Characterization of global ocean turbidity from Moderate Resolution Imaging Spectroradiometer ocean color observations

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115,, doi: /2010jc006160, 2010 Characterization of global ocean turbidity from Moderate Resolution Imaging Spectroradiometer ocean color observations Wei Shi 1,2 and Menghua Wang 1 Received 30 January 2010; revised 1 June 2010; accepted 9 August 2010; published 23 November [1] Seasonal global ocean turbidity is studied and quantified using the diffuse attenuation coefficient at the wavelength of 490 nm, K d (490), derived from measurements of the Moderate Resolution Imaging Spectroradiometer onboard the Aqua satellite. The shortwave infrared based atmospheric correction algorithm and a newly developed K d (490) algorithm have been used to derive K d (490) data for both the global open ocean and coastal turbid waters. The spatial pattern of global open ocean turbidity shows significant seasonal K d (490) variations with highs in the boreal (or austral) spring and summer and lows in the winter for the Northern Hemisphere (or Southern Hemisphere). The clear water with K d (490) 0.1 m 1 covers an average of 95.67% of the global ocean. The modestly turbid waters with K d (490) values ranging from 0.1 to 0.3 m 1 has about 5.12% and 3.07% of the global ocean region in the summer and winter, respectively, with average coverage of 3.59%. Turbid waters with K d (490) over 0.3 m 1 are all located in the coastal regions, river estuaries, and inland lakes with an average global coverage of 0.74%, accounting for 8% to 12% of the total global continental shelf area. The world s major turbid water regions are identified and evaluated in this study. Amazon River Estuary ranks as the world s most turbid region with the mean K d (490) value of 5 m 1. In addition, different mechanisms for the water turbidity in the open oceans and coastal turbid waters are investigated. In the open ocean, variability in the seasonal turbidity is related to the seasonal variation of chlorophyll a concentration, i.e., the seasonal phytoplankton bloom dominates the global geographic perspective of the water turbidity (for waters with K d (490) 0.3 m 1 ). In the coastal region, on the other hand, high turbidity (K d (490) > 0.3 m 1 ) is attributed to the high loading of sediment concentration due to various physical processes, such as sediment resuspension, river runoff, etc. Citation: Shi, W., and M. Wang (2010), Characterization of global ocean turbidity from Moderate Resolution Imaging Spectroradiometer ocean color observations, J. Geophys. Res., 115,, doi: /2010jc Introduction [2] By using the light attenuation in the water column due to the scattering and absorption of the particles and molecules as a measure of water turbidity, the water quality can be related to a quantitative measurement for the light transmission capability. Waters with high turbidity levels are less transparent. An increase of turbidity in the ocean can result from the increase of total suspended matter (TSM), the increase of algae concentration in the water, and the increase of dissolved organic matter (DOM) due to various atmosphere, ocean, and land processes. As an indicator of water clarity, water turbidity can determine the thermal structure of the upper ocean and mixed layer dynamics [Denman, 1973]. Additionally, water turbidity can significantly impact 1 NOAA National Environmental Satellite, Data, and Information Service, Center for Satellite Applications and Research, E/RA3, Camp Springs, Maryland, USA. 2 CIRA, Colorado State University, Fort Collins, Colorado, USA. Copyright 2010 by the American Geophysical Union /10/2010JC the mixed layer dynamics and heat balance [Sathyendranath et al., 1991]. The turbidity increase due to the increase of phytoplankton concentration can lead to significant enhancement of seasonal sea surface temperature (SST) changes [Price et al., 1986]. It has been reported that an increase of the water turbidity due to phytoplankton biomass could lead to 20% amplification of the SST seasonal cycle after incorporating the derived absorption coefficient from ocean color satellite images into the Ocean General Circulation Model (OGCM) [Nakamoto et al., 2000, 2001]. Recent studies show that global, basin wide, and regional SST and the ocean circulation are both sensitive to the water turbidity [Kara et al., 2004, 2005; Subrahmanyam et al., 2008]. [3] In the coastal region and inland waters, such as lakes, marshes, etc., water turbidity is a good indicator of the water quality and can play an important role in understanding the physical, geochemical, and biological processes of the coastal ecosystem. In comparison with the open oceans, the water turbidity in the coastal region is highly dynamic and closely associated with the atmosphere, ocean, and land variability, such as cyclones [Shi and Wang, 2008], algae 1of14

2 blooms [Wang and Shi, 2008], and flood driven river plumes [Shi and Wang, 2009a]. High turbidity waters with a large concentration of TSM in the coastal region can affect water column and benthic processes such as primary productivity [May et al., 2003], coral reef ecosystem [Torres and Morelock, 2002], nutrient dynamics [Mayer et al., 1998], and river dynamics [Nezlin and DiGiacomo, 2005]. [4] In this study, the combined near infrared (NIR) and shortwave infrared (SWIR) atmospheric correction algorithm [Wang, 2007; Wang and Shi, 2005, 2007; Wang et al., 2009b] is used to derive ocean color optical property data for both the open ocean and coastal region waters. Seasonal changes of the global ocean turbidity are characterized and quantified using a newly developed algorithm for the water diffuse attenuation coefficient at the wavelength of 490 nm (K d (490)) [Wang et al., 2009a] from measurements of the Moderate Resolution Imaging Spectroradiometer (MODIS) onboard the Aqua satellite. In the coastal region, some examples are provided for the water turbidity from representatives of the eight most turbid and/or significant regions in the world: the Chinese east coast region, U.S. east coast region, Amazon River Estuary, La Plata River Estuary, Gulf of Martaban and estuaries of Ganges River in the Bay of Bengal, Caspian Sea, North Sea region, and the Gulf of Carpentaria region between Australia and New Guinea. In particular, the interannual variability of the turbidity in the U.S. east coast and China s east coast regions are examined. In addition, mechanisms that cause the change of the water turbidity in the open oceans and coastal ocean regions are discussed. 