Anomalies in 2008 Upwelled Water Properties on the Newport Hydrographic Line

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1 Anomalies in 2008 Upwelled Water Properties on the Newport Hydrographic Line Meghan Flink, Jack Barth, Steve Pierce, Kipp Shearman, Anatoli Erofeev, Justin Brodersen, Laura Rubiano-Gomez Abstract The climatology of the Oregon coastal ocean and shelf is established from historical observations along the Newport Hydrographic (NH) Line. Recent hypoxic events during the upwelling season near the sea floor and interannual variability in ecosystem productivity off Oregon have spurred interest in investigating the extent and cause of deviations from normal water properties off the Oregon coast. Temperature, salinity and potential density data from the 2008 upwelling season provided by autonomous underwater gliders are compared with the multi-year climatology in order to identify anomalies of ocean properties along the NH line. Because the Earth s rotation and southward winds drive warm and relatively fresh surface water offshore being replaced by cold, salty water from below, the onset of the upwelling season is defined in terms of the start of sustained southward winds. A comparison of anomalies in ocean properties and cumulative along-shore wind stress anomalies during the upwelling season aims to quantify how the coastal ocean responds to deviations from normal wind forcing. Introduction In the summer season running from April to September, northerly winds and the Ekman transport cause surface waters to move offshore being replaced by cold, salty, nutrientrich, low-oxygen water from below. This nutrient-rich water drives the production of phytoplankton, which fuels the productive food web from zooplankton to fish, birds, and mammals. However, this seasonal upwelling can cause an accumulation of dangerously low-oxygen waters (hypoxic dead zones ) near the coastal sea floor, if southerly downwelling favorable winds from October to March are not strong enough to drive oxygenpoor waters offshore. Low-oxygen levels are normal for deep, offshore water. Prior to 2000, hypoxia was rare for near-shore coastal waters off Oregon. Since 2002, hypoxic dead zones in coastal waters near the sea floor have become a regular occurrence varying in severity annually (Chan, et al., 2008). These hypoxic dead zones can cause mass die-offs and/or relocation of several species, including salmon and crabs. Predicting the occurrence and severity of hypoxic events is thus important to other scientists and local fisherman. Hence, a comprehensive understanding is needed of seasonal and inter-annual variation of water properties and dynamics, including coastal upwelling, over the continental shelf and slope off the coast of Oregon. The ocean doesn t change by a lot on average year to year because of its shear volume and heat storing capacity. But by comparing monthly anomalies with cumulative upwelling index based on cumulative wind stress, we can begin to better understand how

