Downwelling Kelvin Research Articles

Unraveling the Influence of Equatorial Waves on Post-Monsoon Sea Surface Salinity Anomalies in the Bay of Bengal

In this study, we investigate the connection between planetary equatorial waves, modulated by the Indian Ocean dipole (IOD) and El Niño Southern Oscillation (ENSO), and the interannual variabilities of the salinity distribution in the Bay of Bengal (BoB) in October–December (OND), along with its associated dynamics, using satellite and reanalysis datasets. In OND 2010 and 2016 (1994, 1997, 2006, and 2019), positive (negative) sea surface salinity anomalies (SSSAs) were distributed in the eastern equatorial Indian Ocean (EIO) and Andaman Sea. Moreover, the southward movement of negative (positive) SSSAs along the eastern Indian coast was observed. This phenomenon was caused by large-scale anomalous currents associated with zonal wind over the EIO. During OND 2010 and 2016 (1994, 1997, 2006, and 2019), due to anomalous westerlies (easterlies) over the EIO and anomalous downwelling (upwelling) Kelvin waves, the strengthened (weakened) Wyrtki jet and the basin-scale anomalous cyclonic (anticyclonic) circulation in the BoB gave rise to positive (negative) SSSAs within the eastern EIO and Andaman Sea. In addition, the intensified (weakened) eastern Indian coastal currents led to the southward movement of negative (positive) SSSAs. It is worth noting that downwelling Kelvin waves reached the western coast of India during OND 2010 and 2016, while upwelling Kelvin waves were only confined to the eastern coast of India during OND 1994, 1997, 2006, and 2019. Furthermore, westward salinity signals associated with reflected westward Rossby waves could modulate the spatial pattern of salinity. The distribution of salinity anomalies could potentially influence the formation of the barrier layer, thereby impacting the sea surface temperature variability and local convection.

Atmospheric–Oceanic Processes over the Pacific Involved in the Effects of the Indian Summer Monsoon on ENSO

Abstract This study investigates the physical mechanisms responsible for the impacts of the Indian summer monsoon (ISM) on the evolution of El Niño–Southern Oscillation (ENSO), with a focus on understanding the monsoon-induced Pacific air–sea interactive processes. An observational analysis displays that a weaker-than-normal ISM can strengthen an ongoing El Niño event and weaken a La Niña event, and conversely for a stronger-than-normal ISM. A 1000-yr output from the Community Earth System Model version 2 is capable of reproducing this observed feature and is therefore used to explore the responsible ocean–atmosphere interactive processes involved. Results show that a weak ISM can cause a cyclonic circulation over the western North Pacific via stimulating atmospheric cold Kelvin waves, and conversely for a strong ISM. The westerly (easterly) wind anomalies on the southern flank of the anomalous cyclone (anticyclone) generate eastward (westward) current anomalies in the mixed layer and thus induce anomalous warm (cold) zonal advection. Furthermore, the wind anomalies excite oceanic downwelling (upwelling) Kelvin waves, which deepen (shoal) the thermocline in the equatorial eastern Pacific and result in anomalous warm (cold) vertical advection. A quantitative mixed layer heat budget analysis demonstrates that the influence of the monsoon-induced Pacific wind anomalies on ENSO is mainly achieved by changing the zonal advective feedback and thermocline feedback. This result is confirmed by model sensitivity experiments in which additional monsoon heating or cooling anomalies are imposed over the Indian region during the developing summer of an ENSO event. Significance Statement While previous studies have reported that the Indian summer monsoon (ISM) could influence the evolution of El Niño–Southern Oscillation (ENSO), this study is aimed at understanding the physical mechanisms for the Pacific air–sea interactive processes induced by the monsoon and their impact on ENSO. It is found that a weak and strong ISM can respectively induce an anomalous cyclonic and anticyclonic circulation over the western North Pacific. The wind anomalies on the southern flank of the anomalous circulation mainly change the zonal advective feedback and the thermocline feedback to affect ENSO development. This study provides a detailed physical explanation of how the ISM influences ENSO evolution.

