Statistics of near-surface salinity gradients from historical CTD and Argo data

Transcription

1 Statistics of near-surface salinity gradients from historical CTD and rgo data Oleg Melnichenko, Nikolai Maximenko, James Potemra, and Peter Hacker International Pacific Research Center, SOEST, University of Hawaii cknowledgments. We thank NS Physical Oceanography Program for financial support of this study. We acknowledge the use of freely available rgo data collected by the International rgo Project and the national programs that contribute to it ( We thank the National Oceanographic Data Center (NODC) for providing free access to WOD 09 ( and WOCE ( data. We also thank Jan Hafner and Konstantin Lebedev (IPRC) for technical help with the data collection. Mini-Symposium Ocean Salinity and the Global Water Cycle, IPRC, ugust, 010.

2 rgo float array is anticipated to provide the largest dataset of in situ measurements required for Cal/Val of quarius data. 6 N 3 N 3 S 6 E 1 E 18 1 W 6 W Surface Validation Temperature, o C 0 10 z 0 =[4,6] m Depth, m quarius mission design and sampling strategy Lagerloef et al., Oceanography 008 (Fig.) Salinity, psu

3 6 N 4 N N S 4 S Locations of CTD casts with valid surface (0-1m depth) temperature and salinity measurements from the WOD 09 (blue dots) and WOCE (green dots) data archives. Stations located closer than 400 km to the shoreline were excluded from the analysis Depth (m) Temperature and salinity profiles (example), collected on July 19, 1993 in the North Pacific at o E, o N (WOCE line p14n, station # 49) Temperature ( o C) Salinity

4 6 N 4 N N S 4 S Locations of CTD casts with valid surface (0-1m depth) temperature and salinity measurements from the WOD 09 (blue dots) and WOCE (green dots) data archives. Stations located closer than 400 km to the shoreline were excluded from the analysis Depth (m) ΔS 5 0 = z 0 =[4,6] m Mixed layer For each vertical profile: ΔS5 0 = S(5m) S(0m) ΔT5 0 = T(5m) T(0m) Δσ = σ (5m) σ (0 ) 5 0 m Temperature ( o C) Salinity

5 Historical CTD data Probability P (%) that absolute value of salinity difference, ΔS 5 0, is larger than or equal to a prescribed value for different sub-sets of CTD data. Only 6% of profiles exhibit near-surface salinity differences larger than or equal to 0.1 psu. WOD 09 QC 5std WOCE, flag= Tropics (3 o S-3 o N) No. of profiles Mean Std Mode skew Over the whole dataset, the mean and standard deviation of ΔS 5 0 are equal 0.05 and 0.17 psu, respectively. Probability distributions of ΔS 5 0 are severely skewed toward positive values due to anomalous low-salinity measurements at the surface Off tropics

6 (a) warm SST fresh SSS Historical CTD data Scatter plots of near-surface salinity differences ΔS versus (a), (b), and (c). 5 0 ΔT5 0 ΔT 10 5 ΔS 10 5 ny attempts to extrapolate subsurface profiles onto the surface to predict salinity differences are unlikely to be successful. (b) (c)

7 Probabilistic approach: historical CTD data Probability P (%) that absolute value of salinity difference, ΔS 5 0, is larger than or equal to a prescribed value for different sub-sets of CTD data. Group ll Δσ 5 0 Δσ10 5 Δσ10 5 ΔT10 5 ΔS ΔS 10 5 < No. of profiles Mean Std skew Statistics of ΔS 5 0 for different sub-sets of CTD data

8 Probabilistic approach: historical CTD data Probability P (%) that absolute value of salinity difference, ΔS 5 0, is larger than or equal to a prescribed value for different sub-sets of CTD data. Group ll Δσ 5 0 Δσ10 5 Δσ10 5 ΔT10 5 ΔS ΔS 10 5 < No. of profiles Mean Std skew Statistics of ΔS 5 0 for different sub-sets of CTD data

9 MLD, m Scatter plot of the near-surface salinity differences, ΔS 5 0, versus mixed layer depth. To simulate a typical rgo profile, a search for the mixed layer for each CTD cast was initiated from = [4,6]m. z 0 Salinity difference ΔS 5 0, psu Mixed layer depth range, m Mean( ΔS5 0) Std( ΔS 5 0) psu psu Two groups: 5 MLD <10 10 MLD < / / / /0.086 MLD, m MLD <15 Mean, psu Std, psu MLD < / /0.078 MLD MLD < / /0.071 WOD 09/WOCE

10 6 N 4 N DJF 6 N 4 N MM N N S S 4 S 4 S 6 N 4 N JJ 6 N 4 N SON 14 1 N N 10 8 S 4 S S 4 S Probability (%) of appearance of a complex salinity structure in the near-surface layer identified from rgo float observations. P = NΔS 0. 0 N tot

11 6 N 4 N N DJF Probability (%) of appearance of a complex salinity structure in the nearsurface layer in boreal winter. 8 S 6 4 S N 4 N N DJF DJF S 4 S Seasonal mean precipitation (mm/day) calculated from GPCP daily data ( ) Seasonal mean wind speed (m/sec) 4 0

12 6 N 4 N N Seasonal mean precipitation (mm/day). JJ Seasonal mean wind speed (m/sec) JJ S 4 S N 4 N JJ 14 1 N 10 8 S 4 S Probability (%) of appearance of a complex salinity structure in the nearsurface layer in boreal summer.

13 1 + = P P P P P P + = ) ( μ μ μ )) / ( ) ( ( P P P = μ μ σ μ σ σ where - probability, - mean, and - standard deviation P μ σ y splitting rgo profiles into two mutually exclusive groups and, 0

14 6 N 4 N DJF 6 N 4 N MM N N S S 4 S 4 S 6 N 4 N JJ 6 N 4 N SON N N S S S 4 S Expected standard deviation of salinity differences between in situ rgo observations and quarius SSS retrievals. Subsurface salinity gradient criteria.

15 6 N 4 N DJF 6 N 4 N MM N N S S 4 S 4 S 6 N 4 N JJ 6 N 4 N SON N N S S S 4 S Expected standard deviation of salinity differences between in situ rgo observations and quarius SSS retrievals. Mixed layer depth criteria.

16 Salinity gradient threshold 6 N 4 N DJF 6 N 4 N JJ N N S S 4 S 4 S 6 N 4 N N Expected standard deviation of salinity differences between in situ rgo observations and quarius SSS retrievals. Mixed layer depth threshold DJF JJ 6 N 4 N N S 4 S S 4 S

17 Summary Historical high-resolution CTD profiles are analyzed to characterize vertical salinity differences in the uppermost ocean layer. Globally, the mean and standard deviation of S(5m)-S(0m) are 0.05 psu and 0.18 psu, respectively. Probability distributions of near-surface salinity differences are severely skewed towards positive values. Only about 6% of historical CTD profiles exhibit near-surface salinity differences larger than or equal to 0.1 psu, thus justifying the use of the rgo array for quarius validation. rgo float data from 001 to 009 are used to reconstruct seasonal maps of probability of appearance of a complex salinity structure in the near surface layer. Such probability maps can be easily transformed into maps of other useful statistical characteristics, provided that the global description, inferred from the analysis of historical CTD data, holds locally. To what extent this is true or what adjustments could be made is a subject of ongoing research.

18 Thank you

19 No. of profiles Mean Std Mode skew WOD WOD 09 QC 5std WOCE, flag= Tropics (3 o S-3 o N) Off tropics