Enter Zoom Meeting

OS1.9

EDI
Understanding the Indian Ocean’s past, present and future role in climate variability and predictability

The Indian Ocean is unique among the other tropical ocean basins due to the seasonal reversal of monsoon winds and concurrent ocean currents, lack of steady easterlies that result in a relatively deep thermocline along the equator, low-latitude connection to the neighboring Pacific and a lack of northward heat export due to the Asian continent. These characteristics shape the Indian Ocean’s air-sea interactions, variability, as well as its impacts and predictability in tropical and extratropical regions on (intra)seasonal, interannual, and decadal timescales. They also make the basin particularly vulnerable to anthropogenic climate change, as well as related extreme weather and climate events, and their impacts for surrounding regions, which are home to a third of the global population. Advances have recently been made in our understanding of the Indian Ocean’s circulation, interactions with adjacent ocean basins, and its role in regional and global climate. Nonetheless, significant gaps remain in understanding, observing, modeling, and predicting Indian Ocean variability and change across a range of timescales.

This session invites contributions based on observations, modelling, theory, and palaeo proxy reconstructions in the Indian Ocean that focus on recent observed and projected changes in Indian Ocean physical and biogeochemical properties and their impacts on ecological processes, diversity in Indian Ocean modes of variability (e.g., Indian Ocean Dipole, Indian Ocean Basin Mode, Madden-Julian Oscillation) and their impact on predictions, interactions and exchanges between the Indian Ocean and other ocean basins, as well as links between Indian Ocean variability and monsoon systems across a range of timescales. In particular, we encourage submissions on weather and climate extremes in the Indian Ocean, including marine heatwaves and their ecological impacts. We also welcome contributions that address research on the Indian Ocean grand challenges highlighted in the recent IndOOS Decadal Review, and as formulated by the Climate and Ocean: Variability, Predictability, and Change (CLIVAR), the Sustained Indian Ocean Biogeochemistry and Ecosystem Research (SIBER), the International Indian Ocean Expedition 2 (IIOE-2), findings informed by the Coupled Model Intercomparison Project version 6 (CMIP6) on past, present and future variability and change in the Indian Ocean climate system, and contributions making use of novel methodologies such as machine learning.

Co-organized by BG4/CL2
Convener: Caroline Ummenhofer | Co-conveners: Alejandra Sanchez-FranksECSECS, Peter SheehanECSECS, Yan Du, Muhammad Adnan AbidECSECS, Chunzai Wang, Stephanie A. HendersonECSECS, Roxy Mathew Koll, Cheng Sun
Presentations
| Thu, 26 May, 11:05–11:50 (CEST), 13:20–14:50 (CEST)
 
Room 1.15/16

Thu, 26 May, 10:20–11:50

Chairpersons: Alejandra Sanchez-Franks, Muhammad Adnan Abid

11:05–11:08
Introduction

11:08–11:13
|
EGU22-7416
|
Virtual presentation
Michael Mayer et al.

Accurate forecasts of tropical Indian Ocean variability are crucial for skilful predictions of climate anomalies on a range of spatial and temporal scales. Here we assess the ability of ECMWF’s operational monthly and seasonal prediction systems to represent variability in the Eastern Equatorial Indian Ocean (EEIO), an important center of action especially for the Indian Ocean Dipole (IOD) Mode. Strong air-sea coupling is present in this region. In ECMWF’s currently operational seasonal prediction system, this leads to rapid amplification of a weak cold bias of the oceanic initial conditions in the EEIO, resulting in too frequent occurrences of positive IOD events. Diagnostics show that this is related to winds in the EEIO exhibiting a biased relationship with local and remote sea surface temperatures when compared to reanalysis. The impact of the forecast bias in the EEIO on the skill of ENSO predictions via interbasin interactions is evaluated. We furthermore present results from numerical experiments with, i.a., changed atmospheric model physics and oceanic initial conditions which help to better understand causes of the diagnosed forecast errors as well as mechanisms of interbasin interaction, and provide guidance for model development.

How to cite: Mayer, M., Alonso Balmaseda, M., and Johnson, S.: Understanding and reducing seasonal prediction errors of the ECMWF system in the tropical Indian Ocean, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7416, https://doi.org/10.5194/egusphere-egu22-7416, 2022.

