Transport of NSOW into and within the Irminger

Sea: Insights from model

Xiaobiao Xu1,

William Schmitz Jr.2, Harley E. Hurlburt3, Patrick Hogan3, and Eric P. Chassignet4

1DMS/USM, 2SERF, 3NRL/SSC, 4COAPS/FSU

2ndAMOC annual meeting, June 7-9, 2010, Miami, FL

The area of interest

*** Long-term transport measurements ***

1. TTO: 5.1 (Dickson and Brown,

1994)2. ANG: 7.3[4] (Dickson et al., 2008)3. SECF: 9.0[4.5] (Bacon and Saunders, 2010)4. SEI: 3.2 (Saunders, 1996)5. CGFZ: 2.4 (Saunders, 1994)

Greenland

Iceland

Flow over the Reykjanes Ridge?

Figure 1 of Shor et al. (1980)Figure 6 of Hansen and Osterhus

(2000)

“Saunders (1994) very detailed flux measurement for the passage through the CGFZ seems to be too small to accommodate all the ISOW and this problem is augmented if his value includes a significant fraction of LDW … There is reason to believe that both the overflow flux measurements from SEI and those in the CGFZ do not cover the total flow… On the other hand, there is no reason to assume that all the overflow from the Iceland–Scotland Gap is transported into the Western Basin …”

Bathymetry of Reykjanes Ridge

Bathymetry based on V12.1 of Smith and Sandwell, (1997) with 1min resolution.Red triangles are 3 moorings discussed in Shor et al (1980), in which a westward transport of 2.4 Sv is estimated.

Bight Fracture zone

• HYbrid Coordinate Ocean Model (HYCOM, http://www.hycom.org)

• Atlantic experiment (20°S-80°N)• 0.08° horizontal resolution (eddy-resolving)• 32 layers (in 2) in vertical • Thermobaric effect included• Initialization with zero velocity and T/S from

climatology GDEM• Climatologically (ERA40) forced• 10-year integration, time average of last 5 years

is used as the model-based ‘mean’ state

Model configurations

Flow over the ReykRge

Modeled flow over the ReykRge

Salinity

Velocity (cm/s)

Model-based salinity, normal velocity, and volume transport, over the Reykjanes Ridge (north of CGFZ)

(OSD from WOD2005) data-based salinity along the center of the Reykjanes Ridge and through the CGFZ

Deep circulation in the Iceland Basin

Transport per unit width (m) below of 27.85 kg/m3

Red lines are the crest of Björn, Björnsson, and Gardar Drifts, from Saunders (2001)

SEI mooring array of Saunders (1996)

Southeast of Iceland

Salinity

Velocity

Model Fig. 2b Saunders (1996)

LSW MNAWISOW

While the (three-layer) structure agrees approximately, the simulated salinity of MNAW (max of ~35.3) is higher than observed (max. of ~35.2);The velocity magnitude and distribution are similar to observation;Transport 3.4Sv (≥27.80) based on model and 3.2Sv by Saunders (1996)

Charlie-Gibbs Fracture zone I

Transport per unit width (m) for water of

27.85 kg/m3. Red dots are eight mooring locations of Saunders (1994)

Charlie-Gibbs Fracture zone II

2.4Sv for >27.80

Salinity

Velocity

Obs

erva

tion

Mod

el

2.8Sv based on model

Circulation in the Irminger Sea

≥27.85 27.80 ≤ <27.85

Where do ISOW join DWBC/DSOWAccumulated transport along a NW-SE section in the IrSea

The model-based transports suggest that the crossing, or the confluence of ISOW into DWBC, takes fairly uniformly for layer with 27.85 kg/m3, but in contrast more crossing takes place north of ANG for 27.80<27.85

Across the Irminger Sea

SECF array ANG array

9.8 [5.7 for ≥27.85] Sv in model9.0 [4.5] Sv in observation

8.0 [4.8 for ≥27.85] Sv in model7.3 [4.0] Sv in observation

Salinity Salinity

Velocity profiles of overflow

(a) ANG array: data from Dickson et al. (2008, their Figure 19.6)

(b) SECF array: data from Bacon and Saunders (2010, their table 2)

The model reproduces to some extent the bottom-

intensification of the overflow plume in deeper

slope, but not in the (shallow) continental

slope.

Summary

• Model results help to clarify some unresolved connections on NSOW transports. Results suggest that about 2 Sv of ISOW could flow into Iriminger Sea over the Reykjanes Ridge north of the CGFZ. This helps to resolve the well-known transport balance issue of observed ISOW through CGFZ and along western boundary of Irminger Sea.

• The model-based circulations in the Iceland Basin, through the CGFZ, and in the Irminger Sea are compared to available data. The modeled circulations are semi-consistent (with data) in reproducing transports of overflow waters (defined as ≥27.80 and 27.85 kg/m3) with roughly correct S (and T) characteristics.

• The simulated ISOW through the CGFZ is slightly more saline (denser) than observed, this might influence its spreading pathway west of CGFZ into the Irminger Sea.

• Although the transport along the western boundary of Irminger Sea compares well to observed (within ~1Sv). The vertical profile of the bottom-intensification over the shallow continental slope is not well represented.