Spatio-temporal variability of coastal upwelling using high resolution remote sensing observations in the Bay of Bengal

The work investigated the tempo-spatial variability of coastal upwelling through high-resolution High-Frequency (HF) radar-derived ocean current and other remote sensing observations on the northwest coast of the Bay of Bengal in 2018. Sea Surface Temperature (SST) showed a coastal upwelling signature only during April and May despite upwelling-favorable winds throughout the southwest monsoon. Stratified coastal water and coastal-trapped downwelling Kelvin waves suppressed the upwelling on the western coast from June. Improved fidelity was available from sea surface temperature (SST) and cross-shore surface current (CSSC) components of HF radar-derived ocean surface current, which showed strong consistency in detecting actual coastal upwelling from April to the first week of June. Coastal upwelling events consisted of six phases; three active phases and each followed by three break phases. The active phases of the coastal upwelling from SST lasted for about 15, 6, and 9 days, with a horizontal extension of 105 km, 75 km, and 140 km. The horizontal extension of strong positive CSSC during the first and third phases was over 135 km, while being 75 km during the second active phase. The duration of the active phase and the strong CSSC directly influence the horizontal extension of the upwelled colder water. High resolution (6.25 km) swath data helps to show that the weakening of wind shear near the coast started two days before the start of the second break phase in SST, which could not be captured in 25 km gridded winds. The altimetry track data of sea level anomalies obtained from different sensors for different days also captured the different upwelling phases. A large concentration of Chl-a near the coast, as observed in the chlorophyll-a monthly mean result, suggested the occurrence of coastal upwelling. However, the temporal variability could not be elucidated due to the low temporal coverage of Chl-a data. The active and break phases of the coastal upwelling are explained by associated air-sea interactions. This study suggests that HF radar data is beneficial for monitoring the high spatio-temporal variability of coastal upwelling, and high resolution swath data from the scatterometer is crucial for understanding the variability of coastal upwelling.

Coastal Upwelling in the Western Bay of Bengal: Role of Local and Remote Windstress

Monsoon winds drive upwelling along the eastern coast of India. This study examined the role of coastally trapped Kelvin waves in modulating the seasonal variability of local alongshore windstress (AWS)-driven coastal upwelling along the western Bay of Bengal. The winds generated AWS resulting in a positive cross-shore Ekman transport (ET) from March to the end of September, which forced coastal upwelling along the eastern coast of India. However, coastally trapped Kelvin waves could also modulate this process by raising or lowering the thermocline. Remotely sensed windstress, sea surface temperature (SST), and sea surface height anomaly (SSHA) were used to compute the AWS (the wind-based proxy upwelling index) and an SST-based proxy upwelling index (UISST). A new parametric method of the estimation of coastal angles was developed to estimate the AWS and ET. Coastal upwelling and the Kelvin waves were identified based on the climatology of SSHA, AWS, and UISST, in addition to a complex principal component (CEOF) analysis of the SSHA. The UISST and AWS were found to be closely correlated along the southern section of the east coast of India (between Kavali and Point Calimere), where the coastal upwelling was largely local AWS-driven. However, along the northern section of the coast (between Kashinagara and Kakinada), coastal upwelling was triggered by the first upwelling Kelvin wave, sustained by the local AWS, and then terminated by the first downwelling Kelvin wave. This analysis illustrated that remote equatorial windstress caused coastal upwelling along the northern part of the Indian east coast, while it was primarily locally driven in the southern coast. The findings are helpful in better understanding the mechanisms modulating coastal upwelling along the western Bay of Bengal. These would provide useful insights into the primary productivity and the air–sea interactions in the region.

Spatio-temporal signature of surface biophysical parameters in response to extreme IOD events using remote sensing

