This study investigates the ability of a global ocean reanalysis at 1/12 degrees horizontal resolution, GLORYS12, to represent oceanic processes at intraseasonal and higher-frequency scales. GLORYS12, which includes data assimilation of satellite and multi-instrument in situ observations, is compared to a twin-free simulation (with no assimilation) in the tropical Pacific Ocean. Spectral analyses show that data assimilation improves the realism of sea surface height intraseasonal variability in the entire tropical Pacific Ocean, in both amplitude and phase, with an increase in the amplitude of more than 50% for the 20-90-day band and up to 15% for the 2-20-day band. The improvement is largest along the 5 degrees N/S latitudes, where the magnitude of tropical instability waves is maximum, but is limited along the equator where steric height variability is dominated by intraseasonal oceanic Kelvin waves, already well represented in the free simulation. Wavenumber-frequency spectra show that data assimilation constraint improves both the spatial and temporal scales of intraseasonal waves and their timing. Data assimilation impacts the realism of oceanic simulations in two ways. By modifying the background oceanic stratification, it corrects the phase speed of westward-propagating waves. It is also shown that the intraseasonal component of analysis increments (data assimilation corrections applied) is dynamically consistent and exhibits clear intraseasonal propagation. By demonstrating the benefits of data assimilation for intraseasonal processes in the tropical Pacific Ocean, this study highlights the high value of both in situ and satellite observations to constrain ocean models in a wide range of time scales.

Abstract This study presents the observation and evaluation of a meteotsunami in the Indian Ocean triggered by the Hunga‐Tonga volcanic eruption. The event was detected through tide gauges and bottom‐pressure recordings across the Indian Ocean, with an amplitude of 10–15 cm, lasting for a few days. A numerical model was used to understand the ocean's response to meteotsunami and evaluate the dynamics behind it. The model results show that the sea‐level oscillations result from the ocean waves generated by a propagating Lamb wave. In addition to interaction with bathymetry, refracted and reflected waves also determine the sea‐level variability. Our analysis shows that bathymetric slope plays a vital role in near‐shore processes. The spectral and spatial characteristics of the meteotsunami were reminiscent of seismic tsunamis. Our research on this rare event elucidates the unresolved issues and eventually leads to designing a blueprint for future observation and modeling of meteotsunamis and seismic tsunamis.

Recent work has shown that the Madden–Julian Oscillation (MJO) winds around the Maritime Continent can drive a see-saw in oceanic mass between the Indian and Pacific oceans on intraseasonal time scales. During the boreal winter of 2012–13, this see-saw accounted for about two thirds of an unusually large ( 30 mas, milliarcseconds) fluctuation in the oceanic excitation of Earth’s polar motion about the 90°E meridian. Interestingly, the magnitude of the oceanic influence was nearly at par, but out-of-phase with that of the atmosphere and a factor of 10 larger than effects associated with the hydrological cycle. Here we show that oceanic mass changes and transport anomalies during the 2012–13 boreal winter were indeed most pronounced in the Indo-Pacific basin, and that they possessed a favorable geometry to excite polar motion variations about the 90°E meridian. Phase alignment of the excitation signals from different regions, as well as between mass and motion terms, was a key characteristic of the 2012–13 event, but was far less distinct in other strong see-saw years. Basin-wise, the Indian Ocean acted as a dominant contributor to the 2012–13 polar motion excitation at MJO periods, followed by the Pacific basin. Overall, ocean dynamics in the 10°−65°S latitudinal belt over the Indo-Pacific basin accounted for 93

Ocean monitoring and forecasting systems combine information from ocean observations and numerical models through advanced data assimilation techniques. They are essential to monitor and report on past, present and future oceanic conditions. However, given the continuous development of oceanic models and data assimilation techniques in addition to the increased diversity of assimilated platforms, it becomes more and more difficult to establish how information from observations is used, and to determine the utility and relevance of a change of the global ocean observing system on ocean analyses. Here, a series of observing system simulation experiments (OSSE), which consist in simulating synthetic observations from a realistic simulation to be subsequently assimilated in an experimental analysis system, was performed. An original multiscale approach is then used to investigate (i) the impact of various observing system components by distinguishing between satellites and in situ (Argo floats and tropical moorings), and (ii) the impact of recommended changes in observing systems, in particular the impact of Argo floats doubling and enhancements of tropical moorings, on the fidelity of ocean analyses. This multiscale approach is key to better understand how observing system components, with their distinct sampling characteristics, help to constrain physical processes. The study demonstrates the ability of the analysis system to represent 40-80% of the temperature variance at mesoscale (20-30% for salinity), and more than 80% for larger scales. Satellite information, mostly through altimetric data, strongly constrains mesoscale variability, while the impact of in situ temperature and salinity profiles are essential to constrain large scale variability. It is also shown that future enhancements of Argo and tropical mooring arrays observations will likely be beneficial to ocean analyses at both intermediate and large scales, with a higher impact for salinity-related quantities. This work provides a better understanding on the respective role of major satellite and in situ observing system components in the integrated ocean observing system.

