ekman heat transport. On the other hand, the anomalous surface heat flux tends to cool (warm) the mixed layer, because the warming of the mixed layer by the shortwave radiation becomes less (more) efficient due to the anomalously thick (thin) mixed layer. [ABSTRACT FROM AUTHOR]

Using observational data and outputs from an ocean general circulation model, the growth and decay of the South Atlantic subtropical dipole (SASD) are studied. The SASD is the most dominant mode of interannual variability in the South Atlantic Ocean, and its sea surface temperature (SST) anomaly shows a dipole pattern that is oriented in the northeast-southwest direction. The positive (negative) pole develops because the warming of the mixed layer by the contribution from the climatological shortwave radiation is enhanced (suppressed) when the mixed layer is thinner (thicker) than normal. The mixed layer depth anomaly over the positive (negative) pole is due to the suppressed (enhanced) latent heat flux loss associated with the southward migration and strengthening of the subtropical high. During the decay phase, since the temperature difference between the mixed layer and the entrained water becomes anomalously large (small) as a result of the positive (negative) mixed layer temperature anomaly, the cooling of the mixed layer by the entrainment is enhanced (reduced). In addition, the cooling of the mixed layer by the contribution from the climatological latent heat flux is enhanced (suppressed) by the same thinner (thicker) mixed layer. This paper demonstrates the importance of taking into account the interannual variations of the mixed layer depth in discussing the growth and decay of SST anomalies associated with the SASD. [ABSTRACT FROM AUTHOR]

Using a non-linear statistical analysis called 'self-organizing maps', the interannual sea surface temperature (SST) variations in the southern Indian Ocean are investigated. The SST anomalies during austral summer from 1951 to 2006 are classified into nine types with differences in the position of positive and negative SST anomaly poles. To investigate the evolution of these SST anomaly poles, heat budget analysis of mixed-layer using outputs from an ocean general circulation model is conducted. The warming of the mixed-layer by the climatological shortwave radiation is enhanced (suppressed) as a result of negative (positive) mixed-layer thickness anomaly over the positive (negative) SST anomaly pole. This contribution from shortwave radiation is most dominant in the growth of SST anomalies. In contrast to the results reported so far, the contribution from latent heat flux anomaly is not so important. The discrepancy in the analysis is explained by the modulation in the contribution from the climatological heat flux by the interannual mixed-layer depth anomaly that was neglected in the past studies. [ABSTRACT FROM AUTHOR]

Abstract: The forcing efficiency for the first and the second baroclinic modes by the wind stress in tropical oceans has been discussed by calculating equivalent forcing depth from annual mean, seasonal, and pentadal density profiles of the observational data. In the annual mean field, the first mode is forced preferentially in the western Pacific and the Indian Ocean, whereas the second mode is more strongly excited in the Atlantic and the eastern Pacific. This difference is mostly due to the pycnocline depth; the second mode is more dominantly forced where the pycnocline depth is shallower. We also revealed large seasonal variations of the second mode''s equivalent forcing depth in the western Indian Ocean. The first mode is more dominantly forced during boreal spring and fall in the western Indian Ocean, while the second mode becomes more dominantly forced during boreal summer and winter. Those are due to seasonal variations of both the zonal wind and the pycnocline depth. Moreover, we show that the excitation of the second mode in the western Pacific increases after the late 1970s, which is associated with the decreasing trend of the zonal pycnocline gradient. Revealing the variation of the equivalent forcing depth will be useful for understanding the oceanic response to winds in tropical oceans and the improvement in the predictability of air-sea coupled climate variability in the tropics. [Copyright &y& Elsevier]

