In-situ Observations of the Davidson Current LCDR Ken Wallace OC3570 Winter Cruise 23 – 30 January, 2008 I. Background The thermal energy from the sun drives most of the circulation in the atmosphere and beneath the ocean surface on this planet. This is particularly well illustrated in the North Pacific Ocean. Air heated at the equator rises high into the atmosphere and the rotation of the Earth on its axis moves the heated air mass westward and northward as it rises, then eastward once it cools and descends at higher latitudes, creating a huge, clockwise-rotating mass of air. This rotating air mass frictionally interacts with the ocean surface, driving the major currents that also flow in a clockwise direction. Known as the North Pacific Gyre, this ocean-scale rotating mass of water clearly demonstrates the effects of air-sea interaction on a global scale. Further inspection of this circulation on a regional and local level reveals many spatially and temporally variable fluctuations that occur throughout the year. The California Current System (CCS) forms the eastern branch of the North Pacific Gyre and consists of three primary components: the California Current (CC), the California Undercurrent (CUC) and the Davidson Current (DC). The CC transports cold, subartic surface waters southward from the North Pacific 150 – 1300 km off the U.S. West Coast and from the surface to a depth of approximately 500 m (Collins et al, 2003). The CUC and DC both flow inshore of the CCS and poleward. The CUC flows beneath the surface year round from the continental shelf out to 200 km and generally below the thermocline from 200 m to over 1000 m deep (Pennington et. al, 2007). The DC is a seasonal current that flows at shallower depths of less than 200 m in late Fall and Winter and is the primary focus of this study. Some debate exists among researchers as to the forcing that drives the DC and whether or not it is simply a surface reflection of the CUC (Huyer and Smith, 1974). The CCS is largely influenced by the seasonal wind regimes along the West Coast of the United States. In summer, the subtropical high dominates the weather pattern with predominantly northerly winds and long periods of fair weather. As the seasons change, so too does the behavior of the CCS in response to the variation in the atmospheric forcing. With the onset of winter, the subtropical high retreats equatorward and the Aleutian Low becomes anchored over the subarctic waters off of Alaska. This relaxes the northerly wind stress and allows powerful winter storms with strong southerly flow to frequently influence the flow of coastal California waters. Under these conditions, during the late Fall and Winter, the poleward flowing DC becomes established near the surface over the continental shelf from Northern Baja, CA to Vancouver Island (Chelton, 1982). II. Purpose The purpose of this study is to compare oceanographic and atmospheric data collected from 23 – 30 January, 2008 during a Naval Postgraduate School oceanographic cruise off the Central California Coast and use it to illustrate the characteristics and position of the DC. Additionally, an attempt will be made to make some conclusions about the forces responsible for its annual evolution based on the collected data. Oceanographic and meteorological data will be compared for purposes of explaining the roles of wind stress and wind stress curl in the forcing of the current. This data will also be compared to satellite derived dynamic height contours and moored buoy data. Understanding the physical structure of the DC and the forces that drive it is important in many regards. From a military perspective, a thorough knowledge of the location and movement of water masses with different temperature and salinity (ie. density) characteristics is crucial to understanding how sound propagates through the ocean. This is obviously important when conducting any manner of undersea warfare (USW). The currents also affect ship navigation as well as diving and rescue operations. The principles that govern the intensity, location and duration of the DC are possibly applicable to other currents worldwide where the Navy operates and therefore warrant research and scientific explanation. From an environmental perspective, the DC affects the transport of marine organisms such as plankton and fish larvae which are important indicators of the health of the marine ecosystem. The current can also be a transporter of contaminated ballast water, oil spills, or other pollutants up and down the coast depending on where and at what depth the subject material is introduced