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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 116, C12032, doi:10.1029/2011JC007234, 2011

Spring-neap tidal effects on satellite ocean color observations in the Bohai Sea, Yellow Sea, and East China Sea Wei Shi,1,2 Menghua Wang,1 and Lide Jiang1,2 Received 19 April 2011; revised 5 October 2011; accepted 7 October 2011; published 22 December 2011.

[1] Eight-year ocean color observations between 2002 and 2009 from the Moderate Resolution Imaging Spectroradiometer (MODIS) onboard the Aqua satellite are used to quantitatively assess the spring-neap tidal effects on variability of ocean optical and biogeochemical properties in the Bohai Sea, Yellow Sea, and East China Sea. We demonstrate that spring-neap tidal variation is one of important ocean processes that drive both the synoptic-scale and mesoscale changes of the ocean optical, biological, and biogeochemical properties in the coastal region. Normalized water-leaving radiance spectra (nLw(l)), water diffuse attenuation coefficient at the wavelength of 490 nm (Kd(490)), and total suspended matter (TSM) concentration show significant spring-neap variations in the coastal region within a lunar cycle of 29.53 days. In the open ocean, however, springneap tidal effects on ocean color data are negligible. The entire areal coverage of the turbid waters (Kd(490) > 0.3 m 1) showing significant spring-neap tidal variations is 4–5  105 km2. Similar coverage of moderately turbid waters (0.1 < Kd(490) ≤ 0.3 m 1) is also impacted by the spring-neap tides. The magnitude of the spring-neap tidal effects on the variations of the satellite ocean color properties, e.g., Kd(490) and TSM, is in the same order as the seasonal variations in the coastal region. Highest Kd(490) and largest turbid water coverage lag the new moon (or full moon) about 2–3 days, while the lowest Kd(490) and smallest turbid water coverage are also 2–3 days behind the one-quarter (or three-quarter) moon. This is attributed to the seawater inertia and the friction against the seabed as well as the sediment resuspension process. Citation: Shi, W., M. Wang, and L. Jiang (2011), Spring-neap tidal effects on satellite ocean color observations in the Bohai Sea, Yellow Sea, and East China Sea, J. Geophys. Res., 116, C12032, doi:10.1029/2011JC007234.

1. Introduction [2] Ocean tide is a phenomenon of the sea level rise and fall. Tidal height and tidal current are important factors for the fishery, ship navigation, and coastal zone engineering projects, as well as the tourism and people’s daily life in the coastal region. As the largest source of short-term, widely spread sea level and ocean current changes, ocean tide changes on a time scale from a couple of hours to years. For most of the ocean coastal regions, the largest constituent of the tide is the semidiurnal lunar tide with a period of about 12 h and 25 min. Since tidal change is a result of the combining forces of the Moon, Earth, and Sun, the gravity forces reach maximum when the Moon, Earth, and Sun are in a line during the full and new moons. Likewise, the gravity forces reach minimum when the Moon, Earth, and Sun are at the right angles during the quarter moons. Spring tides with enhanced tidal elevation amplitudes and tidal currents follow 1 Center for Satellite Applications and Research, NOAA National Environmental Satellite, Data, and Information Service, Camp Springs, Maryland, USA. 2 CIRA, Colorado State University, Fort Collins, Colorado, USA.

Copyright 2011 by the American Geophysical Union. 0148-0227/11/2011JC007234

the new/full moons, while neap tides with weakened tidal elevation amplitude and tidal current come with the quarter moons. Thus, a lunar cycle of 29.53 days includes two full spring-neap cycles. Spring and neap tides can lag behind the phases of the moon by a couple of days due to the inertia of the water and the friction of the ocean bottom. Figure 1 schematically shows the spring-neap tide variations in a lunar cycle. [3] High-quality ocean color products have been produced in the global open oceans in the last decade [Bailey and Werdell, 2006; McClain, 2009; McClain et al., 2006; Wang et al., 2005]. Satellite ocean color remote sensing product data, e.g., from the Moderate Resolution Imaging Spectroradiometer (MODIS) [Esaias et al., 1998; Salomonson et al., 1989], have long been used to study the global ocean and atmospheric processes, such as the ocean’s global-scale variability [Behrenfeld et al., 2001; Chavez et al., 1999; Shi and Wang, 2010a], ocean response to a short-term weather event [Shi and Wang, 2007, 2011; Walker et al., 2005], phytoplankton blooms [Babin et al., 2004], harmful algae blooms (HABs) [Tang et al., 2004], floating green algae blooms [Hu, 2009; Shi and Wang, 2009a], river plume [Nezlin et al., 2008; Shi and Wang, 2009b; Warrick and Fong, 2004], ocean optical and biological property variations in the Korean dump site

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Figure 1. Schematic chart about the spring-neap tidal variation in a lunar cycle. Note this chart is reproduced from Wikipedia (http://en.wikipedia.org/wiki/Tide).