2. Method 2.1. The SWIR Based Atmospheric Correction Algorithm [5] The current NASA standard atmospheric correction algorithm for producing the global ocean color product from MODIS uses the Gordon and Wang [1994] algorithm. Specifically, the algorithm uses two NIR bands centered at MODIS 748 and 869 nm to determine aerosol type and estimate the atmospheric effects in the visible by extrapolating the aerosol effect from the NIR into visible bands [Gordon and Wang, 1994]. One assumption for the Gordon and Wang [1994] algorithm is that the NIR ocean contributions are negligible for open oceans. For productive ocean waters, some modifications for estimation of the NIR ocean contributions have been employed [Stumpf et al., 2003]. It has been demonstrated that high quality ocean color products have been produced in the global open oceans with the Gordon and Wang [1994] NIR based atmospheric correction algorithm [Bailey and Werdell, 2006; McClain et al., 2004; Wang et al., 2005]. In the coastal region, however, the sensor derived ocean water leaving radiances at the blue are often biased low and sometimes even go negative. Thus, the ocean color products, such as chlorophyll a concentration, may have significant errors in coastal turbid waters (often biased high). This problem often results from the issue of the NIR ocean contributions in turbid waters [Lavender et al., 2005; Ruddick et al., 2000; Siegel et al., 2000; Wang and Shi, 2005]. On the basis of the fact that the water is strongly absorbing in the SWIR wavelengths [Hale and Querry, 1973] and the ocean is generally still black at the SWIR wavelengths even for very turbid waters [Shi and Wang, 2009b], the SWIR atmospheric correction algorithm for turbid waters has been proposed [Wang, 2007; Wang and Shi, 2005]. Two MODIS Aqua SWIR bands at 1240 and 2130 nm are used for atmospheric correction in deriving the ocean radiance contributions [Wang, 2007], and it has been demonstrated that the SWIR based algorithm can derive the improved ocean color products in the coastal turbid waters [Wang and Shi, 2007; Wang et al., 2007; Wang et al., 2009b]. It should be noted that the issue of strongly absorbing aerosols (e.g., dust, smoke) is still one of the main challenges for accurately deriving satellite ocean color products in the coastal region [IOCCG, 2010]. [6] In this study, the NIR SWIR combined atmospheric correction algorithm [Wang and Shi, 2007; Wang et al., 2009b] (with MODIS Aqua bands of 748 and 869 nm for the NIR algorithm and 1240 and 2130 nm for the SWIR algorithm) is used for generating ocean normalized waterleaving radiance nl w (l) spectra data, which are then used for producing ocean optical and biological properties, e.g., K d (490) product [Wang et al., 2009a] Diffuse Attenuation Coefficient K d (490) From MODIS Observations [7] The diffuse attenuation coefficient at the wavelength of 490 nm, K d (490), is an important water property that can be related to light penetration and availability in the ocean. Thus, it is a good indicator for quantifying the global ocean turbidity and identifying the turbid waters associated with biological processes such as phytoplankton presence in the eutrophic zone [Plattetal., 1988; Sathyendranath et al., 1989; Smith and Baker, 1978] or physical processes such as river plumes [Nezlin et al., 2008; Shi and Wang, 2009a], sediment resuspension [Shi and Wang, 2008], and sediment transportation [Acker et al., 2002]. In a recent study, a new K d (490) model [Wang et al., 2009a] combining the current empirical (or semianalytical) algorithm [Lee et al., 2002; Morel et al., 2007; Mueller, 2000] for open oceans and a newly developed semianalytical K d (490) model for turbid coastal waters [Wang et al., 2009a] has been proposed. For coastal turbid waters, backscattering coefficient at the wavelength of 490 nm can be accurately correlated to the reflectance at red bands. Thus, the K d (490) model for turbid waters is formulated with a semianalytical approach using the MODIS derived normalized water leaving radiance (or reflectance) at wavelengths of 488 and 645 nm (or 488 and 667 nm) to derive K d (490) data in turbid waters. The comparison between MODIS Aqua K d (490) retrievals using the new algorithm and the in situ measurements from open oceans and coastal turbid waters shows that it can accurately estimate diffuse attenuation coefficient K d (490) for both clear and turbid waters, e.g., with a mean ratio value (MODIS derived versus in situ data) of [Wang et al., 2009a]. In particular, compared with the in situ measurements, the new K d (490) algorithm shows considerably improved accuracy in the MODIS retrieved K d (490) product in the coastal turbid waters. For example, for the Chesapeake Bay, the mean ratio value (MODIS derived versus in situ data) was improved to from original of [Wang et al., 2009a]. 2of14

3 along the U.S. east coast and China east coast regions, climatological seasonal variability and their corresponding interannual variability are examined and discussed in detail. The seasonal climatological data (monthly means) are derived from MODIS Aqua measurements from July 2002 to August Figure 1. Histogram of climatological K d (490) in April derived from MODIS Aqua ocean color observations from 2002 to 2008 in the region off the U.S. east coast covering open ocean waters, coastal productive waters, and coastal turbid waters Quantification of Global Ocean Turbidity [8] In this study, the NIR SWIR based atmospheric correction algorithm has been used to derive nl w (l) data for both open oceans and coastal waters. With the derived normalized water leaving radiance spectra data, global distribution of ocean turbidity can then be studied with K d (490) retrievals using the Wang et al. [2009a] model. Global MODIS Aqua measurements in the months of January, April, July, and October 2005, which represent the boreal winter, spring, summer, and fall seasons, are used to study the global turbidity distribution and its seasonal variability. Figure 1 shows an example of the histogram of K d (490) in April derived from 6 years of MODIS Aqua observations from 2002 to 2008 in the southern mid Atlantic Bight off the U.S. east coast region, which includes the Chesapeake Bay, Outer Banks, U.S. northeast continental shelf, and open ocean waters. This region is covered with various types of waters, such as sediment dominated waters, eutrophic (or productive) waters, and oligotrophic waters. Thus, the results are generally representative in the optical features for the global ocean. Featured with different histogram modes, which reflect the