Oceanographic time-series stations provide vital data for the monitoring of oceanic processes, particularly those associated with trends over time and interannual variability. There are likely numerous locations where the establishment of a time-series station would be desirable, but for reasons of funding or logistics, such establishment may not be feasible. An alternative to an operational time-series station is monitoring of sites via remote sensing. In this study, the NASA Giovanni data system is employed to simulate the establishment of two time-series stations near the outflow region of California s Eel River, which carries a high sediment load. Previous time-series analysis of this location (Acker et al. 2009) indicated that remotely-sensed chl a exhibits a statistically significant increasing trend during summer (low flow) months, but no apparent trend during winter (high flow) months. Examination of several newly-available ocean data parameters in Giovanni, including 8-day resolution data, demonstrates the differences in ocean parameter trends at the two locations compared to regionally-averaged time-series. The hypothesis that the increased summer chl a values are related to increasing SST is evaluated, and the signature of the Eel River plume is defined with ocean optical parameters

Acker, James G.

Lee, Zhongping

Leptoukh, Gregory G.

Shen, Suhung

NASA Technical Reports Server

James G. Acker, NASA GES DISC/Wyle IS LLC 
Suhung Shen, NASA GES DISC/George Mason University 
Gregory G. Leptoukh, NASA GES DISC 
Zhongping Lee, University of Massachusetts-Boston  
 
Funding provided by NASA grant NNX09AV57G for the Physics Based Ocean Color Algorithms for Water  
Quality Products of Coastal and Inland Waters project. 
ABSTRACT 
Oceanographic time-series stations provide vital data for the monitoring of oceanic processes, 
particularly those associated with trends over time and interannual variability. There are likely 
numerous locations where the establishment of a time-series station would be desirable, but for 
reasons of funding or logistics, such establishment may not be feasible. An alternative to an 
operational time-series station is monitoring of sites via remote sensing. In this study, the NASA 
Giovanni data system is employed to simulate the establishment of two time-series stations near 
the outflow region of California’s Eel River, which carries a high sediment load. Previous time-
series analysis of this location (Acker et al. 2009) indicated that remotely-sensed chl a exhibits a 
statistically significant increasing trend during summer (low flow) months, but no apparent trend 
during winter (high flow) months. Examination of several newly-available ocean data parameters 
in Giovanni, including 8-day resolution data, demonstrates the differences in ocean parameter 
trends at the two locations compared to regionally-averaged time-series. The hypothesis that 
the increased summer chl a values are related to increasing SST is evaluated, and the signature 
of the Eel River plume is defined with ocean optical parameters.  
Introduction 
     In order to investigate a multitude of oceanic processes with regional significance, the establishment of time- 
series stations at many different sites would be desirable.  However, it is not feasible to maintain a research  
commitment to numerous sites of interest.  An alternative, made feasible by the increased availability and ease- 
of-access to NASA remotely-sensed oceanographic data, is to use these data to simulate time-series stations.   
The NASA Giovanni system (http://giovanni.gsfc.nasa.gov) facilitates time-series investigations by providing  
monthly gridded Level 3 data for many different oceanic variables.  Recently, ocean optical parameters have  
been added to Giovanni, providing enhanced research capability. 
     For this study, the influence of the outflow from California’s Eel River was investigated.  The Eel River, despite its  
relatively small flow, carries a high sediment load. The Eel River plume is thus discernible with ocean optical  
parameters. The study simulated the establishment of two time-series sites, one in a location influenced by the  
seasonal Variability of the Eel River outflow, and the second in a nearby location that is not influenced by the Eel 
River outflow.  The goals of this study were:   a) use remotely-sensed parameters to clearly distinguish the  
Eel River outflow plume; b) compare data trends for the two time-series sites, and c) evaluate increasing chl a  
trends that have been reported for this region in previous studies.  
Methods 
     Time-series data for this study were processed by the NASA Ocean Biology Processing Group (OBPG).  Level 2 
geophysical data parameters are processed to Level 3 gridded products, which are ingested by the NASA Goddard 
Earth Sciences Data and Information Services Center (GES DISC) for use in the Giovanni system.  Input data are 
from the Moderate Resolution Imaging Spectroradiometer (MODIS) on the Terra and Aqua satellites; only sea  
surface temperature data from MODIS-Terra are used.   
     Giovanni was use to create maps of relevant data parameters for site selection (see below).  Giovanni was also  
utilized to generate time-series plots and data output from Level 3 monthly data products.  A 0.1  x 0.1  area  
centered on the simulated time-series site was used to generate the time-series. Data within each area was spatially 
averaged to produce each point in the time series. Both standard and evaluation data products in the Giovanni Water  
Quality data portal were analyzed in this study.  The data output was imported into Excel, allowing linear regression  
and significance F -test analyses to determine trend significance. The Excel ternary diagram plotting spreadsheet  
Tri-Plot (Graham and Midgely 2000) was used for visualization of the ocean optical parameter classification scheme. 
Site Selection 
The general site selected to study the Eel River 
outflow plume is north of Cape Mendocino. Data 
parameter maps were generated with Giovanni 
to determine the approximate areal extent of the 
Eel River outflow in winter (rainy season) months. 
Because the region displayed a prominent area of 
elevated backscatter (bbp) coefficient in January  
2006, maps of five data parameters for the period  
November 2005 – February 2006 were generated.  
RESULTS 
 