The Indian Ocean (IO) evidenced significant effects on regional and global ocean-atmospheric coupling variability in recent decades. Anomalous negative (positive) Indian Ocean Dipole (IOD) events witnessed along tropical IO during 2016 (2019) are one of them. The spatial signature of anomalous IOD events was studied using sea surface temperature anomaly (SSTA), sea surface height, sea surface salinity, chlorophyll-a, zonal and meridional wind components obtained from remote sensing data. The strong easterly wind anomaly blowing along the Sumatra coast causes strong coastal upwelling pioneer for the positive IOD event in 2019. The westward horizontal advection of seawater leads to low (high) sea surface height anomaly along eastern (western) tropical IO. The eastern (western) tropical IO becomes gradually cold (warm) during the mature phase (September–October–November) of positive IOD. Due to strong upwelling high and low sea surface salinity were observed along the eastern and western tropical IO respectively. The prolonged stronger upward (downward) Ekman pumping velocity in east (west) results in divergence (convergence) of seawater responsible for the increase (decrease) of chlorophyll-a and marine primary productivity. The reverse phenomena were characterized by warmer (cooler) SSTA, higher (lower) sea surface height, low (high) sea surface salinity, and decrease (increase) of chlorophyll-a in the eastern (western) tropical IO in the 2016 negative IOD year. The westward (eastward) zonal Ekman mass transport is possible fueling the extreme condition of positive (negative) IOD events. A strong eastward propagating downwelling Kelvin wave was observed due to the reflection of the upwelling Rossby wave from the African coast. Dominated picoplankton (microplankton) to other phytoplankton size classes are responsible for the reoccurrence of open ocean bloom along western (eastern) tropical IO during positive IOD.

The Tropical Pacific Annual Cycle and ENSO in PMIP4 Simulations of the Mid‐Holocene

AbstractWe investigate the tropical Pacific annual cycle and the El Niño/Southern Oscillation (ENSO) in four mid‐Holocene simulations. Our results show that both ENSO variability and the amplitude of the annual cycle of the tropical Pacific cold tongue are reduced under mid‐Holocene forcing, along with a modified annual cycle in ENSO variance. The weakened annual cycle of the cold tongue is attributed to an ocean dynamical response to westerly wind anomalies in the western equatorial Pacific in boreal spring in addition to a thermodynamic response to local insolation changes in the eastern Pacific. The anomalous westerly winds in boreal spring excite an annual downwelling Kelvin wave that deepens the thermocline and propagates eastward along the equator, reaching the central and eastern equatorial Pacific during the development season of ENSO in boreal summer. Upon reaching the eastern Pacific, the downwelling Kelvin wave deepens the near‐surface thermocline, warming the surface ocean and weakening the local ocean‐atmosphere coupling critical to the growth of ENSO events. The westerly wind anomaly is associated with a shift in convection in the western Pacific driven by greater cooling of the Maritime Continent than western Pacific Ocean during the first half of the year (January to June) under tropical insolation forcing. By elucidating a common set of mechanisms responsible for a reduced cold tongue annual cycle and ENSO variability in a diverse range of mid‐Holocene simulations, this work yields important insight into the linkages between the tropical Pacific annual cycle and ENSO that are critical for understanding tropical Pacific climate variability.

Oceanic Rossby Waves Induced Two Types of Ocean–Atmosphere Response and Opposite Indian Ocean Dipole Phases

AbstractThis study analyzed the downwelling Rossby waves in the south Indian Ocean (IO)-induced spring asymmetric mode and the relationship with the Indian Ocean dipole (IOD) event based on observations and reanalysis datasets. The westward downwelling Rossby waves favor significant sea surface temperature (SST) warming in the Seychelles thermocline dome that triggers atmosphere response and the asymmetric mode in spring. The zonal sea level pressure gradient causes anomalous easterly winds in the central and eastern equatorial IO, cooling the SST off Sumatra–Java. Meanwhile, the remainder of the downwelling Rossby waves reach the west coast, transform to northward coastal-trapped waves, and then reflect as eastward downwelling Kelvin waves along the equator. The downwelling Kelvin waves reach the Sumatra–Java coast during late spring to early summer, favoring SST warming in the southeastern tropical Indian Ocean. Thus, there are two types of ocean–atmosphere response almost at the same time along the equator. The final SST status depends on which process is stronger and, as a consequence, triggers a negative or a positive phase of the IOD event in the fall season. The results show four positive and three negative IOD events related to the above processes from 1960 to 2019. The strong downwelling Rossby waves are easier to induce an intense asymmetric mode and negative IOD event, usually associated with preceding strong El Niño in the Pacific. In contrast, the weak downwelling Rossby waves tend to induce a weak asymmetric mode and positive IOD event, usually associated with preceding weak El Niño or anomalous anticyclonic atmospheric circulation in the southeastern IO.

