Vertical Heat Advection Research Articles

Vertical Structures of Marine Heatwaves in the South China Sea: Characteristics, Drivers and Impacts on Chlorophyll Concentration

AbstractMarine heatwaves (MHWs) are characterized as extreme ocean warming events, which have multifaceted impacts on marine ecosystems. The warming of the ocean associated with MHWs can extend into the deep ocean. In the study, five main types of MHWs in the South China Sea (SCS) are identified with various vertical structures, including shallow, surface‐extension‐reversed, surface‐extension‐intensified, deep, and surface‐extension‐intensified‐reversed. Different types of MHWs have different spread patterns and depths. Shallow and surface‐extension‐reversed MHWs are common in coastal areas, and surface‐extension‐intensified MHW events often occur in deep areas. The vertical structures of MHWs are affected by ocean dynamical processes, particularly vertical processes and horizontal heat advection. These MHWs can alter ocean chlorophyll concentration to varying degrees and locations, contingent on their vertical thermal profiles. Furthermore, an analysis of the impacts of a long‐lived MHW event in the SCS in 2020 revealed that the MHWs could alter chlorophyll concentration. These results help to better understand the physical drivers of localized MHWs and their potential impacts in the context of climate change.

Dynamics of a severe summer Shamal wind and its induced dust storm in the Middle East: A diagnostic study based on numerical simulation

This study investigates the formation and dynamics of the Summer Shamal Wind (SSW) and its associated dust storms over the Middle East. A regional climate model, RegCM4, coupled with a dust module, is employed to simulate a severe dust storm event on June 12, 2006. To gain deeper insights, a novel dynamical mechanism is proposed. This mechanism highlights the crucial role of both mechanical and thermal forcing in shaping the regional atmospheric circulation. The vertical heat advection from the Zagros Mountains (ZAG), combined with the mechanical forcing from a shallow incident flow originating from the Turkmenistan anticyclone, triggers upward motion over the ZAG. This synergistic effect leads to the development of a mid-tropospheric anticyclone over the ZAG, which, in turn, induces a westward-propagating Rossby wave. The downward motion and descending air associated with this wave over Mesopotamia establish a terrain-induced local circulation. Consequently, the SSW emerges within a shallow layer at the null-vorticity zone of a coupling-pressure pattern formed by the Arabian anticyclone and the Zagros trough, which is itself a product of the terrain-induced circulation. The study concludes that the intensification and southward movement of the Turkmenistan anticyclone play a pivotal role in the formation of severe SSWs and subsequent dust storms over the Middle East.

Evolution and Drivers of Extreme Subsurface Marine Heatwaves in the Western Tropical Pacific Ocean during 2008 and 2010–11

Abstract Marine heatwaves (MHWs) have adverse impacts on marine ecosystems and fisheries, yet extreme MHWs in the western tropical Pacific Ocean remain largely unexplored. Here, we investigate the evolution and drivers of the two strongest subsurface MHWs in the western tropical Pacific Ocean on record, in 2008 (MHW-08) and 2010–11 (MHW-11). The two events have similar evolutions, as they were both initiated and developed locally in tropical regions during their onset phase and gradually dissipated with multiple cores during the decay phase. We examined the physical processes during the evolution of MHW-08 and MHW-11 and performed a heat budget analysis for the subsurface layer in each case. We find that the onset of both events were mainly controlled by convergence and vertical heat advection due to anomalous Ekman downwelling, while the decay of both events was mainly controlled by horizontal heat advection due to anomalous horizontal currents and withdrawal of anomalous convergence. The relationship between the subsurface MHWs and El Niño–Southern Oscillation is discussed. We suggest that the anomalous heat advections plus the La Niña–related background warming lead to the extreme subsurface MHW events. Our findings may have important implications for environmental change and marine life in the western tropical Pacific Ocean.

Submesoscale Variability and Basal Melting in Ice Shelf Cavities of the Amundsen Sea

AbstractMelting of ice shelves can energize a wide range of ocean currents, from three‐dimensional turbulence to relatively large‐scale boundary currents. Here, we conduct high‐resolution simulations of the western Amundsen Sea to show that submesoscale eddies are prevalent inside ice shelf cavities. The simulations indicate energetic submesoscale eddies at the top and bottom ocean boundary layers, regions with sharp topographic slopes and strong lateral buoyancy gradients. These eddies play a substantial role in the vertical and lateral (along‐isopycnal) heat advection toward the ice shelf base, enhancing the basal melting in all simulated cavities. In turn, the meltwater provides strong buoyancy gradients that energize the submesoscale variability, forming a positive loop that could affect the overall efficiency of heat exchange between the ocean and the ice shelf cavity. Our study implies that submesoscale‐induced enhancement of basal melting may be a ubiquitous process that needs to be parameterized in coarse‐resolution climate models.

