The Effect ofVarying Freshwater Inputs on Regional Ecosystems in

the North Atlantic

Chai, Gangopadhyay and Haidvogel (Co-I: Bisagni, Curchister)

Pan-Regional Synthesis MeetingBoulder, CO

February 18, 2009

GLOBEC – Pan Regional Synthesis

• “The importance of comparative analysis in U.S. GLOBEC for pan-regional synthesis has been recognized from the inception of the program. Comparison of the dynamics of closely related taxa selected as target species in relation to specific physical processes (including stratification, mechanisms of retention and loss, upwelling and downwelling, and cross-front exchange) will be an integral component of the overall synthesis and integration effort in U.S. GLOBEC.”

• (U.S. GLOBEC Synthesis Implementation Plan, 2007).

Proposed Research

• Comparing and contrasting the impact of freshwater influx to the eastern and western sides of the North Atlantic

• Understanding the development and maintenance of a possible three-gyre configuration of Calanus finmarchicus distribution in the North Atlantic

• Predicting the projected trends and variations in the North Atlantic Ocean based on IPCC projections for upcoming decades.

Interannual Variability

Question 1

• “Are the biophysical environments which sustain the populations of Calanus finmarchicus in the three gyres (WNA, NNA and NOR) of the North Atlantic connected to one another, and by what pathways and processes?”

Question 1A

• Are the nutrient and phytoplankton dynamics in these three gyres independent from one another (or not)?

Question 1B

• What are the exchange rates among the three gyres, and do they control plankton dispersal across the North Atlantic Ocean?

Question 2

• “How significant is freshwater variability in determining the stratification and its subsequent impacts on primary productivity in the northern North Atlantic?”

Question 2A

• What are the relative roles of freshwater inputs compared with other local influences on stratification and mixing (e.g., E-P, heating/cooling, winds, etc.)?

Question 2B

• Are the ecosystem impacts of hydrologic variability related to large-scale changes in circulation patterns and the transports of the various North Atlantic current systems, which make up the different boundaries of the three-gyre system?

Question 2C

• What future changes are expected based upon IPCC projections of climate change?

Approach• simulate basin-scale circulation fields for

the GLOBEC decade of 1990-1999 using an eddy-resolving ROMS model extended to include sea ice and riverine inputs of fresh water

• validate these simulations with a new assemblage of hydrographic data and the synthesis study for the decade by Hakkinen and Rhines (2004)

• utilize a 10-component (lower trophic level) CoSINE biogeochemical model to understand the biophysical pathways connecting the three Calanus gyres during 1990-1999

Approach (2)

• forecast the next 20 years of the state of the North Atlantic Ocean using idealized forcing fields (wind curl, heat flux, E-P) representing IPCC scenarios of climate change

• Compile and use datasets on salinity, ice cover, IPCC scenarios, river discharge, hydrography, and Calanus abundances from the eastern and western North Atlantic to examine potential climate-related mechanisms influencing the Gulf Stream system, North Atlantic Current, and Calanus productivity and population dynamics in the North Atlantic Basin.

GLOBEC P4B Results (Gangopadhyay, Bisagni, Batchelder and Gifford)

• The basin-scale North Atlantic model is spun-up using Levitus climatology and forced with adjusted NCEP High and Low NAO fields – 2 validations

• The Gulf Stream position is northward (southward) during High (Low) NAO years

• LSW advection in the GOMGB region during 1997-98 after 1996 Low-NAO

Progress Report and Status

Physical Model simulations are being written up – 1 PhD student; 1 MS student to complete this year.

Biological simulations started

GS mean positions are computed at different depths for both High and Low NAO simulations for comparison and then integrated over the upper 450 m depth.

Advection of Labrador water in the Slope Sea

This four-panel figure shows the density signatures for the month of July during the four years of GLOBEC period in the present model simulation. The yellow region corresponds to the LSW density range of 27.2 through 27.4 σθ. Bluish waters are lighter, and redder waters are heavier than LSW. The advection of LSW in the WNA from 1995 (top-left) through 1998 (bottom-right) is believed to be due to the large drop of the NAO index during 1996. (From Chaudhuri et al., 2008)

Publications• Chaudhuri, A.H., A. Gangopadhyay and

J.J. Bisagni, 2008: Inter-annual Variability of Gulf Stream Warm Core Rings in response to the North Atlantic Oscillation, Accepted in CSR.