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of the Yellow Sea [Son et al., 2011], and other changes of coastal environments [Hu et al., 2004; Shi and Wang, 2008] and inland fresh water environment [Wang et al., 2011]. However, the spring-neap tidal effects on the variation of satellite-derived ocean color products, especially for the coastal regions, have never been studied and addressed. Part of the reason is that the ocean is frequently covered with clouds in daily satellite coverage. Thus, it is difficult to have continuous satellite ocean color observations in a spring-neap tidal cycle for a certain area in order to identify the springneap tidal variability and differentiate it from other changes caused by various ocean and atmospheric processes. [4] The Bohai Sea (BS), Yellow Sea (YS), and East China Sea (ECS), which are located in the western Pacific Ocean, have a total coverage of 1.71  106 km2 (Figure 2). Due to the sediment deposition in these three oceans transported by the rivers, the regions of the BS, YS, and ECS are among the most turbid ocean regions in the world [Shi and Wang, 2010a]. In addition to other ocean processes such as seasonal monsoon winds [Ding, 1994] and river discharge [Lee et al., 2004], tidal current is one of the major processes that drives the ocean physical, optical, biological, and biogeochemical changes in the region. It is estimated that 7%

Figure 2. Map of the Bohai Sea (BS), Yellow Sea (YS), and East China Sea (ECS) with isobaths of 20, 50, and 100 m. The study area is outlined in the box, and locations of the four pseudostations for detailed quantifications are also marked. 2 of 13

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(10 to 12  1010 W) of the global dissipation of tide energy occurs in the shallow water regions east of China [Choi, 1980]. Tide-driven current, mixing, and stratification can cause the enhanced sediment suspension in the Yangtze River Estuary [Yang et al., 2000] and the water property changes over the ocean sand ridges [Li et al., 2009; Shi et al., 2011] in the Bohai Sea. In the Yangtze River Estuary and the Hangzhou Bay of the East China Sea, it was reported that suspended sediment concentration (SSC) showed seasonal and spring-neap tidal cycles [Chen, 2001]. SSCs are higher during the spring tides in the winter season. The SSC during the spring tides is 1.5–2.5 times of the SSC during the neap tides in over a 1-year period. [5] The purpose of this study is twofold. First, we use the BS, YS, and ECS regions as an example to demonstrate that the satellite ocean color observations indeed contain springneap tidal information. This adds another source responsible for the changes observed by satellite sensors, in addition to already known sources such as seasonal and interannual climate variability, extreme weather events (e.g., a hurricane and a river flood), and physical, biological, and biogeochemical processes such as upwelling, sediment resuspension, and phytoplankton bloom. The second purpose is to quantify the effects of the spring-neap tidal cycles on the ocean optical, biological, and biogeochemical properties in these regions and assess the impact of the spring-neap tidal variation on the coastal ecosystem, in comparison to the variations caused by other driving forces such as seasonal climate changes.

2. Tides in the Bohai Sea, Yellow Sea, and East China Sea [6] There have been numerous studies on the tide and tidal current in the BS, YS, and ECS regions using field observations [Larsen et al., 1985; Nishida, 1980; Ogura, 1933], two-dimensional or three-dimensional numerical models [Choi, 1980; Guo and Yanagi, 1998; Yanagi and Inoue, 1994], and satellite altimetry measurements [Yanagi et al., 1997]. These studies show that the tide is predominantly semidiurnal in the region, particularly in the YS and ECS, while the diurnal tide becomes more important in the BS region but its impact remains secondary. The most dominant tidal constitute in the region is the principal lunar semidiurnal constituent M2, which represents the rotation of the Earth with respect to the Moon and has a frequency of about 1.932 cycles per day. According to the corange charts provided in the aforementioned studies, the M2 constituent can reach a tidal range of more than 2.5 m and tidal current of more than 2 m/s near the Yangtze River Estuary and the Hangzhou Bay. [7] The second major player in the region is the principal solar semidiurnal constituent S2, which represents the rotation of the Earth with respect to the Sun and has a frequency of exactly two cycles per day. In this region the magnitude of S2 is about 40% of M2, large enough to produce a strong spring-neap tidal signal. The semidiurnal N2 component, which involves noncircularity of the lunar orbit, has the negligible effect in this study since it has little impact on the phase of spring-neap tide. The major diurnal tides are the lunisolar diurnal constituent K1 and the lunar diurnal constituent O1. Together they represent the effect of the moon’s

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declination and have frequencies of about 1.003 and 0.930 cycles per day, respectively. Their magnitudes are both about 10–20% of M2 in the YS and ECS and become more prominent in the BS region, to about 40% of M2, although previous studies show some extent of disagreement in the BS in terms of K1 and O1 magnitudes [Guo and Yanagi, 1998]. It is noted that K1 and O1 components also have a negligible impact on the phase of spring-neap tide.