11:13–11:18
|
EGU22-9751
|
ECS
Yue Yu

In recent decades, worldwide marine heat wave events have become stronger and more frequent. Especially in the Indian Ocean, where occurs the most significant sea surface temperature warming trend. We use observation and reanalysis data to extract the Indian Ocean marine heatwave events since 1981. And then analyzing the temporal and spatial characteristics of marine heatwave events through feature indicators. According to the different period of the development of the marine heatwave, the sources of predictability from the atmospheric and ocean circulation anomaly are revealed. Then five representative heat wave events will be selected, and multi-member ensemble hindcast with different lead times will be conducted for each event with CESM2 model. Based on the hindcast results, we evaluate the prediction skills for the Indian Ocean marine heatwaves. The capability of models to simulate the sub-seasonal to seasonal signals that affect the heat wave event will be examined eventually.

How to cite: Yu, Y.: The subseasonal to seasoanl predictability of marine heatwave in Indian Ocean, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9751, https://doi.org/10.5194/egusphere-egu22-9751, 2022.

11:18–11:23
|
EGU22-4355
|
ECS
Eddy advection and the evolution of Mixed layer properties: a case study at Southeast Tropical Indian Ocean
(withdrawn)
Marina Azaneu et al.
11:23–11:28
|
EGU22-109
|
ECS
|
|
On-site presentation
Estel Font et al.

High-resolution underwater glider data collected in the Gulf of Oman (2015-16), combined with reanalysis datasets, describe the spatial and temporal variability of the mixed layer during winter and spring. We assess the effect of surface forcing and submesoscale processes on upper ocean buoyancy and their effects on mixed layer stratification. Episodic strong and dry wind events from the northwest (Shamals) drive rapid latent heat loss events which lead to intraseasonal deepening of the mixed layer. Comparatively, the prevailing southeasterly winds in the region are more humid, and do not lead to significant heat loss, thereby reducing intraseasonal upper ocean variability in stratification. We use this unique dataset to investigate the presence and strength of submesoscale flows, particularly in winter, during deep mixed layers. These submesoscale instabilities act mainly to restratify the upper ocean during winter through mixed layer eddies. The timing of the spring restratification differs by three weeks between 2015 and 2016 and matches the sign change of the net heat flux entering the ocean and the presence of restratifying submesoscale fluxes. These findings describe key high temporal and spatial resolution drivers of upper ocean variability, with downstream effects on phytoplankton bloom dynamics and ventilation of the oxygen minimum zone.

How to cite: Font, E., Y. Queste, B., and Swart, S.: Seasonal to intraseasonal variability of the upper ocean mixed layer in the Gulf of Oman, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-109, https://doi.org/10.5194/egusphere-egu22-109, 2022.

11:28–11:33
|
EGU22-3453
YiXuan Li and Xidong Wang

Based on SODA reanalysis data set from 1980 to 2016, this paper combined with a variety of mathematical statistical methods to study the intraseasonal variability characteristics of barrier layer thickness and its physical correlation with climate modes in the Bay of Bengal, and quantitatively explored the dynamic mechanism of intraseasonal variability of barrier layer in different sea areas in the Bay of Bengal by means of Marine dynamic diagnosis method. The relative contributions of different physical processes, such as oceanic advection, Kelvin waves, Rossby waves and freshwater fluxes (rainfall and river runoff), to the barrier layer were evaluated. The physical relationship between the seasonal variation of barrier layer thickness and the Indian Ocean dipole (IOD) is also discussed. The results show that the thickness of the barrier layer varies most obviously in the northern coast of the bay of Bengal and the western coast of Sumatra, and the maximum value of the barrier layer occurs in November ~ December every year, while the variation of the barrier layer in the northern coast is more regular than that in the southern coast. Horizontal advance and entrainment affect the thickness of barrier layer by affecting the salinity of the mixed layer. However, the thickness of barrier layer is mainly caused by the change of isothermal layer due to the obvious stratification of sea surface salinity in the Bay of Bengal. In the southern part of the Bay of Bengal near the equator, during the positive IOD events, the isothermal layer shallowness was caused by the negative anomaly of equatorial zonal wind stress from October to December. In negative IOD events, the equatorial zonal wind stress appears positive abnormality after June, which leads to the increase of isothermal layer in this period. As a result, the thickness of barrier layer In positive IOD years is smaller than that in normal years from October to December, and that in negative IOD years is greater than that in normal years from June to September. However, in the northern Bay of Bengal, the seasonal variability of barrier layer caused by different IOD events was not obvious. At the same time, the net heat flux upward at the air-sea interface will lead to instability and deepen the local mixed layer.