into the ocean. For the fishing industry, the location and timing of the current has obvious implications to where the desired catch may be located, for how long it will be there as well as the health and sustainability of the species in question. Lastly, long-term monitoring of how the current changes over an annual and interannual time frame can provide information about how the ocean circulation is changing and possibly affecting the global climate. III. Data Collection and Methods On 23 January, the R/V Point Sur departed Moss Landing, CA for a week long round trip research cruise from Monterey Bay to San Francisco. The cruise plan consisted of gradually transiting to the north and back to Monterey Bay along a track that made several excursions away from the coast into deeper water (over 2000 m) in order to obtain cross sectional Acoustic Doppler Current Profiler (ADCP) data of both the near shore water and water off the continental shelf (Figure 1). Frequent stops were made for conductivity, temperature and depth (CTD) casts as well as the deployment and recovery of GPS equipped drifting buoys and other oceanographic instruments. As the DC is known to typically flow within 100 km of the coast, this cruise plan was expected to provide data that delineates the waters within the current from the surrounding environment. The primary instrument used for this study was the Teledyne RD Instruments (RDI) Broad Band 75 kHz Acoustic Doppler Current Profiler (ADCP). This instrument uses an acoustic signal and the principle of Doppler shift to measure the velocity of particles (plankton, suspended debris, fish, etc.) within the water column beneath the hull of the ship. The particles scatter this signal back to the ADCP receiver and the velocities are calculated at various depths throughout the water column using the following equation: Fd = 2 Fs (V/C) cos (A) (Gordon, 1996) Fd = Doppler shift frequency, Fs = frequency of sound, V = relative velocity ( between source and receiver), C = sound speed (m/s), A = angle between acoustic beam and scatter velocity (Figure 2). The ADCP is ideal for oceanographic research because it generates low-noise, high resolution data with a minimal consumption of power. The velocities can be depth averaged from the near surface to the lowest depth or separated by depth ranges into specific “bins” for a vertical cross section analysis. The Sea-Bird Electronics, Inc. CTD is used to take water samples and to profile water characteristics from the surface to near bottom depths. In all, 80 CTD casts were made at predetermined locations throughout the cruise as indicated in Figure 1. The primary characteristics are temperature, salinity and pressure (from which density can be derived) as well as dissolved oxygen and a fluorometer which measures the amount of chlorophyll in the water. Additionally, the sea surface height data from the CTD casts can be used to create dynamic topography contours as shown in Figure 3. These contours are derived from horizontal density differences between two locations and are representative of the horizontal pressure gradient force. Reference depths of 200 m and 1000 m, assumed to be levels of “no motion”, were used to determine the geostrophic currents which are referenced to those levels. A program called Surfer 7.0 was used to interpolate the data between stations and to smooth data sparse regions. Pacific Gyre Microstar buoys were deployed during the cruise and served as Lagrangian drifters in the surface current. Each buoy is equipped with a GPS tracking beacon and is tethered to a drogue which essentially attaches the buoy to a parcel of water as it moves along the coast. GPS plots of the buoy tracks and their respective speeds of advance can be determined from the data and give a very good indicator of the movement of the surface waters. Monterey Bay Aquarium Research Institute (MBARI) moorings M1 (36.8 N, 122.0 W) and M2 (36.7 N, 122.4 W), deployed outside of Monterey Bay, use solar and battery powered two-way telemetry to provide real time in-situ data about the water column and atmospheric forcing (Figure 4). Each buoy is equipped with an ATLAS sensor to collect meteorological information as well as sea surface temperature once every ten minutes. An onboard RDI ADCP collects water column speed and direction every fifteen minutes and CTD sensors deployed at 10 and 20 m depths feed information upon request to the onboard computer system. These buoys are particularly useful when examining the effects of wind stress on the properties of the near surface water column over time. Surface current