transport of pure wind current in the area is usually forced against perma- 

 nent flow on the western side of a cyclonic wind field. 



The standard deviation of errors S is 8.33 percent. About one-half of 

 the hindcasts falls within the standard deviation limits. 



CONCLUSIONS 



This method of thermocline depth prediction is mainly limited with 

 respect to time by limitations of weather prediction. With some experi- 

 ence in ocean regions where permanent currents are not present, the pre- 

 dictor can be expected to forecast with an error of less than 10 percent 

 of the observed depth about 70 percent of the time. More exact and regular 

 data are required for improvement of prediction. Present approximations 

 allow the following conclusions: 



The mixed layer during the seasonal thermocline is primarily produced 

 by mechanical mixing due to wind waves and pure wind currents. 



With the same rate of mixing energy, the mixed-layer thickness is 

 inversely proportional to stability in the thermocline. 



Convergence produced by pure wind current increases mixed-layer thick- 

 ness in areas where no convergence formerly existed. Likewise, divergence 

 produced by pure wind current decreases the mixed-layer thickness in areas 

 where no divergence formerly existed. 



Decay of the mixed layer depends on the depth to which existing mechani- 

 cal mixing energy can penetrate under nearly neutral stability conditions. 



The predicted thermocline depth is the mean level of the interface 

 between the mixed layer and the thermocline. 



Predicted distribution of instantaneous depths of the interface in 

 any area will be possible only after sufficient knowledge concerning the 

 spectral distribution of internal waves has been gained. 



Knowledge of the effects of instability mixing acting concurrently 

 with mechanical mixing could improve prediction of the thermocline depth 

 during time of the seasonal thermocline. 



The distribution of the mixed layer and thermocline is a complex prob- 

 lem. Because of this complexity, many different approaches are possible. 

 Other thermocline prediction methods with the same or better approximation 

 will probably be devised. The combination of several methods may lead to 

 dependable prediction of the thermocline depth several days or weeks in 

 advance for all oceans. 



ACKNOWLEDGEMENT 



Help and suggestions rendered by various personnel of the Oceanographic 

 Prediction Division and by the officers and crew of the USCGC CKENCOTEAGUE, 

 whose kind collaboration during September i960 enabled collection of valu- 

 able supplemental data, are gratefully acknowledged. 



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