How to cite: Li, Y. and Wang, X.: Intraseasonal Variability in Barrier Layer Thickness in the Bay of Bengal and its Causes, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3453, https://doi.org/10.5194/egusphere-egu22-3453, 2022.

11:33–11:38
|
EGU22-2149
|
ECS
|
|
Virtual presentation
Shanshan Pang et al.

This study finds that the winter (December–February) barrier layer (BL) in the Bay of Bengal (BoB) acts as a dynamical thermostat, modulating the subsequent summer BoB SST variability and potentially affecting the Indian summer monsoon (ISM) onset and associated rainfall variability. In the years when the prior winter BL is anomalously thick, anomalous sea surface cooling caused by intensified latent heat flux loss appears in the BoB starting in October and persists into the following year by positive cloud–SST feedback. During January–March, the vertical entrainment of warmer subsurface water induced by the anomalously thick BL acts to damp excessive cooling of the sea surface caused by atmospheric forcing and favors development of deep atmospheric convection over the BoB. During March–May, the thinner mixed layer linked to the anomalously thick BL allows more shortwave radiation to penetrate below the mixed layer. This tends to maintain existing cold SST anomalies, advancing the onset of ISM and enhancing June ISM precipitation through an increase in the land–sea tropospheric thermal contrast. We also find that most CMIP5 models fail to reproduce the observed relationship between June ISM rainfall and the prior winter BL thickness. This may be attributable to their difficulties in realistically simulating the winter BL in the BoB and ISM precipitation. The present results indicate that it is important to realistically capture the winter BL of the BoB in air–sea coupled models for improving the simulation and prediction of ISM.

How to cite: Pang, S., Wang, X., Foltz, G. R., and Fan, K.: Contribution of the Winter Salinity Barrier Layer to Summer Ocean–Atmosphere Variability in the Bay of Bengal, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2149, https://doi.org/10.5194/egusphere-egu22-2149, 2022.

11:38–11:43
|
EGU22-656
|
Virtual presentation
Mohammed Salim Poovadiyil et al.

According to Food and Agriculture Organization (FAO), the fisheries sector is a major contributor to coastal economy, ensuring nutritional security and generating employment opportunities is the central Indian Ocean covering Lakshadweep (India), Maldives and Sri Lanka. Harvesting of fish in this region happens mainly in coastal waters up to 100m depth. The fishing pressure on the stock in these waters has increased? Considerably and the deep-sea fishery has become an area for expansion in developed countries (FAO). However, fisheries in high seas pose scientific and technical challenges. High value fish are strongly influenced by the physical environment such as temperature, currents etc. Being able to predict this environment with high degree of accuracy is an invaluable tool for assisting on this expansion.

In order to help forecast the physical environment in the Lakshadweep Sea at medium to high resolutions we have developed two pre-operational data assimilating models at 1/20 (called LD20) and 1/60 (LD60) degrees of resolution based on with NEMO v3.6 as an engine. Both models have 50 geopotential computational levels with full steps in the vertical, they use Smagorinsky scheme for horizontal diffusion, bi-Laplacian viscosity for momentum, and k −epsilon turbulence scheme. The models use time-splitting algorithm with the ratio of baroclinic to barotropic time steps equal to 20. The Galperin parametrization is used to preserve the stratification. The models take initial and boundary conditions as well as data for assimilation from a global model at 1/12 degree resolution available from EU Copernicus Marine Service (CMEMS). The bathymetry is taken from GEBCO_2021. Meteorological forcing comes from the Met Office global model (NWPn768 and NWPn1280), and the tides are forced using OTIS tidal scheme (https://www.tpxo.net/otis). Both models run within Rose/Cylc software environment (https://metomi.github.io/rose/2019.01.2/html/index.html), a toolkit for orchestrating the running models that automatically executes tasks according to their schedules and dependencies.

The LD20 and LD60 models use a novel model-to-model data assimilation scheme (Shapiro and Ondina, 2021) by which the observations are assimilated indirectly, via a data assimilating parent model (CMEMS for LD20 and LD20 for LD60). The models have been run for 5 years from 01.01.2015. As expected, the models reveal more granularity of temperature, salinity and currents, particularly in the coastal areas. The model skill was assessed against The Operational Sea Surface Temperature and Ice Analysis (OSTIA) system. The results show improvement of the bias and Root-mean-square-error in the higher-resolution models compared to the lower-resolution ones. The model outputs can be helpful in the identification of small-scale ocean fronts which are linked to Potential Fishing Zones (Solanki et al, 2005)

References

Shapiro, GI. and Gonzalez-Ondina, JM., 2021. Model-to-model data assimilation method for fine resolution ocean modelling, Ocean Sci. Discuss. https://doi.org/10.5194/os-2021-77, in review.

