Eastern Equatorial Indian Ocean Research Articles

Biases and teleconnections in the Met Office Global Coupled Model version 5.0 (GC5) – insights for seasonal prediction and Australia

The Australian Bureau of Meteorology (The Bureau) has been involved in the package testing and assessment process of the UK Met Office Global Coupled Model Version 5.0 (GC5) configuration. GC5 will underpin the Met Office’s next seasonal prediction system, global coupled numerical weather prediction (NWP) system and Earth System Model. It will also likely be the next version of The Bureau’s seasonal prediction system, and the version to replace the global atmosphere-only NWP system to be the first global coupled NWP system at The Bureau. The GC5 configuration includes a new sea-ice model and substantial updates to almost all areas of model physics. We have evaluated the present-day climate simulation, and compared it to observations and with previous versions GC4 and GC2. Our assessment focuses on the climate mean state and variabilities relevant to Australian seasonal prediction, including the El Niño–Southern Oscillation (ENSO), the Indian Ocean Dipole (IOD), the Southern Annular Mode and the Madden–Julian Oscillation. Notably, in comparison to its predecessor (GC4), GC5 shows significant improvements in the eastern Pacific mean state but a slight degradation in the Indian Ocean in terms of the mean state and variability. These and other results provide us with early insights of the potential performance of the next sub-seasonal or seasonal forecast system. Longstanding issues in the seasonal prediction system associated with the equatorial eastern Indian Ocean biases and an overactive ENSO and IOD will likely remain; however, improvements over the eastern equatorial Pacific in GC5 hold promise of improved prediction skill of ENSO and its teleconnections.

Drivers of Late Miocene Tropical Sea Surface Cooling: A New Perspective From the Equatorial Indian Ocean

AbstractDuring the late Miocene, global cooling occurred alongside the establishment of near‐modern terrestrial and marine ecosystems. Significant (3°C–5°C) sea surface cooling from 7.5 to 5.5 Ma is recorded by proxies at midlatitudes to high latitudes, yet the magnitude of tropical cooling and the role of atmospheric carbon dioxide (pCO2) in driving this trend are debated. Here, we present a new orbital‐resolution sea surface temperature (SST) record spanning the late Miocene to earliest Pliocene (9–5 Ma) from the eastern equatorial Indian Ocean (International Ocean Discovery Program Site U1443) based on Mg/Ca ratios measured in tests of the planktic foraminifer Trilobatus trilobus. Our SST record reveals a 3.2°C decrease from 7.4 to 5.8 Ma, significantly increasing previous estimates of late Miocene tropical cooling. Analysis of orbital‐scale variability shows that before the onset of cooling, SST variations were dominated by precession‐band (19–23 kyr) variability, whereas tropical temperature became highly sensitive to obliquity (41 kyr) after 7.5 Ma, suggesting an increase in high‐latitude forcing. We compare a revised global SST database with new paleoclimate model simulations and show that a pCO2 decrease from 560 to 300 ppm, in the range suggested by pCO2 proxy records, could explain most of the late Miocene sea surface cooling observed at Site U1443. Using our new Site U1443 record as representative of tropical SST evolution, estimated meridional SST gradients suggest a much more modest increase over the late Miocene than previously suggested, in agreement with modeled meridional SST gradients.

Contributions of shortwave radiation to the formation of temperature inversions in the Bay of Bengal and eastern equatorial Indian Ocean: A modeling approach

Contributions of shortwave radiation to the formation of temperature inversions in the Bay of Bengal and eastern equatorial Indian Ocean: A modeling approach

Robust Subsurface Biological Response During the Decaying Stage of an Extreme Indian Ocean Dipole in 2019

AbstractThe Indian Ocean Dipole (IOD) is the predominant interannual climate mode and is critical in regulating the biogeochemical cycles of the equatorial Indian Ocean (EIO). However, the dynamics of nutrient supply and the magnitude of biological responses are less understood. Here, by comparing the biophysical in situ observations across the eastern EIO during the decaying period of a positive IOD in 2019 and neutral condition in 2017, we identify that the shoaling thermocline, initiated by reinforced easterly wind, lifts the nutricline into the euphotic layer under positive IOD condition. Coincidently, the strong turbulent mixing induced by the shear instability between the opposite‐flowing surface current and subsurface Equatorial Undercurrent increases the upward turbulent nutrient flux into the euphotic zone. Their combined effect triggers a vigorous subsurface biological response over the eastern EIO, with approximately twofold higher integrated chlorophyll‐a contents in the water column during a positive IOD than under neutral condition.