changes of K d (490) for various water types as shown in Figure 1, K d (490) is used as a representative of the water turbidity, and we define the global ocean into three categories based on the K d (490) value: (1) clear water with 0<K d (490) 0.1 m 1, (2) modestly turbid waters for 0.1 < K d (490) 0.3 m 1, and (3) turbid waters for K d (490) > 0.3 m 1. Global ocean turbidity is then quantified with the above defined three categories. It should be noted, however, that the characterization of the three categories for the global oceans is more or less in a qualitative sense. [9] In addition to the distribution of global turbidity, the latitudinal variations of clear waters, modestly turbid waters, and turbid waters are also analyzed. In particular, some of the world s most turbid and/or significant regions are identified and analyzed in terms of their seasonal variability, coverage, and spatial pattern of the diffuse attenuation coefficient, as well as optical spectral features. Additionally, 3. Results of K d (490) Distribution 3.1. Global Ocean K d (490) Distribution [10] Figure 2 shows the distribution of K d (490) in the boreal winter, spring, summer, and fall seasons represented with the K d (490) composite images in the months of January, April, July, and October 2005, respectively. Significant seasonal variation in K d (490) distribution can be identified in both hemispheres. The areal coverage of modestly turbid waters with 0.1 < K d (490) 0.3 m 1 is about 8.92, 12.05, 15.84, and km 2 for the months of January, April, July, and October 2005, respectively. In contrast to the seasonal change of the coverage for modestly turbid waters, the coverage of the turbid water (K d (490) > 0.3 m 1 ) shows less seasonal variability with the corresponding coverage of turbid waters, which are about 2.09, 2.27, 2.96, and km 2 for the months of January, April, July, and October 2005, respectively. In general, the Pacific Ocean, Indian Ocean, and Atlantic Ocean are dominated by clear waters with K d (490) less than 0.1 m 1 for all seasons. Specifically, in the central and southern Pacific, central and southern Atlantic, and southern Indian oceans, the waters have less seasonal variability with K d (490), usually < 0.05 m 1 for the entire year. As a comparison, the North Pacific and North Atlantic oceans, especially North Atlantic Ocean, have a considerable seasonal K d (490) variability. Large coverage with relatively high K d (490) can be found in both the spring and summer seasons. K d (490) reaches over 0.1 m 1 in the high latitude North Atlantic in the summer, as well as the West Atlantic and North Sea region in the spring season. [11] In the Arabian Sea and Bay of Bengal in the northern Indian Ocean, however, waters are generally more turbid than in the Pacific and Atlantic oceans with K d (490) > 0.05 m 1. The seasonal variability in the northern Indian Ocean is not as significant as in the northern Pacific and northern Atlantic oceans. In the Southern Hemisphere, however, K d (490) spatial patterns show a reversed seasonal change in the high latitude regions compared with the seasonal changes in the Northern Hemisphere (as expected). Specifically, high K d (490) occurs in the austral summer above 40 S in the Southern Ocean with a large area of K d (490) > 0.1 m 1, while in the austral winter K d (490) becomes uniformly low in most parts of the Southern Ocean. [12] Table 1 provides the breakdown of the detailed areal coverage (%) for the clear, modestly turbid, and turbid waters. It is noted that the total global coverage that MODIS Aqua is able to measure changes seasonally. The total areal coverage observed by MODIS Aqua during the months of January, April, July, and October are 336, 334, 309, and km 2, respectively, in comparison with nominal global ocean area of km 2. Considering that the undetected ocean area is generally located in the high latitude polar regions dominated by sea ice [Rayner et al., 2003], its effect on the calculation of the coverage for various K d (490) 3of14

4 Figure 2. Seasonal distribution of global water turbidity as represented with K d (490) in 2005 during the periods (a) January (winter), (b) April (spring), (c) July (summer), and (d) October (autumn). Eight major turbid regions are marked in Figure 2b. ranges is minimal. Note also that data in Table 1 are based on monthly areal coverage measured by satellite. [13] Table 1 shows that the global ocean is dominated by clear open ocean waters with K d (490) 0.1 m 1. For modestly turbid waters, more than half of the areas in each season are for K d (490) within range of 0.1 to 0.15 m 1, while the coverage of modestly turbid water with 0.25 < K d (490) 0.3 m 1 is minimal, compared with the coverage for other K d (490) ranks. Corresponding to the large coverage of modestly turbid waters in the North Atlantic as shown in Figure 2c, the total coverage is km 2 during the boreal summer. This represents 80% increase of coverage in the modestly turbid water from the annual low of km 2 in the winter. [14] In comparison to a significant seasonal variability of modestly turbid water in terms of the spatial patterns and the K d (490) magnitude, the coverage of the turbid waters shows less variation. As shown in Figure 2, turbid waters are all located in the same regions, i.e., along coastal regions, river estuaries, and inland water bodies, for all seasons even though the extent of the turbid waters might be different by season. Table 1 shows that the largest coverage of turbid waters occurred in July It accounts for 1.01% ocean area, while in the boreal winter the coverage reduced to 0.6%. It is noted that some coastal regions have extremely high water turbidity with K d (490) over 3 m 1. In fact, seasonal water coverage of the extremely turbid water is about 0.13%, 0.12%, 0.19%, and 0.16% of global ocean in the months of January, April, July, and October 2005, respectively. [15] In the coastal and inland waters, significantly enhanced K d (490) can be found with K d (490) > 2 m 1. Most of the significantly enhanced K d (490) are associated with the estuaries in the world s major rivers. These highly turbid and/or significant waters are identified in Figure 2b: (1) China s east coast region (Box A), (2) U.S. east coast region (Box B), (3) Amazon River Estuary (Box C), (4) La Table 1. Global Areal Coverage for Various K d (490) Ranges With Different Seasons Coverage (%) for a Given K d (490) a Range K d (490) a Range January April July October Average > > a Unit in m 1. 4of14

