Estimating the Mean Temperature and Salinity of the Chesapeake Bay Mouth

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1 Estuaries Vol. 25, No. 1, p. 1 5 February 2002 Estimating the Mean Temperature and Salinity of the Chesapeake Bay Mouth RICARDO A. LOCARNINI,LARRY P. ATKINSON*, and ARNOLDO VALLE-LEVINSON Center for Coastal Physical Oceanography, Old Dominion University, Norfolk, Virginia ABSTRACT: We used an extensive temperature and salinity data set to develop a statistically meaningful way of estimating mean temperature and salinity from discrete measurements in the mouth of Chesapeake Bay. From April 1992 to December 1998, the Center for Coastal Physical Oceanography completed 73 monthly hydrographic sections at high spring tide across the mouth of Chesapeake Bay. Time series of area weighted mean bay mouth temperature (MBMT) and salinity (MBMS) were calculated. We found that at any time the temperature at any location in the section correlated with the MBMT with a r 2 of 0.95 or better. A similar analysis for salinity showed that the best correlation was about 0.9 with many locations below 0.8. A correlation between MBMT and temperature at a nearby tide station indicated it was possible to estimate MBMT from the temperature at the tide station to 0.74 C (90% confidence interval). Salinity was not measured at the tide station, but the correlation at a location in the section similar to the tide station indicates that MBMS can be estimated with an error of 1.5 (90% confidence interval). Introduction Management of estuaries of great societal importance such as the Chesapeake Bay depends on analysis of changing conditions and the effectiveness of management decisions. The analysis of estuarine conditions often must reduce complicated data and processes down to manageable indices such as mean temperature and salinity. The mean temperature and salinity of an estuary is a useful concept that expresses other higher level processes such as heat exchange, runoff, and interchange with the adjacent ocean. Though large and growing populations around estuaries cause changes to the system, these changes are difficult to detect and to understand. If data are available that can be statistically reduced to indices, managers are better able to deal with the complex system. To aid in that effort we have gathered and analyzed time series data from the Chesapeake Bay mouth to develop ways of accurately estimating the mean temperature (MBMT) and mean salinity (MBMS) of the Bay mouth region. The analysis will show that observations of temperature and salinity at the Chesapeake Bay Bridge Tunnel (CBBT) tide station operated by the National Oceanic and Atmospheric Administration (NOAA) can be used to estimate MBMT and MBMS. * Corresponding author; tele: 757/ ; fax: 757/ ; atkinson@ccpo.odu.edu. Present address: National Oceanic and Atmospheric Administration, National Oceanographic Data Center, Ocean Climate Laboratory E/OC5, 1315 East-West Highway, Silver Spring, Maryland The Region, Data, and Methods Chesapeake Bay is the largest estuary in the United States (Fig. 1). The Bay mouth is about 25 km wide with shoals 5 to 8 m deep. There are three channels 15 to 25 m deep: Thimble Shoal on the south side, Chesapeake Channel in the middle, and Beach Channel on the north side (Fig. 1). Beach Channel has a sill that limits communication with the adjacent ocean. Twenty CTD (conductivity, temperature, and depth) stations extending across the Bay mouth were occupied nearly every month since April 1992 (Fig. 1). A complete description and discussion of the climatology based on data obtained during this project is presented in Locarnini et al. (2000). The section was completed in about 3 h from Norfolk to the Eastern Shore. All stations were occupied during each cruise with the exception of station 3A in the Thimble Shoal Channel, which was first sampled in September 1994 (47 cruises) and station 18 just off Virginia s Eastern Shore which was first sampled in February 1993 (61 cruises). Temperature, conductivity, and pressure data were collected at each station using a SeaBird SBE 25 Sealogger CTD, except in February 1994 and since July 1998 when a SeaBird SBE 19 SEACAT Profiler was used. Raw data were processed after each cruise using the Seasoft software provided by SeaBird. Salinity was calculated and the data averaged to 1-m depth bins. The instruments were calibrated every 2 years by SeaBird. Sensors were checked every few months locally. The accuracy of the data are C for temperature, for salinity, and 0.25% m for depth. Such accuracy is 2002 Estuarine Research Federation 1

