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)

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

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 36.50 ± 0.10 ‰ (Worthington, 1959)

WOCE/ACCE Profiling Float Array Data and Methods 71 floats (= 60 T-only floats + 11 CTD floats) July, 1997 ~ present Parking depth : 1000 m Surfacing and profiling interval : 10 days More than 8800 temperature profiles and 800 salinity profiles About 150 temperature profiles per month (1998-2001)

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.

EDW in March, 1998 Distribution

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

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

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 (1998-2001) 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

Annual Subduction Rate - Input Data - Production All input data are from profiling floats + NCEP during 1998-2001. Monthly mean data. Integrations start when local MLD reaches its maximum.

Annual Subduction Rate - Results - Production ⇒ Sann = 50 – 150 m/yr ⇒~ 3.9 SV of EDW Production = 1.23 x 1014 m3 ~ 2.24 ± 0.61 x 1014 m3

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

Vertical Diffusion Destruction 514 April profiles ⇒ Numerically integrated for one year K = 10-5 m2/s ⇒ -28.84 ± 2.17 m = 1.00 ± 0.09 x 1014 m3 Too small . K = 10-4 m2/s ⇒　-65.55 ± 3.61 m = 2.27 ± 0.16 x 1014 m3 Comparable to observed .

Why Enhanced Mixing? Destruction ⇒ Susceptible to salt fingering

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.

Conclusions Distribution - Volume : Max in winter, min in fall ⇒ ∆V=2.24 ± 0.61 x 1014 m3. Larger when NAO is lower. - Temperature : Mean = 18.28 ± 0.02 °C. - Heat content : Dominated by volume variability. Production - Annual subduction rate = 50-150 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.)

ARGO Perspectives ⇒ ~300 profiles/month in 20-45 °N, 40-80 °W. ARGO : Profiling float all over the world ocean. - Parking depth : 1000-2000 m - Sampling interval : 10 days - Resolution : ~ 3 ° x 3 ° ⇒ ~300 profiles/month in 20-45 °N, 40-80 °W.

Thank You.