2 the coastal ocean responds to deviations in forcings such as wind stress. The purpose of this study is primarily to identify and understand anomalies within seasonal and interannual variation of water properties such as temperature, salinity and potential density by comparing 2008 Autonomous Underwater Vehicle (glider) data to the historical climatology. Once quantified, the monthly anomalies of T, S, and σ θ are compared to the deviation from the mean of the climatologic cumulative upwelling index. Background On various cruises from 1961 to 1971, physical oceanographic data was regularly collected at discrete depths up to 1000m using Nansen bottles and reversing thermometers at consistent stations along the Newport Hydrographic Line ( N) (Huyer, et al., 2007). Although these observations were during a cool period of Pacific Decadal Oscillation, it is the most complete and seasonally averaged data set to be used for a description of normal coastal ocean conditions (Huyer, et al. 2002). These data were used to describe the physical characteristics, including seasonal variations, of the coastal waters of the Pacific Northwest (Huyer and Smith, 1978). Several investigators are using this historical data as a basis with which to compare current data to look for anomalies within seasonal and inter-annual variation and indications of climate change. However, consistent observation did not resume again until 1997 which was just in time to observe the onset of the El Nino (Smith et al., 2001). The manifestation of the El Niño event in the California Current System (CCS) was seen in the stronger than normal upwelling favorable winds (Huyer, et al., 2002). Features of the typical seasonal upwelling in the CCS are described by Barth et al. (2005). Coastal upwelling along the CCS varies with latitude and thus wind stress. Upwelling conditions in Newport are significantly different from Crescent City (both in the CCS). Because of weaker southward wind stress and a greater influence of the Columbia River plume, the Newport upwelling domain has a warmer, less saline, less dense and thinner surface mixed layer (Huyer, et al., 2005). Observations of such interannual variation in wind stress identified an anomalous upwelling event in 2005 (Pierce et al., 2006). By comparing interpolated means of temperature and salinity with the historical data and analyzing data within the framework of the PDO and ENSO, Smith et al. (2001) observed stronger El Nino events and weaker La Nina events in the 1980s and 1990s than in the previous two decades indicating an overall warming trend of the coastal ocean of central Oregon. In fact, non-seasonal change in temperature and salinity indicate a significant warming trend of the upper waters in the entire California Current (Huyer, et al., 2007). El Niño signals are associated with surface temperature anomalies (Chavez et al., 2002; found in Pierce et al., 2006) Pierce et al (2006) found that surface temperature along the NH line in July 2005 was anomalously warmer than other years even though 2005 was not an El Niño year based on its low MEI value of 0.4. Pierce et al. (2006) also found a significant linear relationship between surface temperature anomalies and the cumulative upwelling index. The onset of the 2005 upwelling season was much later than the

3 2005 average by 38 days (Pierce et al.). While the ENSO signal accounts for interannual variation, the PDO varies on a time scale of decades. The historical data, from was observed in a PDO cool phase and there was a PDO shift in 1977 (Mantua et al., 1997; found in Pierce et al., 2006). In 2008, we saw strong La Nina conditions with strongly negative sea surface temperature anomalies of 2 C in the Pacific Ocean along the US west coast (Spector, 2008). The year 2008 also coincided with a cool phase of the PDO ( According to NOAA s Earth System Research Lab, the average MEI value for 2008 was about (Wolter and Timlin, 1993, 1998: Pierce et al. (2006) also notes an increase in interannual variation after the 1977 PDO shift into a warm phase. In 2006, Jack Barth s glider lab began regular use of Autonomous Underwater Vehicles (gliders), to collect physical and biological oceanographic observations along the Newport Hydrographic line. The gliders collect physical and biological oceanographic data like temperature, conductivity, depth, chlorophyll, backscatter, and dissolved oxygen. In a series of buoyancy driven dives and climbs, the gliders attempt to adhere to the NH line while traveling to about NH-45 and back. Gliders have been able to observe the increasing trend of hypoxia and even anoxia in near shore coastal shelf. The gliders are subject to subsurface currents, which often direct them off the NH line. This necessitated a means of organizing glider data in such a way that it can be consistently compared month to month as well as with the historical data. Peery (2008) created and analyzed three methods to map data points to a cross-shelf section of the NH line by longitude or bottom depth depending on which defined region the data falls in. Materials and Methods The historical data set used as the climatology of the ocean off the Oregon coast is from the Global Ocean Ecosystems Dynamics (GLOBEC). Data collected include means and standard deviations of temperature (T), salinity (S), sigma theta (σ θ ), and specific volume anomaly; and dynamic height. This data was collected regularly from 1961 to 1971 from fixed stations along the Newport Hydrographic Line at standard depths. For stations NH- 5, NH-15, NH-25, NH-35, NH-45, NH-65, NH-85 (named for their distance in nautical miles from shore), depth profiles of seasonal means are available for Winter (Jan. 1, 1962 Feb. 29, 1971), Spring (Apr. 16, 1962 Jun. 20, 1971), Summer (Jun. 21, 1961 Aug. 31, 1971), and Fall (Nov. 1, 1961 Dec. 21, 1971). Monthly means are available for stations out to NH-165. Monthly means of historical data of temperature, salinity and potential density from NH- 5 to NH-45 are interpolated on a grid of distance offshore (km) and pressure (db=m). The horizontal grid size spans 0 km (at the coast) to 80 km (just beyond NH-45) with 1 km increments. The vertical grid size spans 0 m to 200m depth with 5 m increments. Temperature, salinity and potential density fields are smoothed using the average of surrounding data within a box with dimensions 5 meters by 3 kilometers. Producing contour plots of the climatology provides a basis with which 2008 glider data can be compared.

