YEAR 2
Progress Report
Modeling, Assimilating and Predicting Physical-Biological
Climate Variations of the California Current System
Grant Award NNG05GC98G
A proposal funded by:
Modeling, Analysis and Prediction (MAP)
Climate Variability and Change
NNH04ZYS008N Office of Earth Sciences
National Aeronautics and Space Administration
Washington, DC 20546
Principal Investigator:
Emanuele Di Lorenzo, Assistant Professor
School of Earth and Atmospheric Sciences
Georgia Institute of Technology
Atlanta, GA, 30332-0340
edl@eas.gatech.edu, (858) 534-6397
Co-Investigators:
Arthur J. Miller, Research Oceanographer
Climate Research Division
Scripps Institution of Oceanography, UCSD
La Jolla, CA 92093-0224
ajmiller@ucsd.edu.edu, (858) 534-8033
John R. Moisan, Oceanographer
NASA/GSFC Wallops Flight Facility
Wallops Island, VA 23337
jmoisan@osb.wff.nasa.gov, (757) 824-1312
Goals of the project
The focus of this project is to use remotely sensed data (AVHRR / MODIS SST, TOPEX /
ERS-1 altimetry, QuikSCAT / SeaWinds wind fields, and CZCS / SeaWiFS / MODIS ocean color) in
conjunction with in situ CalCOFI data (temperature, salinity, currents, nitrate, chlorophyll-a, bulk
zooplankton) to quantify, diagnose and predict the scales and processes of physical and
biogeochemical variability. Specifically, we are producing a time-dependent picture of the
physical and biogeochemical dynamics during the period 1950-present using individual CalCOFI
cruise observations and associated satellite data combined with the Regional Ocean Modeling
System (ROMS).
Among the Key ESE Research Questions to be addressed here are: How is the ocean
circulation varying on interannual and decadal timescales? How can predictions of climate
variability and change be improved? How well can transient climate variations be understood
and predicted? How well can long-term climatic trends be assessed or predicted? The technical
goals include improving the data assimilation techniques, biological models and dynamical
interpretation of ROMS, which is being used by NASA scientists at JPL in ocean modeling
applications.
Progress on proposal tasks:
During year two we have continued to make progress on the following three tasks: (1)
perform an ensemble of ocean forward model hindcasts of the physics and biology in the
northeast Pacific from 1950-2005, (2) develop and test the data assimilation framework for the
California Current System (CCS) and (3) re-process and analyze the CalCOFI hydrography from
1950-2005 in preparation of the assimilation. We report each task separately.
Motivated by the findings of the first year and second year, we have begun to conduct
research that links decadal variations in the California Current with the entire North Pacific,
including the Kuroshio-Oyashio Externsion region and the tropics. In doing so we have proposed
new hypotheses to explain the dynamics of decadal variability in the Pacific Ocean and how these
may be instrumental in linking ecosystems fluctuations across the basin. These additional
findings, which extend beyond the original scope of the proposal are discussed in section 4.
Publications supported from this grant are highlighted in blue when cited in the text. All figures
are appended at the end of the text.
1. Northeast Pacific physical and biological hindcasts from 19502005
Our first task was to use long-term model integrations in the northeast Pacific region to
investigate the dynamics controlling decadal variations in ocean tracers (e.g. salinity, nutrients).