Solanki HU, Mankodi PC, Nayak SR, Somvanshi VS. 2005. Evaluation of remote-sensing-based potential fishing zones (PFZs) forecast methodology. Continental Shelf Research. 25, (18):2163–2173

How to cite: Poovadiyil, M. S., Ondina, J. M. G., Tu, J., Asif, M., and Shapiro, G. I.: Pre-operational high-resolution ocean models of the Lakshadweep Sea (Indian Ocean), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-656, https://doi.org/10.5194/egusphere-egu22-656, 2022.

11:43–11:50
Q & A

Thu, 26 May, 13:20–14:50

Chairpersons: Caroline Ummenhofer, Peter Sheehan

13:20–13:23
Introduction

13:23–13:28
|
EGU22-4358
|
ECS
|
Virtual presentation
Ying Zhang et al.

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.

How to cite: Zhang, Y., Du, Y., Feng, M., and Hu, S.: Long-Lasting Marine Heatwaves Instigated by Ocean Planetary Waves in the Tropical Indian Ocean During 2015–2016 and 2019–2020, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4358, https://doi.org/10.5194/egusphere-egu22-4358, 2022.

13:28–13:33
|
EGU22-5055
|
Virtual presentation
Yan Du et al.

The tropical Indian Ocean (TIO) basin-wide warming occurred in 2020, following an extreme positive Indian Ocean Dipole (IOD) event instead of an El Niño event, which is the first record since the 1960s. The extreme 2019 IOD induced the oceanic downwelling Rossby waves and thermocline warming in the southwest TIO, leading to sea surface warming via thermocline-SST feedback during late 2019 to early 2020. The southwest TIO warming triggered equatorially antisymmetric SST, precipitation, and surface wind patterns from spring to early summer. Subsequently, the cross-equatorial “C-shaped” wind anomaly, with northeasterly–northwesterly wind anomaly north–south of the equator, led to basin-wide warming through wind-evaporation-SST feedback in summer.

The TIO warming excited a strong and westward extend anomalous anticyclone on the western North Pacific (WNPAC). The WNPAC is usually associated with strong El Niño-Southern Oscillation (ENSO), except for the 2020 case. In 2020, the anomalous winds in the northwestern flank of the WNPAC bring excess water vapor into central China. The water vapor, mainly carried from the western tropical Pacific, converges in central China and result in heavy rainfall. Unlike extreme events in 1983, 1998, and 2016, the extreme rainfall in 2020 was the first and only event during 1979-2020 that followed an extreme positive IOD rather than a strong El Niño. A theory of regional ocean-atmosphere interaction can well explain the processes, called the Indo-Western Pacific Ocean Capacitor (IPOC) effect. This study reveals the importance of IOD in the IPOC effect, which can dramatically influence the East Asian climate even without involving the ENSO in the Pacific.

How to cite: Du, Y., Cai, Y., Chen, Z., and Zhang, Y.: Extreme IOD induced IOB warming and its impacts on western North Pacific anomalous anticyclonic circulation transport in early summer 2020: without significant El Nino influence, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5055, https://doi.org/10.5194/egusphere-egu22-5055, 2022.

13:33–13:38
|
EGU22-6704
|
ECS
|
Virtual presentation
Lianyi Zhang and Yan Du

The El Niño-Southern Oscillation (ENSO) has great impacts on the Indian Ocean sea surface temperature (SST). In fact, two major modes of the Indian Ocean SST namely the Indian Ocean Basin (IOB) and Indian Ocean Dipole (IOD) modes, exerting strong influences on the IO rim countries, are both influenced by the ENSO. Based on a combined linear regression method, this study quantifies the ENSO impacts on the IOB and IOD during ENSO concurrent, developing, and decaying stages. After removing the ENSO impacts, the spring peak of the IOB disappears along with significant decrease in number of events, while the number of events is only slightly reduced and the autumn peak remains for the IOD. By isolating the ENSO impacts during each stage, this study reveals that the leading impacts of ENSO contribute to the IOD development, while the delayed impacts facilitate the IOD phase switch and prompt the IOB development. Besides, the decadal variations of ENSO impacts are various during each stage and over different regions. These imply that merely removing the concurrent ENSO impacts would not be sufficient to investigate intrinsic climate variability of the Indian Ocean, and the present method may be useful to study climate variabilities independent of ENSO.

