Jerome Fiechter

Ocean Sciences Department

University of California, Santa Cruz

ROMS Workshop, Grenoble, 6-8 October 2008

Seasonal and Interannual Ecosystem Variability

in the Coastal Gulf of Alaska

Andy Moore – University of California, Santa Cruz

Chris Edwards – University of California, Santa Cruz

Ken Bruland – University of California, Santa Cruz

Manu Di Lorenzo – Georgia Institute of Technology

Zack Powell – University of California, Berkeley

Al Hermann – NOAA / PMEL

Enrique Curchitser – Rutgers University

Hernan Arango – Rutgers University

Kate Hedstrom – ARSRC, Fairbanks

Collaborators

Outline

• Physical/biological properties of Coastal Gulf of Alaska (CGOA)

• Ocean circulation, ecosystem, and iron limitation models

• Seasonal and interannual ecosystem variability (EOFs)

• Summary

CGOA Physical and Biological Properties

Physical Variability• Downwelling-favorable wind regime (Stabeno et al., 2004)

• AS intrinsic mesoscale variability (Combes and Di Lorenzo, 2007)

• Anticyclonic (Yakutat) eddy passages (Okkonen et al., 2003)

Biological Variability• CGOA: high-productivity shelf, fisheries

• Subarctic Gyre: HNLC region (Lam et al., 2006)

• Iron limitation on primary production (Strom et al., 2006)

Interannual Variability• 1997-1998 El Niño; 1999 La Niña

• 1999 NEP “Cold” Regime Shift (Peterson and Schwing, 2003)

• 2002 NEP Subsurface Cold Event (Curchitser et al., 2005)

Ocean Circulation Model: CGOA-ROMS

• ROMS: ~ 10 km horizontal resolution, 42 sigma levels

• One-way offline nesting with North East Pacific ROMS

• Monthly mean atmospheric and open boundary forcing

• 5-year simulation (1998 through 2002)

Lower Trophic Level Ecosystem Model

• NEMURO: 11 components; 83 parameters (Kishi et al., 2007)

Strom et al., 2007

PS: Nano P

PL: Diatoms

ZS: Ciliates

ZL: Copepods

ZP: KrillFe

NCP

FR p

:

22

2

lim

FekR

RFe

Iron-limited phytop. growth

(Fiechter et al., 2008)

Surface Chlorophyll and Nutrients: Seaward Line, 2001

NEMURO-Fe (x) GLOBEC in situ (squares)

Surface Chlorophyll: NEMURO-Fe vs. SeaWiFS

SeaWiFS

SeaWiFS

SeaWiFS

SeaWiFS

SeaWiFS

NEMURO

NEMURO

NEMURO

NEMURO

NEMURO

Surface Chlorophyll EOFs: NEMURO-Fe vs. SeaWiFS

Surface Chlorophyll EOFs: Diatoms vs. Nanophytop.

Sea Surface Height and Eddy Kinetic Energy EOFs

Alongshore Wind Stress and Wind Stress Curl EOFs

Spring Bloom Variability and Winter Ekman Pumping

Wind Stress Curl, Ekman Pumping, Nutrient Upwelling

Aleutian Low Classification (Bering Sea Winter Climate)

Sea level pressure (hPa)

Rodionov et al., 2005

(La Niña)

(El Niño)

(Split Center Pattern)

Summary

• Ecosystem model reproduces chlorophyll seasonal cycle

and interannual variability, but underestimates fall bloom

• Ecosystem model reproduces phytoplankton community

structure (diatoms on shelf and nanophytoplankton offshore)

• Seasonal cycle and interannual variability account for 80%

of explained variance in model and 40% in observations

• Spring bloom interannual variability correlated with winter

Ekman pumping (i.e., wind stress curl) in northern CGOA

• Connection to Aleutian Low variability in North Pacific?