Kelvin Waves Research Articles

Response of Convectively Coupled Kelvin Waves to Surface Temperature Forcing in Aquaplanet Simulations

AbstractThis study investigates changes in the propagation and maintenance of convectively coupled Kelvin waves (KWs) in response to surface warming. We use a set of three aquaplanet simulations made with the Community Atmospheric Model version 6 by varying the sea surface temperature boundary conditions to represent the current climate as well as warmer (+4 K) and cooler (−4 K) climates. Results show that KWs accelerate at the rate of about 7.1%/K and their amplitudes decrease by 4.7%/K. The dampening of KWs with warming is found to be associated with a weakening of the internal thermodynamic feedback between diabatic heating and temperature anomalies that generates KW eddy available potential energy (EAPE). The phase speed of KWs closely matches that of the second baroclinic mode KW in −4 K, while the phase speed of KWs is approximately that of the first baroclinic mode KW in +4 K. Meanwhile, the coupling between the two baroclinic modes weakens with warming. We hypothesize that in −4 K, as the first and second baroclinic modes are strongly coupled, KWs destabilize by positive EAPE generation within the second baroclinic mode and propagate more slowly, following the second baroclinic mode KW phase speed. In +4 K, as the first and second baroclinic modes decouple, KWs are damped by negative EAPE generation within the first baroclinic mode and propagate faster, following the first baroclinic mode KW phase speed.

Process-based evaluation of intraseasonal oceanic Kelvin waves in the Pacific Ocean in CMIP6 models

Abstract Oceanic intraseasonal Kelvin waves (KWs) help modulate upper-ocean thermal characteristics, providing feedbacks to important coupled air-sea phenomena in the tropics. The recent availability of daily thermocline depth fields from several CMIP6 models makes it possible to evaluate the performance of KWs and identify potential sources of bias. Most models fail to simulate a realistic spatial distribution of KW variability. Models simulate large variability of KWs in the western or eastern Pacific rather than in the central Pacific as observed. The modeled KWs propagate slowly (about 1.5 m/s) compared to observations (about 2.5 m/s). This slow propagation is also identified in wavenumber-frequency spectra for KWs and meridional KW structures, which is more consistent with a second baroclinic mode structure in models compared to the first baroclinic mode structure in observations. An analysis of the relative contributions of vertical wavenumber and background ocean stability to KW phase speeds indicates that the high vertical wavenumber bias in models contributes most to the slow propagation, in which the higher-than-observed vertical wavenumbers implying the biased incorporation of higher baroclinic modes in model KW structure. This finding is further supported by the results of vertical mode decomposition that incorporates background density profiles. These results indicate that a realistic representation of the KW vertical structure is essential to produce realistic KW propagations in models.

The atmospheric aerosol spatial distribution and tropical intra-seasonal oscillations over the South Asian region

Intra-seasonal oscillations (ISO) are well known to modulate the weather phenomena which in turn are known to influence the atmospheric aerosol loading. This study investigates how aerosol loading is modulated in ISO spatio-temporal scales over the Indian region using long-term satellite aerosol optical depth data from Moderate Resolution Imaging Spectroradiometer (MODIS) sensor, onboard Terra Satellite. It is shown that Madden-Julian Oscillation (MJO) and Equatorial Rossby waves (ER) have the highest effect (15–20% of the mean) followed by Mixed-Rossby-gravity and Tropical depressions (MT), and Kelvin wave (KE) (5–15%). Further, a dipolar pattern in aerosol loading was observed, with poles over the Arabian Sea and the Bay of Bengal. These variabilities were found to be mainly driven by anomalous winds associated with the ISOs. Similar to aerosol, dipolar signatures in the atmospheric aerosol radiative forcing (ARF) were also observed with clearer patterns. However, the forcing poles are not centered exactly where aerosol poles were observed, indicating the effect of differential properties of aerosols on the aerosol radiative forcing. Quantitatively, at the surface level, modulation in ARF is up to 3 Wm-2 (15%) for MJO and ER, and up to 2 Wm-2 (5%) for KE and MT; in the atmosphere and at the top of the atmosphere, modulation is up to 2 Wm-2 (15%) for MJO and ER, and up to 1 Wm-2 (5%) for KE and MT.