Correction to: Role of the eastern equatorial Indian Ocean warming in the Indian summer monsoon rainfall trend

Correction to: Role of the eastern equatorial Indian Ocean warming in the Indian summer monsoon rainfall trend

Coral Sr/Ca records provide realistic representation of eastern Indian Ocean cooling during extreme positive Indian Ocean Dipole events

Extreme positive Indian Ocean Dipole (pIOD) events are amplified by non-linear ocean–atmosphere interactions and are characterized by pronounced cooling in the eastern equatorial Indian Ocean. These non-linear feedbacks are not adequately represented in historical products of sea surface temperatures that underestimate the magnitude of extreme pIOD events. Here, we present a sea surface temperature (SST) reconstruction based on monthly coral Sr/Ca ratios measured in two coral cores from Enggano Island (Indonesia), that lies in the eastern pole of the IOD. The coral SST reconstruction extends from 1930 to 2008 and captures the magnitude of cooling during extreme pIOD events as shown in recent satellite and reanalysis data of SST that include ocean dynamics. The corals indicate that the 1961 pIOD event was at least as severe as the 1997 event, while the 1963 pIOD was more comparable to the 2006 event. The magnitude 1967 pIOD is difficult to assess at present due to poor replication between coral cores, and may be comparable to either 1997 or 2006. Cooling during the 1972 pIOD was short-lived and followed by pronounced warming, as seen in the moderate pIOD event of 1982. A combination of coral SST reconstructions and an extension of new reanalysis products of SST to historical time scales could help to better assess the severity and impact of past pIOD events such as the ones seen in the 1960s.

Seasonal Differences of Decadal Thermocline Depth Anomalies in the Tropical Indian Ocean

Abstract The thermocline depth in the tropical Indian Ocean has experienced dramatic decadal variations in recent decades. Using analysis and reanalysis datasets, we find that the decadal thermocline depth anomalies show large seasonal differences. The seasonal differences are modulated by two major modes. The first mode shows a zonal dipole pattern, with opposite thermocline depth anomalies in the equatorial eastern Indian Ocean and western Indian Ocean, and is prominent in summer and winter. The second mode is characterized by marked thermocline depth anomalies in the southern Indian Ocean and is significant in spring and fall. The amplitude of the seasonal oscillation in these two modes has increased substantially in the twenty-first century. Their phase change is in good agreement with the observed thermocline depth anomalies in each season. The results also show that the seasonality of the decadal thermocline depth anomalies arises directly from surface wind variations within the Indian Ocean. The first mode is mainly caused by equatorial zonal wind anomalies. The second mode is dominated by local wind stress curl anomalies. These wind anomalies are both significantly correlated with the ENSO-like SST anomalies in the Pacific Ocean. The findings improve our understanding of the decadal thermocline anomalies, and will help to better evaluate their impact on seasonal phase-locked oceanic and atmospheric processes.

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.

Mixed Layer Salinity Balance in the Eastern Tropical Indian Ocean

AbstractThe mixed layer salinity (MLS) balance in the eastern tropical Indian Ocean is investigated using observations and output from the Estimating the Circulation and Climate of the Ocean model. The results suggest that the annual mean MLS is determined by the balance between the surface freshwater forcing and ocean advection. The seasonal MLS variation in the eastern equatorial Indian Ocean is driven strongly by salt advection associated with the Wyrtki Jet in spring and fall despite weak seasonal cycles of precipitation and evaporation over this region. Off the south Java coast, the MLS is the lowest in summer, due to the advection of freshwater southward associated with the meridional currents, whereas it reaches the maximum in fall, due to strong evaporation. During the negative Indian Ocean Dipole (IOD), the anomalous eastward currents transported high‐salinity water to the east, generating positive MLS anomalies in the eastern tropical Indian Ocean. The regional evaporation minus precipitation anomalies were negative, which reduced the MLS. The MLS balance during the positive IOD is found to be nearly a mirror reflection of that during the negative IOD.