Observation of the Eel River plume optical signature 
    We devised a simple optical classification system using the three apparent optical property parameters now available 
in Giovanni, which are:  adg,  the absorption coefficient of dissolved and detrital matter;  aph, the absorption coefficient  
of phytoplankton, and bbp, the backscatter coefficient.  In order to create a three-component classification system, bbp  
was multiplied by a factor of 10.  This classification system is not a true optical model;  it is simply intended to show  
that the Eel River outflow plume can be distinguished with the available AOPs in Giovanni. 
    For the period July 2002 to December 2011, time-series were generated for several different data parameters in 
Giovanni for the two study sites.  Results of the regression analysis (slope and significance F -test) are shown  
subsequently.  For the optical classification system, adg, aph, and (bbp x 10) were summed, and then the relative  
percentages of each parameter were calculated for each month.  The results were then divided into winter months  
(December-March), non-winter months (April-November) and summer months (May-October).  The relative percentages  
were then plotted in simple ternary diagrams using Tri-Plot.  
    The results demonstrate that the Eel River water is distinguishable during the winter months due to elevated bbp  
Values.  The elevated bbp values are related to the high sediment load carried by the river in the winter, ascribed to  
increased scattering caused by inorganic mineral particles in the outflow plume.  During the summer, the optical  
classification for the water at the Eel River Plume site is very similar to the optical classification at the Coastal Waters  
site. 
adg 
aph bbp 
chl a Zeu 
The maps were used to select initial sites at 40.6  N, 124.8  W within the elevated bbp region, and 41.2  N, 124.5  W  
outside the elevated bbp region.  Time series for the five parameters were examined. This examination revealed that the 
likely influence of the Eel River outflow was not clearly perceived for the initial “in plume” site, and it was moved to  
40.6  N, 124.5  W.  A bbp time-series for this site showed several peaks in the winter, consistent with the high sediment 
load carried by the Eel River in winter  
months.  The two sites used for this study 
were then termed the “Coastal Waters” site 
and the “Eel River Plume” site. 
Nov 2005 – 
Feb 2006 
Nov 2005 – 
Feb 2006 
Nov 2005 – 
Feb2006 
Nov 2005 – 
Feb 2006 
Nov 2005 – 
Feb 2006 
Study sites 
bbp 40.6 N, 124.8 W 
 
July 2002 – Dec 2011 
bbp 40.6 N, 124.5 W 
 
July 2002 – Dec 2011 
Optical water mass classification results for the Coastal Waters site.   Left:  All months. 
Center:  Winter months.   Right:  Non-winter months.  The winter month water is optically 
similar to the non-winter month water at the Coastal Waters site, indicating that the site  
is not influenced by the Eel River outflow.  
Optical water mass classification results for the Eel River Plume site.   Left:  All months. 
Center:  Winter months.   Right:  Summer months.   Due to increased values of bbp caused by 
increased scattering from inorganic mineral particles in the high sediment load carried by the 
Eel River during the rainy winter months, the Eel River water is distinguishable during the  
winter months at this site.  Under the low flow summer conditions, the water at the Eel River 
Plume site is optically similar to the Coastal Waters site. 
Time-series analyses 
 