radar (CODAR) uses high frequency radar signals to measure the flow in the upper 1 meter of the water column. Several of these stations have been installed along the Central Coast from Point Sur to north of San Francisco and provide near-real time analysis of the surface currents (Figure 5). Each CODAR site has two antennas: the first transmits a radio signal out across the ocean surface and the second listens for the reflected radio signal after it has bounced off the ocean's waves. By measuring the change in frequency of the radio signal that returns, the CODAR system determines how fast the water is moving toward or away from the antenna. The system can also determine the height and frequency of the waves near the shore. CODAR was used in this study to locate the DC and estimate its intensity as well as for correlation with observed atmospheric forcing patterns. The geostationary satellite GOES-11 and Quikscat satellites were used to provide information about the synoptic scale weather conditions during the cruise. The GOES-11 satellite is geostationary over the Pacific and provides 1.5 km resolution visible and IR images at half-hourly intervals throughout the day from which the clouds and flow patterns can be determined. The Quikscat scatterometer is a polar orbiting satellite that uses microwave pulses to detect sea surface roughness at a 12.5 km resolution. These roughness measurements are correlated using an algorithm and converted to surface wind stress values displayed by color coded wind barbs over water. Together, the satellite data provides information about the large scale forcing from the dominant wind regimes that impacted sea height conditions during the cruise. IV. DATA Analysis A. ADCP An ADCP with an acoustic frequency of 75 Hz was used to profile the speed and direction of the near surface water particles beneath the research vessel. A review of the 27 – 59 meter depth-averaged lateral transects running perpendicular to the coast of California clearly reveals the position and intensity of a poleward flowing current evident from Monterey Bay to the northernmost transect outside of San Francisco Bay (Figure 6). Off of Monterey Bay, the core of the current appears to be approximately located 30 NM off the coast near 122.25 W and is flowing to the Northwest. It then appears to turn and flow inland slightly as it approaches the mouth of the bay. Once north of the Bay (north of 37 N), the flow then appears to move farther offshore again. A study of the bathymetry in the area reveals that the continental shelf extends further offshore to the north of Monterey Bay (Figure 7), suggesting that the current is influenced somewhat by the extension of the shelf and roughly follows the 200 meter depth contour. The longest transect was directly off of Monterey Bay (line 67) and shows the width of the poleward flow to be approximately 35 NM with a maximum velocity about 10 – 12 NM outside the Bay. Beyond this distance, the magnitude of the current decreases significantly and even reverses direction to the south around 122.6 W. Transects to the North also reflect this decrease in current velocity to the west but are shorter in length and so it is impossible to estimate the true width of the flow of point of reversal in these regions. On the last day of the cruise, approximately 48 hours after the storm had passed and northerly flow associated with a high pressure ridge to the West had been established, a transect was conducted along track 67 (Figure 8). The 27 – 59 m data along this track shows very strong poleward flow with a maximum near 122.2 W and decreasing poleward flow to the west as far as 122.8 W. Additional information about the position and strength of the current can be obtained by viewing the ADCP data in vertically segmented slices. By separating the ADCP data collected on 23 – 25 January outside of Monterey Bay into bins of specified depth ranges from the near surface to deep water, it is obvious that the poleward flow is strongest between the near surface depths to approximately 125 meters (Figures 6 and 7). Evidence of northward flow can be seen down to 325 – 375 meters but at a reduced velocity and over a much narrower lateral distance. The vertical cross section to the north off of San Francisco Bay (SF track) again shows the current to be most intense from near the surface to approximately 125 meters (Figure 9). B. Water Properties Easily measured water properties such as temperature and salinity are often used to distinguish masses of water or currents from the surrounding environment. By displaying the interpolated CTD data