Influence of the Interdecadal Pacific Oscillation on Super Cyclone Activities over the Bay of Bengal during the Primary Cyclone Season

An obvious interdecadal change can be measured in the super cyclones (SCs, categories 4 and 5) that occur from October to November over the Bay of Bengal (BoB). This change may be modulated by the interdecadal Pacific oscillation (IPO). A La Niña-like difference between the 1977–1998 (IP1) and 1999–2014 (IP2) periods forced a local Hadley circulation in the eastern tropical Indian Ocean by strengthening the Walker circulation, which caused plummeting upper-level temperatures and ultimately created favorable thermodynamic conditions to enhance the cyclone intensity. Meanwhile, an equatorial downwelling Kelvin wave caused by heating and westerly wind differences entered the BoB rim along the coast and aptly intensified the cyclone, such that the downwelling Kevin wave and Rossby wave generated by its reflection deepened the thermocline in the BoB. The favorable atmospheric and oceanic conditions in IP2 jointly and preferentially cause far more SC activities from October to November over the BoB compared to IP1.

The Extraordinary Equatorial Atlantic Warming in Late 2019

AbstractSea‐surface temperatures (SSTs) in the eastern equatorial Atlantic are subject to variability on interannual timescales but during the last 20 years, this variability has shown comparatively little activity. In late 2019, however, the warmest event in the satellite observation period developed. Analysis suggests that zonal wind stress anomalies in the western equatorial Atlantic contributed to the development of the warm SST anomalies. Furthermore, wind stress curl anomalies north of the equator generated downwelling Rossby waves that propagated to the western boundary and were reflected into downwelling Kelvin waves that helped to precondition the event. Neither the contemporaneous positive Indian Ocean Dipole nor the El Niño Modoki appears to have contributed substantially to the Atlantic warming, though some uncertainty remains. Based on large‐scale multidecadal variability patterns, a return to enhanced variability is not imminent but careful monitoring will be important.

Role of external forcing on the seasonal and interannual variability of mixed layer depth over the Bay of Bengal using reanalysis datasets during 1980–2015

The monsoon reversal winds in different seasons and high influx of freshwater from various rivers make the Bay of Bengal (BoB) a unique region. Thus, the knowledge of the dynamics of the mixed layer over this region is very important to assess the climatic variation of the Indian subcontinent. A comprehensive study of the role of external forcing on the seasonal and interannual mixed layer depth (MLD) variability over the BoB is carried out for 36 years (1980–2015) using reanalysis products. A weak and strong seasonality of MLD is observed over the northern and the southern BoB (NBoB and SBoB) respectively. The partial correlation suggests that the net heat flux (Qnet) is the major contributor to the deepening of MLD over the NBoB, whereas the wind stress controls the deepening over the SBoB. The seasonal variability reveals the deepening of MLD during summer and winter monsoon and the shallowing during pre- and post-monsoon over the BoB. The relation of the interannual MLD variability and the different phases of the Indian Ocean Dipole (IOD) reveals that the negative phase of IOD is associated with deeper MLD over BoB while the positive phase of IOD depicts shallower MLD. In addition, the opposing characteristic of MLD is highly prominent during October-December. This is majorly contributed by variations related to the second downwelling Kelvin and associated Rossby waves over BoB for the opposing phases of the IOD years.

The Impact of the Madden–Julian Oscillation on Cyclone Amphan (2020) and Southwest Monsoon Onset

Cyclone Amphan was an exceptionally strong tropical cyclone in the Bay of Bengal that achieved a minimum central pressure of 907 mb during its active period in May 2020. In this study, we analyzed the oceanic and surface atmospheric conditions leading up to cyclogenesis, the impact of this storm on the Bay of Bengal, and how the processes that led to cyclogenesis, such as the Madden–Julian Oscillation (MJO) and Amphan itself, in turn impacted southwest monsoon preconditioning and onset. To accomplish this, we took a multiparameter approach using a combination of near real time satellite observations, ocean model forecasts, and reanalysis to better understand the processes involved. We found that the arrival of a second downwelling Kelvin wave in the equatorial Bay of Bengal, coupled with elevated upper ocean heat content and the positioning of the convective phase of the MJO, helped to create the conditions necessary for cyclogenesis, where the northward-propagating branch of the MJO acted as a trigger for cyclogenesis. This same MJO event, in conjunction with Amphan, heavily contributed atmospheric moisture to the southeastern Arabian Sea and established low-level westerlies that allowed for the southwest monsoon to climatologically onset on June 1.