5 Table 2. Location Coverage for the Eight Selected Regions in This Study Location Coverage Region (Box in Figure 2b) Latitude Longitude China s East Coast (Box A) 28.5 N 41.0 N E E U.S. East Coast (Box B) 34.3 N 39.9 N 73.8 W 77.5 W Amazon River (Box C) 2.5 S 5.0 N 45.0 W 53.1 W La Plata Estuary (Box D) 33.0 S 38.0 S 52.0 W 59.0 W Bay of Bengal (Box E) 14.0 N 24.0 N 88.0 W 98.0 W Caspian Sea (Box F) 36.5 N 47.5 N 46.5 E 54.5 E North Sea (Box G) 48.0 N 60.0 N 10.0 W 10.0 E Gulf of Carpentaria (Box H) 4.0 S 18.0 S E E Plata River Estuary (Box D), (5) Ganges River Estuary and Gulf of Martaban in the Bay of Bengal (Box E), (6) Caspian Sea (Box F), (7) North Sea Region (Box G), and (8) Gulf of Carpentaria between Australia and New Guinea (Box H). Table 2 provides location coverage (in latitude and longitude) in this study for these eight regions. The waters in these regions are all highly turbid with over tenfold higher K d (490) than those in the open oceans, representing the world s major highly dynamic coastal and inland water ecosystems Latitudinal Distribution of the Turbid Water Coverage [16] Figure 3 shows the latitudinal distribution of the global ocean water for three K d (490) ranges, i.e., 0 < K d (490) 0.1 m 1, 0.1 < K d (490) 0.3 m 1, and K d (490) > 0.3 m 1, for the months of January (Figure 3a), April (Figure 3b), July (Figure 3c), and October 2005 (Figure 3d). For the four seasons, the distributions of turbid waters (K d (490) > 0.3 m 1 ) show similarity even though there are some differences between the magnitudes of the turbid water coverage at different latitudes in different seasons. Compared to the results in Figure 2, the peaks of the latitudinal turbid water distribution in Figure 3 correspond to the highly turbid regions in Figure 2 for each season. As an example, the high turbid water coverage peaks at the latitudes between 45 N and 50 N in the spring and summer resulted from the turbid waters in the north Caspian Sea and Black Sea. The two peaks between 30 N and 40 N mainly represent the turbid waters along China s coastal region, which includes the East China Sea, Yellow Sea, and Bohai Sea. The enhanced K d (490) magnitude in the month of January suggests that the waters in China s coastal region reach highest turbidity during the winter season [Shi and Wang, 2010]. [17] In contrast to the turbid regions in the Northern Hemisphere, the turbid water coverage in the equator has less seasonal variation. The spatial distribution of global turbid waters as shown in Figure 2 suggests that the peak of turbid water coverage in the equator is linked to the Amazon River estuary, with the coverage of km 2 /(degree latitude) for all four seasons. Similar to the turbid water coverage dominated by the Amazon River estuary in the equatorial region, the highly turbid water located at 35 S Figure 3. Latitudinal distribution of global water turbidity for clear water (K d (490) 0.1 m 1 ), modestly turbid water (0.1 < K d (490) 0.3 m 1 ), and turbid water (K d (490) > 0.3 m 1 ) in 2005 for the months of (a) January, (b) April, (c) July, and (d) October. 5of14

6 Figure 4. Climatology K d (490) distribution (with 6 year composite) in China s coastal region (shown as Box A in Figure 2b and Table 2) for the months of (a) January, (b) April, (c) July, and (d) October. covers a relatively stable area and is associated with the La Plata River estuary marked as Box D in Figure 2b. [18] The latitudinal distribution of the modestly turbid waters with 0.1 m 1 < K d (490) 0.3 m 1 is also quantified in Figure 3 for the months of January, April, July, and October Compared to the results with K d (490) > 0.3 m 1, the latitudinal distributions of the modestly turbid water coverage in these 4 months show some variations. The patterns of K d (490) distribution in the four seasons differ from each other significantly (Figure 2). Enhanced coverage of modestly turbid waters is located in the midlatitudes, with a value of km 2 at 40 N in April, and the modestly turbid waters cover more area in high latitudes at 55 N in July. This large coverage of the modestly turbid waters reflects the enhanced K d (490) in the Northern Hemisphere in the midlatitude and high latitude, especially in the North Atlantic Ocean in these 2 months. [19] In the Southern Hemisphere, peak coverage of modestly turbid waters can be identified in the low latitudes between the equator and 15 S. This peak is elevated during the months of April and July. It might be associated with the pronounced chlorophyll a concentration due to upwelling in the equatorial region. Similar to the large coverage of modestly turbid waters in the Northern Hemisphere during the boreal spring and summer, the coverage of modestly turbid waters in the Southern Hemisphere during the austral spring (October) and summer (January) also increases significantly in the midlatitude and high latitude of the Southern Hemisphere, but the magnitude of the peak only reaches km 2 at 45 N during January, compared to a much more broad extent of modestly turbid waters in the Northern Hemisphere during the months of April and July. The spatial distribution of K d (490) in Figure 2 shows that high K d (490) with values >0.1 m 1 cover a band shaped region traversing the Atlantic, Indian, and east Pacific oceans at 45 S during the austral summer in January. On the other hand, reflecting the large coverage of the modestly turbid water in the Northern Hemisphere in the boreal summer (July), K d (490) spatial distribution (Figure 2c) shows that the northern Atlantic and eastern Pacific over 45 N are dominated with modestly turbid waters. [20] On the other hand, the latitudinal distributions of the clear waters with K d (490) 0.1 m 1 show little seasonal variability, and the areal coverage is, in general, 1 2 orders higher than the turbid and modestly turbid waters as shown in Figure 3. This reflects the dominance of the clear waters in the global ocean and is consistent with the spatial distributions of K d (490) during different seasons as shown in Figure Turbid Waters along China s Coast Region [21] Figure 2 shows that China s coastal region has some of the most turbid waters in the world in terms of coverage and K d (490) magnitude in all seasons with an extended region over 300 km offshore. Using the MODIS Aqua measurements from 2002 to 2008, the climatological seasonal variation of water turbidity in this region has been derived. Figure 4 provides the climatological K d (490) distribution in the winter, spring, summer, and fall as represented with the 6 year composite K d (490) in the months of January, April, July, and October. In general, it shows that China s coastal