obtained along the same transects used to collect ADCP profiles, a concentrated area of poleward flow is easily recognizable. The current is characterized in these profiles as a warmer and less saline anomaly in the upper 200 meters near the coast (Figure 10) along both the line 67 track and the SF track. A notable increase in the dissolved oxygen and fluorometer profiles is also evident in these profiles, characterizing the poleward flowing current as a layer of oxygen and chlorophyll rich water as compared to the rest of the water column. The line 67 profile compares favorably with the ADCP data in positioning the western boundary of the current approximately 30 NM seaward from station #80 in Monterey Bay. The location of the current is more difficult to distinguish along the SF track due to the broad shelf that extends further west of the coast line. This vast expanse of shallow water is more easily mixed and any distinction between water masses could be obscured by strong wind stress and associated mechanical mixing at the surface. The only notable difference in properties of the water in this region is in the salinity profile which is slightly less saline in an area near the coast. As revealed by the ADCP data, the Davidson current appears to be further from the coast near the edge of the continental shelf. This slight difference in salinity could very well be attributed to freshwater run off from the storm that passed over the region from 24 – 26 January. C. Lagrangian Drifters Perhaps the most obvious indicator of the northerly flowing near-surface waters during the cruise is the tracks of the Microstar GPS equipped drifting buoys. From their deployment site near the mouth of the bay, the buoys moved initially to the northeast toward the bay before moving in a steady northwesterly direction along the coast. The first buoy was recovered approximately 15 NM to the Northwest of where it was deployed after almost 48 hours of drifting. The three buoys moved together along a very similar course and at virtually the same speed and were recovered outside of San Francisco Bay after almost 94 hours of drifting on the 27th of January (Figure 11). Their average speed of advance was approximately 40 cm/s. It is important to note that during the time of their northward transit, a powerful storm was impacting the coast with strong southerly winds and associated southerly seas and swell. This alone would obviously move objects near the surface to the north. However, their paths also coincide with the ADCP data which indicated a concentrated area of poleward flow along the coast at depths of over 300 meters. D. CODAR Data Nine coastal radar sites were used for this study: spread out from Point Sur and into Monterey Bay, then scattered up the coast to the north side of the San Francisco Bay outlet. For the most part, the resulting plots show a fair correlation with the observed winds from scatterometry, the MBARI moorings, and the COAMPS wind field plots. The strongest poleward flow is observed on the 27th of January, during the height of the southerly winds produced by the transiting storm. However, even during periods dominated by high pressure and northerly winds at the beginning and end of the cruise, the poleward flow is much weaker but still evident throughout much of the Central Coast and is especially obvious off of Pt Sur (Figure 12), suggesting that the current is not entirely driven by surface winds. E. Meteorological Data It is important to note that the atmospheric forcing prior to and during the period of the cruise encompassed both extremes of the typical seasonal weather. For several days prior to the 23rd of January, a high pressure system dominated the coastal wind flow with light to moderate northwesterly winds. On the second day of the cruise, however, a strong low pressure system approached from the West, generating gale force winds from the southwest that persisted for several days and generated 8 – 11 ft southerly seas (Figure 13). The storm passed to the north by the evening of the 27th as high pressure ridged in from the west and influenced the weather pattern for the remainder of the cruise with predominantly northerly flow. The aforementioned storm that transited the Central California Coast from the 24th through the 28th was very well illustrated in the GOES-11 visible imagery, the Quikscat scatterometry, and the COAMPS fields. The most obvious impact to the area of study was the very strong southerly surface winds that persisted for the duration of the storm. As indicated in Figure 14, these winds were not fetch limited and extended to