Using an ocean–atmosphere coupled general circulation model, air–sea interaction processes associated with the Atlantic meridional mode are investigated from a new viewpoint of its link with the Guinea Dome in the northern tropical Atlantic. The subsurface thermal oceanic dome develops off Dakar from late spring to late fall owing to wind-induced Ekman upwelling. Its seasonal evolution is due to surface wind variations associated with the northward migration of the intertropical convergence zone (ITCZ). Since the upwelling cools the mixed layer in the Guinea Dome region during summer, it is very important to reproduce its variability in order to simulate the sea surface temperature (SST) there. During the preconditioning phase of the positive (negative) Atlantic meridional mode, the dome is anomalously weak (strong) and the mixed layer is anomalously deep (shallow) there in late fall. This condition reduces (enhances) the sensitivity of the mixed layer temperature to the climatological atmospheric cooling. As a result, the positive (negative) SST anomaly appears there in early winter. Then, it develops in the following spring through the wind–evaporation–SST (WES) positive feedback associated with the anomalous northward (southward) migration of the ITCZ. This, in turn, leads to the stronger (weaker) Ekman upwelling and colder (warmer) subsurface temperature in the dome region during summer. It plays an important role on the decay of the warm (cold) SST anomaly through entrainment as a negative feedback. Therefore, simulating this interesting air–sea interaction in the Guinea Dome region is critical in improving prediction skill for the Atlantic meridional mode. [ABSTRACT FROM AUTHOR]

Using a high-resolution ocean general circulation model forced by NCEP/NCAR reanalysis data, the interannual variability of the Guinea Dome is studied from a new viewpoint of its possible link with the Atlantic Meridional Mode (AMM), which is related to the meridional migration of the Intertropical Convergence Zone (ITCZ). The dome develops off Dakar seasonally from late spring to late fall owing to the wind-induced Ekman upwelling; its seasonal evolution is associated with the northward migration of the ITCZ. When the ITCZ is located anomalously northward (southward) from late spring to early summer, as a result of the wind-evaporation-sea surface temperature (SST) positive feedback with positive (negative) SST anomaly over the Northern Hemisphere, the dome becomes unusually strong (weak) in fall as a result of stronger (weaker) Ekman upwelling. This may contribute to the decay of the AMM. Thus, the coupled nature between the AMM and the Guinea Dome could be important in understanding, modeling, and predicting the tropical Atlantic variability. [ABSTRACT FROM AUTHOR]

Using reanalysis data and snow cover data derived from satellite observations, respective influences of Indian Ocean Dipole (IOD) and El Niño/Southern Oscillation (ENSO) on the Tibetan snow cover in early winter are investigated. It is found that the snow cover shows a significant positive partial correlation with IOD. In the pure positive IOD years with no co-occurrences of El Niño, negative geopotential height anomalies north of India are associated with warm and humid southwesterlies to enter the plateau from the Bay of Bengal after rounding cyclonically and supply more moisture. This leads to more precipitation, more snow cover, and resultant lower surface temperature over the plateau. These negative geopotential height anomalies north of India are related to the equivalent barotropic stationary Rossby waves in the South Asian wave guide. The waves can be generated by the IOD-related convection anomalies over the western/central Indian Ocean. In contrast, in the pure El Niño years with no co-occurrences of the positive IOD, the anomalies of moisture supply and surface temperature over the plateau are insignificant, suggesting negligible influences of ENSO on the early winter Tibetan snow cover. Further analyses show that ENSO is irrelevant to the spring/early summer Tibetan snow cover either, whereas the IOD-induced snow cover anomalies can persist long from the early winter to the subsequent early summer. [ABSTRACT FROM AUTHOR]

Abstract: Impacts of the South China Sea Throughflow (SCST) on seasonal and interannual variations of the Indonesian Throughflow are studied by comparing outputs from ocean general circulation model (OGCM) experiments with and without the SCST. The observed subsurface maximum in the southward flow through the Makassar Strait is simulated only when the SCST, which is driven by the large-scale wind, is allowed in the model. The mean volume and heat transport by the Makassar Strait Throughflow are reduced by 1.7Sv and 0.19PW, respectively, by the existence of the SCST in the model. The difference is particularly remarkable during boreal winter when the SCST reaches its seasonal maximum. Furthermore, the SCST is strengthened during El Niño, leading to the weakening in the southward volume and heat transport through the Makassar Strait by 0.37Sv and 0.05PW, respectively. These findings from the OGCM experiments suggest that the SCST may play an important role in climate variability of the Indo-Pacific Ocean. [Copyright &y& Elsevier]