region has significant variations in the seasonal turbidity [Shi and Wang, 2010]. As shown in Figure 4a, winter is the most turbid season in the region with enhanced and enlarged coastal turbid waters. During the winter season, the magnitude of K d (490) in Hangzhou Bay and the coastal region north of the Yangtze River estuary is elevated over 3 m 1, and the turbid water coverage extends over 300 km offshore. In the north, the majority of the Bohai Bay is dominated by the turbid waters with K d (490) over 1 m 1. [22] During the summer, however, the turbidity in the entire region is significantly reduced and the coverage of the turbid region is confined to a region of 100 km offshore north of the Yangtze River, which reduces to 1/3 of the winter coverage. In the Bohai Sea, turbid water only covers a small portion in the north and east. In fact, the turbid water spatial patterns during the spring and fall are quite similar. Even though there is no significant change for the coverage of the turbid water in these two seasons in comparison with that during the winter (Figure 4a), the K d (490) value near the coast is only 1 to2m 1, compared to the value of 3 m 1 in the winter at the same location. In the Bohai Sea, reduced K d (490) pattern is also observed during these two seasons. 6of14

7 Figure 5. Climatology K d (490) distribution (with 6 year composite) in the U.S. east coast region (shown as Box B in Figure 2b and Table 2) for the months of (a) January, (b) April, (c) July, and (d) October Turbid Waters Along the U.S. East Coast Region [23] Although the U.S. east coast region is not among the world most turbid regions, the high population along the U.S. east coast and the concern over the gradual deterioration of its major ecosystems, such as the Chesapeake Bay, make it necessary to evaluate the water turbidity in this region. [24] Figure 5 shows MODIS 6 year climatology of K d (490) spatial distribution during the four seasons, which are represented with the months of January, April, July, and October. For the four seasons, the coverage of the turbid water does not change significantly, i.e., they are primarily located in the Chesapeake Bay, Delaware Bay, and Pamlico Sound within the Outer Banks. January and July correspond to the most and least water turbidity in this region, respectively. Highly turbid waters with K d (490) > 2 m 1 are primarily located at the upper Chesapeake Bay and Delaware Bay, as well as some major river estuaries. In the Pamlico Sound region, K d (490) is high for all four seasons. The seasonal averages of K d (490) in the Pamlico Sound region are 2.28, 1.80, 1.09, and 1.34 m 1 for January, April, July, and October, respectively. In general, the U.S. east coast region is less turbid than the region along China s east coast in terms of both the turbid water coverage and the strength of the turbidity (from a magnitude of K d (490)) Other Major Turbid Water Regions [25] Figure 6 shows some of the world s other most turbid regions, specifically the Amazon River estuary and the Rio De La Plata River estuary between Argentina and Uruguay, as shown as Box C and Box D in the Southern Hemisphere (Figure 2b and Table 2), respectively. In the Northern Hemisphere, in addition to the east coast regions of the United States and China described in sections 3.3 and 3.4, the estuary of the Ganges River and the Gulf of Martaban in the Bay of Bengal (Box E in Figure 2b) also show higher water turbidity. As the largest enclosed body of water on Earth by its areal coverage, the Caspian Sea region, as marked as Box F in Figure 2b, also shows its large coverage of highly turbid waters in comparison with other global ocean regions and inland waters Amazon River Estuary [26] In the Amazon River estuary, the waters show the highest turbidity due to the strongest river runoff with the K d (490) > 4m 1 for most part of the river estuary. Figure 6a shows that most of the extremely highly turbid water is confined within the Amazon River estuary and its tributaries. Some of the river plume waters extend northward about 250 km along the coast. Two fronts can be observed in contrast to the different water turbidity. In the open ocean, the value of K d (490) is 0.04 m 1. Parallel to the coast 120 km offshore, the water turbidity of this region is featured with a band of less extremely high turbid river plume waters with K d (490) values between 1.5 and 2.5 m 1. In the estuary of the Amazon River and its tributaries, the turbidity of the waters gets larger with K d (490) > 5 m 1. It is noted that, unlike the extremely high turbidity of the Amazon River estuary, the mouth of the Torcantins River (south of the Amazon River) shows a relatively low turbidity with the mean K d (490) of 2 m 1. [27] Because of the changes of the dry and wet seasons in the Amazon River region, the water turbidity in this region also experiences seasonal variability corresponding to the strength of Amazon River discharges. Monthly K d (490) composite images (not shown here) reveal that higher water turbidity in this region is found in the austral summer and fall seasons following the wet season, while the waters become clearer in the austral winter and spring seasons following the dry season. The mean values of K d (490) in the Amazon River estuary region during the months of January, April, July, and October 2005 are about 4.72, 4.52, 4.42, and 3.81 m 1, respectively La Plata River Estuary [28] The La Plata River estuary is another region with pronounced water turbidity. Figure 6b shows the mean K d (490) values for the four different seasons represented with the months of January, April, July, and October The magnitude of K d (490) within the La Plata River estuary is over 4 m 1, which is comparable to the value of K d (490) in the Amazon River estuary, and higher than 2 3 m 1 of the mean K d (490) values along the China east coast region. The coverage of the high turbid water in this region is entirely confined within the estuary and does not extend along the coast or to the open oceans. Seasonal maps of the K d (490) (not shown here) show that the highest water turbidity occurs 7of14