4 Gliders provide denser data on a more continuous time scale out to NH-45. Because of the variability and strength of subsurface water current velocities, the gliders tend to veer off their intended course, the NH line. Gliders are used to survey the NH line, thus observations north and south of the transect line must be condensed and mapped to the NH line. However, we cannot simply map all data by longitude to the NH line since the isobaths are not orthogonal to the NH line. On the assumption that the structure of a water column follows the bathymetry, glider data is organized and mapped to the NH line according to the block averaging method in Peery (2008). In Peery (2008), the area the gliders cover is broken up into seven regions which are treated differently based on the bathymetry in each region. In this study, Peery s (2008) method is simplified by not including any glider observations in the southeast region (which is dealt with in Peery (2008) to specifically address the complication of Stonewall Banks). The five regions of data collection used in this study (Figure 1) are defined by the boundaries in Peery (2008). The center region includes all data observations within 4 kilometers of the NH line which can be mapped longitudinally directly to the NH line since the bathymetry in this region does not vary significantly from that of the NH line. The northwest and southwest regions include data observed west of NH-25, and is also mapped longitudinally directly to the NH line since beyond NH-25 the bathymetry is essentially orthogonal to the NH line. The northeast region includes data observed north of the center region and east of NH 25. Because the bathymetry in this region is not orthogonal to the NH line, data in this region is binned by bottom depth then mapped to the position on the NH line with the same bottom depth. Again, the southwest region of observations surrounding Stonewall Banks is not included. In this way, glider data can not only be compared consistently to the climatology, but also to other current monthly observations. After mapping all valid glider data to bins on the NH line, temperature, salinity and potential density glider data for each month of the summer upwelling season of 2008 (April to September) are interpolated on the same grid as the historical data. Then contour plots of the difference between the monthly historical and monthly 2008 glider data provide a qualitative view of the anomalies. The anomalies are then quantified by taking the average difference from climatology of the entire cross section. To avoid the large variability inherent in the surface mixed layer from skewing the average, the difference from the climatology is integrated from 30 meters to 200 meters in depth. Onshore (0-32 km offshore) and offshore (32-80 km offshore) anomalies are also calculated. Additionally, the surface temperature anomaly is calculated integrating from 0 to 5 meters. Like the historical data of ocean properties, historical wind data is taken from the same time period. Buoys operated by NOAA s National Data Buoy Center that are located along the Newport Hydrographic Line provide wind speed and direction data as far back as Reanalysis data for the historically averaged cumulative wind stresses from and the 2008 cumulative wind stress are obtained from the National Center for Environmental Predication. The cumulative wind stress anomalies are calculated by