No evidence for radius inflation in hot Jupiters from vertical advection of heat

Elucidating the radiative-dynamical coupling between the upper photosphere and deeper atmosphere may be key to our understanding of the abnormally large radii of hot Jupiters. Very long integration times of 3D general circulation models (GCMs) with self-consistent radiative transfer are needed to obtain a more comprehensive picture of the feedback processes between dynamics and radiation. Here, we present the longest 3D nongray GCM study to date (86000 d) of an ultra-hot Jupiter (WASP-76 b) that has reached a final converged state. Furthermore, we present a method that can be used to accelerate the path toward temperature convergence in the deep atmospheric layers. We find that the final converged temperature profile is cold in the deep atmospheric layers, lacking any sign of vertical transport of potential temperature by large-scale atmospheric motions. We therefore conclude that coupling between radiation and dynamics alone is not sufficient to explain the abnormally large radii of inflated hot gas giants.

On the Existence of Summer Shamal Wind Induced by the Zagros Mountains in the Middle East

AbstractA new structure is proposed to explain the mechanism governing the summer Shamal wind (SSW) formation in the Middle East (ME). Here, the irreplaceable role of the Zagros Mountains (ZAG) as the primary driver of a localized atmospheric circulation over the ME has been clarified. We show that the SSW is mainly established and maintained due to the local forcing of the ZAG. A background low‐level easterly flow dominates on the east side of the ZAG which is basically from Turkmenistan Anticyclone. The flow in association with a vertical heat advection over the ZAG while generating Iran anticyclone (IA) in the middle troposphere, develops so‐called regional atmospheric circulation over the ME. As a part of the localized circulation, descending air from IA establishes the Arabian anticyclone‐Zagros Trough coupling pattern over Mesopotamia. This coupling pattern is the significant driver of SSW and its induced dust storms over the ME.

Upper-Ocean Processes Controlling the Near-Surface Temperature in the Western Gulf of Mexico from a Multidecadal Numerical Simulation

The upper-ocean processes controlling the near-surface layer temperature in the western Gulf of Mexico (GOM) are examined by estimating the contributing terms in the heat equation based on a 54-year simulation of an eddy-resolving HYbrid Coordinate Ocean Model (HYCOM). An eddy-active region defined by large surface eddy kinetic energy, representing the Loop Current eddies (LCEs) primary trajectory region, is selected for analysis. Both observations and the simulation reveal that the mean net surface heat flux cools the northern GOM and warms the southern GOM. Mean horizontal heat advection contributes to an overall cooling in the eddy-active region. Mean vertical heat advection has a strong seasonal variability associated with the strong seasonal cycle of the mixed layer process: winters tend to have a strong downward heat advection in the eddy-active region and a strong upward heat advection in the rest of the western GOM, while summers tend to have a weak advective heat flux. The downwelling (upwelling) is primarily due to the dominant anticyclonic (cyclonic) wind stress curl. Mean eddy heat flux convergence contributes to the overall warming in the upper ocean of the western GOM. Diffusive flux is not small across the thermocline, and it is expected to have an insignificant influence on the near-surface temperature.

Comparison of ocean heat content estimated using two eddy-resolving hindcast simulations based on OFES1 and OFES2

Abstract. In this study, we have compared the ocean heat content (OHC), estimated using two eddy-resolving hindcast simulations based on Ocean General Circulation Model for the Earth Simulator version 1 (OFES1) and version 2 (OFES2). Results from a global objective analysis of subsurface temperature (EN4) were taken as a reference. Both EN4 and OFES1 suggest that OHC has increased in most regions of the top 2000 m during 1960–2016, which is mainly associated with the deepening of neutral density surfaces and variations along the neutral density surfaces of regional importance. Upon comparing the results obtained from the two OFES hindcasts, we found substantial differences in the temporal and spatial distributions of the OHC, especially in the Atlantic Ocean. A basin-wide heat budget analysis showed that there was less surface heating for the major basins in OFES2. The horizontal heat advection was mostly similar; however, OFES2 had a significantly stronger meridional heat advection associated with the Indonesian Throughflow (ITF) above 300 m. Additionally, large discrepancies in the vertical heat advection were also evinced when the two OFES results were compared, especially at a depth of 300 m in the Indian Ocean. We inferred that there are large discrepancies in the vertical heat diffusion (those that cannot be directly evaluated in this study due to data unavailability), which, along with the different magnitudes of sea surface heat flux and vertical heat advection, were the major factors responsible for the examined differences in OHC. This work suggests that OFES1 provides a reasonable multi-decadal estimate of global and basin-integrated warming trends above 700 m, except for the top 300 m for the Pacific Ocean and between 300–700 m for the Indian Ocean. Although the estimates of the global OHC during 1960–2016 are consistent with observations between 700–2000 m, caution is warranted while examining the basin-wide multi-decadal OHC variations using OFES1. The seemingly suboptimal OHC estimate based on OFES2 suggests that any conclusions on long-term climate variations derived from OFES2 might suffer from large drifts, necessitating audits.