3. MODIS Data Processing and Ocean Color Parameters [8] In this study, all MODIS-Aqua observations over the BS, YS, and ECS between 2002 and 2009 are acquired and processed using the near-infrared (NIR) and shortwave infrared (SWIR) combined atmospheric correction algorithm (with MODIS-Aqua bands of 748 and 869 nm for the NIR algorithm and 1240 and 2130 nm for the SWIR algorithm) [Gordon, 1997; Wang and Shi, 2005, 2007; Wang et al., 2009b] to produce normalized water-leaving radiance spectra nLw(l) [Gordon and Wang, 1994; International Ocean Colour Coordinating Group (IOCCG), 2010]. It has been shown that the ocean black pixel assumption at the SWIR bands for conducting atmospheric correction is generally valid in these regions [Shi and Wang, 2009c]. It has been demonstrated that the SWIR-based algorithm can derive improved ocean color products in the coastal turbid waters and inland waters [Wang, 2007; Wang and Shi, 2007; Wang et al., 2007, 2009b, Wang et al., 2011]. On the other hand, the NIR atmospheric correction for the open ocean nonturbid waters can effectively reduce nLw(l) noise error, thus the NIR-SWIR combined atmospheric correction algorithm for MODIS-Aqua ocean color data processing can provide high-quality nLw(l) data for both the open ocean and the coastal regions in the BS, YS, and ECS. [9] Using the MODIS-Aqua-derived nLw(l) data, water diffuse attenuation coefficient Kd(490) [Morel et al., 2007; Mueller, 2000; Wang et al., 2009a] and upper layer water total suspended matter (TSM) concentration [Zhang et al., 2010] are derived for the BS, YS, and ECS regions. It is noted that the TSM algorithm is a regional model, which was developed using the in situ data made from the 2003 spring and autumn cruises over the YS and ECS [Zhang et al., 2010]. Using the nLw(l), Kd(490), and TSM data, the spring-neap tidal variations in terms of the spatial pattern, optical spectra, turbid water coverage, and seasonal difference are then quantified and further analyzed.

4. Results 4.1. Spring-Neap Tidal Cycle of Kd (490) in the BS, YS, and ECS Regions [10] Using the phases of the moon provided by NASA with Astronomical Algorithms [Meeus, 1998] (http://eclipse.gsfc. nasa.gov/phase/phasecat.html), the time difference between each passing time of MODIS-Aqua over the BS, YS, and ECS regions and the corresponding new moon time was computed, and then all MODIS-Aqua measurements with the time difference falling in the same interval day, i.e., 0–24 h for interval day 1, 24–48 h for interval day 2, etc., were assigned to a certain group. Then a composite Kd(490) image with all

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Figure 3. (a–cc) Climatology Kd(490) images in the Bohai Sea, Yellow Sea, and East China Sea derived from MODIS-Aqua measurements from 2002 to 2009 in a lunar cycle from day 1 (Figure 3a) to day 29 (Figure 3cc).

the granules in each group was computed and used as the representative Kd(490) for the specific interval day in a lunar cycle. [11] Figure 3 shows the Kd(490) spatial distributions with the interval days of 1–29 in a lunar cycle. Significant variations of Kd(490) can be identified in the lunar cycle. Since satellite measurements for generating composite images for

each interval day are also evenly distributed in different seasons with long-term MODIS observations (8 years, 100 lunar cycles), thus the effect of seasonal signal is largely averaged out and negligible. Following the new moon spring tide, enhanced Kd(490) and enlarged high Kd(490) coverage can be found along the coastal regions of the BS, YS, and ECS, as well as the turbid sediment plume in the

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Figure 4. Coverage variations of moderately turbid waters (0.1 < Kd(490) ≤ 0.3 m 1) and turbid waters (Kd(490 > 0.3 m 1) in the BS, YS, and ECS in a lunar cycle. central ECS [Shi and Wang, 2010b] on day 3 (Figure 3c), day 4 (Figure 3d), and day 5 (Figure 3e). In contrast, Kd(490) maps on day 9 (Figure 3i), day 10 (Figure 3j), day 11 (Figure 3k), and day 12 (Figure 3l) show notable decrease in terms of the Kd(490) magnitude and high Kd(490) coverage following the one-quarter neap tide. In comparison to the high Kd(490) turbid water plume in the central ECS after the new moon spring tide, the plume cannot be observed by the satellite anymore. Different from the Kd(490) map after new moon spring tide, highly turbid waters with Kd(490) > 3.0 m 1 are confined to much smaller areas along the coastal region. [12] Following the full moon (days 14 and 15) spring tide, Kd(490) magnitude and high Kd(490) area resurge along the coastal region as well as in the central ECS on day 17 (Figure 3q), day 18 (Figure 3r), and day 19 (Figure 3s). Both the spatial patterns and the magnitudes of Kd(490) shown on days 17, 18, and 19 after the full moon spring tide are similar to the spatial patterns and the magnitudes of Kd(490) on day 3 (Figure 3c), day 4 (Figure 3d), and day 5 (Figure 3e) following the new moon spring tide. Resembling the low Kd(490) magnitudes and coverage areas on day 10 (Figure 3j), day 11 (Figure 3k), and day 12 (Figure 3l), the coasts in the BS, YS, and ECS and the central ECS show the three-quarter (days 22 and 23) neap tide leads to significant ocean turbidity changes represented by both the Kd(490) magnitudes and coverage areas as shown in Figures 3x, 3y, and 3z. [13] The ocean turbidity in the transition phase between the spring and the neap tides over this region can be represented on day 7 (Figure 3g) and day 22 (Figure 3v), while day 14 (Figure 3n) and day 28 (Figure 3bb) show results of the transition from the neap to the spring tides. Both the magnitudes of Kd(490) and the coverage areas of turbid waters are actually moderate between the low-turbidity situations after the first- and three-quarter neap tides and the high-turbidity situations after the new moon and full moon spring tides. [14] Figure 4 further quantifies the coverage areas of the turbid waters with Kd(490) over 0.3 m 1 and moderately turbid waters with Kd(490) ranging between 0.1 and 0.3 m 1. For the turbid waters, the coverage shows a periodic change