5 subtracting mid-month values of the historical cumulative wind stress from 2008 midmonth stresses. The quantified anomalies for each month are then plotted against each other. Results & Discussion The average spring and fall transition dates for , as calculated using NCEP reanalysis wind data, are May 5 and September 14, respectively. According to NCEP reanalysis winds, the upwelling season began earlier than normal in 2008 having the spring transition on March 28 and fall transition on September 23 (Figure 7).However, the climatology of cumulative wind stress is only available for months May through September, so comparison of anomalies are confined to those five months. All cumulative wind stress anomalies are negative indicating more cumulative wind stress in 2008 than normal. The cumulative wind stress anomalies for May through September are: , , , , Most 2008 mid-month cumulative wind stresses are within the standard deviation of the climatology except for September. Evident in the temperature, salinity and density climatology contour plots for each month of the upwelling season are the upward slanting isolines, which are indicative of the upwelling process. These climatologic cross sections for each month provide snap shots of the normal evolution of water properties throughout the upwelling season. (See middle three panels of Figures 2 through 6). Monthly averaged cross sections of 2008 temperature, salinity and potential density for May through September are shown in the top three panels of Figures 2 through 6. A feature to notice is the increase in offshore surface temperatures, especially in August and September. Hereafter, the term than normal refers to the 2008 anomalies as compared with the climatologic average. Cross sections of 2008 monthly anomalies of temperature, salinity and potential density are shown in the bottom three panels of Figures 2 through 6. Beginning in May, we see a relatively cool surface layer with coldest temperature near the coast. Like the climatology, the offshore surface layer gradually warms reaching its anomalously large and warmest temperature in August. Even the near shore surface layer is noticeably warmer in August than any other month. July surface temperature anomaly ( C) shows cooler than normal surface temperature while August surface temperature anomaly (1.517 C) shows warmer than normal surface temperature. September reveals the surface layer beginning to cool down. Surface temperature anomalies were calculated from 0-5 meters depth, but the surface water anomalies extend deeper than 5 meters and the existence of data in the first 0-5 meters is not consistent among all months. A more accurate quantification of surface temperature is needed. Temperature anomalies from cross section ( , , , , C), onshore ( , , , , C) and offshore (-0.623, , , 0.134, C) become less negative and more positive as the upwelling season progresses indicating that 2008 upwelled water is progressively warmer than normal. In September, the onshore temperature anomaly was positive but less than August which is consistent with a greater cumulative wind stress anomaly in September than August.

6 Density depends more on salinity than temperature and we see that reflected in the similarity of the anomaly plots and values for salinity and density in the entire cross section, onshore and offshore anomalies. May June July August September Salinity (Cross Section) Density (Cross Section) Salinity (Onshore) Density (Onshore) Salinity (Offshore) Density (Offshore) A trend is seen in onshore salinity and density anomalies. Beginning in July, there is a small onshore patch of anomalously saltier, denser water that grows in area and magnitude through September. July, August, and September cumulative wind stress anomalies were the largest (-0.906, , N/m^2). Over the same months, onshore salinity ( , 0.051, ) and density ( , , ) anomalies become more positive. This is one trend consistent with what we would expect with increased cumulative wind stress. May anomalies reveal saltier, denser, and cooler than normal subsurface waters, which is probably due to the earlier than normal 2008 spring transition date. A qualitative trend to notice is that anomalies in deeper waters tend to be smaller than anomalies in shallower waters. Since through August, subsurface temperature anomalies of the entire cross section are warmer and salinity and density anomalies are fresher and less dense, this suggests that upwelling in 2008 slowed down in June through August compared with the upwelling rate of the climatology. However, looking onshore at the end of the upwelling season (July through September) when cumulative wind stress anomalies are greater, we see increasing salinity and density anomalies. Onshore temperature anomalies only decrease in September. Perhaps onshore temperature depends on factors other than upwelling metrics. It might also be advantageous to compute potential temperature anomalies instead of in situ temperature. Conclusion This study s aim was to understand how the coastal ocean off Newport responds to deviations in wind stress forcings by investigating and analyzing 2008 monthly changes in water properties. Pierce et al (2006) correlate wind stressing to upwelling metrics. Given the more negative than normal cumulative wind stresses, we expected to see anomalies in water properties to behave accordingly, such that temperature decreased, salinity and density increased with more cumulative wind stress. These preliminary results indicate that the opposite occurred: that increases in cumulative wind stress anomalies correspond to anomalies indicating a dampening of upwelling. Perhaps this wind data is not the most representative of the wind-driven upwelling metrics. Or perhaps, something else is occurring that dominates the wind stress effect on upwelling. However, none of the correlations between water property anomalies and cumulative