Seasonal Cycle of Idealized Polar Clouds: Large Eddy Simulations Driven by a GCM

AbstractThe uncertainty in polar cloud feedbacks calls for process understanding of the cloud response to climate warming. As an initial step toward improved process understanding, we investigate the seasonal cycle of polar clouds in the current climate by adopting a novel modeling framework using large eddy simulations (LES), which explicitly resolve cloud dynamics. Resolved horizontal and vertical advection of heat and moisture from an idealized general circulation model (GCM) are prescribed as forcing in the LES. The LES are also forced with prescribed sea ice thickness, but surface temperature, atmospheric temperature, and moisture evolve freely without nudging. A semigray radiative transfer scheme without water vapor and cloud feedbacks allows the GCM and LES to achieve closed energy budgets more easily than would be possible with more complex schemes. This enables the mean states in the two models to be consistently compared, without the added complications from interaction with more comprehensive radiation. We show that the LES closely follow the GCM seasonal cycle, and the seasonal cycle of low‐level clouds in the LES resembles observations: maximum cloud liquid occurs in late summer and early autumn, and winter clouds are dominated by ice in the upper troposphere. Large‐scale advection of moisture provides the main source of water vapor for the liquid‐containing clouds in summer, while a temperature advection peak in winter makes the atmosphere relatively dry and reduces cloud condensate. The framework we develop and employ can be used broadly for studying cloud processes and the response of polar clouds to climate warming.

The thermodynamic and dynamic control of the sensible heat polynya in the western Cosmonaut Sea

A prominent feature in the west Cosmonaut Sea is the reoccurrence of a sensible heat polynya — the western Cosmonaut Sea Polynya (wCSP), which normally exists in late austral autumn and early winter. The Southern Ocean Sate Estimate (SOSE) reanalysis product is employed to investigate the thermodynamic and dynamic processes controlling the formation and evolution of a real wCSP event in 2009, when the polynya properties revealed from satellite observations are reasonably simulated by SOSE. An oceanic heat budget analysis was conducted for the surface layer above the thermocline, and the results reveal that heat advection is the major term contributing to the surface heat content variation in this area. The precursor of wCSP — an embayment — was formed during cyclonic atmospheric circulations. The negative wind stress curl from a cyclone induced upwelling that brought the warm circumpolar deep water to the surface and presumably melted ice. Meanwhile, the location of the cyclone relative to the wCSP created weaker easterlies over the northern boundary of polynya and stronger easterlies over the southern boundary of polynya. This leads to difference in the meridional oceanic heat transports across the northern and southern boundaries, resulting in net positive meridional oceanic heat transport into the polynya. Both the vertical heat advection and the meridional heat advection contribute significantly to increased heat content in the surface layer of wCSP, causing sea-ice melt and formation of the embayment. The cyclonic wind field promotes convergence of sea ice over the northern area of the embayment and results in the polynya. The formation of wCSP is likely related to the strength and position of Southern Annular Mode on interannual scale, and may potentially affect the biological productivity by controlling the availability of light in the mixed layer.