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following the lunar cycles. Maximum turbid water coverage is found to be on the interval day 3 (following the first spring tide after the new moon) and interval days 18 and 19 (following the second spring tide after the full moon) with coverage areas of 4.6  105 and 4.8  105 km2, respectively. In contrast, the turbid water coverage area drops to 3.8  105 km2 on interval day 10 following the first neap tide after the one-quarter moon, and it drops from 4.8  105 km2 after the second spring tide to 4.2  105 km2 due to the second neap tide after the three-quarter moon. For the moderately turbid waters, the spring-neap tidal effects are still identifiable even though they are not as significant as for turbid waters. There is no notable areal coverage variation during the first spring tide. But the coverage of the moderately turbid waters indeed shows the out-of-phase feature in comparison with the coverage of the turbid waters during the other periods in a moon cycle. It is noted that the total areal coverage for the turbid and moderately turbid waters is, in general, constant. This actually reflects that Kd(490) values of certain areas in these regions change according to the spring-neap tidal phases as shown in Figure 3, leading to the different variations of the moderately turbid and turbid water coverage. 4.2. Ocean Color Properties During Spring and Neap Tides [15] Figure 5 demonstrates the changes of optical and biogeochemical properties represented with nLw(l) at the blue (nLw(443)), green (nLw(555)), red (nLw(645)), and NIR (nLw(859)) wavelengths, Kd(490), and TSM concentration following the new moon (Figures 5a–5f), one-quarter moon (Figures 5g–5l), full moon (Figures 5m–5r), and threequarter moon (Figures 5s–5x). To better quantify ocean properties in a lunar cycle, Figure 5 provides results of the 3 day composite images for the interval days 3–5 (Figures 5a– 5f), 10–12 (Figures 5g–5l), 17–19 (Figures 5m–5r), and 24–26 (Figures 5s–5x). [16] For Kd(490), enhanced amplitude and large coverage of high Kd(490) can be found along the coastal regions of the BS, YS, and ECS after the first (Figure 5e) and second (Figure 5q) spring tides following the new moon and full moon. In the central ECS region, the sediment plume [Shi and Wang, 2010b] is an important feature. In contrast, highly turbid waters are confined to small regions close to the coast with reduced Kd(490) following the first (Figure 5k) and second (Figure 5w) neap tides, which follow the onequarter and three-quarter moons, respectively. The central ECS sediment plume that is clearly shown after the first and second spring tides cannot be identified with the current color scale. On the other hand, different from the significant spring-neap variability in the coastal regions of the YS and ECS, no notable difference of Kd(490) can be identified in the open ocean. [17] TSM patterns as shown in Figures 5f, 5l, 5r, and 5x during the spring and neap-tide periods in the lunar cycles is similar to the Kd(490) spatial patterns. The spring-neap variations of the TSM in the BS, YS, and ECS are important in the coastal region, while the TSM in the open oceans of the YS and ECS is insignificant. Unlike the Kd(490) and TSM, results in Figure 5 show that nLw(l) at the blue band nLw(443) do not have significant changes between the spring and neap tide periods for both the coastal regions and the

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Figure 5. Climatology images of nLw(443), nLw(555), nLw(645), nLw(859), Kd(490), and TSM during the phases of (a–f) days 3–5 for the new moon, (g–l) days 10–12 for the one-quarter moon, (m–r) days 17–19 for the full moon, and (s–x) days 24–26 for the three-quarter moon. open ocean in the BS, YS, and ECS. The spring-neap variations of nLw(l) at the green band nLw(555) from Figure 5 are modest and cannot be clearly identified due to the limitation of the color scale. [18] In comparison to optical properties at the blue and green wavelengths during different spring-neap tidal phases, variations of nLw(l) at the red band nLw(645) (Figures 5c, 5i, 5o, and 5u) and NIR band nLw(859) (Figures 5d, 5j, 5p, and 5v) are significant. The spatial distributions of nLw(645) generally match the Kd(490) and TSM patterns in the same spring-neap tidal phases. This reflects the intrinsic link between the high backscattering at the red band and the elevated sediment loadings in the water column. Enhanced nLw(645) can be observed along the coastal regions as well as in the central ECS after the two spring tides (Figures 5c

and 5o). After the neap tides, significant nLw(645) drops can be observed along the coastal region, and the sediment plumes featured with strengthened nLw(645) in the central ECS cannot be discriminated under the current color scale (Figures 5i and 5u). Similar changes of the ocean optical property can also be found with the NIR nLw(859). It is worth noting that in the open ocean of both the YS and ECS, ocean optical and biogeochemical properties do not show any recognizable changes within a lunar cycle as demonstrated with the comparison between postspring (Figures 5a–5f and 5m–5r) and postneap (Figures 5g–5l and 5s–5x) ocean property patterns. [19] To further quantify and assess the impact of the spring-neap tidal variations on ocean optical and biogeochemical properties in the YS and ECS regions, four