Role of the eastern equatorial Indian Ocean warming in the Indian summer monsoon rainfall trend

Role of the eastern equatorial Indian Ocean warming in the Indian summer monsoon rainfall trend

Evaluation of East Asian Meiyu from CMIP6/AMIP simulations

East Asian Meiyu simulated by 35 global atmospheric models from the 6th Coupled Model Intercomparison Project (CMIP6)/Atmospheric Model Intercomparison Project (AMIP) were systematically evaluated for 1998–2014. The results show that most of the CMIP6/AMIP models can hardly reproduce the observed spatial pattern and interannual variability of East Asian Meiyu. The spatial pattern is relatively better simulated over Southern Korea and Japan where 14 out of 35 models have realistically simulated precipitation, as compared with the lower reaches of the Yangtze River where only 7 out of 35 models can well reproduce the Meiyu precipitation. For the Meiyu interannual variations, GFDL-CM4 and GFDL-ESM4 have the closest variance among the models versus the TRMM observations, while 12 out of 35 models show smaller variances. We explored the relationships of Meiyu precipitation with large-scale circulation and tropical sea surface temperature (SST), and showed that these relationships from CESM2-WACCM-FV2, EC-Earth3-CC, and MPI-ESM1-2-HAM agree well with those based on TRMM precipitation, MERRA2 reanalysis-derived large-scale atmospheric fields, and observed SST. It is found that the models with a better simulation of Meiyu precipitation can capture the relationship between Meiyu precipitation and the SST in the eastern equatorial Pacific and Indian Ocean more realistically. A performance ranking of the 35 individual CMIP6/AMIP models is further provided. It is shown that the top 20% of models based on interannual variability score tend to simulate a more realistic western Pacific subtropical high than the bottom 20% of models. And the top 20% of models based on comprehensive ranking measure simulate a more realistic EAP pattern than the bottom 20% of models.

Impact of air–sea coupling on the simulation of Indian summer monsoon using a high-resolution Regional Earth System Model over CORDEX-SA

A new high-resolution Regional Earth System Model, namely ROM, has been implemented over CORDEX-SA towards examining the impact of air–sea coupling on the Indian summer monsoon characteristics. ROM's simulated mean ISM rainfall and associated dynamical and thermodynamical processes, including the representation of northward and eastward propagating convention bands, are closer to observation than its standalone atmospheric model component (REMO), highlighting the advantage of air–sea coupling. However, the value addition of air–sea coupling varies spatially with more significant improvements over regions with large biases. Bay of Bengal and the eastern equatorial Indian Ocean are the most prominent region where the highest added value is observed with a significant reduction up to 50–500% precipitation bias. Most of the changes in precipitation over the ocean are associated with convective precipitation (CP) due to the suppression of convective activity caused by the negative feedback due to the inclusion of air–sea coupling. However, CP and large-scale precipitation (LP) improvements show east–west asymmetry over the Indian land region. The substantial LP bias reduction is noticed over the wet bias region of western central India due to its suppression, while enhanced CP over eastern central India contributed to the reduction of dry bias. An insignificant change is noticed over Tibetan Plateau, northern India, and Indo Gangetic plains. The weakening of moisture-laden low-level Somalia Jets causes the diminishing of moisture supply from the Arabian Sea (AS) towards Indian land regions resulting in suppressed precipitation, reducing wet bias, especially over western central India. The anomalous high kinetic energy over AS, wind shear, and tropospheric temperature gradient in REMO compared to observation is substantially reduced in the ROM, facilitating the favourable condition for suppressing moisture feeding and hence the wet bias over west-central India in ROM. The warmer midlatitude in ROM than REMO over eastern central India strengthens the convection, enhancing precipitation results in reducing the dry bias. Despite substantially improved ROM’performance, it still exhibits some systematic biases (wet/dry) partially associated with the persistent warm/cold SST bias and land–atmosphere interaction.

Spatiotemporal variation and mechanisms of temperature inversion in the Bay of Bengal and the eastern equatorial Indian Ocean

Spatiotemporal variation and mechanisms of temperature inversion in the Bay of Bengal and the eastern equatorial Indian Ocean

Boreal Summer Negative Correlation Relationship Between Interannual SST and Precipitation Anomalies in the Tropical and Subtropical Western North Pacific

AbstractWestern North Pacific (WNP) is a key region for ENSO development and its effect on East Asian summer monsoon. Different from most other tropical oceanic areas, sea surface temperature (SST) and precipitation anomalies over the WNP are negatively correlated in boreal summer, indicating the ocean is mainly driven by the atmosphere rather than the opposite over there. Two interannual variability modes of such negative correlation relationship are revealed with SVD method, called tropical and subtropical WNP air‐sea coupled modes (WNP‐TCM and WNP‐STCM) in terms of their locations accordingly. Their core regions are both consistent with the near‐zero line of low‐level zonal wind, corresponding to the Northwest Pacific monsoon trough and the west ridge of WNP subtropical high, respectively. In WNP‐TCM, precipitation anomaly is caused by local vorticity anomaly, which is the result of Matsuno‐Gill response to SST anomalies with the reversed signs in central equatorial Pacific and eastern equatorial Indian Ocean. In WNP‐STCM, it is produced by local divergence anomaly, which is remotely forced by SST anomalies with the same signs in equatorial Pacific and Indian Ocean through anomalous Hadley circulation. SST anomalies in the two modes are both controlled by ocean thermal process, especially the solar radiation anomaly. The key factors determining such unique WNP air‐sea interaction are the weak latent heat flux and horizontal SST advection anomalies, which are both closely related to the near‐zero low‐level zonal wind. In addition, WNP‐TCM often happens in ENSO developing summer whereas WNP‐STCM usually appears in ENSO decaying summer.

