MOC related activities at NOC

Joël Hirschi, Elaine McDonagh, Brian King, Gerard McCarthyStuart Cunningham, Harry Bryden, Adam Blaker

SAMOC workshop, Rio de Janeiro 11th – 13th May 2010

Outline

• Some thoughts about SAMOC

• Hydrography in the South Atlantic

• Imprints of eddies and internal waves on MOC

Thoughts about SAMOC

• What is the added value of having an observing system in the South Atlantic?

• How and where should we make measurements?

What value can SAMOC add?

• Heat budget in tropical Atlantic

• Lead/lag relationship between heat fluxes at 26/32°S, 26°N and the development of anomalous heat content/SSTs

• Better understanding of southern vs northern influences on Atlantic MOC

Heat budget of tropical Atlantic?

MOC anomalies: April 2004 – April 2008

MOC anomalies: deseasoned, 2-month running average

Lag correlation between MOC anomalies and SSTs:MOC leads SSTs by 6 months

Influence onNorth AtlanticStorm Track

Origin of NorthAtlantic Hurricanes

Hydrography in the South Atlantic

Focus on Repeat Zonal Hydrography in the Subtropics

• At 24ºS– 2009: JC032– 1983: OC133– 1958: IGY Section

• At 30ºS– 2003 & 1993

Changes at 30ºS: 2003 - 1993

Max. Anom at 13ºC in

Eastern Basin

Salinification of Thermocline and Intermediate Water in the East

Interannual Variability at 30ºS

• Salinity Anomalies with large temporal amplitude are seen in the Eastern Basin – peak to trough comparable with cruise changes

• Salinity anomalies appear to propagate westwards with speeds of ~ 5cm/s recalling anomalies seen highlighted by Biastoch (2008) in satellite and model data

• Salinity Anomalies with large temporal amplitude are seen in the Eastern Basin – peak to trough comparable with cruise changes

Raw Argo salinity on 13ºC

F

S

F

Interannual Variability at 30ºS

• Salinity Anomalies with large temporal amplitude are seen in the Eastern Basin – peak to trough comparable with cruise changes

• Salinity anomalies appear to propagate westwards with speeds of ~ 5cm/s recalling anomalies seen highlighted by Biastoch (2008) in satellite and model data

Interannual Variability at 30ºS

• Salinity Anomalies with large temporal amplitude are seen in the Eastern Basin – peak to trough comparable with cruise changes

• Salinity anomalies appear to propagate westwards with speeds of ~ 5cm/s recalling anomalies seen highlighted by Biastoch (2008) in satellite and model data

Interannual Variability at 30ºS

Imprint of eddies and internal waves on MOC

NEMO (Nucleus of European Modelling of the Ocean)

• NEMO ¼° global

1958 to 2001 run

• 64 Vertical levels

• 6-hourly forcing for

wind, heat and

freshwater fluxes

Madec (2008) http://www.nemo-ocean.eu

Two passes through 1958 to 2001 forcing:Influence of initial conditions

Pass 1 Pass 2

Atlantic MOC at 26°S at 1000 m depth

Red: pass 2Blue: pass 1

Atlantic MOC 1100 m depth: 26N

MOC in 1/4° NEMO spinup: mean MOC 1976-2001 [Sv]

Standard deviation of MOC 1976-2001 (5-day averages)

Sv

Sv

Sv

Snapshots of sea surface height in passes 1 and 2

Pass 1, 21-25/12/1980 Pass 2, 21-25/12/1980m

SSH difference between passes 2 and 1 [m]

Std. dev .of MOC variability due to eddies & waves

Sv

Sv

Sv

Ratio between chaotic and total MOC variability

• Importance to clearly define benefits of SAMOC

• Hydrographic measurements at 30°S suggest large interannual variability – difficult to detect trends

• Imprint of eddies and internal waves varies from 1-2 Sv std.dev. (Tropics to mid-latitudes) to more than 5 Sv std.dev. at the equator

• Model results suggest similar “chaotic” variability at 25°S and 32°S

• Inferred “chaotic” variability likely to be a lower limit as eddies are likely to be too “predictable” in ¼° model

Conclusions