Using outputs from the “twentieth-century climate in coupled models” (20c3m) control run of the Coupled Model Intercomparison Project, phase 3 (CMIP3), coupled GCMs, the authors have examined how seasonal variations of the Seychelles Dome (SD) are simulated in the southwestern Indian Ocean. The observed SD shows a dominant semiannual signal due to the semiannual variation in the local Ekman upwelling resulting from a combination of two terms related to the wind stress curl and the zonal wind stress. However, all models fail to reproduce this important mechanism. In particular, the latter contribution—that determined by the seasonal variation of the zonal wind stress associated with the Indian monsoon—is not well simulated. Successful models need to reproduce the asymmetric nature of the monsoon: a shorter and stronger summer monsoon and a longer and weaker winter monsoon. Possible remedies for the model bias are also discussed. [ABSTRACT FROM AUTHOR]

The tropical Indian Ocean climate variability is investigated using an artificial neural network analysis called self-organizing map (SOM) for both observational data and coupled model outputs. The SOM successfully captures the dipole sea surface temperature anomaly (SSTA) pattern associated with the Indian Ocean Dipole (IOD) and basin-wide warming/cooling associated with ENSO. The dipole SSTA pattern appears only in boreal summer and fall, whereas the basin-wide warming/cooling appears mostly in boreal winter and spring owing to the phase-locking nature of these phenomena. Their occurrence also undergoes significant decadal variation. Composite diagrams constructed for nodes in the SOM array based on the simulated SSTA reveal interesting features. For the nodes with the basin-wide warming, a strong positive SSTA in the eastern equatorial Pacific, a negative Southern Oscillation, and a negative precipitation anomaly in East Africa are found. The nodes with the positive IOD are associated with a weak positive SSTA in the central equatorial Pacific or positive SSTA in the eastern equatorial Pacific, a positive (negative) sea level pressure anomaly in the eastern (western) tropical Indian Ocean, and a positive precipitation anomaly over East Africa. The warming in the central equatorial Pacific appears to correspond to El Niño Modoki discussed recently. These results suggest usefulness of SOM in studying large-scale ocean–atmosphere coupled phenomena. [ABSTRACT FROM AUTHOR]

Using an ocean general circulation model (OGCM), seasonal variation of the Seychelles Dome (SD) is investigated for the first time. The SD is an oceanic thermal dome located in the southwestern Indian Ocean, and its influence on sea surface temperature is known to play an important role in the Indian monsoon system. Its seasonal variation is dominated by a remarkable semiannual cycle resulting from local Ekman upwelling. This semiannual nature is explained by different contributions of the following two components of the Ekman pumping: one term that is proportional to the planetary beta and the zonal wind stress and the other term that is proportional to the wind stress curl. The former is determined by the seasonal change in the zonal component of the wind stress vector above the SD; it is associated with the Indian monsoon and causes downwelling (upwelling) during boreal summer (boreal winter). The latter, whose major contribution comes from the meridional gradient of the zonal wind stress, also shows a clear annual cycle with strong upwelling during boreal summer and fall. However, it remains almost constant for 5 months from June to October, even though the zonal wind stress itself varies significantly during this period. The above overall feature is due to the unique location of the SD; it is located between the following two regions: one is dominated by the seasonal variation in wind stress resulting from the Indian monsoon, and the other is dominated by the southeasterly trade winds that prevail throughout a year. The above uniqueness provides a novel mechanism that causes the strong semiannual cycle in the tropical Indian Ocean. [ABSTRACT FROM AUTHOR]

