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

2. 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

3. MLD Dynamics are Complicated

4. 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

5. 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: deg C
Conductivity Accuracy: psu equivalent
Pressure Accuracy: dbar

6. 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

7. Glider Mission Buoy 46042 Dates of Operation:
01/30/2009 – 02/04/2009
Programmed collect data in between two points:
N W NW
N 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

8. MLD vs. Time

9. MLD vs. Time

10. MLD Comparison

11. MLD Statistics PDDiff (0.125) Mean = 48.7123 StdDev = 17.0047
Variance =
Dens Function
Mean =
StdDev =
Variance =
Covariance =
Correlation Coefficient r2 =

12. MLD vs. Time

13. MLD and Temperatures

14. MLD and Heat Flux

15. MLD and Winds

16. MLD and Waves

17. Temperature vs. Salinity

18. High Salinity Anomaly

19. 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

20. 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.

21. QUESTIONS?