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A marine heatwave (MHW) is typically defined as an anomalous warm event in the surface ocean, with wide-ranging impacts on marine and socio-economic systems. The surface warming associated with MHWs can penetrate into the deep ocean; however, the vertical structure of MHWs is poorly known in the global ocean. Here, we identify four main types of MHWs with different vertical structures using Argo profiles: shallow, subsurface-reversed, subsurface-intensified, and deep MHWs. These MHW types are characterized by different spatial distributions with hotspots of subsurface-reversed and subsurface-intensified MHWs at low latitudes and shallow and deep MHWs at middle-high latitudes. These vertical structures are influenced by ocean dynamical processes, including oceanic planetary waves, boundary currents, eddies, and mixing. The area and depth of all types of MHWs exhibit significant increasing trends over the past two decades. These results contribute to a better understanding of the physical drivers and ecological impacts of MHWs in a warming climate. The authors identify four main types of vertical structures of marine heatwaves, with different impact depths and spatio-temporal distributions, that are influenced by multiscale ocean dynamical processes. [ABSTRACT FROM AUTHOR]

To investigate the South Equatorial Current (SEC) in the Indian Ocean, 18 drifters were deployed off Sumatra‐Java during boreal spring 2019. These drifter observations revealed a large SEC meander that swung southward about 1,100 km during 40 days along ∼100°E. The southward motion occurred on the eastern edge of the Seychelles‐Chagos thermocline ridge, accompanied by an anticyclonic eddy/Rossby wave from the east, indicated by the geostrophic velocity induced by the zonal pressure gradient. Meanwhile, the ageostrophic velocity induced by the southeasterly wind also contributes to this southward motion. This is verified by a Lagrangian Trajectory Model in which drifter trajectories are well reproduced using Chinese‐French Oceanic SATellite winds products. Further analysis of the historical drifters with similar trajectories reveals that the geostrophic and ageostrophic currents jointly drive drifters to move southward. Ageostrophic component could contribute up to 45% of the southward movement of the SEC meander. Plain Language Summary: The South Equatorial Current (SEC) is westward in the tropical Indian Ocean. During boreal spring 2019, 18 drifters were deployed off Sumatra‐Java, and their trajectories show a large SEC meander, with a typical drifter moved southward over 1,100 km during 40 days along ∼100°E. The geostrophic component, associated with the Seychelles‐Chagos thermocline ridge and an anticyclonic eddy/Rossby wave, and ageostrophic component, induced by southeasterly wind, jointly drive drifters to move southward. Further diagnoses of historical drifters show that the ageostrophic component could take up to 45% contribution in the southward movement in the SEC meander. Key Points: South Equatorial Current in the Indian Ocean advected drifters southward more than 1,100 km within 40 days (∼32 cm s−1) in 2019Combined effects of the zonal pressure gradient and the southeasterly wind contribute to the southward movementSouthward movement usually occurs in spring and summer, contributing from ageostrophic current up to ∼45% [ABSTRACT FROM AUTHOR]

Marine heatwaves (MHWs) in the tropical Indian Ocean (TIO) showed remarkable increases in duration and frequency during the satellite observing era, responding to rising sea surface temperature. Long‐lasting MHWs were found in three upwelling regions of the TIO in 2015–2016 and 2019–2020, closely related to persistent downwelling oceanic planetary waves. In 2015, a prolonged MHW (149 days) in the western TIO was initiated by the downwelling Rossby waves associated with the co‐occurring super El Niño and positive Indian Ocean dipole (IOD) events. In the following year, the negative IOD sustained the longest MHW (372 days) in the southeastern TIO, prompted by the eastward‐propagating equatorial Kelvin waves. In 2019–2020, the two longest MHWs recorded in the southwestern TIO (275 days in 2019 and 149 days in 2020) were maintained by the downwelling Rossby waves associated with the 2019 extreme IOD. This study revealed the importance of ocean dynamics in long‐lasting MHWs in the TIO. Plain Language Summary: Marine heatwaves (MHWs) are extreme sea surface warming events, which can have destructive impacts on marine ecosystems and socio‐economy. The tropical Indian Ocean (TIO) has experienced the most significant sea surface temperature warming among the global oceans in the satellite observing era since the 1980s. The increases in duration and frequency of MHWs were found statistically significant in the TIO, and further increases can be expected under global warming. Several long‐lasting MHWs occurred in recent years, associated with extreme climate modes. Internal ocean waves triggered by anomalous winds deepened the thermocline through the convergence of warm upper‐ocean water and reduced upwelling of subsurface cold water, resulting in a surface warm that facilitated the MHW formation. Thus, the consecutive propagating waves during the extreme climate events sustained the prolonged MHWs in the TIO. These results expand our understanding of the physical mechanisms responsible for MHWs in the TIO under the Indo‐Pacific climate variability and global warming. Key Points: Long‐lasting marine heatwaves (MHWs) occurred in the upwelling regions of the tropical Indian Ocean during 2015–2016 and 2019–2020The recent long‐lasting MHWs are related to the super El Niño in 2015–2016 and extreme Indian Ocean dipole in 2019Oceanic downwelling waves induced‐thermocline warming plays a crucial role in the long‐lasting MHWs [ABSTRACT FROM AUTHOR]