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Figure 6. Spring-neap variations of (a) climatology Kd(490) and (b) climatology TSM at station A (33.51°N, 121.24°E) in the YS, station B (35.91°N, 123.74°E) in the central YS, station C (30.49°N, 121.81°E) in the Hangzhou Bay, and station D (31.79°N, 124.86°E) in the central ECS.

pseudostations as shown in Figure 2 were selected to study the Kd(490) and TSM, as well as nLw(l) spectra in this region. Station A is located in the coastal turbid waters of the YS at (33.51°N, 121.24°E), while station B is in the central YS at (35.91°N, 123.74°E). Since the Hangzhou Bay is known for its spectacular tides, pseudostation C at (30.49°N, 121.81°E) is chosen to evaluate its spring-neap tidal effects on the variations of water optical and biogeochemical properties. Pseudostation D is located at (31.79°N, 124.86°E) in the middle of the central ECS plume as shown in Figures 5e and 5q following the two spring tides. [20] Figure 6 shows the spring-neap tidal effects on the variations of climatology Kd(490) (Figure 6a) and TSM (Figure 6b) at these four stations. The climatology Kd(490) and TSM in each day of a spring-neap cycle at the four stations were derived from the specific group of MODISAqua measurements corresponding to the same interval day as described in section 4.1. Kd(490) varies from 3 m 1 after the two neap tides to 4 m 1 following the spring tides at stations A and C along the coastal regions. More significant variations following the spring-neap tidal cycle are also found at station D in the central ECS. Kd(490) value is 0.4 m 1 after the neap tide, compared with 1.1–1.2 m 1

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(more than doubled) after the spring tide. In contrast, Kd(490) keeps almost constant in a lunar cycle at station B in the open ocean of the central YS. [21] Similar to the spring-neap variability of Kd(490) as shown in Figure 6a, variations of TSM at stations A, C, and D also follow the phase of a lunar cycle (Figure 6b). Specifically, at station A TSM changes from less than 50 g m 3 after the neap tides to 80 g m 3 after the spring tides. Similar spring-neap changes also occur to station C in the Hangzhou Bay. It is noted that TSM at station C is actually lower than that at station A even though Kd(490) values are comparable at these two stations. This suggests that variations of these two parameters are, in general, consistent with each other, but they are not interexchangeable. The discrepancy of Kd(490) and TSM variation might reflect the fact that the sediment grain size, type, composition and texture are different at these two stations since the sediment deposition along the YS coastal region comes from the ancient Yellow River, while the Yangtze River is the main source for the sediment in the Hangzhou Bay [DeMaster et al., 1985; McKee et al., 1983]. [22] TSM concentration at station D within the central ECS plume also shows periodic variations in a lunar cycle (Figure 6b). However, the change is not as significant as the spring-neap change of Kd(490) at this station as shown in Figure 6a. This implies that Kd(490) is more sensitive than TSM to the changes at this station. Similar to the Kd(490), no TSM changes can be identified following the spring-neap tidal variations in a lunar cycle at station B in the open ocean of YS region. [23] Figure 7 demonstrates the spectral variations at the four stations in the BS, YS, and ECS regions following the two spring and two neap tide periods as defined in Figure 5. Considerable discrepancy between the normalized waterleaving reflectance spectra, rwN(l), for the spring and neap tides occurs at stations A and C in the coastal regions (Figures 7a and 7c). The normalized water-leaving reflectance is defined as rwN(l) = p nLw(l)/F0(l), where F0(l) is the extraterrestrial solar irradiance [Gordon and Wang, 1994; IOCCG, 2010]. In general, rwN(l) for these two stations are almost identical at the green and blue wavelengths for both the spring and neap tides. Major differences in rwN(l) are found at the red (645 nm) and NIR (859 nm) wavelengths. During the spring tides, the red rwN(645) reaches 0.12 at station A, while its value is 0.09–0.10 during the neap tides. On the other hand, the NIR rwN(859) values are 0.045 and 0.035 for the spring and neap tides, respectively. Even larger differences in rwN(645) and rwN(859) between the spring and neap tides can be found at station C in the Hangzhou Bay. [24] Figure 7b shows that rwN(l) spectra are almost identical at station B in the central YS for the spring and neap tides. This indicates that effects of the spring-neap tides on variations of ocean properties are trivial and cannot be observed by satellite sensors in the open ocean. At station D in the central ECS plume, however, considerable differences in rwN(l) can be found between the spring and neap tides (Figure 7d). These are especially true for rwN(l) at the red and NIR wavelengths. As an example, rwN(645) values are 1.8% for the neap tides, while for the spring tides rwN(645) values reach 2.9%.