Using outputs from a high-resolution OGCM, seasonal and interannual variations of the Angola Dome (AD) are revisited. Although the AD was previously considered to be one large cold tongue extending from the West African coast, it is shown that two cold domes exist. These two domes have remarkably different mechanisms for their seasonal variation. The weak dome, whose center is located at 6°S, 1°E, develops from May to September owing to the divergence of heat transport associated with upwelling. The strong dome, on the other hand, extends from the west coast of Africa between 20° and 15°S, and develops from April to August by the surface heat flux. The interannual variation of the weak dome is strongly influenced by the Atlantic Niño. An unusual relaxation of easterly wind stress in the central equatorial Atlantic Ocean associated with the Atlantic Niño triggers second baroclinic downwelling equatorial Kelvin waves, which propagate eastward along the equator and poleward along the coast after reaching the African coast as coastal Kelvin waves. Then, downwelling Rossby waves radiate away from the coast and cause significant warming in the weak dome region. The interannual variation of the South Equatorial Undercurrent may be associated with that of the AD; its transport decreases by 0.6 Sv, and its core shifts equatorward by 0.2° when the AD is anomalously weak. [ABSTRACT FROM AUTHOR]

Using outputs from the SINTEX-F1 coupled GCM, the thermodynamics of ENSO events and its relation with the seasonal cycle are investigated. Simulated El Niño events are first classified into four groups depending on during which season the Niño-3.4 sea surface temperature anomaly (SSTA) index (5°S–5°N, 120°–170°W) reaches its peak. Although the heat content of the tropical Pacific decreases for all four types, the tropical Pacific loses about twice as much during an El Niño that peaks during winter compared with one that peaks during summer. The surface heat flux, the southward heat transport at 15°S, and the Indonesian Throughflow heat transport contribute constructively to this remarkable seasonal difference. It is shown that the Indonesian Throughflow supplies anomalous heat from the Indian Ocean, especially during the summer El Niño–like event. Changes in the basic state provided by the seasonal cycle cause differences in the atmospheric response to the SSTA, which in turn lead to the difference between the surface heat flux and the meridional heat transport anomaly. [ABSTRACT FROM AUTHOR]

The decadal variation in the tropical Indian Ocean is investigated using outputs from a 200-yr integration of the Scale Interaction Experiment-Frontier Research Center for Global Change (SINTEX-F1) ocean–atmosphere coupled model. The first EOF mode of the decadal bandpass- (9–35 yr) filtered sea surface temperature anomaly (SSTA) represents a basinwide mode and is closely related with the Pacific ENSO-like decadal variability. The second EOF mode shows a clear east–west SSTA dipole pattern similar to that of the interannual Indian Ocean dipole (IOD) and may be termed the decadal IOD. However, it is demonstrated that the decadal air–sea interaction in the Tropics can be a statistical artifact; it should be interpreted more correctly as decadal modulation of interannual IOD events (i.e., asymmetric or skewed occurrence of positive and negative events). Heat budget analysis has revealed that the occurrence of IOD events is governed by variations in the southward Ekman heat transport across 15°S and variations in the Indonesian Throughflow associated with the ENSO. The variations in the southward Ekman heat transport are related to the Mascarene high activities. [ABSTRACT FROM AUTHOR]

Abstract: Using an output from 200-year integration of the Scale Interaction Experiment of EU project-F1 model (SINTEX-F1), the annual ENSO reproduced in the coupled general circulation model is investigated, suggesting the importance of reproducing an annual cycle in realistically simulating ENSO events. Although many features of the annual ENSO are reproduced, the northward expansion of sea surface temperature anomaly (SSTA) in the eastern tropical Pacific stays south of the equator. It is suggested that this model bias is due to the excitation of the too strong Rossby waves in the southeastern tropical Pacific, which reflect at the western boundary and intrude into the eastern equatorial Pacific. The zonal wind stress anomaly along the equator also plays an important role in generating the equatorial Kelvin waves. The amplitude of SSTA for the annual ENSO mode is reproduced, but its variance is only 20% of the observation; this is again due to the lack of northward migration of seasonal SSTA in the equatorial region and weaker coastal Kelvin waves along South America. Remedies for the model bias are discussed. [Copyright &y& Elsevier]