A Large Subsurface Anticyclonic Eddy in the Eastern Equatorial Indian Ocean

AbstractSubsurface eddies are pervasive in the ocean, but those in the eastern equatorial Indian Ocean are not well understood due to the lack of in‐situ observations. Using multiple‐platform observations and reanalysis product, we investigate the characteristics, evolutionary process, origin, and underlying generation mechanism of a subsurface anticyclonic eddy (SAE) in the eastern equatorial Indian Ocean during January‒February 2020. Results show that the horizontal radius of the SAE was slightly greater than 250 km, and the SAE core was located at ∼100‐m depth near the 24.7σ0 isopycnal. This warm, saline and oxygen‐rich SAE was weakly stratified and characterized by tilted lens structures. Further water‐mass tracing analysis indicates that the SAE shifted westward along 4°N with a mean propagation speed of 0.18 m/s from the northwest coast of Sumatra Island. The hydrographic characteristics of the SAE are similar to those of the strong Equatorial Undercurrent, it is speculated that the SAE was probably generated by the north branch flow as the eastward undercurrent reached Sumatra Island. Both lateral shear of undercurrent and conservation of potential vorticity were attributed to the formation of the observed SAE.

Mixed layer heat budget in the eastern equatorial Indian Ocean during the two consecutive positive Indian Ocean dipole events in 2018 and 2019

Mixed layer heat budget in the eastern equatorial Indian Ocean during the two consecutive positive Indian Ocean dipole events in 2018 and 2019

Biotic responses of deep-sea benthic foraminifera in the equatorial Indian Ocean during the Quaternary: Influence of the ballasting effect on organic matter by calcareous plankton skeletons

We investigated Quaternary benthic foraminifera in core GPC1803 in the eastern equatorial Indian Ocean since ~740 ka to examine the relationship between benthic foraminiferal fauna and ballasting of particulate organic matter (POM) by calcareous plankton skeletons. Because plankton skeletons affect POM sinking in the water column as ballasts, ballasting of POM by biominerals may play an important role to linkage between the surface and deep-sea paleoceanography. We consider this scheme, based on faunal analysis of benthic foraminifera. From multidimensional scaling (MDS) of fossil benthic foraminifera, we recognized two MDS axes. Based on the scores of MDS axis 1, we subdivided our study period into five intervals and identified two major faunal transitions at ~370 ka and ~ 210 ka. After ~210 ka, the food supply to the seafloor increased with an increase in the past flux of calcareous plankton skeletons. A positive correlation between the benthic foraminifera accumulation rate and the carbonate mass accumulation rate existed during ~600–370 ka and after ~210 ka. It is reasonable to suppose that the increase in calcareous plankton skeletons stimulated more efficient vertical transport of POM through the water column due to the positive ballasting effect on POM. The ~370–210 ka period seemed to represent a transitional phase in food supply. Prior to ~370 ka, alternations among three seasonal food supply-related species were generally observed along with glacial–interglacial variations probably under lower food supply, compared with the after ~210 ka period; however, alternations among these three species became obscure and intermittent after ~370 ka. The nearly continuous common occurrences of these seasonal food supply-related species before ~370 ka might have been enhanced by a lower past flux of calcareous plankton skeletons. Scenario of ballasting of POM by biomineral might explain long-term changes of carbonate corrosivity in the Indian Ocean, similarly in the Atlantic and eastern equatorial Pacific Oceans, during the mid-Brunhes dissolution interval. • Quaternary deep-sea benthic foraminifera in the equatorial Indian Ocean • Positive relationship between BFAR and carbonate MAR after ~210 ka • Common occurrences of seasonal food supply-related species before ~370 ka • Changes in the influence of ballasting of POM by calcareous plankton

Impact of Indian Ocean surface temperature gradient reversals on the Indian Summer Monsoon

