Assessment of the Surface Mixed Layer Using Glider and Buoy Data LT Jeremiah Chaplin Operational Oceanography Winter 2009

Objectives Make observations concerning the relationship between Mixed Layer Depth (MLD), Atmospheric Forcing Mechanisms, and Waves Compare two methods for calculating MLD Simple Density Difference of 0.125 Curvature method developed by Scripps

MLD Dynamics are Complicated

Mixed Layer Dynamics Simplified Wind Stress (τ) at the Ocean Surface Vertical Velocity Shear Waves Langmuir Circulations Turbulent Mixing Ocean Surface Density Changes Surface Cooling Positive Heat Flux (Atmosphere) More Dense above Less Dense Negative Buoyancy Flux Surface Heating Negative Heat Flux (Atmosphere) Less Dense above More Dense Positive Buoyancy Flux

Webb Research Corporation Glider Specifications Weight: 52 kg Hull Diameter: 21.3 cm Vehicle Length: 1.5 meters Depth Range: 4–200 meter Speed: 0.4 m/sec horizontal average Endurance: Typically 30 days, depending on measurements and communication Range: 1500 km Navigation: GPS, magnetic compass, altimeter, subsurface dead reckoning Sensor Package: Conductivity, Temperature, Depth Communications: RF modem, Iridium satellite, ARGOS, Telesonar modem Temperature Accuracy: 0.002 deg C Conductivity Accuracy: 0.005 psu equivalent Pressure Accuracy: 0.2 dbar

Station 46042 - MONTEREY - 27NM West of Monterey Bay, CA Owned and maintained by National Data Buoy Center 3-meter discus buoy ARES payload 36°47'19" N 122°24'15" W Site elevation: sea level Air temp height: 4 m Anemometer height: 5 m Barometer elevation: sea level Sea temp depth: 0.6 m below Water depth: 1574 m Watch circle radius: 1248 yards

Glider Mission Buoy 46042 Dates of Operation: 01/30/2009 – 02/04/2009 Programmed collect data in between two points: 36 38.65N 122 07.25 W NW 36 38.20N 122 04.00 W SE Dive/Ascent Angle 26° Glider surfaced for 5 min after each dive/ascent Target Depth 194 meters Collects Pressure, Temperature, and Salinity continuously at 0.5Hz ~30 km WRC Glider

MLD vs. Time

MLD vs. Time

MLD Comparison

MLD Statistics PDDiff (0.125) Mean = 48.7123 StdDev = 17.0047 Variance = 289.1598 Dens Function Mean = 26.6593 StdDev = 21.5653 Variance = 465.0604 Covariance = 190.6155 Correlation Coefficient r2 = 0.5198

MLD vs. Time

MLD and Temperatures

MLD and Heat Flux

MLD and Winds

MLD and Waves

Temperature vs. Salinity

High Salinity Anomaly

Correlation r2 Wind related data seem to correlate the best Correlation Coefficients r2 (sigmaT difference of 0.125) Variable   r2 Wind Stress 0.38 Wind Speed 0.39 Wind Direction 0.25 Friction Velocity 0.41 Wind Wave Height Swell Wave Height 0.28 Significant Wave Height -0.21 Mean Wave Direction 0.44 Air Temperature -0.31 Air/Sea Temperature Difference -0.24 Latent Heat Flux Sensible Heat Flux 0.21 Correlation Coefficients r2 (Density Function) 0.27 0.26 0.18 0.23 0.12 -0.25 0.35 -0.16 -0.13 Wind related data seem to correlate the best As wind increases the MLD deepens (increases) Wave data less correlated except for Wind Waves  As wind increases, wind waves increase thus the MLD deepens (increases) Temperatures correlated well  As air temp increases the MLD decreases surface heating The Trends are the same for both estimates of MLD

Conclusions Both methods of calculating MLD perform well independently, and when compared to each other, both statistically and observationally. It is difficult to quantify the relationships between atmospheric forcing mechanisms and the MLD. However, qualitatively the relationships are clearly evident in the observations.

QUESTIONS?