Using the high-resolution (9km, 15km) ocean model ensembles (Figure 1 shows the spatial
domain) we have made further progress in understanding the dynamics underlying the North
Pacific Gyre Oscillation (NPGO), which we had reported last year [Di Lorenzo et al., 2008a]. The
NPGO emerges as the 2nd dominant mode of sea surface height variability (2nd EOF SSH) in the
Northeast Pacific in both model and satellite. NPGO is significantly correlated with previously
unexplained fluctuations of salinity, nutrients and chlorophyll-a measured in long-term
observations in the California Current (CalCOFI). We use the term NPGO because its fluctuations
reflect changes in the intensity of the central and eastern branches of the North Pacific gyre
circulations as evident from the NPGO SSHa anomalies. Further analyses have revealed that
NPGO also explains salinity and nutrient fluctuations recorded in the Gulf of Alaska (Line P) and
at the Hawaii station (Figure 1) [Di Lorenzo et al., 2008b, in prep.]. The NPGO thus provides a
strong indicator of fluctuations in the mechanisms driving physical and ecosystem dynamics. A
more detail analysis of the oceanic budget associated with the surface expression of the NPGO
and the Pacific Decadal Oscillation (PDO) was performed during year 2 [Chhak et al., 2008] and
revealed how the oceanic patterns typical of these modes are shaped by anomalies in the surface
wind-driven currents that display the mean tracer gradients. In this study we also isolate the
atmospheric forcing of the NPGO, which is directly related to a well know pattern of sea level
pressure variability termed the North Pacific Oscillation [Walker and Bliss, 1932]. The combined
action of the atmospheric forcing of the PDO and NPGO contribute to modulations in the
upwelling cell along the entire North Pacific eastern boundary (see next section).
1.1 Changes in upwelling and ecosystem along the North Pacific eastern boundary
A parallel task of this proposal was to diagnose how the eddy field and upwelling system
respond to decadal and interannual changes in the forcing. To this end we have completed a
systematic analysis of how the North Pacific eastern boundary upwelling cell changes during
different phases of the PDO and NPGO mode. Using the ROMS adjoint model in combination with
a passive tracer we conducted inverse calculations to track decadal changes in the depth of the
upwelling cell [Chhak and Di Lorenzo., 2007]. We find that decadal changes in upwelling along the
CCS are not uniform in space and reflect forcing associated with the different large-scale climate
modes. While the upwelling cells north of 38°N reflect PDO variability, the cells south of 38°N are
controlled by changes in wind stresses associated with the NPGO [Di Lorenzo et al.,
2008a](Figure 2). These findings are relevant in light of the recent evidence that links changes in
upwelling in the California Current with the occurrence of different regimes in the
sardine/anchovies [Rykaczewski and Checkley, 2008].
Motivated by the findings in the CCS we also concluded passive tracer experiments in the Gulf
of Alaska to show how changes in the eddy statistics affect the offshore transport iron-rich
coastal water, which are critical for explaining the ecosystem in the Alaskan gyre. The findings of
these experiment show that predominant downwelling conditions associated with the positive
phase of the PDO drive a stronger eddy field and stronger offshore transport [Combes and Di
Lorenzo, 2008](Figure 3).
1.2 Large-scale structure and dynamics of the NPGO in the Pacific
Using satellite data of SSH we have begun investigating the large-scale structure of the NPGO.
We find that the NPGO pattern extends beyond the North Pacific and is part of a global-scale
mode of climate variability that is evident in global sea level trends and sea surface temperature
[Di Lorenzo et al., 2008a] (Figure 4). The sea surface temperature (SST) footprint reveals an
equatorially symmetric structure and ENSO-like shape suggesting that coupled tropical dynamics
are involved in the dynamics of the NPGO.
For year 2008 we plan to further explore the dynamics underlying these structures using the
outputs of the Earth Simulator Global Eddy resolving model hindcast from 1950 to present
(OFES model), the NASA GISS coupled model and of the GFDL coupled climate model.
1.3 NPGO links decadal variations in the California Current with the Kuroshio-Oyashio Extension
The NPGO is locally forced by surface wind variability associated with the North Pacific
Oscillation (NPO) (Figure 5), which is a fluctuation in sea level pressure characterized by high
pressure over Hawaii and low pressure in the Gulf of Alaska [Walker and Bliss, 1932]. Therefore
the NPGO is the oceanic expression of the NPO. We find that the oceanic sea level anomalies
forced by the NPO trigger an oceanic adjustment with radiation of westward propagating Rossby
wave signals that reach the North Pacific western boundary and modulate decadal variations in
the KOE (Figure 5). Although the role of Rossby waves in modulating decadal variations in the
KOE has been noted before (Qiu et al., 2005; Miller et al., 2000; Taguchi et al. 2007), the link
between the eastern and western boundary was not established. Therefore the NPGO provides
an important link between prominent decadal variations in the eastern and western boundaries.