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Figure 7. Comparisons of the normalized water-leaving reflectance spectra rwN(l) following the new/ full moon spring tides and one-quarter/three-quarter neap tides at (a) station A, (b) station B, (c) station C, and (d) station D.

4.3. Seasonal Variations of Kd(490) During Spring and Neap Tides [25] The ocean optical and biogeochemical properties of the BS, YS, and ECS have a significant seasonal variation. Using diffuse attenuation coefficient Kd(490), the springneap variations in all four seasons are characterized and quantified in Figure 8. It is noted that time frames for generating seasonal Kd(490) composite images for the new moon spring tide, one-quarter moon neap tide, full moon spring tide, and three-quarter moon neap tides are the same as in Figure 5. [26] Figures 8a–8d show Kd(490) distributions during the spring season (March–May) for the new moon spring tide (Figure 8a), one-quarter moon neap tide (Figure 8b), full moon spring tide (Figure 8c), and three-quarter moon neap tide (Figure 8d). In the open ocean, such as the central YS and vast ECS offshore waters, Kd(490) does not show changes in a lunar cycle during the spring season. In contrast, significant spring-neap changes can be found in most of the BS, as well as continental YS and ECS regions. During the spring season, enhanced Kd(490) and expanded high Kd(490) coverage occur in the two spring tides. Specifically, the central ECS sediment plume is notable during the spring tides, while it is not observed during the neap tides. It is also interesting to note that the effect of the full

moon spring tide is more significant in terms of the increased Kd(490) magnitude and the expanded high Kd(490) coverage. [27] In comparison to the spring season, the spring-neap tidal changes during the summer season (June–August) are less pronounced (Figures 8e–8h). Same as the spring season, no changes of Kd(490) are identified in the open ocean in a lunar cycle. In the coastal region, the BS region shows little difference between the Kd(490) for the two spring tides (Figures 8e and 8g) and two neap tides (Figures 8f and 8h). There is also no central ECS sediment plume as observed with strengthened Kd(490) for both the spring and neap tides. However, some differences between Kd(490) results for the spring and neap tides indeed exist. Moderately enhanced Kd(490) values and slightly enlarged high Kd(490) coverage during the two spring tides are located in the regions of the YS coast and the Hangzhou Bay. [28] In general, the Kd(490) results in the autumn are similar to the spring season for both the spatial coverage of pronounced Kd(490) and Kd(490) magnitudes. Large coverage areas of the turbid waters and enhanced Kd(490) can be found for the periods following the new moon spring tide (Figure 8i) and full moon spring tide (Figure 8k). For the periods corresponding to the one-quarter (Figure 8j) and the three-quarter (Figure 8l) neap tides, considerable decrease of

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Figure 8. Seasonal variations of Kd(490) for the four different moon phases in the season of (a–d) spring, (e–h) summer, (i–l) fall, and (j–m) winter. The four phases are the same as defined in Figure 5.

turbid water coverage and drop of Kd(490) values occur in the BS and coastal YS and ECS regions. The central ECS plume is no longer observable from MODIS-Aqua Kd(490) data following the two neap tides. [29] During the winter season, the Kd(490) variations following the spring-neap tides of a lunar cycle are significantly strengthened. Same as for the other seasons, there is no spring-neap variability in the open ocean of the YS and ECS. For the periods corresponding to the two spring tides, both the turbid water coverage and Kd(490) values reach the maxima in a year (Figures 8m and 8o). In the central ECS region, the sediment plume with pronounced Kd(490) also becomes observable following the two neap tides (Figures 8n and 8p). In comparison, this sediment plume cannot be identified following the two neap tides in the other seasons.

Specifically, Kd(490) values at station A in the YS coastal regions are 4.7 and 4.5 m 1 for the new moon and full moon tides during the winter season, respectively. These values are considerably larger than 3.8 m 1 of the annual mean Kd(490) as shown in Figure 6a. Kd(490) values are 1.9– 2.0 m 1 and 0.9–1.1 m 1 at station D within the central ECS plume for the spring and neap tides, respectively. In contrast, for the ECS region the annual Kd(490) mean values are 1.2 and 0.4 m 1 for the two spring and two neap tides, respectively.