How to cite: Zhang, L. and Du, Y.: Revisiting ENSO impacts on the Indian Ocean SST based on a combined linear regression method, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6704, https://doi.org/10.5194/egusphere-egu22-6704, 2022.

13:38–13:43
|
EGU22-3397
|
On-site presentation
Tamas Bodai et al.

“Decadal influence” on the El Nino--Southern Oscillation-Indian summer monsoon (ENSO-ISM) teleconnection have been much studied but with plurality and ambiguity about the concept of influence. We provide formal definitions of the apparent influence of a specific factor which enable us to test them as null-hypotheses. Using the recently released Community Earth System Model v2 (CESM2) Large Ensemble (LE) data, we show that a 50% chance for the detection of the apparent Indian Ocean (IO) influence under stationary conditions might take 2000 years of data. However, we find that this influence is mostly apparent indeed, as it originates from fluctuations of the decadal apparent -- as opposed to climatological -- ENSO variability, which causally influences an IOD-like apparent mean state. We also show that no unattributed so-called “decadal influence”, reflected in a deviation from a linear regression model of the teleconnection as a null-hypothesis, can be detected in 20th c. observations even regionally.  Only the LE data is sizable enough to reveal this effect.

How to cite: Bodai, T., Sundaresan, A., Lee, J.-Y., and Lee, S.-S.: Indian Ocean influence on the ENSO-Indian monsoon teleconnection is mostly apparent , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3397, https://doi.org/10.5194/egusphere-egu22-3397, 2022.

13:43–13:48
|
EGU22-9122
|
On-site presentation
Fred Kucharski et al.

The role of the Indian Ocean heating anomalies in the ENSO teleconnection to South Asia and North Atlantic/European regions are investigated in the early winter season. Using re-analysis data, CMIP5 simulations and idealized numerical model experiments it is shown that the ENSO teleconnections in early winter in these regions are dominated by an ENSO-induced heating dipole in the Indian Ocean region. The Indian Ocean heating dipole leads to a Gill-type response in the South Asian region through Sverdrup balance. For a warm ENSO event, this response is a cyclonic upper-level anomaly that shifts the subtropical South Asian jet southward and increases precipitation in the that region. The cyclonic anomaly is the starting point of a stationary Rossby wavetrain that traverses the North Pacific and North American region and eventually reaches the North Atlantic. Here transient eddy feedbacks are likely to strengthen a response that spatially projects on the positive phase of the NAO and negative phase of the Atlantic ridge patterns. For cold ENSO events these anomalies are roughly opposite. The importance of the Indian Ocean heating dipole decreases towards late Winter due to a southward shift of the Indian Ocean rainfall climatology and a more dominant direct wavetrain from the central Pacific region.

How to cite: Kucharski, F., Abid, M. A., Joshi, M. K., Ashfaq, M., and Evans, K. J.: Role of Indian Ocean heating anomalies in the early winter ENSO teleconnection to the South Asian and North Atlantic regions, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9122, https://doi.org/10.5194/egusphere-egu22-9122, 2022.

13:48–13:53
|
EGU22-6567
|
ECS
|
On-site presentation
Muhammad Adnan Abid et al.

In the current study, we analyzed the predictability of the tropical Indian Ocean precipitation anomalies and the North Atlantic European (NAE) circulation anomalies during the boreal early winter season using the ECMWF System-5 seasonal (SEAS5) prediction dataset. The observational analysis show that the boreal Autumn Indian Ocean dipole (IOD) conditions are the pre-courser for the early winter precipitation anomalies in the Tropical Western-Central Indian Ocean (TWCIO) region, which is well represented in the ECMWF-SEAS5 prediction system. Moreover, the ECMWF-SEAS5 skillfully predicts the Indian Ocean (IO) precipitation anomalies with some biases during the early winter. These biases tend to weaken the IO teleconnections to the NAE Region during the boreal early winter, mimicking the prediction skill of the NAE circulation anomalies. Furthermore, the positive TWCIO heating anomalies tend to favor the above normal Surface Air temperature (SAT) conditions over the NAE region, indicating to the mild early winter conditions over the region. The ECMWF-SEAS5 system shows a significant prediction skill of the surface temperature anomalies over the NAE region.

How to cite: Abid, M. A., Kucharski, F., and Molteni, F.: Predictability of the Indian Ocean and North Atlantic European circulation anomalies during early winter, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6567, https://doi.org/10.5194/egusphere-egu22-6567, 2022.

