# Presentation: 136865

## Freshwater Transports of the Global OceanLynne Talley, Joseph L. Reid and Paul E. Robbins   NOAA COSC  April 2004Scripps Institution of Oceanography, UCSD,   La Jolla, CA, USA  ,     ltalley@ucsd.edu/Fax: 1-858-534-9820

- Surface circulations  (Reid, 1994, 1997, 2003). **  **Circulations are absolute geostrophic flow from top to bottom, adjusted at each station pair for mass balance on closed sections (equal to the net transport through the section - e.g. Bering Strait or Indonesian Throughflow).  Bottom velocity from the adjusted solution for each station pair used for the overturning transports.

**Circulation and transports - background (Reid, 1994, 1997, 2003; Talley, 2003****)**

- Atlantic

- Maximum for NADW overturn is 18 Sv in pressure coordinates, but much larger in density coordinates (28 Sv).  AABW is 10 Sv in density coordinates and nonsense in pressure.

- Pacific

- South Pacific: LCDW cell upwelling of 10 Sv.  North Pacific feature may be an artefact of the data sets: deep northern boundary downwelling cell of 8 Sv.

- Indian

- CDW upwelling cell of 18 to 20 Sv.  Also a weak northern boundary downwelling cell .

- Meridional overturning streamfunction (mass transport)

**Southern Ocean overturn: 30°S     Two cells****: **
**Upper ocean, producing mode and Antarctic Intermediate Water - carries MOST of the freshwater transport and also most of the heat transport**
**Deep ocean, producing Antarctic Bottom Water from Atlantic, Pacific, Indian Deep Waters - very little freshwater transport**

**9 Sv**
**0.59 FSv**

**27 Sv**
**0.01 FSv**

**7 Sv**
**0.02 FSv**

**3 Sv**
**0.01 FSv**

**18 Sv ****0.03 FSv**

**-0.23 FSv (LSW)**
**-0.29 FSv (NADW)**

**Summary: Meridional overturns below the subducted layer:**
- Highly dependent on water mass and latitude.
- Thermocline - Subantarctic Mode Water - Antarctic Intermediate Water formation carries the most total freshwater transport across 30°S (complete circumpolar section)
- Labrador Sea Water and NADW formation in North Atlantic carry southward freshwater transport (since feeding surface waters there are much saltier than outflow!  )
- Other deep overturns have modest freshwater transports.

**19 Sv**

- Salinity at 24°N in the N. Atlantic

- Example: Gulf Stream, northward Ekman transport and southward interior flow at 24°N in the N. Atlantic.
- All Southern Hemisphere FW transports are equatorward - this means saltier waters advect poleward (western boundary currents), are freshened on average, and fresher waters are returned equatorward (see arrows in South Atlantic).
- North Pacific: similar picture (equatorward FW transport)
- North Atlantic: southward flow is actually saltier than northward flow because section is at 24°N and large evaporation area lies north of it.  (See surface salinity, and also NCEP E-P.)

- {

- Gulf Stream

- Surface salinity

**Direct transport estimates**
**1. Ocean’s geostrophic flow field - from Reid (1994, 1997), Robbins and Toole (1997) - and temperature , salinity - for individual zonal hydrographic sections using station profiles**
**2. Ocean’s Ekman transport and upper layer temperature and salinity.  From NCEP annual mean wind climatology and temperature-salinity climatology (Levitus).**
**3. Freshwater “transport” through a section is relative to an arbitrary mean salinity. When mass is balanced (enclosed region), then the FW divergence within is balanced by surface E-P**

**Freshwater transport divergence**
- Positive values: net precipitation
- Negative values: net evaporation

**Freshwater transports for each section:**
- Total: meaningful
- Basin absolute values: are meaningless (relative to reference salinity, mass not balanced because of Bering Strait and ITF)

**Freshwater transports for the shallow overturning cells (subducting subtropical waters)**

**Conclusions**
- Salinity affects climate by regulating the ocean’s overturn (inhibits or enhances)
- Ocean salinity is governed by surface evaporation/precipitation/runoff and also by freshwater transport (circulation)
**Upper ocean subtropical gyres balance most of the local strong evaporation**
**Intermediate and deep water overturn are not a trivial part of the budget**
**Net evaporation in the Indian as well as Atlantic is an important part of the NADW “conveyor” **
**Net precipitation (freshening) in the Pacific**
**Bering Strait is not a mystical part of the NADW freshwater budget. In fact Arctic runoff is likely more important for helping regulate NADW salinity**
**Benguela salty and AAIW fresh source waters nearly balance in salinity contribution to NADW: weakening or strengthening of either one could change balance for NADW**
**Southern Ocean: upper cell has smaller overturning volume transport than deeper cell, but much larger freshwater transport (impact of direct air-sea flux versus diapycnal diffusion)**

- Freshwater Transports

- Indonesian throughflow (ITF): carry 10 Sv (34.5S) northward through S. Pacific sections, southward through S. Indian sections.

- Bering Strait: carry 1 Sv (32.5S) northward through all Pacific sections, 1 Sv southward through all Atlantic sections

- 0.5 Sv of the FW transport that enters the N. Atlantic from the north  is from the Arctic and 0.06 from Bering Strait.  These freshen the NADW by **1 psu** compared with input salty subtropical surface water.
- Therefore the Atlantic freshening might be almost all due to Arctic (runoff and precip) and only 10% due to Bering Strait throughput.  (Equivalently: if Bering Strait were shut, the NADW salinity would increase by only about  **0.1 psu**.)

**Bering Strait versus Arctic impact on North Atlantic freshwater balance**

**Freshwater transports associated with meridional overturns - water mass production and upwelling**

**Salinity distribution and stratification**

- Salinity at sea surface and at 1500 meters (Levitus, 1994)

- Density difference surface to 1500 dbar
- With actual salinity                      Assuming 34.9 throughout

- Density difference if salinity is assumed uniform (i.e. this only depends on temperature) (note that N. Pacific, Arctic, much more of Antarctic would overturn)

- Density difference from the surface to 1500 meters:
- Arrows show largest differences - coincide with deep convection regions

**8 Sv SAMW upwells in tropics**

**14 Sv downwells in Gulf Stream & NAC, joins 4 Sv AAIW**

**8 Sv AABW upwells into NADW in  tropics**

**18 Sv into NADW in north**

**Freshwater transport for conversion of ****salty Benguela**** and ****fresher AAIW**** into NADW: ****-0.04**** + ****0.06**** Sv = ****0.02 Sv**
**So there is a very weak northward freshwater transport here.  If AAIW were considerably weaker, then net FW transport would be southward.**

**fresher**

**saltier**

- Red: northward FW transport.  Blue: southward FW transport