Transient Response of the Southern Ocean to Idealized Wind and Thermal Forcing across Different Model Resolutions

AbstractThe Southern Ocean has undergone significant climate-related changes over recent decades, including intensified westerly winds and increased radiative heating. The interplay between wind-driven cooling and radiative warming of the ocean is complex and remains unresolved. In this study, idealized wind and thermal perturbations are analyzed in a global ocean–sea ice model at two horizontal resolutions: nominally, 1° and 0.1°. The sea surface temperature (SST) response shows a clear transition from a wind-driven cooling phase to a warming phase. This warming transition is largely attributed to meridional and vertical Ekman heat advection, which are both sensitive to model resolution due to the model-dependent components of temperature gradients. At higher model resolution, due to a more accurate representation of near-surface vertical temperature inversion and upward Ekman heat advection around Antarctica, the anomalous SST warming is stronger and develops earlier. The mixed layer depth at midlatitudes initially increases due to a wind-driven increase in Ekman transport of cold dense surface water northward, but then decreases when the thermal forcing drives enhanced surface stratification; both responses are more sensitive at lower model resolution. With the wind intensification, the residual overturning circulation increases less in the 0.1° case because of the adequately resolved eddy compensation. Ocean heat subduction penetrates along more tilted isopycnals in the 1° case, but it orients to follow isopycnal layers in the 0.1° case. These findings have implications for understanding the ocean response to the combined effects of Southern Hemisphere westerly wind changes and anthropogenic warming.

Importance of Regional‐Scale Auroral Precipitation and Electrical Field Variability to the Storm‐Time Thermospheric Temperature Enhancement and Inversion Layer (TTEIL) in the Antarctic E Region

AbstractA dramatic thermospheric temperature enhancement and inversion layer (TTEIL) was observed by the Fe Boltzmann lidar at McMurdo, Antarctica during a geomagnetic storm (Chu et al. 2011, https://doi.org/10.1029/2011GL050016). The Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model (TIEGCM) driven by empirical auroral precipitation and background electric fields cannot adequately reproduce the TTEIL. We incorporate the Defense Meteorological Satellite Program (DMSP)/Special Sensor Ultraviolet Spectrographic Imager (SSUSI) auroral precipitation maps, which capture the regional‐scale features into TIEGCM and add subgrid electric field variability in the regions with strong auroral activity. These modifications enable the simulation of neutral temperatures closer to lidar observations and neutral densities closer to GRACE satellite observations (~475 km). The regional scale auroral precipitation and electric field variabilities are both needed to generate strong Joule heating that peaks around 120 km. The resulting temperature increase leads to the change of pressure gradients, thus inducing a horizontal divergence of air flow and large upward winds that increase with altitude. Associated with the upwelling wind is the adiabatic cooling gradually increasing with altitude and peaking at ~200 km. The intense Joule heating around 120 km and strong cooling above result in differential heating that produces a sharp TTEIL. However, vertical heat advection broadens the TTEIL and raises the temperature peak from ~120 to ~150 km, causing simulations deviating from observations. Strong local Joule heating also excites traveling atmospheric disturbances that carry the TTEIL signatures to other regions. Our study suggests the importance of including fine‐structure auroral precipitation and subgrid electric field variability in the modeling of storm‐time ionosphere‐thermosphere responses.

Volume and Heat Budgets in the Coastal California Current System: Means, Annual Cycles, and Interannual Anomalies of 2014–16

AbstractThe data-assimilating California State Estimate (CASE) enables the explicit evaluation of spatiotemporally varying volume and heat budgets in the coastal California Current System (CCS). An analysis of over 10 years of CASE model output (2007–17) diagnoses the physical drivers of the CCS mean state, annual cycles, and the 2014–16 temperature anomalies associated with a marine heat wave and an El Niño event. The largest terms in the mean mixed layer (from−50 to 0 m) volume budgets are upward vertical transport at the coast and offshore-flowing ageostrophic Ekman transport at the surface, the two branches of the coastal upwelling overturning cell. Contributions from onshore geostrophic flow in the Southern California Bight and alongshore geostrophic convergence in the central CCS balance the mean volume budgets. The depth-dependent annual cycle of vertical velocity exhibits the strongest upward velocity between −40- and −30-m depth in April. Interannual volume budgets show that over 50% of the 2013.5–16.5 time period experienced downwelling anomalies, which were balanced predominantly by alongshore transport convergence and, less often, by onshore transport anomalies. Mixed layer temperature anomalies persisted for the entirety of 2014–16, reaching a maximum of +3° in October 2015. The mixed layer heat budget shows that intermittent high air–sea heat flux anomalies and alongshore and vertical heat advection anomalies all contributed to warming during 2014–16. A subsurface (from −210 to −100 m) heat budget reveals that in September 2015 anomalous poleward heat advection into the Southern California Bight by the California Undercurrent caused deeper warming during the 2015/16 El Niño.