In the first step of the time-series analysis, the complete time-series for the Eel River Plume site were generated for 
the period July 2002 to December 2011.  All of the data products are derived from MODIS-Aqua observations, which 
commenced in July 2002.  
In the time series shown  
at left for the Eel River  
Plume site, the influence  
of the Eel River outflow is  
clearly discerned in the  
elevated values of bbp and  
remote sensing reflectance  
at 555 nanometers (Rrs555),  
which occur during the winter  
rainy season.  Surprisingly,  
euphotic depth minima do  
Not correspond to bbp or  
Rrs555 maxima, indicating  
that absorption dominates  
over reflection with regard  
to water clarity.  MODIS sea  
surface temperature displays 
a decreasing trend over the  
time-series period at the  
Eel River Plume site. 
Time-series trends at the Coastal Waters site 
Using the regression analysis function in Excel, the linear slope for the data parameter time series and the significance 
F value were calculated.  The six parameters analyzed were adg, aph, chlorophyll a (chl a), normalized fluorescence line 
height (nFLH), euphotic depth (Zeu) and total absorption (a).  The slope and significance F value are shown below.   
A significance F value < .05 indicates a significant trend at the 95% confidence level. 
Coastal Waters 
Eel River Plume 
Time-series trends at the Eel River Plume site 
Using the regression analysis function in Excel, the linear slope for the data parameter time series and the significance 
F value were calculated for the summer months at the Eel River Plume site. This analysis was conducted to determine 
if trends in the summer at this site, when the influence of the Eel River water is low, were similar to the trends at the 
Coastal Waters site. This analysis extends the earlier work of Acker et al. (2009). 
Slope = -.0115 
Significance F = 0.045 Slope = -.0008 
Significance F = 5.86 x 10-7 
Slope = -1.14 x 10-5 
Significance F = 0.0084 
Slope = -4.79 x 10-5 
Significance F = 0.023 
Slope =  + .045 
Significance F = 0.028 
Slope = -.003 
Significance F = 0.0048 
Slope =  -.0017 
Significance F = 0.0011 
Slope = -.0003 
Significance F =  0.5 
Slope = -.034 
Significance F =  0.12 
Slope = -.00005 
Significance F = 0.29 Slope = + .011 
Significance F = 0.66 
Slope = -.0064 
Significance F = 0.01 
DISCUSSION 
 
 
    This simulation has provided highly interesting results worthy of further 
investigation.  Two goals of the study, to identify the optical signature of 
the Eel River outflow and to compare trends at the two sites, have been 
demonstrated.  The third goal is to evaluate these results in comparison 
to previous studies.  
    At both the Coastal Waters site and the Eel River Plume site, MODIS  
sea surface temperature exhibits a statistically significant negative trend  
(significance F = .008 and .01, respectively).  Summer month SSTs in the  
Eel River plume site did not exhibit a statistically significant trend.  The  
declining trend in SST can be attributed to an increased temperature  
difference between land and ocean temperatures, causing an increase in upwelling-favorable alongshore winds.  These 
conditions would be expected to increasing oceanic upwelling and thus increase phytoplankton chlorophyll concentrations 
due to increased nutrient availability. (Bakun 1990; Snyder et al. 1993) 
    Unlike previous studies, however, this time-series site simulation showed decreasing chl a concentration trends and  
decreasing trends in related optical parameters, which is not consistent with the decreasing trend in SST, provided that 
upwelling has actually increased.  The study sites are located in a “trap” region (Largier 2010), and are not in either the 
Cape Blanco or Cape Mendocino upwelling regions.  It is possible that increased irradiance, possibly related to decreased 
cloud cover or coastal fog, is allowing enhanced photodegradation of organic matter in this region.  
    Acker et al. (2009) reported an increasing trend in SeaWiFS chl a for the Eel River outflow region, for an area slightly 
further offshore than the Coastal Water and Eel River plume sites.  The figure above shows that SeaWiFS chl a shows  
an increasing trend in the Cape Mendocino upwelling region for the 1998-2007 time period studied by Acker et al., while  
MODIS chl a shows a decreasing trend from 2002 to present, concomitant with an increasing trend in Zeu, which is based on 
MODIS data.  Therefore, differences in spaceborne instrumentation and data processing must be considered as a possible 
explanation as well.  In order to determine if the trends observed in this study are real, comparison to in situ data would be 
advisable, thus underscoring the need for sea-truth data to analyze trends observed in remotely-sensed data.  The NOAA 
Central and Northern California Ocean Observation System (CeNCOOS) may be capable of providing such data. 
   
This presentation is dedicated to the memory of 
Gregory G. Leptoukh, October 13, 1953 - January 12, 2012  
Cape Mendocino upwelling region 
Acker, J.G. , E. McMahon,  S. Shen, T. Hearty, and N.  Casey (2009)  Time- 
series analysis of remotely-sensed SeaWiFS chlorophyll in river-influenced  
coastal regions. EARSeL eProceedings, 8(2), 114-139. 
 
Bakun, A.  (1990) Global climate  change and intensification of coastal  
upwelling. Science, 247, 198-201.  
 
Graham, D.J. and N.G. Midgley (2000) Graphical representation of particle  
shape using triangular diagrams: an Excel spreadsheet method.  Earth Surf. 
Process. Landforms, 25, 1473-1477. 
 
Largier, L. (2010)  Oceanography of the MLPA  North Coast Study Region.   
Presentation to the MLPA Blue Ribbon Task Force, January 14, 2010.  
 
Snyder,  M.A., L.C. Sloan, N.S. Diffenbaugh, and J.L. Bell (2003)  Future  
climate change and upwelling in the California Current.  Geophysical  
Research Letters,  30(15),  1823, doi: 10.1029/2003GL017647 
https://ntrs.nasa.gov/search.jsp?R=20120009279 2019-08-30T20:24:51+00:00Z


Using NASA's Giovanni System to Simulate Time-Series Stations in the Outflow Region of California's Eel River

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