5. Discussion and Conclusion [30] In this study, we use the BS, YS, and ECS regions as an example to demonstrate and quantify the spring-neap

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Figure 9. Time delays (in hours) of the spring-neap tides corresponding to the phases of new/full moons and onequarter/three-quarter moons at around the stations B (central YS) and C (Hangzhou Bay).

tidal effects of the lunar cycle on the satellite ocean color observations. This study shows that the spring-neap tidal variation is also one important ocean process that drives both the synoptic-scale and mesoscale changes of ocean optical, biological, and biogeochemical properties in addition to some known processes, such as seasonal climate change [Shi and Wang, 2010b], ENSO event [Behrenfeld et al., 2001; Chavez et al., 1999], tropical storm [Shi and Wang, 2007, 2008, 2011], Rossby wave [Cipollini et al., 2001], etc. [31] This study shows that spring-neap tidal effects of the lunar cycle on the satellite ocean color observations are significant in the coastal regions and negligible for the open oceans. At stations A and B near the coast, Kd(490) values range between 3 and 4 m 1 within the spring-neap tidal cycle. Significant changes of Kd(490) in the spring-neap cycle also occur at station D in the central ECS plume. In comparison, the seasonal variation of Kd(490) ranges 2.2– 4.4 m 1 for station A and 3.1–4.2 m 1 for station B. This suggests that the spring-neap tidal effects on the variation of the satellite ocean color retrievals, e.g., Kd(490) and TSM, are in the same order as the seasonal variations, which account for about two thirds of the entire long-term monthly Kd(490) and TSM variance budgets. Since the spring-neap tidal effects on the ocean optical, biological, and biogeochemical property changes are always embedded in the satellite ocean color observations, the significance of the spring-neap tidal variability in satellite ocean color products implies that the phase of the lunar cycle and the impact of the spring-neap tides need to be considered when satellite ocean color observations are interpreted. [32] In the central ECS, the plume shows significant variability within a spring-neap cycle. This demonstrates that this plume is locally generated [Shi and Wang, 2010], and tidal current, not the ocean cross-shelf circulation [Yuan et al., 2008], is one of the major ocean processes that drive the ocean turbidity changes in the ECS. At around station D in the central ECS, mean tidal current amplitudes during spring tides are 0.80, 0.67, and 0.56 m/s at water depths of 5, 20, and

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35 m, respectively, following the tidal current harmonics [Guo and Yanagi, 1998]. On the other hand, the mean tidal current amplitudes during neap tides are 0.34, 0.31, and 0.23 m/s at the corresponding depths of 5, 20, and 35 m. Consequently, the kinetic energy in the spring tide is approximately 4–6 times of that in the neap tide. In comparison, the spring-neap variation of the Kd(490) changes from 0.5–0.6 m 1 in the neap tide to 1.0–1.1 m 1 in the spring tide at this location, while the TSM varies between 3–4 and 6–7 g/m3 in a spring-neap tidal cycle. This further demonstrates that spring-neap cycles of the Kd(490) and TSM in this region are attributed to the tidal energy change in a spring-neap tidal cycle. It is also noted that the variation of the normalized water-leaving reflectance rwN(l) in a springneap cycle reflects not only the change of the sediment concentration in the water column, but also the possible changes of the sediment optical properties [Bowers and Binding, 2006] and the particle size [Bowers et al., 2007] due to different turbulent energy in a spring-neap cycle. [33] In addition, this study also shows that the spring-neap tidal changes as observed by MODIS-Aqua ocean color measurements are not in phase with the phase of the moon. For example, the highest Kd(490) and largest turbid water coverage lag the new moon (or full moon) by about 2 or 3 days, while the lowest Kd(490) and smallest turbid water coverage are also 2–3 days behind the one-quarter (or three-quarter) moon. Two reasons are attributed to these phase delays. First, the effects of the seawater inertia and the friction against the seabed can cause the lag of the springneap tides by 1–3 days. The time when the spring-neap tides exactly occur can be estimated at any location as long as the phases of the tidal harmonic constituents M2 and S2 of the location are known, because the spring-neap tides occur when M2 and S2 tides are in phase (spring) or out of phase by 180 degrees (neap). Using the phase information of M2 and S2 observed near the mouth of the Hangzhou Bay and central Yellow Sea [Ogura, 1933; Yanagi et al., 1997], and the NASA lunar phase table, the time lags of the spring-neap tides after the new (full) and quarter moon between 2001 and 2010 are computed (Figure 9). For the location near the Hangzhou Bay (station C), the lag ranges from 15–54 h, with a mean of 34.5 h or about 1.4 days. For other locations, the pattern shown in Figure 9 will not change, and the only difference is the lag range. For example, at the station near the central Yellow Sea (station B), the lag ranges from 18– 57 h, and its histogram is just a shift of the previous one to the right by 3 hours (Figure 9), indicating that station B has a spring-neap response lag of 3 hours after the station near the Hangzhou Bay. In the open ocean, the lag is much shorter, with the value ranges from 0 to 38 h for a station at the shelf break near (29°N, 129°E) (not shown). [34] Another reason for the time lag of the MODIS-Aqua observations is the sediment resuspension process. Since sediment resuspension is caused by the friction between the current flow and the ocean bottom [Dyer, 1986], stronger tidal currents associated with the spring tides can lead to high sediment concentrations at the ocean bottom. However, TSM in the upper layer water column, which is observed by satellite, is not in phase with the tidal current variation. Actually, some studies [Uncles et al., 1994, 2002; Yang et al., 2000] show that TSM in the water column is correlated