We used multiple data sets to investigate the decadal and longer time scale variability of sea surface partial pressure of CO2 (pCO2) over the equatorial Pacific Ocean from 1990 to 2019. Unlike the increasing trend in the global oceanic pCO2, both the Niño 3.4 and warm pool regions featured distinct decadal variabilities. A pCO2 increasing stagnation, a hiatus stage, was identified in the Niño 3.4 region from 2010 to 2019. Further analysis demonstrated that low‐frequency pCO2 was negatively correlated with the Interdecadal Pacific Oscillation (IPO) index. Both the 2009/10 and 2015/16 El Niño events and an increasing phase of IPO contributed to this long pCO2 stagnation. Correspondingly, a weaker upwelling was induced in the central and eastern tropical Pacific as the trade winds weakened and thus stagnated the increase in pCO2 in the Niño 3.4 region. Our results imply a significant impact of climate variabilities on sea surface pCO2 in the equatorial Pacific. Plain Language Summary: The equatorial Pacific Ocean is one of the largest natural sources of carbon dioxide (CO2) to the atmosphere. It can partially reduce the CO2 absorbed by large areas of the ocean. Here we use a series of data sets to investigate the long‐term variations in the sea surface partial pressure of CO2 over the equatorial Pacific Ocean. Unlike the increasing trend in the global ocean surface pCO2 due to the continuous increase in atmospheric CO2 impacted by human activities, the equatorial Pacific Ocean features significant long‐term variabilities. The pCO2 observations and products that we used support a pCO2 increasing hiatus at the Niño 3.4 region (located in the central and eastern equatorial Pacific Ocean) since 2010. Further analysis found that the hiatus is related to El Niño events and the large‐scale, long period oscillation of the Pacific basin. Our results imply the impact of natural climate variabilities on sea surface pCO2 in this important sea area of the equatorial Pacific Ocean. Key Points: The pCO2 in the equatorial Pacific features a significant long‐term trend and decadal to interdecadal variabilityThe increasing pCO2 in the Niño 3.4 region shows stagnation since 2010 due to El Niño events and the increasing phase of the Interdecadal Pacific OscillationPacific natural climate variability mitigates the large CO2 source in the tropical Pacific Ocean [ABSTRACT FROM AUTHOR]

Submesoscale density fronts and the associated processes of frontogenesis and symmetric instability (SI) are investigated in the northwest Pacific subtropical countercurrent (STCC) system by a high-resolution simulation and diagnostic analysis. Both satellite observations and realistic simulation show active surface fronts with a horizontal scale of ~20 km in the STCC upper ocean. Frontogenesis-induced buoyancy advection is detected to rapidly sharpen these density fronts. The direct straining effect of larger-scale geostrophic flows is a primary influence on the buoyancy-gradient frontogenetic tendency and frontal baroclinic potential vorticity (PV) enhancement. The enhanced lateral buoyancy gradients in conjunction with atmospheric forced surface buoyancy loss can produce a negative Ertel PV and trigger frontal SI in the STCC region. Up to 30% of the mixed layer (ML) inside a typical eddy has negative PV in the high-resolution simulation. As a result, the cross-front ageostrophic secondary circulations tend to restratify the surface boundary layer and induce a large vertical velocity reaching ~100 m day−1, substantially facilitating the vertical communication of the STCC system. At the same time, the SI is identified to be responsible for a forward cascade of geostrophic kinetic energy in the STCC region, despite the coexistence of ML eddies and SI in the deep winter ML. Therefore, these active density fronts and their SI-associated submesoscale processes play important roles in the enhanced vertical exchanges (e.g., heat, nutrients, and carbon) and energy transfer to smaller scales in the eddy-active STCC upper ocean, as well as triggering phytoplankton blooms at the periphery of eddies. [ABSTRACT FROM AUTHOR]