Northern Bay of Bengal (BoB) receives a significant amount of freshwater, from precipitation and river discharge, which makes it comparatively low saline, specifically during August/September. However, the factors that determine the dispersal of this freshwater within the BoB remain relatively less addressed. Analysis of the near-surface salinity data from a moored buoy located in the northern BoB during 2011–2019 supplemented with satellite and model data showed prominent interannual variability viz. time of occurrence, persistence and magnitude of freshening. The surface salinity exhibit significant lowering during 2011, 2015 and 2017 with unique prolonged freshening in 2017. The offshore advection of low salinity water is initiated with sustained low wind stress further favoured by existing surface circulation and mesoscale eddies contributing to the unique characteristics. In addition, the maximum freshening observed in 2011 coincides with positive Indian Ocean Dipole event, whereas that of the prolonged freshening in 2017 is favored with anomalously low wind stress.

Recent discovery of a see-saw in intraseasonal barotropic sea level driven by boreal winter Madden-Julian Oscillation winds in the tropical Indo-Pacific basin has renewed interest in the barotropic dynamics in the tropics, which was otherwise known to be prominently a region of baroclinic dynamics at intraseasonal timescales. In this study, using a reference model simulation and several sensitivity experiments, we provide comprehensive details of the dynamics associated with this see-saw. It is found that the narrow Indonesian straits are instrumental in relaying barotropic excitations from one basin to the other. The intraseasonal barotropic circulation associated with this see-saw dynamics appears well organized at basin scale, both in the Indian Ocean and in the Pacific Ocean. During the positive phase of the see-saw, the Madden-Julian Oscillation winds over the Maritime Continent drive a basin-wide anti-clockwise circulation of ∼2 Sv around the Australian Continent with the eastern arm of this circulation located along the west African coast. Concurrently, it also drives a clockwise circulation in the southern Pacific Ocean. This circulation pattern reverses during the negative phase of the see-saw. In the Indonesian Throughflow region, the relatively narrow Ombai Strait and Lombok Strait facilitate this circulation in comparison to the wider Timor Passage. We discuss and point out the significant differences in this flow pattern with the mean flow across the Indonesian seas. We also address how some regions in the northern Pacific Ocean are decoupled from this large-scale dynamics. We show that it takes about a week for both the basins to adjust and to establish the see-saw, once the Madden-Julian Oscillation winds reach the Maritime Continent.

The study investigates the temporal change in tides under the background of rapid sea level rise, for a highly vulnerable coastal region. It highlights the existence of non-astronomic tidal variability at seasonal and secular time-scales in the Ganga-Brahmaputra-Meghna (GBM) delta, along with increasing low and high tidal levels. The observed variability in semi-diurnal tides was found to be coherent to the mean sea level changes. M2 tide can be a proxy for sea level changes as they show a direct relationship. Steric changes were introduced in the barotropic ADCIRC (ADvanced CIRCulation) model and the results proved that change in water depth modulates the energetics of the tidal wave thereby causing amplification. A regionally varying tidal response was noticed, such that the regions associated with the GBM delta showed maximum amplification. The factors that can cause tidal amplification are region-dependent and coastal geomorphology plays an important role in it. Tidal prediction based on changing sea level was able to capture the observed seasonal modulation of tides and increasing trend in low and high tidal levels. Incorporating the effect of changing tides can improve the accuracy of tidal predictions and will help advance studies regarding regional sea level change, coastal flooding and tide-surge interaction.

An intraseasonal see-saw has been observed in the Indo-Pacific barotropic sea level anomaly during boreal winters. This see-saw carries a significant amount of energy and is crucial for the tropical sea level and angular momentum budget. Here, we evaluate the performance of several state-of-the-art ocean general circulation models (OGCMs), including the Modular Ocean Model (MOM), the Nucleus for European Modeling of the Ocean (NEMO), Massachusetts Institute of Technology general circulation model (MITgcm), and the HYbrid Coordinate Ocean Model (HYCOM) in reproducing the see-saw. Regardless of differences in model physics, forcings, setup, and resolution, all OGCMs simulate see-saw in the Indo-Pacific oceanic mass, making it a robust oceanic phenomenon. The models with horizontal resolutions ranging from 25 to 9 km, particularly those with higher resolution, are successful at simulating the qualitative characteristics of the see-saw. We show that a proper representation of the Indonesian straits is vital for a reasonable simulation of the see-saw. Furthermore, we show that the inclusion of the polar ocean in the models has little impact on the see-saw structure, implying that OGCMs with semi-global domain are appropriate tools for capturing the see-saw dynamics.