We are in the process of submitting a manuscript [Di Lorenzo et al., 2008c, in prep.] that shows
how decadal variations in the Kurioshio and in the California Current are driven by a common
forcing -- the NPO.
1.4 Coherent Ecosystem variations in the Pacific and NPGO
An important aspect of this project is to understand how physical variability drive resposes
in the ecosystems. In the CCS we find that changes in upwelling associated with the NPGO exert
the dominant control. However the NPGO exhibit a large-scale structure in SSH and SST, which is
also evident in satellite Chl-a from SeaWiFS (Figure 6). This suggests that NPGO type variability
may be invoked to explain ecosystem variations throughout the Pacific. Although this finding is
beyond the scope of the proposed research, which was limited to the CCS, we intend to pursue
this hypothesis with a combined analysis of the SeaWiFS satellite data and the OFES model
output.
1.5. Mechanism of Pacific Decadal Variability: a hypothesis
The NPO drives the NPGO and a downstream decadal response in the KOE [Di Lorenzo et al.,
2008c, in prep.]. The NPO has also been linked to ENSO-like expression in the tropics [Vimont et
al. 2003]. Preliminary analysis suggests that NPO may have a quasi-deterministic component that
originates in the western tropical Pacific. This precursor pattern may be associated with ENSOlike coupled dynamics in the tropics. If true, these causal links may provide a mechanism for a
quasi-decadal oscillation. We intend to explore this hypothesis, which is summarized in Figure 7,
in the NASA GISS coupled model and the GFDL 2.0.
2. Data assimilation and predictability dynamics in the CCS
One of the goals of this project is to combine the ocean model with the CalCOFI data to produce a
reanalysis of the CCS state from 1950-2004 and build a coastal ocean prediction system.
2.1 Data Assimilation and CCS Reanalysis
After completing the development of a 4D variational data assimilation platform (4DVAR)
of the ROMS model [Di Lorenzo et al., 2007] we have completed the first data assimilation
experiments using CalCOFI in situ TS data and the satellite merged products from AVISO
[Haidvogel et al., 2008; Muccino et al., 2008]. As we progress towards a systematic application of
the data assimilation framework to produce a reanalysis of the ocean physical observations for
the CCS we are in process of building a website to post the reanalysis product. This website will
be instrumental to promptly distribute the data as well as the modeling platform used in the
assimilation. This will allow the users not only to analyze the model data but also to perform
additional experiments with the model (e.g sensitivity, ecosystem simulations, etc.). Preliminary
distribution of the model data is available on the Georgia Tech DODS server
(http://dods.o3d.org:8080). These simulations have led to collaborations in two external
investigations. The first uses the model output to diagnose the dynamics that lead to a shoaling
of the CCS oxygen minimum layer [Bograd et al. 2008], while a second investigation uses the
model advection field to explore the biological connectivity along the southern California coast
[Rasmussen et al., 2008]
In the next year we plan to complete the data assimilation model-observation integration
and post on the website a fully referenced reanalysis product from the period 1950-2005.
2.2 Predictability of mesoscale flows in the coastal ocean
During this second year we have begun to explore more systematically the timescale and
the dynamics controlling the predictability of the CCS. To this end we have performed an
ensemble of high-resolution regional ocean model simulations, which selectively introduce
uncertainties in the initial conditions and external surface forcing. We find that the timescales of
predictability of the upper ocean show a strong dependence on the phase of the seasonal cycle.
During the summer season, when uncertainties in the external forcing functions shows their
annual minimum, the predictability in the upper ocean is strongly driven by errors in the initial
condition field. This condition is reversed during the winter season, due to the strong
uncertainties in the forcing, which in this period of the year controls the predictability of the
mesoscale circulation. In contrast, below the thermocline the predictability is driven by errors in
the initial conditions during all the year, without any meaningful seasonal dependance. The
timescales associated with a Nash-Sutcliffe predictability skill of 0.75 ranges from 30 to 15 days
for the upper ocean, and are about 15 days for the lower levels. The timescale of the persistence
of the ocean state associated with the same measure skill ranges from 15 to 7 days in all the
vertical layers. This implies that the ocean model has dynamics skill in the time range of 7-30
days when properly initialized with data assimilation. These results are currently being prepared
for publication [Mosca and Di Lorenzo, 2008].