Based on ERA-interim reanalysis dataset from 1979 to 2018, this study revisits the global and regional water cycle, as well as the water vapor transport between ocean basins. The surface of the Earth is divided into five continents and four ocean basins, water vapor budget, and its seasonal variability of each continent and ocean basin are calculated. The Indian Ocean and the Atlantic are major net water vapor source regions, ~ − 0.47 Sv and ~ −0.82 Sv, respectively, while the Pacific and the Arctic are weak net water vapor sink regions, ~ 0.04 Sv and ~ 0.06 Sv, respectively. A total of 85% of evaporation and 78% of precipitation occur at the ocean-atmosphere interface while 15% of evaporation and 22% of precipitation occur at the land-atmosphere interface. About 1.19 Sv water vapor is carried out from oceans to lands. The water vapor transport is about 0.07 Sv from the Indian Ocean to the Pacific, and 0.22 Sv from the Atlantic to the Pacific through the segments. At low latitudes, both the Atlantic and the Indian Ocean transport water vapor to the Pacific, with a total of 0.29 Sv. Those results update the estimate of global and regional water vapor budget. [ABSTRACT FROM AUTHOR]

The 2019 positive Indian Ocean Dipole (IOD) was the strongest event since the 1960s which developed independently without coinciding El Niño. The dynamics is not fully understood. Here we show that in March–May, westward propagating oceanic Rossby waves, a remnant consequence of the weak 2018 Pacific warm condition, led to anomalous sea surface temperature warming in the southwest tropical Indian Ocean (TIO), inducing deep convection and anomalous easterly winds along the equator, which triggered the initial cooling in the east. In June–August, the easterly wind anomalies continued to evolve through ocean‐atmosphere coupling involving Bjerknes feedback and equatorial nonlinear ocean advection, until its maturity in September–November. This study clarifies the contribution of oceanic Rossby waves in the south TIO in different dynamic settings and reveals a new triggering mechanism for extreme IOD events that will help to understand IOD diversity. Plain Language Summary: The Indian Ocean Dipole (IOD) is an ocean‐atmosphere coupled climatic phenomenon which can cause severe social and economic losses in the surrounding regions such as drought in the Maritime Continent/Australia and flooding in East Africa. The IOD features a see‐saw structure accompanied by an anomalous sea surface temperature gradient, winds, and oceanic adjustments. The El Niño–Southern Oscillation in the Pacific is an important trigger to a strong IOD event. However, an extreme positive IOD event occurred in 2019 without a concurrent or ensuing El Niño. We show that the thermocline warming associated with anomalous ocean downwelling in the southwest tropical Indian Ocean triggered atmospheric convection, inducing anomalous easterly winds along the equator and hence, positive feedbacks associated with an IOD event. This study may help to understand the evolution of extreme IOD and improve IOD predictability. Key Points: The oceanic downwelling Rossby waves in the south tropical Indian Ocean is key to the 2019 extreme positive Indian Ocean Dipole (IOD)The Rossby waves induced thermocline warming, triggering wind‐evaporation‐SST feedback thus anomalous easterly winds along the equatorThe easterly wind anomalies further triggered the Bjerknes feedback and other positive feedbacks and established an extreme IOD [ABSTRACT FROM AUTHOR]

This study identifies a new triggering mechanism of the Indian Ocean Dipole (IOD) from the Southern Hemisphere. This mechanism is independent from the El Niño‐Southern Oscillation (ENSO) and tends to induce the IOD before its canonical peak season. The joint effects of this mechanism and ENSO may explain different lifetimes and strengths of the IOD. During its positive phase, development of sea surface temperature cold anomalies commences in the southern Indian Ocean, accompanied by an anomalous subtropical high system and anomalous southeasterly winds. The eastward movement of these anomalies enhances the monsoon off Sumatra‐Java during May–August, leading to an early positive IOD onset. The pressure variability in the subtropical area is related with the Southern Annular Mode, suggesting a teleconnection between high‐latitude and midlatitude climate that can further affect the tropics. To include the subtropical signals may help model prediction of the IOD event. Plain Language Summary: An Indian Ocean Dipole (IOD) at its positive phase featuring anomalously high and low sea surface temperature (SST) in the west and east equatorial Indian Ocean, respectively, shifts atmosphere convection westward, causing severe floods and droughts in surrounding west and east Indian Ocean‐rim regions. Known triggering mechanisms, such as the external El Niño‐Southern Oscillation (ENSO), cannot explain development or intensity of many IOD events. Here we find a novel triggering mechanism of the IOD from the Southern Hemisphere, in which the subtropical high pressure and wind anomalies forcing cool SST anomalies evolve to trigger onset of an IOD event via enhancing the monsoon off Sumatra‐Java. This Southern Hemisphere Mechanism can operate independently from the ENSO and commences earlier than the ENSO forcing, thus providing explanation of different IOD characteristics and a longer prediction lead time. Key Points: A triggering mechanism of the IOD originated from the Southern Hemisphere is identifiedThe Southern Hemisphere Mechanism is independent from ENSO and associated with subtropical and high‐latitude climatic variabilityThe joint effects of such mechanism and ENSO produce different characteristics of the IOD [ABSTRACT FROM AUTHOR]