This study investigates the variability and sources of Quasi-Biweekly (QB) oscillation (10-20 days) in coastal sea level along the western boundary of the Bay of Bengal (BoB) using tide-gauge data and simulations from a very high-resolution regional model. Observations show most significant spectral energy in the QB band (similar to 12 days) exists in the northernmost location (Paradeep) with an amplitude of about 5-10 cm in sea level and energy in this band decreases equatorward along the coast. The QB oscillations in sea level contribute up to 16%-36% of the total subtidal sea-level variability in the western BoB and these oscillations are more pronounced during June-November. It is observed that sea-level oscillations in the QB band propagate equatorward along the east coast of India as coastal-trapped waves with a phase speed of about 2-4 ms(-1) and this speed is consistent with the theoretical estimates. Our study revealed that the strong QB winds found in the northern BoB are one of the forcing factors that excite QB oscillation in sea level in the western BoB. Further analysis using numerical simulations shows that the QB sea level propagating from the equatorial Indian Ocean and QB oscillation generated by strong winds associated with the tropical cyclones formed in the BoB also contribute to QB coastal sea-level fluctuations in this region.

Strong large-scale winds can relay their energy to the ocean bottom and elicit an almost immediate intraseasonal barotropic (depth independent) response in the ocean. The intense winds associated with the Madden-Julian Oscillation over the Maritime Continent generate significant intraseasonal basin-wide barotropic sea level variability in the tropical Indian Ocean. Here we show, using a numerical model and a network of in-situ bottom pressure recorders, that the concerted barotropic response of the Indian and the Pacific Ocean to these winds leads to an intraseasonal see-saw of oceanic mass in the Indo-Pacific basin. This global-scale mass shift is unexpectedly fast, as we show that the mass field of the entire Indo-Pacific basin is dynamically adjusted to Madden-Julian Oscillation in a few days. We find this large-scale ocean see-saw, induced by the Madden-Julian Oscillation, has a detectable influence on the Earth's polar axis motion, in particular during the strong see-saw of early 2013. Intense winds over the Maritime Continent associated with the Madden-Julian Oscillation lead to a large-scale redistribution of oceanic mass in the Indo-Pacific basins, and have a noticeable impact on the Earth's angular momentum.

Strong large-scale winds can relay their energy to the ocean bottom and elicit an almost immediate intraseasonal barotropic (depth independent) response in the ocean. The intense winds associated with the Madden-Julian Oscillation (MJO), over the tropical interface between the Indian Ocean and the Pacific Ocean (popularly known as Maritime Continent) generate significant basin-wide intraseasonal barotropic sea level variability in the tropical Indian Ocean. Here we show, using an ocean general circulation model and a network of in-situ bottom pressure recorders, that the concerted barotropic response of the Indian and the Pacific Ocean to these winds leads to an intraseasonal see-saw of oceanic mass in the Indo-Pacific basin. This global-scale mass shift is unexpectedly fast, as we show that the mass field of the entire Indo-Pacific basin is dynamically adjusted to MJO in a few days. We also explain how this near-global-scale MJO-induced oceanic phenomenon is the first signature from a climate mode that can be isolated into the Earth polar axis motion, in particular during the strong see-saw of early 2013.

Changes in sea level may be attributed either to barotropic (involving the entire water column) or baroclinic processes (governed by stratification). It has been widely accepted that barotropic sea level changes in the tropics are insignificant at intraseasonal time scales (periods of 30–80 days). Based on bottom pressure records, we present evidence for significant basin-wide barotropic sea level variability in the tropical Indian Ocean during December–April with standard deviations amounting to ∼30–60% of the standard deviation in total intraseasonal sea level variability. The origin of this variability is linked to a small patch of wind over the Eastern Indian Ocean, associated with boreal winter Madden–Julian Oscillations (MJO). These large fluctuations are likely to play a prominent role in the intraseasonal sea level and mass budgets. Because of their much faster propagation than baroclinic processes, they allow the basin to adjust to climatic perturbations much more rapidly than was previously thought.

The link between North Indian Ocean (NIO) high swell events and the meteorological conditions over the Southern Indian Ocean (SIO) is explored in this article, using a combination of in situ measurements and model simulations for the year 2005. High waves, without any sign in the local winds, sometimes cause severe flooding events along the south-west coast of India, locally known as the Kallakkadal events and cause major societal problems along the coasts. In situ observations report 10 high swell events in NIO during 2005. Our study confirms that these events are caused by the swells propagating from south of 30 degrees S. In all cases, 3-5 days prior to the high swell events in NIO, we observed a severe low pressure system, called the Cut-Off Low (COL) in the Southern Ocean. These COLs are quasistationary in nature, providing strong (similar to 25 ms(-1)) and long duration (similar to 3 days) surface winds over a large fetch; essential conditions for the generation of long-period swells. The intense equator ward winds associated with COLs in the SIO trigger the generation of high waves, which propagate to NIO as swells. Furthermore, these swells cause high wave activity and sometimes Kallakkadal events along the NIO coastal regions, depending on the local topography, angle of incidence, and tidal conditions. Our study shows that such natural hazards along the NIO coasts can be forecasted at least 2 days in advance if the meteorological conditions of the SIO are properly monitored.