13:53–13:58
|
EGU22-5207
|
|
Virtual presentation
Chao-jie Du and Dao-yi Gong

In this study, the possible associations between the precipitation in the Southeastern Africa (SEAF, in this study area between 10°S to 25°S and 25°E to 53°E,) and the Antarctic Oscillation (AAO) in seasons from October to March (DJFM) was investigated. A statistically significant three-month lag correlation between them was found. After removing the El Niño/Southern Oscillation and Indian Ocean dipole signals, AAO from August to October (ASO-AAO) and DJFM-precipitation was significantly correlated, and the interannual correlation coefficients calculated by CMAP, GPCP, CRU, and GPCC were +0.63, +0.42, +0.59, and +0.53 (p<0.05), respectively. The positive correlation suggests that an enhancing (weakening) ASO‐AAO could be conducive to increases (decreases) of DJFM-precipitation in SEAF in austral summer. Further analyze the corresponding water vapor and circulation conditions. The responses of local and regional meteorological conditions to the ASO‐AAO support the AAO-precipitation links. During positive ASO-AAO years, in the troposphere low level is a cyclonic flow field in the high level is an anticyclonic circulation, accompanied by an enhanced ascending motion, and such a structure is favor to rainfall. A preliminary mechanism analysis shows that a positive ASO-AAO may induce a sea surface temperature warming tendency in Western Equatorial Indian Ocean.  This warming then enhances the regional ascending motion in SEAF and enhances the convection precipitation on the northwest SEAF. Moreover, the anomalous sensible and latent heating, in turn, intensifies the cyclone through a Gill-type response of the atmosphere. Through this positive feedback, the tropical atmosphere and SST patterns sustain their strength from spring to summer and eventually the SEAF precipitation.Note that’s for simplicity, the AAO index was multiplied by −1 throughout this study.

How to cite: Du, C. and Gong, D.: Influence of Antarctic Oscillation on the Southeastern Africa summer precipitation during 1979-2018, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5207, https://doi.org/10.5194/egusphere-egu22-5207, 2022.

13:58–14:03
|
EGU22-4578
|
Virtual presentation
Frederick Bingham et al.

The sea surface salinity (SSS) maximum of the South Indian Ocean (the SISSS-max) is a large, oblong, high-salinity feature centered at 30degS, 90degE, at the center of the South Indian subtropical gyre. It is located poleward of a region of strong evaporation and weak precipitation. Using a number of different satellite and in situ datasets, we track changes in this feature since the beginning of the Argo era in the early 2000's. The centroid of the SISSS-max moves seasonally north and south, furthest north in late winter and farthest south in late summer. Interannually, the SISSS-max has moved on a northeast-southwest path about 1500 km in length. The size and maximum SSS of the feature vary in tandem with this motion. It gets larger (smaller) and saltier (fresher) as it moves to the northeast (southwest) closer to (further from) the area of strongest surface freshwater flux. The area of the SISSS-max almost doubles from its smallest to largest extent. It was maximum in area in 2006, decreased steadily until it reached a minimum in 2013, and then increased again. The seasonal variability of the SISSS-max is controlled by the changes that occur on its poleward, or southern, side, whereas intereannual variability is controlled by changes on its equatorward side. The variations in the SISSS-max are a complex dance between changes in evaporation, precipitation, wind forcing, gyre-scale ocean circulation and downward Ekman pumping. Its motion correlated with SSS changes throughout the South Indian Ocean and is a sensitive indicator of changes in the basin's subtropical circulation.

How to cite: Bingham, F., Gordon, A., and Brodnitz, S.: Seasonal and Interannual Variability of the South Indian Ocean Sea Surface Salinity Maximum, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4578, https://doi.org/10.5194/egusphere-egu22-4578, 2022.

14:03–14:08
|
EGU22-1749
|
ECS
|
|
On-site presentation
Marie Montero et al.

The Bay of Bengal is under the influence of the monsoon and has a highly contrasted and variable Sea Surface Salinity (SSS). In situ salinity data is however too sparse to reconstruct interannual SSS variability of the Bay of Bengal prior to synoptic SSS mapping of SMOS launched in 2009.