Using a high-resolution OGCM result, multiscale variations of the Mindanao Dome (MD) are discussed. The MD is generated by local Ekman upwelling as the positive curl of the Asian winter monsoon increases over the western tropical Pacific, as discussed in the literature. It is shown, however, that the MD decays owing to the Pacific basinwide annual cycle; the warm anomaly that propagates from the eastern tropical Pacific plays an important role in the attenuation of the MD. Also, the negative wind stress curl associated with the northward shift of the ITCZ in the eastern Pacific and the easterly component of the trade winds in the central Pacific strengthens the warm anomaly. The interannual variation of the MD is governed by variations in both the Ekman upwelling forced locally and the downwelling forced remotely in the east. At its northern fringe, the MD is attenuated by the westward propagating warm eddies on the North Equatorial Current, which are triggered by a negative wind stress curl and amplified through a baroclinic conversion process. This suggests the importance of oceanic internal variabilities in discussing climate variations in the tropical western Pacific. [ABSTRACT FROM AUTHOR]

The Indian Ocean Dipole (IOD) is an interannual climate mode of the tropical Indian Ocean. Although it is known that negative sea surface temperature (SST) anomalies in the eastern pole during the positive IOD are stronger than positive SST anomalies during the negative IOD, no consensus has been reached on the relative importance of various mechanisms that contribute to this asymmetry. Based on a closed mixed layer heat budget analysis using a regional ocean model, here we show for the first time that the vertical mixing plays an important role in causing such asymmetry in SST anomalies in addition to the contributions from the nonlinear advection and the thermocline feedback proposed by previous studies. A decomposition of the vertical mixing term indicates that nonlinearity in the anomalous vertical temperature gradient associated with subsurface temperature anomalies and anomalous vertical mixing coefficients is the main driver of such asymmetry. Such variations in subsurface temperature are induced by the anomalous southeasterly trade winds along the Indonesian coast that modulate the thermocline depth through coastal upwelling/downwelling. Thus, the thermocline feedback contributes to the SST asymmetry not through the vertical advection as previously suggested, but via the vertical mixing. [ABSTRACT FROM AUTHOR]

Ningaloo Niño/Niña is the dominant interannual sea surface temperature (SST) variability off Western Australia with the former typically stronger than the latter. Although a recent study suggested that oceanic teleconnections from the Pacific partly contribute to the amplitude asymmetry, this seems counterintuitive as La Niña, which often induces the Ningaloo Niño, is generally weaker than El Niño. Here, mechanisms of the amplitude asymmetry in the oceanic teleconnection are investigated by analyzing reanalysis data and conducting ocean model simulations. Sensitivity experiments using a linear continuously stratified model reveal that stronger easterly wind anomalies in the western equatorial Pacific contribute to the stronger oceanic teleconnection during the Ningaloo Niño. Furthermore, a mixed layer heat budget analysis with a regional ocean model shows that negatively skewed SST anomalies in the central equatorial Pacific related to the skewed wind anomalies in the western equatorial Pacific, are mainly induced by vertical advection/diffusion and entrainment. Plain Language Summary: Ningaloo Niño is a phenomenon with anomalously warm sea surface temperature off the Western Australia coast in austral summer, while Ningaloo Niña is associated with anomalously cool sea surface temperature. These anomalous events are known to affect the marine ecosystems and Australian precipitation. It has been pointed out that the Ningaloo Niño is generally stronger than the Ningaloo Niña. A recent study showed that the oceanic teleconnection from the tropical Pacific partly contributes to this asymmetric nature. Based on numerical simulations using a simplified ocean model, it is shown that stronger easterly wind stress anomalies in the western equatorial Pacific contribute to the stronger oceanic teleconnection during the Ningaloo Niño. A heat budget analysis for the upper ocean using outputs from a more realistic regional ocean model reveals that sea surface temperature anomalies in the central‐western tropical Pacific related to this asymmetric wind stress variability are partly caused by asymmetry in oceanic vertical processes. Key Points: Ningaloo Niño/Niña show amplitude asymmetry in sea surface temperature (SST) anomalies due partly to oceanic teleconnection from the PacificStronger easterly wind anomalies in the western equatorial Pacific contribute to amplitude asymmetry in the oceanic teleconnectionThe skewed zonal wind anomalies are related to larger negative SST anomalies in the central equatorial Pacific induced by vertical processes [ABSTRACT FROM AUTHOR]