Local identification of equatorial Kelvin waves in real‐time operational forecasts

AbstractEquatorial Kelvin waves (KWs) are associated with a variety of equatorial atmospheric phenomena, namely, tropical convection, cloud and precipitation variability, tropical cyclogenesis, the onset of monsoon season over Africa and India, and even the quasi‐biennial oscillation and the Madden–Julian oscillation. As such, if operational forecasts are to improve near the Tropics, it is important that they correctly represent KWs. This article aims at developing a novel methodology for identifying KWs in real‐time operational forecasts at specific longitudes using only the meridional structures of the solutions to the free Laplace tidal equations, known as Hough vector functions. The main advantages of this newly proposed methodology are that (a) it can identify KWs at a specific longitude and (b) very low computational cost is needed to apply it to real‐time operational forecasts. The application of the method to 2015–2017 European Centre for Medium‐range Weather Forecasts (ECMWF) analysis and operational forecast data reveals a systematic bias in the representation of KWs by the ECMWF operational forecast model.

Kelvin Wave Propagation over a Sloping Interface and Relationships with El Niño Southern Oscillation

Internal Kelvin Wave (KW) propagation is studied about variations in the sea surface temperature anomaly (SSTA) over the tropical Pacific. Temperature and Salinity (TS) observations have been used to define the vertical structure of the ocean about the propagation properties of KWs. Changes in the vertical structure of the water column determine consistent zonal variations in the wave velocity, with values varying, roughly, from 1.8 to 2.6 m/s. The authors document that KWs are formed regularly at the western boundary of the tropical Pacific, but, in these cases, never overcome the dateline. Occasionally, KWs are generated in the region comprised between 170oE and 170oW, and, on all these occasions, a positive phase of the El Niño Southern Oscillation (El Niño) event is recorded. A model, named Sloping Interface Model (SIM), is proposed to relate changes in the pycnocline depth, associated with transiting KWs, and SST anomaly variations. In the SIM, whose equations are consistent with the Recharge/Discharge paradigm, the ocean is described as a two-layer system and the climatological state, represented by a sloping pycnocline, is maintained by a constant easterly wind stress. Using the SIM and coherently with the Recharge/Discharge paradigm, the authors show that changes in the averaged SSTA over El Niño 3, 3.4 and 4 regions are nearly perfectly correlated to pycnocline displacements due to transiting KWs.

Dual Wave Energy Sources for the Atlantic Niño Events Identified by Wave Energy Flux in Case Studies

AbstractThe evolution of equatorial sea surface temperature anomalies during Atlantic Niño events has demonstrated its diversity in both intensity and timing. To investigate the mechanism responsible for this diversity, this study focuses on ocean responses to atmospheric forcing, manipulating the wind forcing in both equatorial and off‐equatorial regions to excite linear ocean models for three types of events that occurred in 1999, 2019, and 2021 respectively. The results reveal the dual wave energy sources for the equatorial Kelvin waves (KWs): one is the local wind forcing in the western tropical basin; the other is the reflection due to the off‐equatorial Rossby waves in the western boundary. The reflected KWs can precondition the events when wind‐forced KWs insufficiently displace the thermocline (e.g., the boreal winter of 2019 and 2021). The participation of off‐equatorial wave energy hence leads to the diversity of the Atlantic Niños.

Upper-Ocean Circulation and Tropical Atlantic Interannual Modes

Abstract The impact of tropical Atlantic Ocean variability modes in the variability of the upper-ocean circulation has been investigated. For this purpose, we use three oceanic reanalyses, an interannual forced-ocean simulation, and satellite data for the period 1982–2018. We have explored the changes in the main surface and subsurface ocean currents during the emergence of Atlantic meridional mode (AMM), Atlantic zonal mode (AZM), and AMM–AZM connection. The developing phase of the AMM is associated with a boreal spring intensification of North Equatorial Countercurrent (NECC) and a reinforced summer Eastern Equatorial Undercurrent (EEUC) and north South Equatorial Current (nSEC). During the decaying phase, the reduction of the wind forcing and zonal sea surface height gradient produces a weakening of surface circulation. For the connected AMM–AZM, in addition to the intensified NECC, EEUC, and nSEC in spring, an anomalous north-equatorial wind curl excites an oceanic Rossby wave (RW) that is boundary-reflected into an equatorial Kelvin wave (KW). The KW reverses the thermocline slope, weakening the nSEC and EUC in boreal summer and autumn, respectively. During the developing spring phase of the AZM, the nSEC is considerably reduced with no consistent impact at subsurface levels. During the autumn decaying phase, the upwelling RW-reflected mechanism is activated, modifying the zonal pressure gradient that intensifies the nSEC. The NECC is reduced in boreal spring–summer. Our results reveal a robust alteration of the upper-ocean circulation during AMM, AZM, and AMM–AZM, highlighting the decisive role of ocean waves in connecting the tropical and equatorial ocean transport.