Previous studies have demonstrated the ability of X minus C-band measurements, such as those of AMSR-E (May 2002-Oct 2011), to track SSS changes in high-contrast regions and at high Sea Surface Temperature (SST). Here, we apply this approach to reconstruct the Bay of Bengal SSS before 2010. We remove the effects of other geophysical variables such as SST, surface wind, and atmospheric water content using an empirical approach. SSS is then retrieved based on another empirical fit, trained on the ESA Climate Change Initiative (CCI) SSS dataset, over the AMSR-E and CCI common period (Jan 2010 to Oct 2011). Our first results are encouraging: spatial contrast between the low post-monsoon SSS values close to estuaries and along the west coast of India are reproduced. Our algorithm, however, tends to overestimate low SSS and underestimate high SSS values, possibly due to data contamination near the coast and/or a suboptimal removal of the signals from other geophysical variables. Nevertheless, the first results show a correct representation of the recognizable Indian Ocean Dipole (IOD) phenomena. Furthermore, we are currently creating and studying the use of a neuronal network with the intention to include more parameters in the algorithm.

The long-term goal of this work is to merge the C-, X-, and L-band data with in-situ measurements thus providing a long-term reconstruction of monthly SSS in the Bay of Bengal with a ~50 km resolution This dataset will be used to explore the physical processes that drive interannual SSS variability in regions where it is strong, such as near major river estuaries or along the west coast of India.

How to cite: Montero, M., Reul, N., de Boyer Montégut, C., Vialard, J., and Tournadre, J.: Towards long-term (2002-present) reconstruction of northern Indian Ocean Sea Surface Salinity based on AMSR-E and L-band Radiometer data, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1749, https://doi.org/10.5194/egusphere-egu22-1749, 2022.

14:08–14:13
|
EGU22-6852
|
ECS
|
Highlight
|
|
Virtual presentation
Sahil Sharma et al.

The weakening of zonal atmospheric circulation, a widely accepted projection of climate change in response to global warming, features a weakening of the Indian Ocean Walker circulation (IWC), with an anomalous ascending motion over the western and anomalous descending motion over the eastern Indian Ocean.  The projected IWC weakening has previously been attributed to slower warming in the east than the west, that is, to a positive Indian Ocean Dipole (IOD)-like warming pattern.  However, such a warming pattern can also be induced by IWC weakening. As a result, the cause-and-effect relationship cannot be easily determined, and the projected change is poorly constrained and highly uncertain. Here, using a suite of coupled climate model simulations under a high-emission scenario, we find that the IWC slowdown is accompanied by not only a positive IOD-like warming pattern but also anomalous meridional circulation that is associated with anomalous descending motion over the eastern Indian Ocean. We further show that the anomalous local meridional circulation is closely linked to enhanced land-sea thermal contrast and is unlikely to result from the positive IOD-like warming pattern, suggesting that the IWC weakening is in part driven by the anomalous local meridional circulation. Our findings underscore the important role of local meridional circulation changes in modulating future IWC changes. 

How to cite: Sharma, S., Ha, K. J., Cai, W., Chung, E.-S., and Bódai, T.: Local meridional circulation changes contribute to a projected slowdown of the Indian Ocean Walker circulation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6852, https://doi.org/10.5194/egusphere-egu22-6852, 2022.

14:13–14:18
|
EGU22-10745
|
ECS
|
|
Virtual presentation
Badarvada Yadidya and Ambarukhana Devendra Rao

The Andaman Sea, located in the Indian Ocean's northeastern region, is well known for its large-amplitude internal waves. The Indian Ocean Dipole, according to recent research, has a significant impact on the interannual variability of density stratification and internal wave activity in this region. The global climate model CanESM5 has demonstrated a reasonable ability to capture the variability of the Indian Ocean Dipole in its historical simulations. As a result, the long-term variability of internal waves is investigated using the CanESM5 density stratification. The stratification showed an increasing trend in the upper 100 m since 1900 due to radiative forcing. Internal wave activity is expected to increase in the twenty-first century, altering the effects of climate change on coastal ecosystems. Additionally, model simulations utilizing the three-dimensional Massachusetts Institute of Technology general circulation model are conducted to investigate the impact of increasing stratification on internal tides. Variations in the generation, propagation, and dissipation of internal tides along with their basic characteristics are quantified.  

How to cite: Yadidya, B. and Devendra Rao, A.: The effect of climate change on internal wave activity in the Andaman Sea , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10745, https://doi.org/10.5194/egusphere-egu22-10745, 2022.

14:18–14:23
|
EGU22-4602
|
ECS
Chetankumar Jalihal et al.

