This seminar presentation discusses the role of air-sea forcing on the variability of mixed layer depth and hydrography in the Arabian Sea and Bay of Bengal. The presentation includes two case studies and discusses variables like near surface winds, incoming shortwave radiation, freshwater flux, and their impact on mixed layer depth, sea surface temperature, and salinity. Sensitivity experiments were performed using an ocean circulation model to analyze the influence of winds, shortwave radiation, and freshwater flux on these regions. The results show that winds and shortwave radiation have the greatest influence on mixed layer depth and sea surface temperature, particularly during summer months.

1. SEMINAR PRESENTATION
The Role of Air-Sea Forcing on the variability
Mixed layer Depth (MLD), hydrography in Arabian
Sea and Bay of Bengal
Presented by:
Md Rony Golder
ID No. MS-190660
Fisheries and Marine Resource
Technology Discipline
Khulna University, Khulna,
Bangladesh
19 May 2019 Coastal and Marine Science

2. Case study 1 Case study 2
19 May 2019 Coastal and Marine Science
Srivastava et al., 2017 Prasad, 2004

3. Learning objectives
• Near surface zonal meridional winds
• Incoming short wave radiation
• Freshwater flux
• Mixed layer Depth variation
• Hydrography ( SST, SSS)
• Air sea interaction
• Contribution of near surface winds
19 May 2019 Coastal and Marine Science

4. Introduction
• The near surface air sea forcing greatly influence the
ocean variable
• Influence of winds on the variability of mixed layer in the
northern Indian ocean during different phases of summer
monsoon. ( Gopalkrshna et al., 1988)
• Physical mechanisms governing the seasonal evolution of
MLD and SST along meridional sections in AS and BoB.
(Prasad, 2004)
19 May 2019 Coastal and Marine Science

5. Introduction….
• Duncan and Han (2009) investigated intraseasonal SST
variability in the Indian Ocean during boreal summer with a
series of experiments using the (HYCOM).
• Da-Allada et al. (2015) suggested that the seasonal cycle of
mixed layer salinity in the tropical Indian ocean.
• There have not been many studies of the role of air sea forcing
on the variability of hydrography, mixed layer depth in these
regions.
19 May 2019 Coastal and Marine Science

6. Methodology
19 May 2019 Coastal and Marine Science
The ocean general circulation
model, which we use for the study,
is the MITgcm
configuration
of the MITgcm
in a limited
area around
the region
65°—95°E,
5°—22°N
The MITgcm is a z-coordinate model and solves the incompressible Navier—
Stokes equation. We use the Boussinesq and hydrostatic approximations in our
model setup. The model space is discretized using staggered Arakawa C grid.
A third order direct space-time advection scheme is employed for temperature
and salinity.
We performed 4 sensitivity experiement ( CTRL run, WWINDS, WSWAVE and WFWF)

7. Results and Discussion
Seasonal variability of the SST of the region during DJF, MAM, JJA, and SON months
Winter Pre-monsoon Monsoon Post monsoon
Increase
Decease
Very small
influence
19 May 2019 Coastal and Marine Science

8. Results and Discussion……
Seasonal SST difference map between CTRL run and sensitivity experiments
Significantly
negative
Positive
Very
small
change
with
above
• Maximum influence of SWR is observed in MAM (summer) season
• Absence of incoming SR has a direct influence on mixed layer depth.
19 May 2019 Coastal and Marine Science

9. Results and Discussion……
Seasonal variability of the SSS of the region during DJF, MAM, JJA, and SON months
Decease
Decease
Different in
different
location
19 May 2019 Coastal and Marine Science

10. Results and Discussion……
Seasonal SSS difference map between CTRL run and sensitivity experiments
Minimum Highest Highest
Positive in
all Season
• The SSS differences in the BoB are higher than AS in all the seasons
• In the absence of near surface winds the surface salinity of the region decreases
Either (0) or
sig. (+)
19 May 2019 Coastal and Marine Science
Lot of
seasonal
variability

11. 19 May 2019 Coastal and Marine Science
Results and Discussion……
Maps of MLD difference between CTRL run and different sensitivity experiments
Highly Positive
• In the absence of near surface wind forcing, the MLD of the region drastically decreases
during DJF due to lack of momentum transfer, vertical mixing, and associated physical
processes.
• the absence of freshwater flux forcing (WFWF) results into an increase in the MLD of the AS
and BoB in all season except DJF
Negative

12. 19 May 2019 Coastal and Marine Science
Model derived mixed‐layer
depth (MLD, m), sea surface
temperature (SST, °C) and
net heat flux (NHF, W m−2)
along (left panel) 64.5°E
(right panels) 88.5°E (which
are representative of the
open‐ocean Arabian Sea
and Bay of Bengal) from the
control run (CR). The MLD is
defined as the depth at
which the density (σt) is
0.25 Kg m−3 greater than
the surface value.
Journal of Geophysical Research: Oceans, Volume: 109, Issue: C3, First published: 20 March 2004, DOI: (10.1029/2003JC002000)
A comparison of mixed‐layer dynamics between the Arabian Sea and Bay of
Bengal: One‐dimensional model results

13. 19 May 2019 Coastal and Marine Science
Same as previous figure
but from the observations.
The MLD is computed from
the monthly mean
climatology of Conkright et
al. [1998] temperature and
salinity; the SST and NHF
are taken from the SOC
climatology
Journal of Geophysical Research: Oceans, Volume: 109, Issue: C3, First published: 20 March 2004, DOI: (10.1029/2003JC002000)
A comparison of mixed‐layer dynamics between the Arabian Sea and Bay of
Bengal: One‐dimensional model results
Model‐Data Comparison
The summer and winter monsoons deep MLD and cool SST, particularly in the
Arabian Sea, are reproduced in the model. The stronger (weaker) winter cooling in
the Arabian Sea (Bay of Bengal) is in close agreement with the SOC climatology.

14. 19 May 2019 Coastal and Marine Science
The MLD obtained with
various constant wind
stress experiments (top
panel) τ = 0.138 N m−2
(wind speed = 8 m s−1)
and (bottom panel) τ =
0.216 N m−2 (wind
speed = 10 m s−1).
Contour interval is 10
m.
Journal of Geophysical Research: Oceans, Volume: 109, Issue: C3, First published: 20 March 2004, DOI: (10.1029/2003JC002000)
A comparison of mixed‐layer dynamics between the Arabian
Sea and Bay of Bengal: One‐dimensional model results
Constant Wind Stress Experiment
Seasonal variation of the wind stress forcing has a greater impact on the mixed‐layer evolution
in the Arabian Sea than in the Bay of Bengal.

15. 19 May 2019 Coastal and Marine Science
Thank You