8 Figure 6. Annual mean K d (490) distribution as represented by the composite image of K d (490) in January, April, July, and October 2005 for the regions of (a) Amazon River estuary, (b) La Plata River estuary, (c) Bay of Bengal, (d) Caspian Sea, (e) North Sea, and (f) Gulf of Carpentaria. These six regions are marked as Boxes C, D, E, F, G, and H as shown in Figure 2b and in Table 2 for the corresponding coverage. Note that a log scale is used to accommodate large K d (490) variations from various regions. in the austral fall and winter, while the lowest K d (490) exists in the austral spring. As an example, the monthly mean K d (490) values in the La Plata River estuary during the months of January, April, July, and October are 3.88, 4.62, 4.58, and 3.55 m 1, respectively. Similar to the coverage of the turbid waters in the Amazon River estuary, however, the coverage of the turbid waters in this region does not show significant seasonal variations Bay of Bengal [29] In the Northern Hemisphere, the Bay of Bengal and the Caspian Sea are the other two regions featured with highly turbid waters. In the Bay of Bengal, the two major turbid water regions are the estuary of the Ganges River and the Gulf of Martaban (Figure 6c). In these two highly turbid regions, however, the annual mean turbidity is smaller than those in the Amazon River and La Plata River estuaries with the mean K d (490) ranging from 3 to4m 1. Both the coverage of the highly turbid waters and the magnitude of K d (490) in these two regions do not show significant seasonal variations with the highest K d (490) occurring during the boreal summer and fall and the lowest one during the boreal winter. For example, the mean K d (490) value at a location of (22.18 N, E) within the Ganges River estuary was 3.42 m 1 in the summer of 2005, while for the same location the mean K d (490) value dropped to 2.95 m 1 in the winter of Caspian Sea [30] The Caspian Sea has the largest inland waters on Earth and accounts for over 40% of the global total inland water. Figure 6d shows the annual distribution of K d (490) in Clearly, this inland lake is dominated by significantly different water types. For the central and southern Caspian Sea regions, K d (490) is dominated by modestly turbid waters with K d (490) ranging from 0.1 to 0.2 m 1 and with little seasonal variability. The K d (490) values are similar to the bloom waters in the North Atlantic and East Pacific in the summer as shown in Figure 2c. On the other hand, the northern Caspian Sea region features highly turbid waters. Even though K d (490) is not as high as in the Amazon River and La Plata River estuaries, K d (490) is usually > 2m 1 and makes the turbidity in the northern Caspian Sea in the same magnitude as vast turbid regions along China s east coast region. Seasonal K d (490) (results not shown here) suggests that the highest turbidity in the northern Caspian Sea occurs during the fall season. The mean K d (490) values in the northern Caspian Sea for the months of January, April, July, and October 2005 are about 2.40, 1.97, 2.03, and 3.53 m 1, respectively, with the corresponding coverage (regions with K d (490) > 2.0 m 1 ) of 2.42, 2.13, 1.55, and km 2, respectively. 8of14

9 Figure 7. MODIS Aqua derived time series of K d (490) and chlorophyll a concentration in the central North Atlantic with a 2 2 box centered at a location of (43 N, 34 W) (location marked in Figure 2c) North Sea and Gulf of Carpentaria [31] Figures 6e and 6f show the annual mean patterns of water turbidity in the North Sea (Box G in Figure 2b) and the Gulf of Carpentaria between Australia and New Guinea (Box H in Figure 2b). In comparison with the other turbid regions, such as the Amazon River estuary and China s east coastal region, these two regions are less turbid with less coverage of highly turbid waters. The highly turbid waters with K d (490) > 1.0 m 1 are all confined near the coast in these two regions. For most parts of these two regions, K d (490) ranges from 0.15 to 0.5 m Mechanisms for Global Ocean Turbidity 4.1. Mechanisms for Open Ocean Turbidity [32] As shown in Figure 2, the turbidity of the vast global ocean experiences significant seasonal variability. This is especially true for the North Atlantic Ocean with pronounced K d (490) in the boreal summer (Figure 2c) and reduced K d (490) in the winter. This basin wide seasonal change of water turbidity can be attributed to seasonal biological variability associated with the North Atlantic bloom [Siegel et al., 2002] during the spring and early summer. In fact, for both the boreal winter and summer seasons, the spatial distributions of chlorophyll a concentration in January and July (results not shown) are similar to those of K d (490) (Figures 2a and 2c). [33] Figure 7 provides a MODIS Aqua measured time series of K d (490) and chlorophyll a concentration from 2002 to 2008 obtained with an area of 2 2 centered at a location of (43 N, 34 W) in the North Atlantic (location indicated in Figure 2c), showing that the turbidity change in the open ocean is closely related to the chlorophyll a variation. At this location, changes of the chlorophyll a concentration and the diffuse attenuation coefficient are synchronous with each other in terms of both temporal variation and their corresponding magnitudes. As an example, both of these two parameters peak in mid and late spring, attributing to the North Atlantic phytoplankton blooms, while K d (490) and the chlorophyll a concentration reduce to an annual low during the boreal winter season. In particular, during 2003, K d (490) and chlorophyll a concentration reached their highest values of 0.11 m 1 and 0.8 mg/m 3 at this location in April, respectively, while these values dropped to 0.04 m 1 and 0.2 mg/m 3, respectively, during the winter in that same year. Interannual changes of K d (490) and chlorophyll a concentration also show similarity with the highest peaks occurring in 2003 and the lowest in 2008 for both K d (490) and chlorophyll a concentration. These results are consistent with those from some previous studies such as Morel et al. [2007] Mechanisms of the High Turbidity in the Coastal Region [34] Unlike the modest increase of K d (490) in response to the elevation of chlorophyll a concentration following a phytoplankton bloom such as the North Atlantic bloom as shown in Figure 7, some coastal and estuarial regions, such as along China s east coast region and regions shown in Figure 6, are waters featuring extremely high turbidity. Different from the turbidity in the open oceans and productive waters, the extremely enhanced water turbidity results primarily from the high concentration of suspended sediments in the coastal regions, especially river estuaries, instead of biological activities in the open oceans or productive waters. [35] Using China s east coast region as an example, Figure 8 demonstrates different mechanisms of highly turbid waters in the coastal region and river estuaries. Different from the optical properties of open ocean waters and productive