Upwelling along the western boundary of the Arabian Sea and the Indian summer monsoon rainfall are positively correlated in modern observations. Upwelling transports nutrients into the euphotic zone and thus controls primary productivity. Therefore, primary productivity in the region of upwelling has been used to reconstruct monsoons of the distant past. Such reconstructions suggest that monsoons lag insolation by about 9 kyrs (nearly out-of-phase), contrary to several speleothem-based reconstructions that indicate a more in-phase relation of monsoon with insolation. Using results from transient as well as time-slice experiments, we have shown that factors other than the Indian monsoon affect upwelling on the precession time scales. These factors modulate the spatial extent of upwelling, resulting in the precession-scale variability in primary production. This is in contrast with modern observations, where most of the variations in primary productivity are a result of changes in the intensity of upwelling. We find that the spatial extent of upwelling is nearly out-of-phase with insolation. Thus, primary productivity lags insolation. We conclude that primary productivity in the Arabian Sea is not a good proxy for the Indian summer monsoon rainfall.

How to cite: Jalihal, C., Srinivasan, J., and Chakraborty, A.: Precession-scale variability of upwelling in the Arabian Sea and its implications for proxies of Indian summer monsoon, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4602, https://doi.org/10.5194/egusphere-egu22-4602, 2022.

14:23–14:28
|
EGU22-2632
|
ECS
|
|
On-site presentation
Anna Arrigoni et al.

The Middle Pleistocene Transition (MPT) represents a critical rearrangement in the Earth’s climate state, expressed as a switch from obliquity-dominated glacial/interglacial patterns towards the quasi-periodic 100 kyr cyclicity that characterized the Earth’s recent climatic history. This fundamental reorganization in the climatic response to orbital forcing occurred without comparable changes in the astronomical rhythms before or during the MPT. Although the MPT has been intensely studied, the triggering mechanisms still remain poorly understood.

High-resolution records from the equatorial to mid-latitude shelf areas are to date rarely considered. For this reason, we investigated an expanded MPT section from International Ocean Discovery Program (IODP) Expedition 356 Site U1460A (eastern Indian Ocean, 27°22.4949′S, 112°55.4296′E, 214.5 mbsl). At Site U1460A, we combine high-resolution records of shallow marine productivity and organic matter flux (Auer et al., 2021) with new benthic and planktonic foraminifera records. By implementing this multi-proxy approach, we aim to better define the response of the Leeuwin Current System over the MPT on tropical shelf regions.

We will investigate benthic foraminifera assemblages at Site U1460A to reconstruct the bottom water community response to the Leeuwin Current System variations during the MPT. At the same time, the benthos/plankton (B/P) ratio of U1460A will be used to constrain the local impact of sea-level changes. Presently work is in progress to generate a B/P ratio for the MPT interval to better assess the impact of sea-level changes on a highly dynamic shelf setting on the western coast of Australia. Shallow coastal areas are markedly sensitive to the glacial/interglacial connected sea-level oscillations. Monitoring the variation in the B/P ratio can provide a preliminary overview of local sea-level changes along the Australian shelf which could be linked to the glacial/interglacial changes of the MPT. Higher values in this ratio indicate lowstand phases, while lower values are characteristic of higher sea level phases. The foraminifera data will be compared to a multi-proxy dataset (Auer et al., 2021) to constrain the local sea-level-driven environmental change over the MPT. Using this we will be able to untangle the impact of local versus global climatic change over the MPT.

Taxonomic identifications are underway following an extensive washing procedure developed for the sample material. Benthic foraminifera show moderate to good preservation, while the planktonic assemblage exhibits moderate to very good preservation. Foraminiferal tests appear white, opaque with apertures, and pores moderately covered by sediment. Some individuals are chipped or partially broken. Specimen preservation (plankton and benthos) decreases during glacial intervals where the abundance of planktonic foraminifera is low.

Finally, we recorded the presence of Globorotalia tosaensis at the top of our study interval at a depth of 61.72 mbsf (corresponding to sample U1460A-14F-3W, 20-24 cm). The continuous presence of this taxon indicates an age older than 0.61 Ma (Wade et al., 2011) at the top of our study interval, and therefore supports the age model of Auer et al. (2021).

How to cite: Arrigoni, A., Auer, G., Petrick, B., Mamo, B., and Piller, W. E.: Multi-proxy study of the Leeuwin Current System evolution along the northwestern coast of Australia during the Middle Pleistocene Transition , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2632, https://doi.org/10.5194/egusphere-egu22-2632, 2022.

14:28–14:50
Q & A