CTD/XBT Comparison, Quality of JJYY Data and XBT Data Analysis of the Mixed Layer Depth by LT Mike Roth OC3570 20MAR01

Purpose XBT/CTD temperature comparison Value lost though the use of JJYY data points Determine impact of mechanical and thermal forcing on the MLD

XBT-CTD Comparison Insruments Sea Bird CTD T-7 Sippican XBTs

XBT-CTD Comparison Location of collocated XBTs and CTDs 1-7 cruise one 8 –10 cruise three

XBT-CTD Comparison Data Processing Converted CTD dbars to m (Saunders, 1981) Data deeper than deepest depth of collocated instrument discarded XBT had smaller sampling interval (0.7 m: 1.98 m) Linear interpolation of XBT to standard CTD levels 383 levels / 3,326 data pts each/ total=6,652

XBT-CTD Comparison Quality Control of the data Causes Spikes due to XBT copper wire hitting hull Bad XBTs Upper 4 m (16.28 %) 3 Stage Process 0.2° C criteria to flag points Visual Final re-run

XBT-CTD Comparison Quality Control of the data Example of a bad XBT

XBT-CTD Comparison Quality Control of the data Results of QC Reduced sample size to 9 CTD/XBTs None of CTD data flagged Removed: 501 pts (1,002) 2,825 pts (5,650); or 85% remained for statistical analysis

XBT-CTD Comparison Data Analysis XBT subtracted from CTD temp at each depth for each collocated XBT/CTD

XBT-CTD Comparison Data Analysis Composite Plot

XBT-CTD Comparison Findings XBT generally > CTD Greatest T-diff in upper 100 m 60-80 m was largest T-diff (-0.2915 ºC at 69.5 m) with max variability as well (0.5007 ºC at 67.5 m) CTD>XBT (+ T-diff) between 90-105 m (max 0.0538 ºC at 91.3 m) 105 to 760 m mean T-diff = -0.077 ºC and smaller std

XBT-CTD Comparison Comparison to Similar Studies NPS R/V Point Sur   NPS R/V Point Sur February 2001 July-August 2000 Schmeiser (2000) Heinmiller et al. (1983) CTD – XBT Temp. Diff. Depth (m) Mean ( C) Std ( C) 25-125 -0.0907 0.1779 -0.2198 0.3598 -0.17 0.08 250-350 -0.0731 0.0903 -0.1076 0.2194 -0.10 0.10 175-350 -0.0810 0.0951 -0.1171 0.1975 0.11 175-375 -0.0851 0.0960 -0.1212 0.1981 -0.13 0.16 Mean -0.0783 0.1047 -.1549 0.2151

JJYY-XBT/CTD Comparison Results of QC Much easier to perform XBT-6 salvaged for upper 100 m Only had 7 collocated JJYY-XBT/CTD Started with 2,247 pts (4,494) after interpolation Ended up with 2,245 pts (4,490); 99% remained for statistical analysis

JJYY-XBT/CTD Comparison Results of Data Analysis NAVO SVPG software with user interface performs a pre-Quality control of the data and With a linear interpolation of the data points does perform well as representing the full XBT profile ….

Analysis of MLD using XBTs

Analysis of MLD using XBTs

Analysis of MLD using XBTs

Conclusions Good Quality Control is a necessity, start below 4m Sometimes you are limited to small sample sizes which can have a significant impact on final results XBT/CTD comparison did reveal similar results as past studies (ie. XBT>CTD bias, upper 125 m) JJYY-XBT/CTD comparison showed SVPG is a useful tool in representing the original XBT profile and makes QC much easier Ungridded XBT data is not ideal for MLD analysis, but does show effects of thermal and mechanical forcing