An analysis of Argo data reveals that the subantarctic mode water (SAMW) in the South Indian Ocean (SIO), characterized by a vertical potential vorticity (PV) minimum, decreases by 10% in volume from 2004 to 2018. Most of this decrease occurs at the 26.8–26.9 kg m−3 density range which forms southwest of Australia, while a slight volume increase occurs at 26.6–26.8 kg m−3. Further analysis indicates that the weakening of the Mascarene High and westerly winds in the SIO reduces the evaporation‐precipitation, surface heat flux, and Ekman pumping and shoals the mixed layer southwest of Australia, which leads to a volume decrease at 26.8–26.9 kg m−3 in approximately 3 years. West of 90°E, the parameters exhibit the opposite change, leading to a volume increase at 26.6–26.8 kg m−3. This result suggests that surface winds play an important role in the variability of the SIO SAMW volume during the Argo period. Plain Language Summary: During austral winter in the subantarctic zone, a water mass called the subantarctic mode water with homogeneous properties (temperature, salinity) forms, sinks, and moves to the interior of the ocean. The formation and transport of the mode water are crucial for the climate, as the water takes up heat, freshwater, carbon, oxygen, and nutrients into the ocean interior. We find a distinct volume loss of the mode water in the South Indian Ocean from 2004 to 2018. Although this mode water forms in broad density ranges in different regions, the volume loss only occurs in the dense mode water south of Australia. Moreover, the volume of the light mode water, which forms in the center of the South Indian Ocean, slightly increases. We find that surface winds control the surface heat flux and water convergence, which determine the volume change here. These winds tend to vary oppositely in the two different mode water formation regions and make the volume vary in opposition phases. This study reveals the layer dependence of the mode water volume variation and reinforces the importance of atmospheric forcing, which would possibly help interpret the heat uptake in the Southern Ocean. Key Points: Argo data reveal a subantarctic mode water volume loss in the South Indian Ocean over the period 2004–2018Most of the volume loss occurs in the density range of 26.8–26.9 kg m−3, while a volume increase occurs at 26.6–26.8 kg m−3Changes in the SAMW volume are controlled by surface forcing, which is closely related to the Mascarene High variation [ABSTRACT FROM AUTHOR]

The south tropical Indian Ocean (TIO) warms following a strong El Niño, affecting Indo-Pacific climate in early boreal summer. While much attention has been given to the southwest TIO where the mean thermocline is shallow, this study focuses on the subsequent warming in the southeast TIO, where the mean sea surface temperature (SST) is high and deep convection is strong in early summer. The southeast TIO warming induces an anomalous meridional circulation with descending (ascending) motion over the northeast (southeast) TIO. It further anchors a "C-shaped" surface wind anomaly pattern with easterlies (westerlies) in the northeast (southeast) TIO, causing a persistent northeast TIO warming via wind–evaporation–SST feedback. The southeast TIO warming lags the southwest TIO warming by about one season. Ocean wave dynamics play a key role in linking the southwest and southeast TIO warming. South of the equator, the El Niño–forced oceanic Rossby waves, which contribute to the southwest TIO warming, are reflected as eastward-propagating oceanic Kelvin waves along the equator on the western boundary. The Kelvin waves subsequently depress the thermocline and develop the southeast TIO warming. [ABSTRACT FROM AUTHOR]