the south and west for approximately 1000 NM. The impacts of this kind of surface stress are three fold: movement of the layer of water in contact with the wind, mechanical mixing of water near the surface to some depth, and the effects of wind stress curl (Figure 15). The MBARI M1 and M2 buoy wind, temperature and salinity time series plots show a very clear and direct correlation between the increased southerly winds and the deepening of the mixed layer. As would be expected, there is a slight lag between the atmospheric forcing and the associated affects within the water column but the relationship is consistent for both the northerly and southerly wind regimes. Winds from the north leading up to the cruise had clearly upwelled cooler, subsurface waters and stratified the water column and the opposite affect occurred with the strong southerly winds during the storm (Figure 16). An additional but less obvious affect associated with the wind speed and direction is forcing due to wind stress curl. The wind stress initially moves surface water in the direction of the winds but Coriolis due to the rotation of the earth accelerates the water proportionally and to the right in the Northern Hemisphere. The resulting Ekman transport is 90 degrees to the right of the wind direction. The vertical motion within the Ekman layer (WE) can be represented as the curl of the wind stress: Where Ty and Tx are the North-South and East-West wind stresses, f is the Coriolis parameter and rho is the density. From this equation, we can see that positive wind stress curl (northerly winds along the coast of California) cause divergence in the Ekman layer and negative (southerly winds) wind stress curl causes convergence. Convergence at the surface, as with southerly winds, would essentially pile water up along the coast and divergence would tend to cause upwelling. This correlates nicely with the mooring time series plots which indicated the vertical movement of the mixed layer. F. Dynamic Height The dynamic topography for the central coast was determined from the CTD casts from Point Sur north to approximately 37.5 N. Figure 3 depicts the dynamic topography in the upper layer which gives a first guess of what the surface currents should look like. Clearly, the highest heights are located along the coast and in Monterey bay, decreasing offshore to the west and southwest. Overall, a maximum difference of approximately 0.5 J/kg is observed. This difference implies a westward pressure gradient force, which, when combined with the Coriolis effect as the water parcels are accelerated would result in a southerly flow of water at the surface (as observed). The 0/1000 depiction, which assumes the 1000 m depth is a level of “no motion” and compares the overall density difference of the entire water level down to this level. The result is very similar to but with even less variation that that of the 0/200 depiction and again illustrates higher heights along the coast and in the bay, decreasing steadily offshore. The maximum difference is approximately 0.4 J/kg, suggesting that a density difference exists deeper than just the surface layer. The pressure gradient force is the same as at the 0/200 level and again results in a northward geostrophic current, as observed. V. Conclusions No one would argue after a review of all of the data above, that surface and sub- surface poleward flow is indeed evident off of the central coast during the timeframe of the cruise. The ADCP indicates that this flow extends to over 300m in the water column and is continuous from south to north throughout the region surveyed. The CODAR data, as a non-survey ship source, consistently confirms the movement of the surface layer water to the north although with some variability. The dynamic height contours derived from the CTD casts also match what would be expected of geostrophic forcing for a poleward flow. Although little distinction can be made between the CUC and the DC, it is difficult to imagine that the wind stress alone could be responsible for the poleward flow below depths of 50m. For waters nearer to the surface, however, it makes sense that the widespread southerly winds associated with the powerful storm system that hit the coast could have driven much of the motion of the layer. Therefore, whether or not the surface flow is actually an extension of the CUC or an entirely separate and distinct current with different forcing functions is a complicated argument to make. From a dynamic topography perspective, both opinions can be argued. One would expect that for a northerly current to exist in the