3. Observational analysis of the CCS ocean climate
A detailed analysis of the raw 55-year CalCOFI data confirms that thermocline
temperature, Tthermo, warms after the 1976-77 climate regime shift (Figure 8). But the analysis
clearly reveals that the thermocline depth, Dthermo, remains essentially constant across the
regime shift (Figure 8), although it does vary on decadal timescales. The surface intensified
warming after the 1976-77 shift resulted in an increased temperature difference across the
thermocline, a higher buoyancy frequency, Nmax, and a more stable upper ocean (Kim and
Miller, 2007).
Roemmich and McGowan (1995) suggested that the surface warming increased the temperature
difference across the thermocline resulting in less lifting of the thermocline by the coastal
upwelling. Both thermocline depth and stratification strength (the buoyancy frequency at the
pycnocline) affect the nutrient flux due to coastal upwelling because they influence the source of
upwelled water. As the surface heating changed the strength of stratification, it also changed the
slope of the nitrate-temperature relation (Figure 9) for the mid-depth waters (roughly 30m to
200m). The 12C isotherm, which many previous studies have used as a proxy of thermocline
depth, is associated with increased nitrate values after the shift. But since the stratification also
increased, the depth from which upwelled waters are mixed to the surface may have changed.
Further study is needed to determine how these changes might have interacted to affect
upwelling, mixing, cross-shelf transport of nutrient, and vertical nutrient fluxes to the euphotic
zone. The temperature-nitrate relationship of the mid-depth waters may be a key indicator of the
quality of upwelled water that contributes to sustaining primary production in the euphotic zone
(Kim and Miller, 2007).
4. Outreach and Education Activities
This proposal supports the education of two graduate students: Jason Furtado (Georgia
Tech) and Hey-Jin Kim (Scripps).
During the course of this project we have distributed the results at the following invited
talks/seminars:
 Basin-scale climate feedback and biological implications of North Pacific eastern
boundary dynamics, ICES/PICES Early Career Scientist Conference, Baltimore,
Maryland. 6/2007.
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North Pacific Gyre Oscillation links ocean climate to ecosystem change PICES Victoria
16th Annual Meeting, Victoria, British Columbia. 10/2007.
North Pacific Gyre Oscillation links ocean climate to ecosystem change PICES
Workshop on Climate Projections, Victoria, British Columbia. 10/2007.
North Pacific Gyre Oscillation links ocean climate to ecosystem change Invited
Seminar, University of California Santa Cruz. 10/2007.
North Pacific Gyre Oscillation links ocean climate to ecosystem change Invited
Seminar, University of Hawaii. 11/2007.
Linking the tropic to extra-tropical variability: North Pacific Gyre Oscillation Invited
Talk, AGU Fall 2007 Session, San Francisco. 12/2007.
The North Pacific Gyre Mode Invited Talk, AGU Fall 2007 Session, San Francisco.
12/2007.
North Pacific Gyre Oscillation: linking decadal variations in the tropics and extratropics, University of California Santa Cruz. 02/2008
Results from the modeling activity are also shared with other researcher working in the
NSF US GLOBEC projects and the California Current NSF-LTER site.
We have also created a website to disseminate the results from this research on the NPGO
(http://o3d.org/npgo) and the model data (http://dods.o3d.org:8080).
Publications Acknowledging this NASA Grant
( published, in press and submitted )
Bograd, S. J., C. G. Castro, E. Di Lorenzo, D. M. Palacios, H. R. Bailey and W. Gilly, 2008: The
shoaling of the hypoxic boundary in the California Current. Geophysical Research Letters,
submitted.