Link Between Equatorial Wind Anomalies and Intraseasonal Eddies in the Northeastern Bay of Bengal

AbstractFeatures and mechanism of intraseasonal cyclonic and anticyclonic eddy generation in the northeastern Bay of Bengal (NE‐BoB) are investigated using satellite observations, ocean reanalysis, and related wind forcing data. Our results suggest that the intraseasonal cyclonic (anticyclonic) eddies generated in the NE‐BoB can be primarily attributed to intraseasonal easterly (westerly) wind anomalies in the equatorial Indian Ocean, which provokes the upwelling (downwelling) Kelvin wave (KW) traveling along the equator and subsequently the eastern boundary through the Preparis Channel into the NE‐BoB. Anomalous equatorial zonal wind can result in a strong intraseasonal subsurface flow along the KW waveguide located on the continental slope, and this KW‐associated subsurface flow is essential to the formation process of both anticyclonic and cyclonic eddies in the NE‐BoB. A seasonal difference in the response of sea level anomaly (SLA) to the KW‐associated subsurface flow is revealed, which can be explained by the seasonal variability of stratification along the waveguide. The eddy related SLA shows a considerable correlation coefficient of −0.58 with the accumulated transport caused by KW‐associated subsurface flows through the Preparis Channel when it lags 18 days, indicating a possible eddy generation mechanism that the KW‐associated subsurface flow dominated net horizontal inflow (outflow) causes vertical stretching (shrinking) of the subsurface layer, which in turn triggers an anticyclonic (cyclonic) eddy.

Projected future changes in equatorial wave spectrum in CMIP6

The simulation of the Madden–Julian Oscillation (MJO) and convectively coupled equatorial waves (CCEWs) is considered in 13 state-of-the-art models from phase 6 of the Coupled Model Intercomparison Project (CMIP6). We use frequency–wavenumber power spectra of the models and observations for Outgoing Longwave Radiation (OLR) and zonal winds at 250 hPa (U250), and consider the historical simulations and end of twenty-first century projections for the SSP245 and SSP585 scenarios. The models simulate a spectrum quantitatively resembling that observed, though systematic biases exist. MJO and Kelvin waves (KW) are mostly underestimated, while equatorial Rossby waves (ER) are overestimated. Most models project a future increase in power spectra for the MJO, while nearly all project a robust increase for KW and weaker power values for most other wavenumber–frequency combinations, including higher wavenumber ER. In addition to strengthening, KW also shift toward higher phase speeds (or equivalent depths). Models with a more realistic MJO in their control climate tend to simulate a stronger future intensification.

Three-Dimensional Structure of the Equatorial Kelvin Wave: Vertical Structure Functions, Equivalent Depths, and Frequency and Wavenumber Spectra

Abstract The wavenumber filtering of the Kelvin waves (KWs) is performed by a multivariate projection of the ERA5 data on the horizontal structures of the KWs from linear theory and vertical structure functions (VSFs) spanning the troposphere and stratosphere. The associated equivalent depths are consistent with solutions expected for the bounded atmosphere. The zonal wavenumber spectra of the KW energy and temporal variance are shown to be continuous and red with the rotational kinetic energy being dominant over the divergent kinetic energy at zonal wavenumber k = 1. Spatial and temporal variance in wavenumber space is analyzed in terms of periods from linear theory. The strongest variability is found in periods of 8–12 days at k = 1 and VSFs with a single zero-crossing in the troposphere and a structure resembling upward propagating waves across the tropopause and higher up. Secondary maxima are associated with VSFs that have multiple zero crossings in the troposphere and small equivalent depths. A comparison of the wavenumber and wavenumber–frequency methods shows a disagreement between frequencies from the Fourier time series and linear theory. A phase-locking experiment, in which the KW phases are replaced by linear theory estimates, demonstrates that power spectra from wavenumber–frequency filtering in the troposphere are largely defined by wave phases.