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to the integration of the tidal currents in a semidiurnal/diurnal cycle as well as the water depth. In particular, the water depth (bathymetry) plays important role for the satellite observations of spring-neap tidal effects. In the three seas, the MODIS-Aqua observations only show significant springneap variability in the region with water depth less than 50 m. Since both the delays caused by the ocean hydrodynamics and the sediment resuspension processes depend on the coastal geography, continental bathymetry, and settling velocities of the particles associated with the sediment grain sizes, types and compositions, the lags of satellite ocean color observations, e.g., Kd(490), TSM, at different locations in the BS, YS, and ECS, are not exactly the same. Thus, it is important to note that the spring-neap tidal effects on the variation of ocean properties are generally different for different coastal regions. [35] The effect of the spring-neap cycle revealed from this study is not an aliasing signal from daily satellite overpass. First, some studies [Uncles et al., 1994, 2002; Yang et al., 2000] show that TSM in the water column is correlated to the integration of tidal currents in a semidiurnal/diurnal cycle. Thus, satellite-derived Kd(490) and TSM data actually reflect the strength of the tidal current in the entire semidiurnal/diurnal cycle. This suggests that highs and lows of Kd(490) and TSM in a spring-neap cycle are not really correlated to the specific phase of the semidiurnal/diurnal tides. Next, MODIS-Aqua overpasses in the region actually have up to around 2 hours difference in a repeated 16 day cycle. At stations B and C (Figure 2), it was found that, compared the satellite overpass time to the M2 tidal phase for the group of MODIS measurements (2002–2009) within 3–5 days interval (Spring 1 tide), satellite ocean color data are actually sampled quite randomly in a phase range from 50– 210° (i.e., no correlation between MODIS measurements and M2 tidal phase). Thus, it indicates that sample aliasing has a negligible impact on the observed spring-neap tidal effects from MODIS-Aqua ocean color observations. Finally, results of the spring-neap tidal effects in TSM derived from satellite observations from this study are consistent with those from in situ measurements at the Yangtze River estuary [Chen, 2001]. It provides an additional evidence that MODISAqua-observed spring-neap cycle variations in TSM and Kd(490) indeed reflect the truth in the Bohai Sea, Yellow Sea, and East China Sea. [36] This study provides a quantitative assessment on the spring-neap tidal effects on the variations of ocean optical and biogeochemical parameters in the BS, YS, and ECS. The entire areal coverage of the turbid waters (Kd(490) > 0.3 m 1) with significant spring-neap tidal variations is 4–5  105 km2. Similar coverage of moderately turbid waters is also impacted by the spring-neap tides (Figure 3). Shi and Wang [2010a] show that turbid waters with Kd(490) > 0.3 m 1 are all located in the coastal regions, river estuaries, and inland lakes with an average global coverage accounting for 8% to 12% of the total global continental shelf area in a year. As a major ocean process that drives the global ocean, especially the coastal oceans, spring-neap tidal variations can also have extensive effects on the other continental shelf areas, such as the Bay of Fundy [Amos and Tee, 1989] and San Francisco Bay [Schoellhamer, 2002]. A fortnightly cycle in the Tagus estuary turbid plume off the west coast of

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Europe has been reported using MODIS-Aqua-measured nLw(l) at 551 nm [Valente and da Silva, 2009]. In the southern San Francisco Bay, about one half the variance of TSM is caused by the spring-neap tidal cycle, and TSM lags the spring-neap cycle by 2 days [Schoellhamer, 1996]. Also, both Tamar (located to the west of Plymouth in England) and Weser (in Germany) estuaries exhibit a hysteresis in TSM during the falling and rising spring–neap cycle [Grabemann et al., 1997]. In addition, in the Scheldt estuary of Belgium, the tide averaged mud concentration is 1.3– 1.7 times higher during a spring tide than that during a neap tide [Fettweis et al., 1998]. [37] With the capability of frequent, synoptic, and nearsurface views of the global ocean, satellite ocean color observations indeed are an effective tool to quantitatively evaluate the global coastal environment changes driven by the spring-neap tidal variations. On the other hand, the regional differences of the tide, coastal wave activities, river flow, coast shape and ocean bathymetry, sediment characteristics and distribution at the ocean bottom, etc., determine that the impacts of the spring-neap tides are complex and highly different for various global coastal regions. All of these provide further challenges for assessing and monitoring the spring-neap tidal effects on the coastal ecosystems. [38] Acknowledgments. This research was supported by NASA and NOAA funding and grants. We thank B. G. Bowers, B. Jones, and another anonymous reviewer for their critical and constructive comments. MODIS L1B data were obtained from the NASA/GSFC MODAPS Services website. The views, opinions, and findings contained in this paper are those of the authors and should not be construed as an official NOAA or U.S. Government position, policy, or decision.

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Zhang, M., J. Tang, Q. Dong, Q. Song, and J. Ding (2010), Retrieval of total suspended matter concentration in the Yellow and East China seas from MODIS imagery, Remote Sens. Environ., 114, 392–403, doi:10.1016/j.rse.2009.09.016. L. Jiang, W. Shi, and M. Wang, Center for Satellite Applications and Research, NOAA National Environmental Satellite, Data, and Information Service, E/RA3, Room 102, 5200 Auth Rd., Camp Springs, MD 20746, USA. ([email protected])

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