Chhak, K. and E. Di Lorenzo, 2007: Decadal variations in the California Current upwelling cells.
Geophys. Res. Lett., 34, L14604, doi:10.1029/2007GL030203.
Chhak, K., E. Di Lorenzo, N. Schneider and P. Cummins, 2008: Forcing of low-frequency ocean
variability in the Northeast Pacific. Journal of Climate, submitted.
Combes, V. and E. Di Lorenzo, 2007: Intrinsic and forced interannual variability of the Gulf of
Alaska mesoscale circulation. Progress in Oceanography,
doi:10.1016/j.pocean.2007.08.011.
Combes, V. and E. Di Lorenzo, 2008: Interannual and decadal variations in cross-shore mixing in
the Gulf of Alaska. Journal of Physical Oceanography, submitted.
Di Lorenzo E., Schneider N., Cobb K. M., Chhak, K, Franks P. J. S., Miller A. J., McWilliams J. C.,
Bograd S. J., Arango H., Curchister E., Powell T. M. and P. Rivere, 2008: North Pacific Gyre
Oscillation links ocean climate and ecosystem change. Geophys. Res. Lett.,
doi:10.1029/2007GL032838.
Di Lorenzo, E., Moore, A., H. Arango, Chua, B. D. Cornuelle, A. J. Miller, B. Powell and Bennett A.,
2007: Weak and strong constraint data assimilation in the inverse Regional Ocean
Modeling System (ROMS): development and application for a baroclinic coastal upwelling
system. Ocean Modeling, doi:10.1016/j.ocemod.2006.08.002.
Haidvogel D., H. Arango, W.P. Budgell, B.D. Cornuelle, E. Curchitser, E. Di Lorenzo, K. Fennel, W.R.
Geyer, A.J. Hermann, L. Lanerolle, J. Levin, J.C. McWilliams, A.J. Miller, A.M. Moore, T.M.
Powell, A.F. Shchepetkin, C.R. Sherwood, R.P. Signell, J.C. Warner, J. Wilkin, 2008: Ocean
forecasting in terrain-following coordinates: Formulation and skill assessment of the
Regional Ocean Modeling System. Journal of Computational Physics, 227, 3595-3624
Kim, H-J. and A. J. Miller, 2007: Did the thermocline deepen in the southern California Current
after the 1976-77 climate regime shift? Journal of Physical Oceanography, in press.
Moore, A., H. Arango, E. Di Lorenzo, B. D. Cornuelle and A. J. Miller, 2008: An Adjoint Sensitivity
Analysis of the Southern California Current Circulation and Ecosystem. Part I: The Physical
Circulation , Journal of Physical Oceanography, in revision.
Muccino, J., A. Bennett, B. Cornuelle, B. Chua, E. Di Lorenzo, et al., 2008: The Inverse Ocean
Modeling System. II: Applications. Journal of Atmospheric and Ocean Technology, in press.
Rasmussen L. L., B. D. Cornuelle, L. A. Levin, J. L. Largier and E. Di Lorenzo, 2008: Effects of smallscale features and local wind forcing on tracer dispersion and estimates of population
connectivity in a regional scale circulation model . Journal of Geophys. Res., submitted.
Seo, H., A. J. Miller and J. O Roads, 2007: The Scripps Coupled Ocean-Atmosphere Regional
(SCOAR) model, with applications in the eastern Pacific sector, Journal of Climate, in press.
( in preparation )
Di Lorenzo E., Schneider N. and Ceballos L., 2008: Linking physical-biological decadal variation
in the North Pacific western and eastern boundaries. Journal of Climate, in prep.
Di Lorenzo E., Schneider N., Franks P. J. S., Miller A. J., Bograd S. J., and Thomas A., 2008: Coherent
variations of nutrients and salinity in the Gulf of Alaska and California Current. Geophys.
Res. Lett., in prep.
Mosca, C. and E. Di Lorenzo, 2008: Predictability of mesoscale coastal flow in the California
Current. Dynamicsâ¨of Atmospheres and Oceans, in prep.