EXAMINING FEATURES OF ENHANCED ......Summer monsoon June-Sep WICC EICC SC GW SE SMC SMC LL UNIQUE...
Transcript of EXAMINING FEATURES OF ENHANCED ......Summer monsoon June-Sep WICC EICC SC GW SE SMC SMC LL UNIQUE...
Helga do Rosário Gomes and Joaquim I. Goes
Lamont Doherty Earth Observatory at Columbia University, Palisades, NY, 10964, USA
Sergio deRada
Naval Research Laboratory, Stennis Space Center, MS 39529, USA
Fei Chai
University of Maine, Orono, ME 04469, USA
Joji Ishizaka
Division for Land-Ocean Ecosystem Research, Institute for Space-Earth Environmental Research (ISEE), Nagoya University, Nagoya, Japan
EXAMINING FEATURES OF ENHANCED PHYTOPLANKTON
BIOMASS IN THE BAY OF BENGAL USING A COUPLED
PHYSICAL-BIOLOGICAL MODEL
WHY STUDY THE BAY OF BENGAL?
The Bay of Bengal is surrounded by the most populous countries
Yet one of the least studied regions in the world
Its atmosphere-ocean dynamics is linked to the El Niño
December 5th 2015
Four named tropical cyclones each year causing huge damage to
life and economy because of poor disaster management strategies
Summer monsoon June-Sep
WICC EICC
SC
GW
SE
SMC
SMC
LL
UNIQUE FEATURES OF THE BAY OF BENGAL
The Bay of Bengal is a semi-enclosed basin in
the North Indian OceanAnother major distinguishing feature is the seasonal
reversal of currents which are anti-clockwise during
summer monsoon
• Winds are very strong and from the southwestTremendous amounts of freshwater from rivers
(2.95 x 1012 m3 yr-1) and from precipitation which
is responsible for strong vertical stratification
of the upper layers.
Winter monsoon Nov-Feb
UNIQUE FEATURES OF THE BAY OF BENGAL
SC
During the winter monsoon the currents are
in an anti-clockwise direction
Less river run off so salinity increases
EICC
NMC
WICC
Physical processes and biological productivity are still not well known
because no large-scale ocean expeditions have been conducted here unlike
in the Arabian Sea
Physical measurements have improved because of Argo floats but there is
a severe paucity of simultaneous chemical, phytoplankton biomass and
productivity estimates
Ocean color data limited because of the presence of a perennial cloud
cover
WHY DOES THE BIOLOGY OF BAY OF BENGAL REMAIN
UNDERSTUDIED?
From 1951 to 2006, there exist only about 350 coastal profiles of
Chlorophyll a and even fewer of primary productivity
A substantial fraction of phytoplankton biomass is at depth and not
captured by satellites
The Bay of Bengal is also constantly under cloud cover because of
excessive precipitation so phytoplankton are light limited
The presence of the freshwater layer in the BoB is a major
impediment for the influx of nutrients from the deep layers into the
euphotic column so phytoplankton are nutrient limited
WHAT IS THE CURRENTLY KNOWN ABOUT THE BAY
OF BENGAL?
The Bay of Bengal is considered to be much less productive
compared to the Arabian Sea which is one of the most productive
regions in the world because of large-scale upwelling
EFFECT OF FRESH WATER ON OPEN OCEAN WATERS
Temp SalinityBAY OF BENGAL PROCESS
STUDIES (BOBPS)
Vertical distribution of Temperature (oC), Salinity (psu) in
the upper 300m and Nitrate (µM) in the upper 120m along
the open ocean (88oE) . Dashed line indicates the depth
of mixed layer and thick line indicates 1 µM contour
(source Prasanna Kumar et al., JPR, 2007)
PHYSICAL PROCESSES THAT CAN ERODE THE
HALOCLINE AND CAUSE NUTRIENT INFLUXES INTO
THE EUPHOTIC COLUMN
Weather-related events that are unique to the Bay of Bengal are Low Pressure
Systems or cyclones
Closer to the shelf region, the East Indian Coastal Current (EICC) plays a
significant role in bringing nutrients into the upper ocean through upwelling,
eddies and recirculation domes
Riverine discharge may contribute nutrients close to the mouths of the
rivers
Cyclonic domes around Sri Lanka formed by open ocean Ekman pumping in
response to monsoonal winds
CAN WE CIRCUMVENT THE PROBLEM USING AN
ECOSYSTEM MODEL?