Ningaloo Niño/Niña is the dominant interannual sea surface temperature (SST) variability off Western Australia with the former typically stronger than the latter. Although a recent study suggested that oceanic teleconnections from the Pacific partly contribute to the amplitude asymmetry, this seems counterintuitive as La Niña, which often induces the Ningaloo Niño, is generally weaker than El Niño. Here, mechanisms of the amplitude asymmetry in the oceanic teleconnection are investigated by analyzing reanalysis data and conducting ocean model simulations. Sensitivity experiments using a linear continuously stratified model reveal that stronger easterly wind anomalies in the western equatorial Pacific contribute to the stronger oceanic teleconnection during the Ningaloo Niño. Furthermore, a mixed layer heat budget analysis with a regional ocean model shows that negatively skewed SST anomalies in the central equatorial Pacific related to the skewed wind anomalies in the western equatorial Pacific, are mainly induced by vertical advection/diffusion and entrainment. Plain Language Summary: Ningaloo Niño is a phenomenon with anomalously warm sea surface temperature off the Western Australia coast in austral summer, while Ningaloo Niña is associated with anomalously cool sea surface temperature. These anomalous events are known to affect the marine ecosystems and Australian precipitation. It has been pointed out that the Ningaloo Niño is generally stronger than the Ningaloo Niña. A recent study showed that the oceanic teleconnection from the tropical Pacific partly contributes to this asymmetric nature. Based on numerical simulations using a simplified ocean model, it is shown that stronger easterly wind stress anomalies in the western equatorial Pacific contribute to the stronger oceanic teleconnection during the Ningaloo Niño. A heat budget analysis for the upper ocean using outputs from a more realistic regional ocean model reveals that sea surface temperature anomalies in the central‐western tropical Pacific related to this asymmetric wind stress variability are partly caused by asymmetry in oceanic vertical processes. Key Points: Ningaloo Niño/Niña show amplitude asymmetry in sea surface temperature (SST) anomalies due partly to oceanic teleconnection from the PacificStronger easterly wind anomalies in the western equatorial Pacific contribute to amplitude asymmetry in the oceanic teleconnectionThe skewed zonal wind anomalies are related to larger negative SST anomalies in the central equatorial Pacific induced by vertical processes [ABSTRACT FROM AUTHOR]

Influences from the tropical Pacific and Indian Oceans and atmospheric internal variability on the South China Sea (SCS) atmospheric circulation and cold tongue (CT) variabilities in boreal winter and the relative roles of remote forcings at interannual time scales are studied using observational data, reanalysis products, and coupled model experiments. In the observation, strong CT years are accompanied by local cyclonic wind anomalies, which are an equatorial Rossby wave response to enhanced convection over the warmer-than-normal western equatorial Pacific associated with La Niña. Also, the cyclonic wind anomalies are an atmospheric Kelvin wave response to diabatic cooling anomalies linked to both the decaying late fall negative Indian Ocean dipole (IOD) and winter atmospheric internal variability. Partially coupled experiments reveal that both the tropical Pacific air–sea coupling and atmospheric internal variability positively contribute to the coupled variability of the SCS CT, while the air–sea coupling over the tropical Indian Ocean weakens such variabilities. The northwest Pacific anticyclonic wind anomalies that usually precede El Niño–Southern Oscillation–independent negative IOD generated under the tropical Indian Ocean air–sea coupling undermine such variabilities. [ABSTRACT FROM AUTHOR]