• Chaudhuri, A.H., A. Gangopadhyay, and J.J. Bisagni, 2009: Response of the western North Atlantic basin to characteristic high and low phases of the North Atlantic Oscillation, Submitted ms.

• 3 other short papers are in preparation.

Proposed Simulations

• (i) run the high-resolution NAB model for 1990-1999 with sea ice and rivers

• (ii) compare this run with the existing Phase IVB run for both ENA and WNA

• (iii) run with active biogeochemistry

• (iv) validate the property fields while addressing biophysical subsets of Q1 and Q2

Proposed Simulations (2)

• Connectivity of the three-gyre system from a Lagrangian viewpoint – use ROMS floats

• Forecast studies based upon IPCC scenarios with a focus on surface forcing and freshwater input

• Simulations to be made available to the community

FecalPellet

Sinking PhysicalModel

Nitrate[NO3]

Advaction& Mixing

SmallPhytoplankton

[P1]NO3

Uptake

Micro-Zooplankton

[Z1]

Grazing

Ammonium[NH4]

Excretion

NH4Uptake

Detritus-N[DN]

Silicate[Si(OH)4]

Diatoms[P2]

SiUptake

N-Uptake

Meso-zooplankton

[Z2]

Sinking

Detritus-Si[DSi]

GrazingFecalPellet

Sinking

Predation

Lost

Total CO2[TCO2]

BiologicalUptake

Air-Sea Exchange

Physical-Biogeochemical Model

IronIron

IronIron

FecalPellet

Sinking PhysicalModel

Nitrate[NO3]

Advaction& Mixing

SmallPhytoplankton

[P1]NO3

Uptake

Micro-Zooplankton

[Z1]

Grazing

Ammonium[NH4]

Excretion

NH4Uptake

Detritus-N[DN]

Silicate[Si(OH)4]

Diatoms[P2]

SiUptake

N-Uptake

Meso-zooplankton

[Z2]

Sinking

Detritus-Si[DSi]

GrazingFecalPellet

Sinking

Predation

Lost

Total CO2[TCO2]

BiologicalUptake

Air-Sea Exchange

Physical-Biogeochemical Model

IronIron

IronIron

Figure 7. Schematic diagram of the CoSINE biogeochemical ecosystem model to be used in the proposed modeling activities over the North Atlantic basin. Black arrows show the path of N; dashed red arrows show the path of Si; and dotted blue arrows show the path of CO2. Indicated points show uptakes that are a function of iron availability.

First Six Months

• (i) reconfigure the NAB model for its enhanced domain size and inclusion of riverine inputs and sea ice; Run with SODA initial and Boundary conditions

• (ii) run low and/or high-resolution ROMS with linked CoSINE sub-model;

• and (iii) introduce freshwater forcing variability via the open boundary condition options within ROMS.

1/6 degree ROMS

Arctic-Atlantic ROMS

(modified from Budgell, 2005)

Forcing and IBC Fields

• Initial condition from SODA 1985

• Boundary Condition – monthly SODA fields (1985-2008)

• CORE2 Forcing

• Ice and Rivers (Budgell, 2005)

• IPCC – 3 scenarios: A1F1(High), A1B(medium) and B1(low)

SODA fields

SODA – 1985 fields -- Temp

Salinity

SODA -- Currents

Winter Summer

NAO Low

NAO High

SALINITY

Future Work

• Pan-Regional Synthesis on

• Understanding

• The Effect ofVarying Freshwater Inputs on Regional Ecosystems in the North Atlantic

Seasonality – Boundary Condition

Salinity Boundary Condition - 1985

Figure 8: Surface nitrate (mmol Nm-3) concentration for the winter season from observations (upper panel) and our coupled physical (ROMS)-ecosystem simulation (lower panel). The observations derive from NODC climatology, while the model results are an average of five model years. The high concentrations of nitrate are located in the subpolar gyre.