Ocean Analysis and Modelling Laboratory, IITBBS

Featured projects (2)

Assessment of the Scatsat-1 winds Investigate the multi-scale ocean processes using ROMS model and winds from Scatsat-1
To identify the seasonal and intraseasonal variabilities of Ocean and their roles on the monsoon and cyclones

Featured research (13)

The present study focuses on climatological distributions of Monsoon Depressions (MDs) during different phases of Monsoon Intraseasonal Oscillation (MISO) and their relationship with different observed meteorological and oceanic parameters from buoys. The MISO is represented as a cyclic process with eight phases to show the northward propagation of the rainfall band. Almost 60% of MDs occur during the third and fourth phases of MISO over the north Bay of Bengal (BoB) and central India. Interestingly, a similar climatological composite of SST for different MISO phases does not precisely match the spatial precipitation pattern over the BoB. Instead, the oscillation is shown over a confined area near the east coast and to the north of 15∘\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^\circ$$\end{document} N. The lagged (6–12 days) impact of SST is well recognized in central and west BoB. The characteristics of MISO are explained through a detailed investigation of two contrasting years (2017 and 2018) in terms of the number of MDs. The analysis showed possible impacts of intensity and track of depressions on the MISO in terms of its intensity and successive phases. The intraseasonal signal of surface salinity is mostly negative during the weak phases of MISO (7, 8, and 1) with a slight lag (10–15 days) because of a freshwater flux resulting from the previous wet period. Similarly, during the active phases (phase 3 to phase 6), the salinity signal becomes positive for the lagged impact of higher evaporation during the current and prior phases. A consecutive repetition of phases 4 to 6, associated with different MDs concurrence with increasing ocean heat content (OHC), is noted during both years. The ISO in subsurface (25–60 m) temperature is stronger in 2018, and upward propagation of temperature anomaly helps in formations of more depressions than in 2017. In addition, six years (2013–2018) of subsurface temperature showed that MDs formation followed the warmer subsurface. The ISO signals for different meteorological parameters, Air Temperature, Sea Level Pressure, and Wind Speed are comparatively stronger (by amplitude) in 2018 than in 2017, resulting in higher variability of MISO and more MDs.
The present work analyzed the high frequency radar-derived surface current data for seasonal variation of tidal and non-tidal currents in the Gulf of Khambhat, India during 2018. The analysis showed that the M2 tidal (time period 12.42 hours) current is the strongest tidal constituent of magnitude from 1.0 m/s to 1.5 m/s in different seasons. This M2 currents is more than three times higher than the K1 (time period 23.93 hours) tidal current, which varies from 0.2 to 0.5 m/s. The meridional component than the zonal majorly drives the total currents. The de-tided surface currents is much less than the tidal current. The percentages of the tidal current to the total current is around 80%. The inclination of the tidal ellipse varies between 60° – 70° in different seasons, indicate the direction of tidal flow. A convergence (divergence) of surface currents is noted during flood and ebb currents around 21 °N due to downslope on both the sides. In February 2018, a strong surface current with a magnitude of around 0.5 m/s was seen flowing from the Arabian sea to the Gulf on the western and southern Gulf. A flow with 0.1 to 0.3 m/s is noted on the eastern Gulf from the shallower bathymetry to deep regions near the Tapti River. A cyclonic circulation flow in noted in August. A time-series analysis of the daily currents showed that oscillations of 5-, 10-, 15- days exist in the current observation. This study also highlighted the need of subsurface ocean observation for studying the ocean dynamics in the Gulf.
