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