waters, sediment dominated waters are featured with significantly enhanced normalized water leaving radiance spectra (nl w (l)) in the red and NIR wavelengths [Shi and Wang, 2008, 2009a, 2009b; Wang et al., 2007]. In fact, studies have shown that [Miller and McKee, 2004; Shi and Wang, 2009a] in the Mississippi River estuary the normalized water leaving radiance at the red band 645 nm, nl w (645), is proportional to the concentration of total suspended matters (TSM). On the other hand, the nl w (645) value for the open oceans is usually less than 0.1 mw cm 2 mm 1 sr 1, and the increase of the nl w (645) value due to the chlorophyll a concentration in the productive water is trivial, compared with nl w (l) values in the blue and green bands. Indeed, the radiance spectra of the North Atlantic waters (not shown here) show that nl w (645) only increases from 0.05 to 0.10 mw cm 2 mm 1 sr 1 from the nonbloom season to bloom season. In the coastal region, however, even though high productivity is often associated with high sediment concentration, the contribution of the phytoplankton particles to the normalized water leaving radiance at the red and NIR wavelengths is much smaller than those contributed from the sediment particles. The different optical features of the sediment waters, productive waters, and open ocean waters in the red and NIR wavelengths suggest that nl w (l) in red bands, such as nl w (645) (or nl w (667)) from MODIS Aqua measurements, can actually act as a surrogate for the amount of suspended particles in the coastal waters in order to investigate its role of impacting on coastal turbidity. [36] It is found that the spatial distribution of nl w (645) along China s east coast region is almost identical to the spatial pattern of K d (490), with low nl w (645) and K d (490) in the open ocean and farther offshore regions, while in China s east coast regions, such as Hangzhou Bay, the Yangtze River estuary, and the Yellow Sea coastal region, nl w (645) and 9of14

10 Amazon River estuary at a location of (0.63 N, W) (location shown in Figure 6a with Data Analysis ) and the Ganges River estuary at a location of (22.18 N, E) (location shown in Figure 6c with Data Analysis ) during the months of January, April, July, and October 2005, respectively. Corresponding to the small seasonal changes of K d (490) in the Ganges River estuary, the r wn (l) spectra are quite stable during the four seasons. In the Amazon River estuary, however, the change of r wn (l) spectra is obviously reflecting the significant K d (490) seasonal variations. In addition to the difference of the magnitudes of r wn (l) in these two regions, the r wn (l) spectral shapes for these two regions are also different. The reflectance spectrum at the Ganges River estuary peaks in the green band at 555 nm, while the reflectance spectrum at the Amazon River estuary peaks in the red band at 645 nm. These reflectance spectra results reflect mainly the change in reflectance magnitudes spectrally, i.e., the maximal reflectance is shifted to red with more sediment in suspension in the Amazon River estuary, compared with a maximal value at the green for the less turbid Ganges River estuary. It might also be attributed Figure 8. MODIS Aqua derived time series of (a) K d (490) and (b) nl w (645) from a location of (30.51 N, E) in Hangzhou Bay as marked in Figure 4a. Monthly climatology data (dotted lines) of K d (490) and nl w (645) are corresponding monthly mean values derived from MODIS Aqua measurements from July of 2002 to August of K d (490) are both significantly enhanced. Using Hangzhou Bay as an example, time series of nl w (645) and K d (490) at a location of (30.51 N, E) within Hangzhou Bay (location indicated in Figure 4a with Data Analysis ) shows synchronized seasonal and interannual variability (Figure 8). Consistent with the K d (490) spatial distributions in different seasons, the highest turbidity is observed in the winter and the lowest in the fall with the K d (490) magnitude ranging from 0.7 to 3 m 1 (Figure 8a). Correspondingly, nl w (645) at this location also changes seasonally with the high over 4 mw cm 2 mm 1 sr 1 during the winter and low below 1mWcm 2 mm 1 sr 1 in the fall (Figure 8b). For a comparison, in the open ocean, K d (490) and nl w (645) are typically 0.06 to 0.13 m 1 and 0.05 to 0.15 mw cm 2 mm 1 sr 1, respectively, with little seasonal variation. 5. Optical Properties of Turbid Waters 5.1. Regional Dependent Optical Spectra Data [37] Figure 9 shows the seasonal normalized water leaving reflectance spectra, r wn (l), for different turbid waters in the Figure 9. Comparison of the seasonal normalized waterleaving reflectance spectra derived for the months of January, April, July, and October of 2005 and for the regions of (a) Amazon River estuary at a location of (0.63 N, W) (marked in Figure 6a) and (b) Ganges River estuary at a location of (22.18 N, E) (marked in Figure 6c). 10 of 14

11 Figure 10. Annual mean nlw(859) distributions as represented by the composite nlw(859) images from January, April, July, and October of 2005 for the regions of (a) Amazon River Estuary, (b) La Plata River Estuary, (c) Bay of Bengal, and (d) Caspian Sea. These four regions are marked as Boxes C, D, E, and F in Figure 2b. to regional differences for these two regions in the water optical, biological, and geochemical properties Normalized Water Leaving Radiance in the Red and NIR Wavelengths [38] Since turbid waters are featured with the enhanced nlw(l) (or rwn(l)) in the red and NIR wavelengths, the increase of water turbidity is also reflected with elevated levels of nlw(l) in the red and NIR bands. Figure 10 shows the corresponding annual mean nlw(l) at the wavelength of 859 nm, nlw(859), in some extremely turbid regions as shown in Figure 6, i.e., Amazon River, La Plata River estuary, Bay of Bengal, and Caspian Sea. Consistent with low water turbidity in the offshore areas in these regions, nlw(859) is nearly 0, but remarkable nlw(859) is found in the turbid waters of these four regions. It is evident that both the magnitude and spatial patterns of nlw(859) follow Kd(490) as shown in Figure 6 in these four regions. For example, the nlw(859) reaches over 3 mw cm 2 mm 1 sr 1 in the middle of the Amazon River estuary, where Kd(490) is 5 m 1 at the same location. Strong ocean contribution at the NIR wavelength in the turbid region indicates that the SWIR based atmospheric correction is necessary for deriving accurate ocean color products in these regions. [39] Figure 11 provides examples of the relationships between MODIS Aqua derived Kd(490) and nlw(645) (or nlw(859)) in the Amazon River estuary. Results in Figure 10 show that nlw(645) increases monotonically with the increase of Kd(490). In the open ocean, nlw(645) is only 0.1 mw cm 2 mm 1 sr 1 with Kd(490) < 0.1 m 1. When Kd(490) reaches 4 m 1 in the central Amazon River estuary, nlw(645) becomes significantly larger at 6.0 mw cm 2 mm 1 sr 1. On the other hand, the scatterplot of Kd(490) versus nlw(869) in Figure 11 shows a significant deviation from a quasi linear Kd(490) nlw(645) relationship. When Kd(490) is less than 3 m 1, nlw(859) is more or less proportional to Kd(490) linearly. But for the extremely turbid waters with Kd(490) > 3 m 1, the linearity between Kd(490) 11 of 14