absence of atmospheric forcing, a pressure gradient force would still be necessary and, hence, higher dynamic topography would have to be observed inshore. On the other hand, a storm with considerable southerly winds would naturally be expected to pile up water to the right of the direction of the wind flow, which, in this case, would also be along the coast. With only a week of data to rely on, no conclusive evidence of either case can be demonstrated. VI. Future Work In light of the above discussion, it is obvious that long-term data collection from in-situ sensors at various depths and for prolonged periods of time would be beneficial for better modeling the complex interactions of the CCS. Use of glider and RAFOS floats at appropriate depths in the water column could significantly improve the amount and resolution of data collected, further enhancing the understanding of the dynamics involved with the DC. A multiyear time series comparison of dynamic topography contours and measurements of both the subsurface and surface currents, while filtering out the seasonal effects of changing wind stress regimes, could also help resolve the question of what other factors are involved with driving the currents. Seasonal effects of the changing weather patterns could also be factored in to match the actual observed currents. REFERENCES Chelton, Dudley B., 1982: Large Scale Response of the California Current to Forcing by the Wind Stress Curl. CalCOFI Rep., Vol. XXII. Collins et al., 2003: The California Current system off Monterey, California: physical and biological coupling. Deep-Sea Research, 50, 2389 - 2404. Huyer, A. and R. L. Smith, 1974. A subsurface ribbon of cool water over the continental shelf off Orgeon. J. Phys. Oceanogr., 4, 381-391. Pennington, J. T. et al., 2007: Ocean Observing in the Monterey Bay National Martine Sanctuary: CalCOFI and the MBARI time series. A report to the Sanctuary Integrated Monitoring Network (SIMoN) Monterey Bay Sanctuary Foundation. OC3570, Pacific Ocean Activities 23-26 Jan. 2008 (Leg I) 27-30 Jan. 2008 (Leg II) 38.00 = CTD = XBT = Sonde S17 = KiteSonde 37.75 S8 = Mooring 59 60 61 37.50 X78 S18 62 North Latitude X79 63 Drifters 37.25 67 X83 * = 6, S3, S6, X70 X80 64 X82 Glider 66 68 X81 37.00 65 X84,S19 69 S7 70,X85,S20 79 1 200, 1000, 2000, 2,X66 S1 80 36.75 and 3000m 71,X86 77,X91 7 78,15,X77 78, isobaths shown 72,X87 5,X69 3,X67 * 76 4,S2,X68 73,X88 S21 X90 75 14 8,X71 36.50 74,X89 13,S5,X76 9,X72 12,X75 S4 10,X73 11,X74 HARP 36.25 123.75 123.50 123.25 123.00 122.75 122.50 122.25 122.00 121.75 West Longitude Figure 1. OC3570 Winter Cruise Track Figure 2. Principles of ADCP operation illustrated (Gordon, 1996). Figure 3. Dynamic Height Contours derived from CTD casts. Figure 4. MBARI Moorings (M1 and M2). Figure 5. CODAR site at Point Sur, CA. Mean 27-59 m, OS75 VMADCP, OC3570 Winter 2008 37.6 37.4 37.2 Latitude, N 37 36.8 36.6 36.4 30 cm/s 36.2 -123.4 -123.2 -123 -122.8 -122.6 -122.4 -122.2 -122 -121.8 Longitude, W Figure 6. ADCP Transects 23 – 30 January, 2008. Figure 7. California shelf extension off the coast north of Monterey Bay. Mean 27-59 m, OS75 VMADCP, OC3570 Jan 30 2008 0630-1842 Line 67 37.6 37.4 37.2 Latitude, N 37 36.8 36.6 36.4 30 cm/s 36.2 -123.4 -123.2 -123 -122.8 -122.6 -122.4 -122.2 -122 -121.8 Longitude, W Figure 8. Line 67 ADCP transect, 30 January, 2008. Line SF 29 Jan 2008 0 10 40 -30 -5020 0 10 -40 0 10 50 0 10 10 20 -1 20 20 20 20 240 -30 -10 -20 -40 10 -2 -1 0 -3 0-4 0 0 210 30 10 30 -100 80 -5 0 - - 20 20 0 -5 0 0 4 -3 00 -10 10 0 0-50 20 0 -5 -4 1 10 30 20 1 00 -3 0 20 -4 2 10 20 60 10 -2 -200 20 -10 10 20 0 10 20 -30 0 -4010 40 2 0 -- 10 Depth, m 0 -300 10 10 10 -10 -20 -30 -30 -50-40 10 -5 0 10 0 20 0 -50 -20 0 3-0 0 0 2 0 10 0 0-50 --4 0 -400 10 0 0 10 -40 0 0 -1 0 10 10 -10 20 -4 0 0 -10 -5 0 --3 -3 0 -5 0 0 -2 4-0 -20 -500 10 50 10 0 20 -1 0 -4-0 -30 -0 -4 0 0 -110 0 -32 0 0 -20 10 1-00 0 5 2 -10 -600 -40 -123.4 -123.35 -123.3 -123.25 -123.2 -123.15 -123.1 -123.05 Longitude, W Figure 9. Vertical cross section of SF ADCP track. Figure 10. Smoothed CTD profile along track 67. Figure 11. Lagrangian drifter tracks. Figure 12. January 23rd CODAR image showing northerly flow off of Pt. Sur despite northerly wind flow. Figure 13. Quikscat imagery from 25 January, illustrating the large storm that impacted the Winter cruise. Figure 14. COAMPS field indicating the wind strength of the storm. Figure 15. Wind Stress Curl defined. Figure 16. MBARI M2 buoy profile showing downwelling associated with the southerly storm winds January 25 – 28th.
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