2 2 R. A. Locarnini et al. Fig. 1. Bathymetry (m) of the mouth of Chesapeake Bay. are CTD (conductivity, temperature, and depth) stations along a line occupied since April 1992 by the Center for Coastal Physical Oceanography. (36 58 N, 76 7 W) indicates the location of the oceanographic and meteorological station operated by the National Oceanic and Atmospheric Administration at the Chesapeake Bay Bridge Tunnel (CBBT). very high considering the short-term variations in temperature and salinity during a CTD cast in estuaries. Most of the data used here have been reported in Valle-Levinson et al. (1995a,b,c) and Reyes-Hernandez and Valle-Levinson (1997a,b) in addition to the previously mentioned report on the climatology. The Oceanographic Products and Services Division of the NOAA operates an oceanographic and meteorological station at the CBBT (Fig. 1). The water temperature sensor is about 2.3 m below mean lower low water or about 3.4 m below the water surface during high spring tides. Measurements of water temperature obtained at this station (accuracy 0.1 C) at the high tide corresponding to each of the monthly hydrographic cruises were extracted and are used here. Results and Discussion A section across the Bay mouth (Fig. 2) shows the typical structure of an estuary with a wide mouth where Coriolis force becomes important. Note the high salinity in the deeper parts of the channels and on the northern side where inflow predominates and the low salinity surface layer on the south side where outflow predominates. In this case temperatures are lower over the shoals and in the deeper parts of channels. Each dot in the vertical section represents a point in a grid across the Chesapeake Bay mouth where temperature and salinity measurements were made. From these data we calculated the area weighted mean bay mouth temperature and salinity. For example, the MBMT and MBMS on February 27, 1998 were 7.64 C and 20.32, respectively. The correlation between each grid point in the section and MBMT and MBMS were calculated using the following scheme. All data were formed into time series: temperature T(x,z,t), salinity S(x,z,t), MBMT(t), and MBMS(t), where x represents the 20 stations across the Bay mouth (Fig. 1), z represents the depth that ranged from 1 to 22

3 Mean Chesapeake Bay Mouth T-S Conditions 3 Fig. 2. Vertical sections of temperature ( C) and salinity constructed with data collected at high spring tide on February 27, 1998 at stations across the Chesapeake Bay mouth. See Fig. 1 for stations locations. TSC stands for Thimble Shoal Channel, CC for Chesapeake Channel, and BC for Beach Channel. m, and t represents one of the 73 samplings of the Bay mouth. The linear correlation was calculated between each point (x, z, t) and the corresponding MBMT(t) and MBMS(t). Only grid points where measurements were obtained for the 73 cruises up to December 1998 were considered for further analysis. Figure 3 (upper panel) shows the distribution of the temperature r 2. The higher correlation in the upper layers is no doubt because those waters are uniformly responding to air/sea heat fluxes that have large spatial scales. The lower layers in the channels correlate less to the Bay mouth mean because the temperature is more controlled by advection from offshore. Nearby coastal waters are influenced by the same air/sea heat fluxes so the correlations are quite similar. The salinity correlation (Fig. 3, lower panel) varied from about 0.4 to 0.6 in the deeper parts of the channels to near 0.88 at mid-depth at stations 3 4, 9 10, 12 14, and 17. Salinity is more dependent on the influx of individual rivers and the mixing patterns within the Fig. 3. Vertical sections of the square of the linear correlation coefficients (r 2 ) obtained by calculating linear regressions between the mean Bay mouth temperature and salinity and the actual measurements of temperature and salinity at discrete locations across the Bay mouth (indicated by dots) during 73 high spring tides hydrographic cruises completed between April 1992 and December See Fig. 1 for stations locations. TSC stands for Thimble Shoal Channel, CC for Chesapeake Channel, and BC for Beach Channel. Bay. The upper layers correlate reasonably well with mean salinity because they are influenced relatively uniformly by the overall runoff, precipitation, and evaporation in the Bay. Salinity in the channels is less well correlated because, as was the case for temperature, advection from offshore is important. Since station 3 on the southern shoulder of the Thimble Shoals Channel is located similarly with respect to the channel as is the CBBT station we would like to see how well temperature and salinity there correlate with the MBMT and MBMS. If they are well correlated we may be able to use the CBBT temperature and salinity (if it is installed) as a measure of the MBMT and MBMS. The correlation of temperature and salinity data at 3-m depth at station 3 and MBMT and MBMS are shown in Fig. 4. The 90% confidence interval for the estimates of