On the rapid intensification for Typhoon Meranti (2016): convection, warm core, and heating budget

Through a cloud-resolving simulation of the rapid intensification (RI) of Typhoon Meranti (2016), the convections, warm core, and heating budget are investigated during the process of RI. By investigating the spatial distributions and temporal evolutions of both convective-stratiform precipitation and shallow-deep convections, we find that the inner-core convections take mode turns, from stratiform-precipitation (SP) dominance to convective-precipitation (CP) prevalence during the transition stages between pre-RI and RI. For the CP, it experiences fewer convections before RI, and the conversion from moderate/ moderate-deep convections to moderate-deep/deep convections during RI. There is a clear upper-level warm-core structure during the process of RI. However, the mid-low-level warming begins first, before the RI of Meranti. By calculating the local potential temperature (θ) budget of various convections, the link between convections and the warm core (and further to RI via the pressure drop due to the warming core) is established. Also, the transport pathways of heating toward the center of Meranti driven by pressure are illuminated. The total hydrostatic pressure decline is determined by the mid-low-level warm anomaly before RI, mostly caused by SP. The azimuthal-mean diabatic heating is the largest heating source, the mean vertical heat advection controls the vertical downwards transport by adiabatic warming of compensating down-drafts above eye region, and then the radial θ advection term radially transports heat toward the center of Meranti in a slantwise direction. Accompanying the onset of RI, the heating efficiency of the upper-level warming core rises swiftly and overruns that of the mid-low-level warm anomaly, dominating the total pressure decrease and being mainly led by moderate-deep and deep convections. Aside from the characteristics in common with SP, for CP, the eddy component of radial advection also plays a positive role in warming the core, which enhances the centripetal transport effect and accelerates the RI of Meranti.

Modelling study of transformations of the exchange flows along the Strait of Gibraltar

Abstract. Vertical transfers of heat, salt and mass between the inflowing and outflowing layers at the Strait of Gibraltar are explored basing on the outputs of a three-dimensional fully nonlinear numerical model. The model covers the entire Mediterranean basin and has a very high spatial resolution around the strait (1/200∘). Another distinctive feature of the model is that it includes a realistic barotropic tidal forcing (diurnal and semi-diurnal), in addition to atmospheric pressure and heat and water surface fluxes. The results show a significant transformation of the properties of the inflowing and outflowing water masses along their path through the strait. This transformation is mainly induced by the recirculation of water, and therefore of heat and salt, between the inflowing and outflowing layers. The underlying process seems to be the hydraulic control acting at the Espartel section, Camarinal Sill and Tarifa Narrows, which limits the amount of water that can cross the sections and forces a vertical recirculation. This results in a complex spatio-temporal pattern of vertical transfers, with the sign of the net vertical transfer being opposite in each side of the Camarinal Sill. Conversely, the mixing seems to have little influence on the heat and salt exchanged between layers (∼2 %–10 % of advected heat and salt). Therefore, the main point of our work is that most of the transformation of water properties along the strait is induced by the vertical advection of heat and salt and not by vertical mixing. A simple relationship between the net flux and the vertical transfers of water, heat and salt is also proposed. This relationship could be used for the fine-tuning of coarse-resolution model parameterizations in the strait.

On the Responses of Mesosphere and Lower Thermosphere Temperatures to Geomagnetic Storms at Low and Middle Latitudes

AbstractObservations from lidars and satellites have shown that large neutral temperature increases and decreases occur in the middle and low latitudes of the mesosphere and lower thermosphere region during geomagnetic storms. Here we undertake first‐principles simulations of mesosphere and lower thermosphere temperature responses to storms using the Thermosphere Ionosphere Mesosphere Electrodynamics General Circulation Model to elucidate the nature and causes of these changes. Temperature variations were not uniform; instead, nighttime temperatures changed earlier than daytime temperatures, and temperatures changed earlier at high latitudes than at low ones. Furthermore, temperatures increased in some places/times and decreased in others. As the simulation behaves similar to observations, it provides an opportunity to understand physical processes that drive the observed changes. Our analysis has shown that they were produced mainly by adiabatic heating/cooling that was associated with vertical winds resulting from general circulation changes, with additional contributions from vertical heat advection.