Evolution of individual equatorial atmospheric Kelvin waves in the stratosphere from FORMOSAT-7/COSMIC-2 temperatures

Temperatures obtained from Formosa Satellite-7/Constellation Observing System for Meteorology, Ionosphere and Climate-2 (FORMOSAT-7/COSMIC-2) in the stratosphere are analysed to investigate equatorial atmospheric Kelvin waves (KW) during the period from October 2019 to March 2021. Least square fitting followed by a two-dimensional fast Fourier transform are employed to extract the characteristics of these waves. Comparison with ERA5 mean zonal winds clearly indicates wave-mean flow interactions. KW activity was stronger from November 2019 to July 2020 and observed in the entire stratosphere due to the prevailing westward wind. After July 2020, although the ambient winds were favourable, wave activity was weaker and is attributed to the La Nina state of El Nino Southern Oscillation (ENSO) in the equatorial Pacific. The emphasis of the current study is on individual KW events and their evolution as they propagate eastward and upward. Waves of many periods are excited simultaneously in the lower atmosphere and as they propagate upward with different speeds, disperse along the path. During November–December 2019, KW periods ranged from 8 to 21 days. Vertical wavelength of the slow waves (> 15 days) is found to be ~ 3 km just above the tropopause that increased to 10–12 km at ~ 40 km. This increase in vertical wavelength is due to Doppler effect of the mean wind on the wave. For the fast waves of periods 8, 6.5 and 4.5 days, the wavelengths are larger and 18, 24 and 30 km, respectively. COSMIC-2 data, with its high vertical resolution, provided this opportunity to investigate the fine structure of the KW delivering many insights into the atmospheric dynamics involved.

Energetics of Interactions between African Easterly Waves and Convectively Coupled Kelvin Waves

Abstract Perturbation kinetic and available energy budgets are used to explore how convectively coupled equatorial Kelvin waves (KWs) impact African easterly wave (AEW) activity. The convective phase of the Kelvin wave increases the African easterly jet’s meridional shear, thus enhancing the barotropic energy conversions, leading to intensification of southern track AEWs perturbation kinetic energy. In contrast, the barotropic energy conversion is reduced in the suppressed phase of KW. Baroclinic energy conversion of the southern track AEWs is not significantly different between Kelvin waves’ convective and suppressed phases. AEWs in the convective phase of a Kelvin wave have stronger perturbation available potential energy generation by diabatic heating and stronger baroclinic overturning circulations than in the suppressed phase of a Kelvin wave. These differences suggest that southern track AEWs within the convective phase of Kelvin waves have more vigorous convection than in the suppressed phase of Kelvin waves. Barotropic energy conversion of the northern track AEWs is not significantly different between Kelvin waves’ convective and suppressed phases. The convective phase of the Kelvin wave increases the lower-tropospheric meridional temperature gradient north of the African easterly jet, thus enhancing the baroclinic energy conversion, leading to intensification of northern track AEWs perturbation kinetic energy. In contrast, the baroclinic energy conversion is reduced in the suppressed phase of KW. These results provide a physical basis for the modulation of AEWs by Kelvin waves arriving from upstream.

Variation of Radar-Observed Precipitation Characteristics in Relation to the Simultaneous Passages of a Madden–Julian Oscillation Event and Convectively Coupled Equatorial Waves during the Years of the Maritime Continent Pilot Study