Numerical Simulation of Tropical Cyclone Mora Using a Regional Coupled Ocean-Atmospheric Model

Tropical storms (TS) are highly sensitive to the behavior of nearby oceans, and the only way to improve their predictability is to include all the possible factors contributing to their genesis and motion. The present study evaluates the response of air–sea coupling on TS Mora, which occurred during the pre-monsoon season of 2017. Mora is simulated using a non-hydrostatic regional atmospheric modeling system coupled to a regional ocean modeling system model. The observations show a low-level Rossby wave in the atmosphere during the storm genesis. There was also evidence of increased sea surface height simultaneously in the form of a downwelling Kelvin wave at the west coast of the Bay of Bengal. The present study emphasizes the relationship between near-surface oceanic and atmospheric circulation patterns. We have tried to elucidate the process with the help of dynamical equations and model solutions. We also analyze the pivotal role of the atmospheric Rossby wave in the formation of the coastal Kelvin wave. The Kelvin wave seems to produce a warm pool required to form a low-pressure system, which further gave rise to the tropical storm Mora. The changes in the temperature and salinity profile within the ocean subsurface layers during storm passage are further used for inferences regarding the vertical structure of the upper ocean due to wind-generated mixing. The model analysis reveals that the ocean and atmospheric counterparts responsible for storm genesis are well recorded in the coupled model when compared to the observations. It was found that air–sea coupling in the model made it possible to realistically capture ocean and atmospheric responses.

Warming Trends in the Central Equatorial Indian Ocean and the Associated Coupled Feedback Processes

Mohapatra, S. and Gnanaseelan, C., 2020. Warming trends in the central equatorial Indian Ocean and the associated coupled feedback processes. In: Sheela Nair, L.; Prakash, T.N.; Padmalal, D., and Kumar Seelam, J. (eds.), Oceanic and Coastal Processes of the Indian Seas. Journal of Coastal Research, Special Issue No. 89, pp. 39-45. Coconut Creek (Florida), ISSN 0749-0208.The recent increasing trends in the tropical Indian Ocean (TIO) temperature have strong impact on the regional as well as global climate variability. In this work we studied the changes in the boreal summer monsoon winds and the associated monsoon currents on modulating the surface and subsurface temperature of the equatorial Indian Ocean. The reduction in the meridional sea surface temperature (SST) gradient and strengthening of zonal SST gradient over the Indian Ocean reduced the northward propagation of south-westerly wind, thereby enhancing the surface westerlies over the equator. The strengthened surface westerlies increased the equatorial downwelling Kelvin waves and deepened the thermocline. The present study unravels the causes for the observed confinement of strong SST trend over the central equatorial Indian Ocean (CEIO) even though surface westerlies and the associated downwelling extend upto the eastern equatorial Indian Ocean (EEIO). In contrast to the CEIO surface warming, EEIO experienced strong subsurface warming at thermocline. Our analysis suggests that the upper ocean stratification plays an important role in the observed warming trend of both CEIO surface and EEIO subsurface. The eastward equatorial currents bring warm salty water from Arabian Sea and WEIO towards east and warms CEIO, however the high saline surface water is found to sink in the EEIO, thereby warming the subsurface instead of surface. The composite of temperature anomaly for the period 1978-2002, the period of warm Pacific Decadal Oscillation, shows patterns similar to temperature trends, which clearly suggests the dominance of decadal variability on the TIO temperature trends.

Contribution of oceanic wave propagation from the tropical Pacific to asymmetry of the Ningaloo Niño/Niña

Ningaloo Nino/Nina 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 Nino/Nina. The latter forcing includes the atmospheric teleconnection and oceanic wave propagation through the Indonesian archipelago, mainly associated with the El Nino/Southern Oscillation (ENSO). One of the most important characteristics of the Ningaloo Nino/Nina is their asymmetry; the Ningaloo Nino is stronger than the Ningaloo Nina. 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 Nino/Nina 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 Nina (El Nino) that often co-occurs with the Ningaloo Nino (Nina) are stronger (weaker) and contribute to the asymmetry.