4 4 R. A. Locarnini et al. Fig. 5. Time series of mean Chesapeake Bay mouth temperature ( C, open squares) and of the corresponding water temperature ( C, solid squares) measured at the National Atmospheric and Oceanic Administration (NOAA) oceanographic and meteorological station at the Chesapeake Bay Bridge Tunnel (CBBT). Fig. 4. Linear regressions between mean bay mouth temperature ( C) and water temperature ( C) measured at station 3 at a depth of 3 m (upper panel; 0.85 C, 90% confidence interval), and mean bay mouth salinity and salinity measured at station 3 at a depth of 3 m (lower panel; 1.5, 90% confidence interval). Measurements were obtained during high spring tides. Station 3 is located on the southern shoulder of the Thimble Shoal Channel, about 5-km west-northwest of the National Atmospheric and Oceanic Administration oceanographic and meteorological station at the Chesapeake Bay Bridge Tunnel; see Fig. 1 for the locations. MBMT and MBMS using these regressions is about 0.85 C and 1.5, respectively. The NOAA CBBT station is about 5 km to the east-southeast of our CTD station 3 (Fig. 1). The two locations are similarly located with respect to the controlling bathymetry: the Thimble Shoals Channel (Fig. 1). The time series of MBMT and CBBT water temperature (Fig. 5) shows a good visual correlation. Correlation analysis (Fig. 6) shows that MBMT can be estimated from CBBT measurements to 0.74 C at the 90% confidence interval. We expect that salinity measurements at the CBBT would yield an accuracy similar to that found at station 3: about 1.5 (90% confidence interval). With this information, statistically based statements can be made regarding the temperature and salinity state of the Bay mouth. With these correlations, statements regarding the temperature and salinity state of the Bay mouth can be determined using currently available data from the CBBT station ( click on PORTS and Chesapeake Bay). Statements regarding the departure of the Bay mouth from mean temperature and salinity conditions can be scientifically documented. After a hot dry spell accurate statements about the apparent high water temperatures and salinity can be quantified. Or, after a hurricane, accurate statements can be made regarding the low salinity in the Bay because of high rainfall. Fig. 6. Linear regression between mean bay mouth temperature and water temperature measured at the National Atmospheric and Oceanic Administration oceanographic and meteorological station at the Chesapeake Bay Bridge Tunnel (CBBT).

5 Mean Chesapeake Bay Mouth T-S Conditions 5 ACKNOWLEDGMENTS We thank R. C. Kidd for his work maintaining and operating the CTD system. We also thank Captain Robert N. Bray and Donnie Padgett of the R/V Linwood Holton. Thanks are extended to all Old Dominion University colleagues who participated and collaborated in the monthly Chesapeake Bay cruises. We thank Kate Bosley (NOAA) for her helpful comments on an early version of this manuscript, and Jerry Miller (Naval Research Laboratory) who supervised the Bay mouth cruises for several years. Support for the Chesapeake Bay mouth monitoring program was provided by the Department of Ocean, Earth and Atmospheric Sciences and the Center for Coastal Physical Oceanography at Old Dominion University. LITERATURE CITED LOCARNINI, R. A., A. VALLE-LEVINSON,L.P.ATKINSON,R.C.KIDD, AND J. L. MILLER Climatology of the Chesapeake Bay Mouth. Technical Report CCPO TR Center for Coastal Physical Oceanography, Old Dominion University, Norfolk, Virginia. REYES-HERNANDEZ, C. AND A. VALLE-LEVINSON. 1997a. Monthly hydrography in the lower Chesapeake Bay, Data report, CCPO TR Center for Coastal Physical Oceanography, REYES-HERNANDEZ, C. AND A. VALLE-LEVINSON. 1997b. Monthly hydrography in the lower Chesapeake Bay, Data report, CCPO TR Center for Coastal Physical Oceanography, VALLE-LEVINSON, A., J. L. MILLER, AND M. PARASO. 1995a. Monthly hydrography in the lower Chesapeake Bay, Data report, CCPO TR Center for Coastal Physical Oceanography, VALLE-LEVINSON, A., J. L. MILLER, AND M. PARASO. 1995b. Monthly hydrography in the lower Chesapeake Bay, Data report, CCPO TR Center for Coastal Physical Oceanography, VALLE-LEVINSON, A., J. L. MILLER, AND M. PARASO. 1995c. Monthly hydrography in the lower Chesapeake Bay, 1994 Data report, CCPO TR Center for Coastal Physical Oceanography, Received for consideration, November 27, 2000 Accepted for publication, June 1, 2001

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