AbstractIn this study, we examined the variations of precipitation morphology and rainfall in relation to the simultaneous passages of a Madden–Julian oscillation (MJO) event and convectively coupled equatorial waves (CCEWs) observed during the Years of the Maritime Continent pilot study. We utilized globally merged infrared brightness temperature data and the radiosonde and radar data observed aboard the Research Vessel Mirai at 4°4′S, 101°54′E. As well as the observed MJO event, equatorial Rossby waves (ERWs), Kelvin waves (KWs), and mixed Rossby–gravity waves (MRGWs) were identified. The radar data exhibited high-frequency variation, mainly caused by KWs and MRGWs, and low-frequency variation, mainly caused by the MJO and ERWs. The MRGWs predominantly modulated convective echo areas and both convective and stratiform volumetric rainfall. In contrast, the MJO event had little influence on the variance of convective echoes. Moreover, stratiform echo areas and volumetric rainfall were more strongly modulated by the combined effects of the MJO, ERWs, KWs, and MRGWs than their convective counterparts. The intense development of stratiform echo areas and volumetric rainfall was coherent with the superimposition of the active phases of the MJO event and all the analyzed CCEWs. The strongest development and a significant reduction of convective echo-top heights before and after the peak MJO date, respectively, were coherent with the passages of ERWs and MRGWs, which were the dominant wave types in modulating echo-top heights. Thus, it appears that the superimposition of the CCEWs on the MJO event exerted complex modulations on the convective activities within the MJO event.

The vertical structure of annual wave energy flux in the tropical Indian Ocean

A recently developed energy flux diagnosis scheme, which incorporates a smooth connection between the tropical and subtropical zones, is used in the present study to investigate vertically propagating waves in the tropical Indian Ocean (IO) based on the result of a linear, continuously stratified ocean model driven by climatological wind forcing. This extended diagnosis reveals deep-reaching eastward energy fluxes at the equator which develop four times per year and are associated with equatorial Kelvin waves (KWs) generated by semiannual winds. The authors find that the downward transfer of wave energy is particularly deep in the southern Bay of Bengal (SBoB). This downward flux is attributed to off-equatorial Rossby waves and appears four times per year, maximizing its amplitude during November–December. Southwesterly winds in the Arabian Sea intensify eastward energy flux of KWs at mid-depth, which maximizes in amplitude in August. This is contrastive to KW energy flux at the surface which peaks in May. These mid-depth equatorial KW packets subsequently arrive at the eastern boundary of the IO and are diffracted poleward to produce downward energy flux in November and December detected in the SBoB.

A Simulation Study of Kelvin Waves Interacting with Synoptic Events during December 2016 in the South China Sea and Maritime Continent

Abstract This study evaluates the model simulation of interaction between convectively coupled tropical disturbances in the South China Sea (SCS) and Maritime Continent (MC). The Model for Prediction Across Scales (MPAS) is used to simulate the major interaction events in December 2016 with a fixed 60-km horizontal resolution and a variable 60–15-km resolution. Compared with an observational analysis, the overall spatial and temporal evolution of simulated rainfall and circulation reveals the capability of MPAS for reproducing equatorial Kelvin waves (KWs), and the interactions with equatorial Rossby waves and off-equatorial mixed Rossby–gravity (MRG)/TD-type waves up to a 5–7-day lead in both fixed 60-km and variable 60–15-km resolutions. Two interaction events are further examined. One involves an MRG/TD wave, prevailing northeasterlies, and a Borneo vortex developed in SCS during 6–11 December. The other involves a KW converging with the easterly trade wind that led to an MRG/TD-type wave and the formation of Typhoon Nock-ten during 16–20 December. The MPAS 60–15-km resolution tends to produce stronger precipitation and more coherent vorticity structures in both interaction events. Increasing the resolution to 15 km contributes to better representation of finer spatial vorticity and rainfall structures.

The Impact of Kelvin Wave Activity during Dry and Wet African Summer Rainfall Years

This study highlights the influence of convectively coupled Kelvin wave (KW) activity on deep convection and African easterly waves (AEWs) over North Africa during dry and wet boreal summer rainfall years. Composite analysis based on 25 years of rainfall, satellite observed cold cloud temperature, and reanalysis data sets show that KWs are more frequent and stronger in dry Central African years compared with wet years. Deep convection associated with KWs is slightly more amplified in dry years compared with wet years. Further, KW activity over North Africa strengthens the lower level zonal flow and deepens the zonal moisture flux in dry years compared with wet years. Results also show that enhanced KW convection is in phase with above-average AEW variance in dry years. However, enhanced KW convection is out-of-phase with average AEW activity in wet years. In general, this study suggests that KW passage over Africa enhances convective activity and more strongly modulates the monsoon flow and moisture flux during the dry years than wet years.