7 wind stress anomalies are significant, so a more likely suspect is the small sample size of anomaly data being correlated. A conclusive relationship between water property anomalies and cumulative wind stress anomalies from NCEP reanalysis winds cannot be determined. Adding more anomaly data points from inclusion of previous and subsequent years may begin to reveal a relationship. Despite inconclusive quantitative results, qualitative analysis of the anomaly plots reveals a trend (increasing salinity and density anomalies from July to September) consistent with increased cumulative wind stress. Comparing 2008 monthly anomalies with other factors that may affect water property during upwelling like Columbia River plume and the MEI and PDO indices may provide more understanding of whether the anomalies can be associated with distant ENSO or PDO signals or stronger, local forcings. Another way to analyze this data might be to look at the change in upwelled water properties between the climatology months and between the current year months, then look at the differences in month to month changes. The month to month differences in climatology represent a normal evolution based on normal wind stress. Similarly, month to month changes in current year upwelling properties might reflect anomalies in cumulative wind stress.

8 Figure 1. Glider transects along the NH line for each month over bathymetry contours. Black triangles mark (right to left) the NH stations: 5, 15, 25, 35. Vertical and horizontal dashed lines divide the area into the 5 regions specified in the binning procedure: center, northeast, northwest, southeast and southwest.

9 Figure 2. (Bottom) May temperature, salinity and potential density anomalies calculated from the May 2008 averages (top) minus the historical multi-year May averages (middle). Black carrots at the top of each plot mark the locations (right to left) of stations NH-5, NH-15, NH-25, and NH-35.

10 Figure 3. (Bottom) June temperature, salinity and potential density anomalies calculated from the June 2008 averages (top) minus the historical multi-year June averages (middle). Black carrots at the top of each plot mark the locations (right to left) of stations NH-5, NH-15, NH-25, and NH-35.

11 Figure 4. (Bottom) July temperature, salinity and potential density anomalies calculated from the July 2008 averages (top) minus the historical multi-year July averages (middle). Black carrots at the top of each plot mark the locations (right to left) of stations NH-5, NH-15, NH-25, and NH-35.

12 Figure 5. (Bottom) August temperature, salinity and potential density anomalies calculated from the August 2008 averages (top) minus the historical multi-year August averages (middle). Black carrots at the top of each plot mark the locations (right to left) of stations NH-5, NH-15, NH-25, and NH-35.

13 Figure 6. (Bottom) September temperature, salinity and potential density anomalies calculated from the September 2008 averages (top) minus the historical multi-year September averages (middle). Black carrots at the top of each plot mark the locations (right to left) of stations NH-5, NH-15, NH-25, and NH-35.

14 Figure 7. (Top) Daily averaged meridional wind stress from for Blue bars represent southward wind stress and dashed lines indicate the spring (March 28) and fall (September 23) transition dates. (Bottom) In red is the averaged meridional cumulative wind stress from with standard deviations in black. The 2008 cumulative wind stress is in blue. Lengths of the green bars are the anomalies taken at mid-month values (the 15 th, except the September value is taken at the 14 th since the historical climatology ends September 14 th ). All wind data is from the NCEP reanalysis.

15 Figure 8. Linear regressions of temperature (30 to 200 meters depth), surface temperature (0 to 5 meters depth), salinity and potential density anomalies versus cumulative wind stress anomalies. Ocean anomalies are calculated as an average of gridded values across the entire cross section of each month s anomaly plot. Negative cumulative wind stress anomaly represents more wind stress in 2008 than the climatology.

16 Figure 9. Linear regressions of temperature (30 to 200 meters depth), surface temperature (0 to 5 meters depth), salinity and potential density anomalies versus cumulative wind stress anomalies. Offshore ocean anomalies (left) are calculated as an average of gridded values from 32 to 80 km offshore. Onshore ocean anomalies (right) are calculated from 0 to 32 km offshore.

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