Indian Summer Monsoon (ISM) precipitation is the main determinant of livelihood in a densely populated world region. The interannual variability of the ISM is influenced by several modes of climate variability, including anomalous seasonal sea surface temperature (SST) gradient reversals between the eastern, western, and northeastern Indian Ocean. With global warming, the frequency of zonal and meridional Indian Ocean's SST gradient changes is projected to increase but its impact on the ISM is debated. Here we present a 25,000-year proxy record of SST and inferred Ganges-Brahmaputra-Meghna (GBM) River runoff that provides a spatially integrated measure of ISM precipitation changes. This record indicates a monotonic deglacial strengthening of the ISM system when the SST gradient between the Bay of Bengal surface water and the eastern equatorial Indian Ocean was reversed. We posit that the reversal in the meridional SST gradient reduced the impact of Heinrich Event 1 and Younger Dryas on the low elevation part of the ISM domain. Furthermore, the proxy record shows that the strongest Holocene ISM strengthening occurred between 7900±470 and 5700±360 years before present, coinciding with and causally linked to the reversal of the Indian Ocean zonal SST gradient and ensuing changes in the wind fields, a sequence of events that is inferred from and supported by the results of our climate simulation. Our study demonstrates that changes in the Indian Ocean's zonal and meridional thermal gradient strongly shaped the timing of Holocene monsoon strengthening and the response of ISM to the last deglacial freshwater forcing.

Simulation of Indian summer monsoon rainfall, interannual variability and teleconnections: evaluation of CMIP6 models

We analyse the performance of global climate models of 6 $$\mathrm{th}$$ generation of Coupled Model Intercomparison Project (CMIP6) in simulating climatological summer monsoon rainfall over India, interannual variability (IAV) of all-India summer monsoon rainfall (ISMR) and its teleconnections with rainfall variability over equatorial Pacific and Indian Oceans. The multimodel ensemble mean (MME) of 61 CMIP6 models shows the best skill in simulating mean monsoon rainfall over India compared to the MMEs of 6 $$\mathrm{th}$$ generation atmosphere-only models (AMIP6) and the previous generations of Atmospheric and Coupled Model Intercomparison Projects (AMIPs and CMIPs). Systematic improvement and reduction in bias are evident from lower to higher AMIPs/CMIPs. Still, there exists dry bias over a narrow region of the monsoon zone of central India besides wet and cold bias over the surrounding oceans. The persistence of errors in atmosphere-only models hints that the source of errors could be with atmosphere models. Fifteen CMIP6 models selected through objective criteria, perform the best in simulating mean monsoon, IAV of ISMR, the strong inverse relationship between ISMR and Boreal summer El Niño-Southern Oscillation (ENSO), and the inverse relationship between all-India rainfall and north–west tropical Pacific rainfall in June. Several models reproduce the dipole structure of Equatorial Indian Ocean Oscillation (EQUINOO) with the centres over western and eastern equatorial Indian Ocean. But, ISMR-EQUINOO relationship in many of them is opposite to the observed. Our analysis implies the need for capturing ISMR-EQUINOO link to improve the simulation of IAV of ISMR which is crucial for reliable monsoon prediction and projection.

Surface paleoceanography of the eastern equatorial Indian Ocean since the latest Miocene: Foraminiferal census and isotope records from ODP Hole 758A

The surface hydrography of the Indian Ocean (north of the equator) is influenced by the seasonal wind reversals, which affect the thermocline depth and mixed layer thickness as well as sea surface temperature (SST). We analyzed the surface-subsurface hydrographic changes in the Tropical Eastern Indian Ocean (TEIO) during the late Neogene and Quaternary, using planktic foraminiferal census data from ODP Hole 758A combined with surface-dwelling planktic foraminiferal stable oxygen and carbon isotope ratios. The occurrence of dominant thermocline-dwelling species and high surface productivity during the latest Miocene - Early Pliocene (between 6 and 3.4 Ma) reflects an intense Indian summer monsoon (ISM). Deepening of the thermocline and thickening of the mixed layer in the TEIO were driven by the weakening of the ISM after 3.4 Ma, causing fluctuations in surface productivity. Heat budget capacity increased after 3.4 Ma in the TEIO due to the thickening of the mixed layer, which may have also reduced the heat transport towards the South Pole. Further, the hydrography of the TEIO was affected by the advection of nutrient-rich saline water from the western Indian Ocean by the strong equatorial eastward flow between 2.7 and 1.2 Ma. We suggest that the development of the equatorial zonal Westerlies (Walker circulation) across the tropical Indian Ocean was a gradual process starting at ~3.4 Ma, which intensified at 2.7 Ma due to regional strengthening of the Indian winter monsoon.