Ecosystem models can augment in situ and satellite data especially in under-
sampled regions like the Bay of Bengal
They provide not only horizontal context but also vertical which is not possible
using remote sensing. Our in situ studies have shown that a large fraction of
phytoplankton resides below the upper 10m
Model data for the Bay of Bengal can provide long term hindcast simulations
that can help understand the biological response to climate indices such as
the Indian Ocean Dipole (IOD) and El Niño Southern Oscillation (ENSO);
something not possible with the present relatively short remotely sensed Chl a
record
INDIAN OCEAN CIRCULATION MODEL
• Navy Coastal Ocean Model (NCOM) configured for Indian Ocean (30S-30N, 30.5-
121.5E) with Mercator grid and 1/8° equatorial horizontal resolution
• 40-level σ/z hybrid vertical grid with 19 sigma (σ) levels at the top and 21 depth (z)
levels at the bottom
• Higher vertical resolution from surface to about 137m in the mixed layer and the
euphotic zone where most of the biological interactions take place
• Bathymetry derived from the Naval Research Laboratory’s 2-minute database
withland/sea boundary set at the 5 meter isobath
• Initial and boundary conditions from 1/8º Global NCOM physical fields, WOCE
2009 and Carbon Dioxide Information Analysis Center (CDIAC) biogeochemical
fields
• Atmospheric forcing by momentum and heat fluxes from NASA's (MERRA) dataset
• Relaxation to monthly climatology temperature and salinity from Generalized
Digital Environmental Model (GDEM version 3) all other state variables are free
running
• Model integration was conducted from 1980 to 2012; the period of MERRA data
availability
Si
Uptake
Nitrate
[NO3]
Advection
& Mixing
NO3
Uptake
Micro-Zooplankton
[Z1]Grazing
Ammonium
[NH4]
Excretion
NH4
Uptake
Detritus-N
[DN]
Fecal
Pellets
Sinking
Silicate
[Si(OH)4]
SmallPhytoplankton
[P1]
Diatoms
[P2]
N-U
ptak
e
Meso-Zooplankton
[Z2]
Sinking
Detritus-Si
[DSi]
Fecal
Pellets
Sinking
Pre
dati
on
Lost
Carbon Dioxide
[TCO2]
Oxygen
[O2]
Phosphate
[PO4]
Alkalinity
[ALK]
Air-Sea exchange
Respiration
Pho
tosy
nthe
sis
PhysicalModel
Solar
Rad
Light
model PAR
PO4
Uptake
PO4
Uptake
Att
enua
tion
Att
enua
tion
Photosynthesis
Respiration
Respiration
Photosynthesis
Gra
zing
Carbonate
Chemistry
13-component Carbon, Silicate, Nitrogen Ecosystem
Model (CoSiNE) developed by F. Chai
Small
Phytoplankton
[P1]
DIATOMS
[P2]
MICRO ZOO
[Z1]
MESO ZOO
[Z2]
DETRITUS-N
[DN]
DETRITUS-Si
[DSi]
Ammounium
[NH4]
Nitrate
[NO3]
Carbon dioxide
[TCO2]
Phosphate
[PO4]
Alkalinity
[ALK]
MERRA hourly
irradiance
SALINITY FIELDS FROM NCOM-COSiNE MODEL
COMPARISON OF MONTHLY MEAN CHLOROPHYLL FROM MODEL
versus REMOTEDLY SENSED BY SATELLITES
SUMMER MONSOON AND FORMATION OF SRI LANKA
DOME
• The most prominent feature is the Sri Lanka
Dome which appears east of Sri Lanka in May,
matures in July and disappears in Sept.