Impact of river runoff on seasonal sea level, Kelvin waves, and East India Coastal Current in the Bay of Bengal : A numerical study using ROMS

High riverine freshwater discharge makes the salinity distribution highly heterogeneous in the Bay of Bengal (BoB). This paper investigates the impact of freshwater discharge from ten major rivers on the seasonal sea surface variability and the coastal Kelvin wave characteristics in the BoB using the Regional Ocean Modeling System (ROMS). Two numerical experiments were conducted (i) with- and (ii) without-incorporating monthly climatology of river runoff into the model under the same forcing conditions. The model was forced by Comprehensive Ocean-Atmosphere DataSet (COADS) climatology for both the experiments. This study demonstrates that incorporation of river discharge into the model simulates a realistic picture of sea surface salinity, seasonal sea level variations, Kelvin wave activities, and equatorward flowing East India Coastal Current (EICC). The amplitude of the second downwelling Kelvin wave is increased most (greater than 4 cm at the Odisha coast and approximately 2 cm near the Chennai coast) due to river discharge. Apart from that, we also found that the equatorward EICC is strengthened and more organized when river runoff is considered. The volume transport by equatorward EICC becomes almost double when river runoff is included in the model in most of the region.

Anomalous warming of the western equatorial Indian Ocean in 2007: Role of ocean dynamics

In this study, observational data, together with the simulations from a high-resolution numerical ocean model, are used to identify the processes responsible for the anomalous warm sea surface temperature (SST) in the equatorial Indian Ocean during late spring/early summer in 2007. Our analysis suggests that the SST in the western equatorial Indian Ocean (WEIO) remained unusually warm owing to the combined influence of the wind induced and reflected Rossby waves in the equatorial Indian Ocean. Anomalous westerly wind burst in the equatorial Indian Ocean in early April generated an eastward propagating downwelling Kelvin wave, which got reflected from the eastern boundary as a downwelling Rossby wave and propagated to the WEIO. The easterlies appeared during the second fortnight of April generated another downwelling Rossby wave, which also propagated to the WEIO. Thus a couple of downwelling Rossby waves, reflected and direct wind induced, propagated to the WEIO, deepened the thermocline and prohibited cooling in the WEIO during the early months of summer monsoon season. This study demonstrates the importance of the equatorial winds and associated oceanic dynamics in modulating the SST variability in the WEIO.

Role of interannual equatorial forcing on the subsurface temperature dipole in the Bay of Bengal during IOD and ENSO events

Role of equatorial forcing on the thermocline variability in the Bay of Bengal (BoB) during positive and negative phases of the Indian Ocean Dipole (IOD) and El Nino Southern Oscillation (ENSO) was investigated using the Regional Ocean Modeling System (ROMS) simulations during 1988 to 2015. Two numerical experiments were carried out for (i) the Indian Ocean Model (IOM) with interannual open boundary conditions and (ii) the BoB Model (BoBM) with climatological boundary conditions. The first mode of Sea Surface Height Anomalies (SSHA) variability showed a west-east dipole nature in both IOM and altimetry observations around 11°N, which was absent in the BoBM. The vertical section of temperature along the same latitude showed a sharp subsurface temperature dipole with a core at ~ 100 m depth. The positive (negative) subsurface temperature anomalies were observed over the whole northeastern BoB during NIOD (PIOD) and LN (EN) composites due to stronger (weaker) second downwelling Kelvin Waves. During the negative phases of IOD and ENSO, the cyclonic eddy on the southwestern BoB strengthened due to intensified southward coastal current along the western BoB and local wind stress. The subsurface temperature dipole was at its peak during October–December (OND) with 1-month lag from IOD and was evident from the Argo observations and other reanalysis datasets as well. A new BoB dipole index (BDI) was defined as the normalized difference of 100-m temperature anomaly and found to be closely related to the frequency of cyclones and the surface chlorophyll-a concentration in the BoB.