12 Figure 11. Scatter plot of MODIS Aqua measured nl w (645) and nl w (859) versus K d (490) obtained from the Amazon River estuary. and nl w (859) is not valid anymore. Instead, nl w (869) increases almost exponentially with the increase of K d (490). The nonlinearity between nl w (859) and K d (490) is due to nonlinear relationships of nl w (l) at wavelengths among red, NIR, and SWIR bands for the extremely turbid waters as described by Shi and Wang [2009b]. 6. Discussions and Conclusion [40] It has long been known that the current NIR based atmospheric correction algorithm leads to errors of ocean color retrievals in turbid coastal regions since the black pixel assumption for the NIR bands is no longer valid [Lavender et al., 2005; Ruddick et al., 2000; Siegel et al., 2000; Stumpf et al., 2003; Wang and Shi, 2005]. On the other hand, the current operational K d (490) algorithm based on the blue green normalized water leaving radiance ratio [Mueller, 2000] is accurate only for clear open ocean waters but not applicable for optically complex waters [Son et al., 2005; Wang et al., 2009a]. In this study, the NIR SWIRbased atmospheric correction algorithm [Wang, 2007; Wang and Shi, 2007] is used to derive ocean water leaving radiance spectra data, and the water diffuse attenuation coefficient data are obtained from the MODIS derived radiance data using a newly developed K d (490) algorithm [Wang et al., 2009a]. It has been demonstrated that the NIR SWIRbased atmospheric correction algorithm for ocean color data processing can derive improved ocean color products in the coastal turbid regions [Wang and Shi, 2007;Wang et al., 2007; Wang et al., 2009b], and a new K d (490) algorithm can be reliably used for producing global K d (490) data [Wang et al., 2009a]. Thus, the K d (490) data can be used to study the global ocean (including both open oceans and coastal waters) turbidity distribution. [41] This study is different from some other studies [Lee and Hu, 2006; Morel and Belanger, 2006]. The study of Morel and Belanger [2006] is focused on identifying turbid waters, while the work of Lee and Hu [2006] is for identifying Case 1 waters. Both studies have not directly addressed global ocean turbidity. No ocean bio optical model such as the one of Lee and Hu [2006] is used in this study. Instead of using qualitative analysis with turbid water flag and focus on turbid water detection algorithm, we have conducted a systematic analysis to quantify global ocean turbidity using a water diffuse attenuation coefficient at the wavelength of 490 nm, K d (490), as a representative of the water turbidity in this study. In the work of Morel and Belanger [2006], for the sediment dominant water the turbid water detection is based on a reflectance threshold comparison at the green band between the sensor measured and the model based reflectance estimation from the chlorophyll a value. It is expected that the global turbid water map from Morel and Belanger [2006] should be more or less consistent with the coverage derived for high K d (490) values. On the other hand, Lee and Hu [2006] suggested that the region with high backscattering (also enhanced K d (490)) in the Southern Hemisphere during Autumn 2003 might be attributed to the abundance of ocean s suspended calcium carbonate due to coccolith blooms [Balch et al., 2005]. [42] Four months of MODIS Aqua global ocean data are processed to produce the seasonal change of global ocean turbidity. Interannual variability of global ocean biosphere has been documented and associated with the global ocean and climate processes [Behrenfeld et al., 2006; Yoder and Kennelly, 2003]. Because of the relationship between global ocean turbidity and chlorophyll a concentration, which is related to ocean productivity and phytoplankton bloom, it is consequently deducted that interannual variability of global water turbidity should follow the changes of the ocean biosphere. In the regional scale, in addition to the global climate variability, some major climate events, and the variability of the oceanic and atmospheric processes such as tropical storms and hurricanes [Hu and Muller Karger, 2007; Shi and Wang, 2008], river flooding caused by extreme rainfall [Shi and Wang, 2009a; Walker et al., 1994] can also trigger significant interannual changes of the marine ecosystem and anomalous water turbidity in the coast regions. Since the focus of this study is the seasonal change of global ocean turbidity, this long term variability of ocean turbidity in both global and local scales is beyond the scope of this research. [43] This study demonstrates that global ocean turbidity has seasonal variations with the highest coverage of modestly turbid waters for K d (490), ranging from 0.1 to 0.3 m 1 in the boreal spring, and the lowest in the boreal winter. The modestly turbid waters are mostly located in the open oceans and offshore regions and account for about and km 2 global ocean areas in the summer and winter, respectively. Most of the turbid waters with K d (490) over 0.3 m 1 are located in the coastal regions, river estuaries, and inland lakes. Seasonal change of the turbid water coverage is not significant. The coverage of the turbid waters is about 8% 12% of the total km 2 global continental shelf coverage [Borges et al., 2005]. Different mechanisms for the offshore, modestly turbid waters and the coastal turbid waters are investigated. The offshore, modestly turbid waters are attributed to the seasonal turbidity increase due to the seasonal phytoplankton blooms, such as the spring and early summer North Atlantic phytoplankton blooms responding to seasonal climate changes. In the coastal regions, on the other hand, high turbidity is caused by the resuspension of sediment particles. Spectral analysis of different turbid waters also suggests that particle properties such as size, size distribution, 12 of 14