Variability of oceanic salinity, an indicator of the global hydrological cycle, plays an important role in the basin-scale ocean circulation. In this study, interannual to decadal variability of salinity in the upper layer of the Indian Ocean is investigated using Argo observations since 2004 and data assimilating model outputs (1992–2015). The southeastern Indian Ocean shows the strongest interannual to decadal variability of upper-ocean salinity in the Indian Ocean. Westward propagation of salinity anomalies along isopycnal surfaces is detected in the southern Indian Ocean and attributed to zonal salinity advection anomalies associated with the Indonesian Throughflow and the South Equatorial Current. Composite and salinity budget analyses show that horizontal advection is a major contributor to the interannual to decadal salinity variability of the southern Indian Ocean, and the local air–sea freshwater flux plays a secondary role. The Pacific decadal oscillation (PDO) and El Niño–Southern Oscillation (ENSO) modulate the salinity variability in the southeastern Indian Ocean, with low salinity anomalies occurring during the negative phases of the PDO and ENSO and high salinity anomalies during their positive phases. The Indonesian Throughflow plays an essential role in transmitting the PDO- and ENSO-related salinity signals into the Indian Ocean. A statistical model is proposed based on the PDO index, which successfully predicts the southeastern Indian Ocean salinity variability with a lead time of 10 months. [ABSTRACT FROM AUTHOR]

The Indo-Pacific Region is a noteworthy area with the sophisticated ocean and atmosphere system. Here, we focus on the low-frequency signals related to the ocean and atmosphere variation and its impact on dynamic and thermodynamic processes in this region. Dynamically, the sea level rise signals in the tropical west Pacific (WTP) and the southeast Indian Ocean (IO) are closely linked to each other through the pathways of oceanic waveguide within the Indonesian Seas. The low-frequency westward-propagating wave originated in the tropical Pacific is the major cause for the southeast IO sea level change. At this linkage, the interannual wave signal is dominated by the wind variability along the equatorial Pacific and the interdecadal signal is driven mainly by the wind curl off the equatorial Pacific, respectively associated with the El Nino and the Pacific Decadal Oscillation. The sea level variability in the southwest IO is mainly related to the local wind. Thermodynamically, we find a contrasting trend pattern of sea surface salinity between the WTP and the southeastern tropical IO since mid-1990s, with significant salinity increase in the WTP and freshening in the southeastern tropical IO. These trends were associated a strengthening trend of the Walker Circulation over the tropical Indo-Pacific. Precipitation, surface level pressure and wind also underwent a decadal shift in mid-1990s. Understanding low-frequency processes in the tropical Indo-Pacific will better inform on how the tropical hydrological cycle will be affected by the natural variability and a warming climate. [ABSTRACT FROM AUTHOR]

An unusual positive Indian Ocean Dipole (IOD) event occurred in 2015 associated with the 2015/16 extreme El Niño. Unlike the canonical IOD, sea surface temperature (SST) warming in the west-central tropical Indian Ocean (TIO) dominated the strong zonal SST gradient as cooling off Sumatra-Java was weak. Over the southeastern TIO, deeper thermocline has suppressed the upwelling cooling since 2012. Such deepened thermocline related to a low-frequency adjustment and curtailed cool anomalies in the 2015 positive IOD but favored warm anomalies in the 2016 negative IOD. Based on statistical analyses, ocean assimilation data confirm that an IOD-like pattern exists in the TIO on decadal time-scale. During a negative decadal IOD-like phase, thermocline is deeper in the southeastern TIO; the thermocline-SST feedback is unfavorable for positive IOD occurrence and intensity, but conducive to negative IOD events. Thus, we propose that the 2015-2016 IOD events are modulated by the low-frequency variability of thermocline. [ABSTRACT FROM AUTHOR]

Remarkable warming of the Southern Indian Ocean during the recent two decades is assessedusing a heat budget analysis based on the Estimating the Circulation and Climate of theOcean version 4 release 3 model results. The annual mean temperature averaged in theupper-700 m Southern Indian Ocean during 1998-2015 has experienced significant warmingat a rate of 1.03×10−2∘C/year. A heat budget analysis indicates that the increase is mostlydriven by decreased cooling from net air-sea heat flux and increased warming from heatadvection. Increased Indonesian Throughflow advection is the largest contributor to warmingthe upper 700 m of the Southern Indian Ocean, while the reduction of surface turbulent heatflux is of secondary importance. These results expand our understanding of the decadalheat balance in the Indian Ocean and of Indo-Pacific decadal climate variability. [ABSTRACT FROM AUTHOR]