Ocean Dynamics Shapes the Structure and Timing of Atlantic Equatorial Modes

AbstractA recent study has brought to light the co‐existence of two distinct Atlantic Equatorial Modes during negative phases of the Atlantic Multidecadal Variability: the Atlantic Niño and Horse‐Shoe (HS) mode. Nevertheless, the associated air‐sea interactions for HS mode have not been explored so far and the prevailing dynamic view of the Atlantic Niño has been questioned. Here, using a forced ocean model simulation, we find that for both modes, ocean dynamics is essential to explain the equatorial SST variations, while air‐sea fluxes control the off‐equatorial SST anomalies. Moreover, we demonstrate the key role played by ocean waves in shaping their distinct structure and timing. For the positive phase of both Atlantic Niño and HS, anomalous westerly winds trigger a set of equatorial downwelling Kelvin waves (KW) during spring‐summer. These dKWs deepen the thermocline, favouring the equatorial warming through vertical diffusion and horizontal advection. Remarkably, for the HS, an anomalous north‐equatorial wind stress curl excites an upwelling Rossby wave (RW), which propagates westward and is reflected at the western boundary becoming an equatorial upwelling KW. The uKW propagates to the east, activating the thermocline feedbacks responsible to cool the sea surface during summer months. This RW‐reflected mechanism acts as a negative feedback causing the early termination of the HS mode. Our results provide an improvement in the understanding of the TAV modes and emphasize the importance of ocean wave activity to modulate the equatorial SST variability. These findings could be very useful to improve the prediction of the Equatorial Modes.

Revisiting MJO, Kelvin waves, and El Niño relationships using a simple ocean model

Mechanisms governing interactions between the Madden–Julian Oscillation (MJO), Kelvin wave activity, and El Nino development are reexamined using the oceanic component of the Zebiak–Cane (ZCocn) model of the Pacific basin. Prescribed wind stress from a free run of the super-parameterized Community Climate System Model version 4 (SP-CCSM4) is used to force ZCocn and the simulated El Nino events are analyzed with respect to their relationship with the MJO wind forcing. Composites of El Nino events strongly influenced by the MJO show the earlier onset of a flattened, El Nino-like state of the thermocline. In contrast, the composites of El Nino events not influenced by the MJO winds show a later onset, and are dominated by a transient-like thermocline along with periods of upwelling Kelvin wave activity. Sensitivity experiments performed to identify whether modifying MJO wind stress and oceanic Kelvin wave activity influences these features show that although MJO contributes to the development of these features, it is not necessarily the primary driver. The relative phasing between MJO and oceanic Kelvin wave activity seems to be the most important factor governing the influence of MJO on El Nino. When in phase and collocated with Kelvin wave activity, MJO westerly wind stress contributes to the amplification of preexisting downwelling Kelvin waves, leading to earlier onset and greater strength of the resulting El Nino events. The out-of-phase interactions between MJO and oceanic Kelvin waves explain the observed lack of influence of MJO onto some El Nino events.

El Niño–Like Physical and Biogeochemical Ocean Response to Tropical Eruptions

AbstractThe oceanic response to recent tropical eruptions is examined in Large Ensemble (LE) experiments from two fully coupled global climate models, the Community Earth System Model (CESM) and the Geophysical Fluid Dynamics Laboratory Earth System Model (ESM2M), each forced by a distinct volcanic forcing dataset. Following the simulated eruptions of Agung, El Chichón, and Pinatubo, the ocean loses heat and gains oxygen and carbon, in general agreement with available observations. In both models, substantial global surface cooling is accompanied by El Niño–like equatorial Pacific surface warming a year after the volcanic forcing peaks. A mechanistic analysis of the CESM and ESM2M responses to Pinatubo identifies remote wind forcing from the western Pacific as a major driver of this El Niño–like response. Following eruption, faster cooling over the Maritime Continent than adjacent oceans suppresses convection and leads to persistent westerly wind anomalies over the western tropical Pacific. These wind anomalies excite equatorial downwelling Kelvin waves and the upwelling of warm subsurface anomalies in the eastern Pacific, promoting the development of El Niño conditions through Bjerknes feedbacks a year after eruption. This El Niño–like response drives further ocean heat loss through enhanced equatorial cloud albedo, and dominates global carbon uptake as upwelling of carbon-rich waters is suppressed in the tropical Pacific. Oxygen uptake occurs primarily at high latitudes, where surface cooling intensifies the ventilation of subtropical thermocline waters. These volcanically forced ocean responses are large enough to contribute to the observed decadal variability in oceanic heat, carbon, and oxygen.