• The genesis of the dome is a strong cyclonic
curl in the wind field and upward Ekman
pumping which brings cooler water to the near-
surface waters
• The southern flank is the nutrient –rich Western
India Coastal Current which moves northwards
forming a cyclonic gyre
• The decay of the Sri Lanka dome is caused by
the arrival of warm Rossby waves from the east
MONTHLY MODEL OUTPUTS OF SURACE CHLOROPHYLL FOR
SUMMER 1993
• Coastal upwelling in the south
• Small increase in Chl a at
mouth of rivers
• Largest increase in Chl a
around Sri Lanka
• Bloom first starts south of Sri
Lanka
• High Chl a in the path of the
Southwest Monsoon Current
BINNED LEVEL 2 MODIS OCEAN COLOR CHL A IN THE REGION OF
THE SRI LANKA DOME
• Ocean Color data cannot
show the Sri Lanka bloom
in its entirety because of
acute cloud cover
• Increase in Chl a seen
south of Sri Lanka in June
• Increased Chl a east of Sri
Lanka
• Chl a enhancement in the
path of the Southwest
monsoon Current
SURFACE NITRATE CONCENTRATIONS (µM) IN THE BAY OF BENGAL
DURING THE SUMMER MONSOON (JUNE-SEP)
MODEL DERIVED VERTICAL PROFILES OF CLIMATOLOGICAL
TEMPERATURE DURING SUMMER MONSOON
Upwelling south of Sri Lanka
during the summer monsoon
shoaling of colder waters (24-
26oC) from 100 m to the surface
MODEL DERIVED VERTICAL PROFILES OF CLIMATOLOGICAL
CHL A DURING SUMMER MONSOON
Upwelling related Chl a began in
May
No sub-surface Chl a maxima
indicating light limitation from
extreme cloud cover
Bloom up to 60 m but decreased
as summer progressed
WINTER UPWELLING IN THE SOUTHWESTERN BAY OF
BENGAL
Winter Monsoon Current brings low
saline water from BoB into the Arabian
Sea
EICC flow is equatorward along entire
coast
Upwelling due to Ekman pumping
associated with large scale cyclonic
circulation. This dome called the Bay of
Bengal Dome survives through Jan.
The Bay of Bengal dome is shallower
than the SLD but spread over a much
larger area
WINTER NITRATE ENHANCEMENT FROM BAY OF BENGAL DOME AND
RIVER RUNOFF
NO3 at 10m ( Winter)
NO3 at 50m ( Winter)
NO3 at 20m ( Winter)
NO3 at 100m ( Winter)
Source Narvekar and Prasanna Kumar (Biogeosciences, 2014)
DEVELOPMENT OF THE WINTER PHYTOPLANKTON BLOOM IN THE BAY OF
BENGAL DOME
No winter convective
cooling and blooms in the
north like in the Arabian
Sea
Large scale blooms north-
east of Sri Lanka
Dissipation by Jan 1997
Ocean cloud data unable to
provide a complete image
unlike the ocean color
image
INTERANNUAL VARIABILITY IN CHLOROPHYLL DURING
WINTER(NOV-FEB) FROM MODIS-AQUA
2003 2004 2005 2006
2007 2008 2009 2010
2012
DISAPPEARANCE OF THE WINTER PHYTOPLANKTON BLOOM IN THE BAY
OF BENGAL DOME DURING THE FOLLOWING IOD YEAR
SUMMARY
• Nutrient inputs and phytoplankton biomass enhancement are greatest in
the southwestern Bay of Bengal due to the two cyclonic domes, the Sri
Lanka Dome and the Bay of Bengal Dome
• Other nutrients inputs into the Bay of Bengal include upwelled nutrients
brought by the due Western India Coastal Current and the Summer
Monsoon Current
• The onset of IOD events are responsible for the large interannual
variability observed in the Bay of Bengal
• We have developed a coupled physical-biological model for the Bay of
Bengal that is capable of reproducing bio-physical features observed in
limited shipboard and satellite datasets