This study focuses on the impact of varying landfall timing in a tidal cycle (i.e., the spring-neap phase) and varying wind speeds (i.e., cyclone intensities) on the surge tides for the tropical cyclone Fani in the Bay of Bengal using a hydrodynamic finite element-based 2D (ADvanced CIRCulation) ADCIRC model setup. For atmospheric forcing, the Cyclostrophic Symmetric Holland wind Model (H80) and Generalized Asymmetric Holland Model (GAHM) model are used to estimate the wind fields from the IMD best track data. Comparisons with in-situ winds from moored buoys within the proximity of cyclone track showed that H80 simulated winds underestimate the observed winds in terms of magnitude followed by a mismatch in the wind directions as well. In contrast, the GAHM simulated wind fields, which are statistically better in terms of both magnitude and direction are used for different wind experiments. To understand the impact of the varying landfall timing and varying wind speeds on storm surges, a series of sensitivity experiments have been performed during a tidal cycle with modulated high and low winds along the cyclone track. The experiments considering varying landfall timing during a tidal cycle indicate the strongest surge tides (1.99 m) during the spring high tide phase, whereas the lowest surge tide of 0.94 m is observed during spring low tide. However, the surge tide at the actual time of landfall is 1.20 m which is during the transition from low tide to high tide. On the other hand, the combined impact of wind speeds and varying landfall timing indicated the strongest surge tides of 2.25 m during high wind conditions associated with spring high tides. In contrast, the surge tides decrease significantly during low tide and low wind conditions. This study confirms the importance of both winds and landfall timing on the storm surges, which will be crucial to forecast the storm surges associated with the tropical cyclones.
In recent years, the seasonal patterns of Tropical Cyclones (TC) in the Bay of Bengal have been shifting. While tropical depressions have been common in March–May (spring), they typically have been relatively weaker than the TCs during October–December. Here we show that the spatial pattern of recent warming trends during the last two decades in the southwestern Bay has allowed for stronger springtime pre-monsoon cyclones such as Amphan (May 2020, Super Cyclone) and Fani (April–May 2019, Extremely Severe Cyclone). The tracks of the pre-monsoon cyclones shifted westward, concurrent with an increasing rate of warming. This shift allowed both Fani and Amphan tracks to cross the northeastward warm Western Boundary Current (WBC) and associated warm anti-cyclonic eddies, while the weaker Viyaru (April 2013, Cyclonic Storm) did not interact with the WBC. A quantitative model linking the available along-track heat potential to cyclone’s intensity is developed to understand the impact of the WBC on cyclone intensification. The influence of the warming WBC and associated anti-cyclonic eddies will likely result in much stronger springtime TCs becoming relatively common in the future.
The 30-60 days bandpass filtered winds, sea surface temperature (SST), and rainfall from both the satellites and moored buoys have indicated the existence of active and break phases of MISOs with a periodicity of 10-12 days. All the parameters show a northward propagation of the MISO signals from the southern to the northern BoB with stronger magnitude on the north of 12 • N. The warmer SST causes the high wind and precipitation in an active phase after 4-5 days. During active phases, SST dropped, and break phase occurs with less wind and precipitation after 10-12 days. Prominent signatures of the MISOs are also observed along the ocean subsurface from the temperature, salinity, and current profiles. The 23 • C isotherm (D23) deepens during the active phases of the MISOs to make the surface warm. The D23 shoals during the break phases, indicating cooling of the ocean surface. The in-phase relationship of 100 m temperature and wind speeds together indicate an important role of the surface winds during the different phases of MISO. Deepening and shoaling of mixed layer are observed in the upper ocean during the different MISO phases with varying characteristics in the northern and southern BoB. The sub-surface signatures of MISOs are strong near 100 m for temperature, but for salinity and currents, the signatures are restricted within 50 m depth.

Lab head

Sourav Sil
  • School of Earth, Ocean and Climate Sciences
About Sourav Sil
  • Sourav Sil currently works at the School of Earth, Ocean and Climate Sciences, Indian Institute of Technology Bhubaneswar. Sourav does research in Physical Oceanography. Their current project is Multiscale Circulation Variability in the Bay of Bengal using numerical models and observations.

Members (6)

Samiran Mandal
  • Indian Institute of Technology Delhi
Saikat Pramanik
  • Indian Institute of Technology Delhi
Dipanjan Dey
  • University of Southampton
Abhijit Shee
  • Indian Institute of Technology Bhubaneswar
Arkaprava Ray
  • Indian Institute of Technology Bhubaneswar
Hitesh Gupta
  • Indian Institute of Technology Bhubaneswar