Measured Horizontal Temperature Gradients Constrain Heat Transfer Mechanisms in Greenland Ice

AbstractIce in the ablation zone of the Greenland ice sheet is known to contain vertical temperature gradients that arise from conduction at the boundaries, the addition of strain and latent heat, and advective heat transport. A three‐dimensional array of temperature measurements in a grid of boreholes reveals horizontal ice temperature gradients that challenge the present conceptualization of heat transfer. We measure two distinct types of temperature variability in the horizontal direction, one impacting a confined region where ice temperatures span a range of 5°C, and another with temperatures consistently varying by approximately 2°C across the entire 3‐D block. We suggest the first demonstrates the localized and limited nature of latent heat input, and the second demonstrates that vertical heat advection outpaces diffusion. These findings imply that ice flow is highly variable over sub‐ice‐thickness length scales, which in turn generates contrasts in ice temperature that may impact ice deformation and fracturing.

CO2‐Induced Ocean Warming of the Antarctic Continental Shelf in an Eddying Global Climate Model

AbstractOcean warming near the Antarctic ice shelves has critical implications for future ice sheet mass loss and global sea level rise. A global climate model with an eddying ocean is used to quantify the mechanisms contributing to ocean warming on the Antarctic continental shelf in an idealized 2xCO2 experiment. The results indicate that relatively large warm anomalies occur both in the upper 100 m and at depths above the shelf floor, which are controlled by different mechanisms. The near‐surface ocean warming is primarily a response to enhanced onshore advective heat transport across the shelf break. The deep shelf warming is initiated by onshore intrusions of relatively warm Circumpolar Deep Water (CDW), in density classes that access the shelf, as well as the reduction of the vertical mixing of heat. CO2‐induced shelf freshening influences both warming mechanisms. The shelf freshening slows vertical mixing by limiting gravitational instabilities and the upward diffusion of heat associated with CDW, resulting in the buildup of heat at depth. Meanwhile, freshening near the shelf break enhances the lateral density gradient of the Antarctic Slope Front (ASF) and disconnect isopycnals between the shelf and CDW, making cross‐ASF heat exchange more difficult. However, at several locations along the ASF, the cross‐ASF heat transport is less inhibited and heat can move onshore. Once onshore, lateral and vertical heat advection work to disperse the heat anomalies across the shelf region. Understanding the inhomogeneous Antarctic shelf warming will lead to better projections of future ice sheet mass loss.

Interannual Variability of Sea Surface Temperature in the Southwest Pacific and the Role of Ocean Dynamics

Abstract This paper investigates the mechanisms causing interannual variability of upper ocean heat content and sea surface temperature (SST) in the southwest Pacific. Using the ECCOv4 ocean reanalysis it is shown that air–sea heat flux and ocean heat transport convergence due to ocean dynamics both contribute to the variability of upper ocean temperatures around New Zealand. The ocean dynamics responsible for the ocean heat transport convergence are investigated. It is shown that SSTs are significantly correlated with the arrival of barotropic Rossby waves estimated from the South Pacific wind stress over the latitudes of New Zealand. Both Argo observations and the ECCOv4 reanalysis show deep isotherms fluctuate coherently around the country. The authors suggest that the depth of the thermocline around New Zealand adjusts to changes in the South Pacific winds, modifies the vertical advection of heat into the upper ocean, and contributes to the interannual variability of SST in the region.

Mean Subsurface Upwelling Induced by Intraseasonal Variability over the Equatorial Indian Ocean

AbstractA possible formation mechanism of mean subsurface upwelling along the equator in the Indian Ocean is investigated using a series of hierarchical ocean general circulation model (OGCM) integrations and analytical considerations. In an eddy-resolving OGCM with realistic forcing, mean vertical velocity in the tropical Indian Ocean shows rather strong upwelling, with its maximum on the equator in the subsurface layer below the thermocline. Heat budget analysis exhibits that horizontal and vertical heat advection by deviations (i.e., due to deviations of velocity and temperature from the mean) balances with vertical advection caused by mean equatorial upwelling. Horizontal heat advection is mostly associated with intraseasonal variability with periods of 3–91 days, while contributions from longer periods (>91 days) are small. Sensitivity experiments with a coarse-resolution OGCM further demonstrate that such mean equatorial upwelling cannot be reproduced by seasonal forcing only. Adding the intraseasonal wind forcing, especially meridional wind variability with a period of 15 days, generates significant mean subsurface upwelling on the equator. Further experiments with idealized settings confirm the importance of intraseasonal mixed Rossby–gravity (MRG) waves to generate mean upwelling, which appears along the energy “beam” of the MRG wave. An analytical solution of the MRG waves indicates that wave-induced temperature advection caused by the MRG waves with upward (downward) phase propagation results in warming (cooling) on the equator. This wave-induced warming (cooling) is shown to balance with the mean equatorial upwelling (downwelling), which is consistent with simulated characteristics in the OGCM experiments.