Cusp-shaped fluctuations of the sea surface temperature (SST) front in the tropical Pacific, now known as tropical instability waves (TIWs), were discovered by remote sensing in the 1970s. Their discovery was followed by both theoretical and analytical studies, which, along with in situ observations, identified several possible generation mechanisms. Although modeling studies have shown that TIWs strongly influence the heat budget, their influence on local variations of realistically initialized predictions is not yet understood. We here evaluate a series of medium-range (up to ~ 10 days) coupled atmosphere–ocean predictions by a coupled model with different horizontal resolutions. Observational SST, surface wind stress, heat flux, and pressure data showed that representation of temporally and spatially local variations was improved by resolving fine-scale SST variations around the initialized coarse-scale SST front fluctuations of TIWs. Our study thus demonstrates the advantage of using high-resolution coupled models for medium-range predictions. In addition, analysis of TIW energetics showed two dominant sources of energy to anticyclonic eddies: barotropic instability between equatorial zonal currents and baroclinic instability due to intense density fronts. In turn, the eddy circulation strengthened both instabilities in the resolved simulations. This revealed feedback process refines our understanding of the generation mechanisms of TIWs. [ABSTRACT FROM AUTHOR]

Wintertime sea surface temperature (SST) anomalies that disappear in summer and recur in the following winter are known as "reemergence." The role of reemergence in recurrent SST anomalies in an area in the central North Pacific is investigated quantitatively for the first time by conducting an online mixed layer heat budget analysis with a realistic ocean model simulation. In contrast to past studies that have emphasized the importance of vertical entrainment of subsurface temperature anomalies, it is shown that several mechanisms are operating in recurrence of SST anomalies during 1977–1999. In particular, anomalous Ekman meridional advection of the mean meridional temperature gradient induced by zonal wind stress anomalies plays an important role in many years. Also, coincidence in the sign of SST anomalies in winter and surface heat flux anomalies in the following autumn leads to recurrence of SST anomalies. Plain Language Summary: Sea surface temperature anomalies formed in a winter tend to reemerge in the subsequent winter. This process known as reemergence is considered as a key oceanic mechanism of decadal climate modes that have large impacts on global climate and ecosystems. Based on quantitative analyses of a realistic ocean model simulation, we show that recurrence of sea surface temperature occurs through two mechanisms: (1) wind‐induced anomalous meridional advection in autumn favoring reappearance of anomalously warm or cool surface water and (2) coincidence in the sign of sea surface temperature anomalies in winter and surface heat flux anomalies in the following autumn. Key Points: The role of reemergence in the North Pacific is investigated for the first time with a completely closed mixed layer heat budgetRecurring sea surface temperature is not necessarily due to entrainment of anomalous subsurface temperature formed in the previous winterAnomalous Ekman meridional advection and coincidence in the sign of surface heat flux anomalies are the key for recurrence [ABSTRACT FROM AUTHOR]

Ningaloo Niño/Niña is the dominant mode of interannual variability of sea surface temperature (SST) in the southeastern Indian Ocean. According to previous studies, both local air-sea interaction and remote forcing contribute to generation and amplification of the Ningaloo Niño/Niña. The latter forcing includes the atmospheric teleconnection and oceanic wave propagation through the Indonesian archipelago, mainly associated with the El Niño/Southern Oscillation (ENSO). One of the most important characteristics of the Ningaloo Niño/Niña is their asymmetry; the Ningaloo Niño is stronger than the Ningaloo Niña. Using a regional ocean modeling system (ROMS), the impact of oceanic wave propagation on the amplitude and asymmetry of SST anomalies associated with the Ningaloo Niño/Niña is investigated. For these purposes, a sensitivity experiment in which oceanic lateral boundary conditions are replaced by the monthly climatology is conducted. It is shown that the oceanic teleconnection transmitted from the western tropical Pacific can explain about 30% of the observed amplitude asymmetry in SST anomalies. Results from composite and heat budget analyses suggest that coastal downwelling (upwelling) Kelvin waves from the western tropical Pacific associated with La Niña (El Niño) that often co-occurs with the Ningaloo Niño (Niña) are stronger (weaker) and contribute to the asymmetry. [ABSTRACT FROM AUTHOR]