1. Observations of the North Atlantic Subtropical Mode Water
Young-Oh Kwon
Stephen C. Riser
(University of Washington)
First ARGO Science Workshop
(November 12-14, 2003)

2. Mode Waters in the World Oceans
Background and Objectives
Mode Waters in the World Oceans
(Hanawa and Talley, 2001)

3. Eighteen Degree Water (= North Atlantic Subtropical Mode Water)
Background and Objectives
Eighteen Degree Water (= North Atlantic Subtropical Mode Water)
17.9 ± 0.3 °C and ± 0.10 ‰ (Worthington, 1959)

4. WOCE/ACCE Profiling Float Array
Data and Methods
71 floats (= 60 T-only floats + 11 CTD floats)
July, 1997 ~ present
Parking depth : m
Surfacing and profiling interval : 10 days
More than 8800 temperature profiles and 800 salinity profiles
About 150 temperature profiles per month ( )

5. Objective Mapping of EDW
Data and Methods
Mapping upper 900m temperature using each month’s observations (Jan,1998 – Dec, 2001).
Identify EDW from each objectively mapped temperature profiles.
(Criteria : 17°C  T  19°C and dT/dZ  0.006°C/m)
Mapping EDW thickness, temperature and depth of upper boundary.
Calculate time series of EDW volume, temperature and heat content.

6. EDW in March, 1998
Distribution

7. Time Series (Jan,1998-Dec,2001)
Distribution

8. Production Vwinter– Vfall = 2.24 ± 0.61 x 1014 m3
⇒ Annual production/destruction rate.

9. Annual Subduction Rate - Definition -
Production S
ann = - W Ek b f ò d
vdz + 1 h m 2 T ( )
Vertical Pumping
Lateral Induction
(Huang and Qiu, 1994)
WEk : Ekman pumping velocity
← 　Monthly mean NCEP reanalysis wind stress ( )
u, v : Geostrophic velocity
←　Monthly mean absolute geostrophic velocity from profiling floats
hm,1, hm,2 : MLD in the 1st and 2nd winter
←　Monthly mean objectively mapped MLD from profiling floats
T : Averaging period = 365 days
d : Depth of the particle following the calculated Lagrangian trajectory

10. Annual Subduction Rate - Input Data -
Production
All input data are from profiling floats + NCEP during
Monthly mean data.
Integrations start when local MLD reaches its maximum.

11. Annual Subduction Rate - Results -
Production
⇒ Sann = 50 – 150 m/yr
⇒~ 3.9 SV of EDW Production
= 1.23 x 1014 m3
~ ± 0.61 x 1014 m3

12. Destruction
Which process is responsible for the observed annual EDW destruction of 2.24 ± 0.61 x 1014 m3 ?
- Vertical diffusion
- Lateral diffusion

13. Vertical Diffusion
Destruction
514 April profiles ⇒ Numerically integrated for one year
K = 10-5 m2/s
⇒ ± 2.17 m
= ± 0.09 x 1014 m3
Too small .
K = 10-4 m2/s
⇒　 ± 3.61 m
= ± 0.16 x 1014 m3
Comparable to observed .

14. Why Enhanced Mixing?
Destruction
⇒ Susceptible to salt fingering

15. Lateral Diffusion K = 500 – 3000 m2/s
Destruction
Meridional thickness profiles ⇒ Numerically integrated for one year
K = 500 – 3000 m2/s
⇒ 7 – 11 % of initial volume destroyed.
Observed : 48% of winter EDW volume destroyed annually.

16. Conclusions Distribution
- Volume : Max in winter, min in fall ⇒ ∆V=2.24 ± 0.61 x 1014 m3.
Larger when NAO is lower.
- Temperature : Mean = ± 0.02 °C.
- Heat content : Dominated by volume variability.
Production
- Annual subduction rate = m/yr ⇒ ~1.23 x 1014 m3.
Destruction
- Vertical diffusion with K=10-4 m2/s eroded comparable amount of
EDW to the observed.
- Salt fingering could be responsible for the enhanced mixing.
- Lateral diffusion plays only a secondary role.
(~15-23 % of the observed destruction.)

17. ARGO Perspectives ⇒ ~300 profiles/month in 20-45 °N, 40-80 °W.
ARGO : Profiling float all over the world ocean.
- Parking depth : m
- Sampling interval : 10 days
- Resolution : ~ 3 ° x 3 °
⇒ ~300 profiles/month in °N, °W.

18. Thank You.