The Climatological Horizontal Pattern of Energy Flux in the Tropical Atlantic as Identified by a Unified Diagnosis for Rossby and Kelvin Waves

AbstractThis study investigates the horizontal distribution of wave energy flux in the tropical Atlantic Ocean using shallow‐water model experiments for three gravest baroclinic modes forced by climatological winds. This is the first attempt to apply a unified diagnostic scheme to the analysis of energy fluxes associated with both Rossby waves (RWs) and Kelvin waves (KWs) in the tropical Atlantic Ocean. Those analyses were difficult in previous studies owing to the difference of equatorial and quasigeostrophic dynamics. The scheme yields the transfer route (i.e., trajectories by group velocity vector) of wave energy originating from both local wind forcing and boundary reflection. The meridional flux of wave energy crossing two zonal sections (at 3°N and at 2°S) is examined for understanding where the equatorial and off‐equatorial regions interconnect. Equatorward energy fluxes into the basin interior are found mainly on the zonal section in the Northern Hemisphere (at 3°N), indicating the radiation of RWs from the Guinea coast. At the equator, seasonal transition in May from eastward wind anomaly to westward anomaly yields a significant energy input at the central basin that radiates both RWs and KWs. The energy flux of these RWs arrives at the western boundary in September, followed by reflection of KWs to yield eastward transbasin energy fluxes in October‐December. The analysis also identifies an unexpected wind input in May at 4°S which induces both RWs and inertial gravity waves.

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.

The Influences of Convectively Coupled Kelvin Waves on Multiscale Rainfall Variability over the South China Sea and Maritime Continent in December 2016

ABSTRACTWe studied the scale interactions of the convectively coupled Kelvin waves (KWs) over the South China Sea (SCS) and Maritime Continent (MC) during December 2016. Three KWs were observed near the equator in this month while the Madden–Julian oscillation (MJO) was inactive. The impacts of these KWs on the rainfall variability of various time scales are diagnosed, including synoptic disturbances, diurnal cycle (DC), and the onset of the Australian monsoon. Four interaction events between the KWs and the westward-propagating waves over the off-equatorial regions were examined; two events led to KW enhancements and the other two contributed to the formation of a tropical depression/tropical cyclone. Over the KW convectively active region of the MC, the DC of precipitation was enhanced in major islands and neighboring oceans. Over the land, the DC hot spots were modulated depending on the background winds and the terrain effects. Over the ocean, the “coastal regime” of the DC appeared at specific coastal areas. Last, the Australian summer monsoon onset occurred with the passage of a KW, which provided favorable conditions of low-level westerlies and initial convection over southern MC and the Arafura Sea. This effect may be helped by the warm sea surface temperature anomalies associated with the La Niña condition of this month. The current results showcase that KWs and their associated scale interactions can provide useful references for weather monitoring and forecast of this region when the MJO is absent.

Is the boreal spring tropical Atlantic variability a precursor of the Equatorial Mode?

The Equatorial Mode (EM) governs the tropical Atlantic inter-annual variability during boreal summer. It has profound impacts on the climate of adjacent and remote areas. However, predicting the EM is one of the most challenging and intriguing issues for the scientific community. Recent studies have suggested a possible connection between the boreal spring Meridional Mode (MM) and the EM through ocean wave propagation. Here, we use a set of sensitivity experiments with a medium-resolution ocean model to determine the precursor role of a MM to create equatorial SST variability. Our results demonstrate that boreal summer equatorial SSTs following a MM, are subject to two counteracting effects: the local wind forcing and remotely-excited oceanic waves. For a positive MM, the anomalous easterly winds blowing along the equator, shallow the thermocline, cooling the sea surface via vertical diffusion and meridional advection. Anomalous wind curl excites a downwelling Rossby wave north of equator, which is reflected at the western boundary becoming an equatorial Kelvin wave (KW). This downwelling KW propagates eastward, deepening the thermocline and activating the thermocline feedbacks responsible for the equatorial warming. Moreover, the local wind forcing and RW-reflected mechanism have a significant and comparable impact on the equatorial SST variability. Changes in the intensity and persistence of these distinct forcings will determine the equatorial SST response during boreal summer. Our results give a step forward to the improvement of the EM predictability.