Estimating the Meridional Overturning Circulation Estimating the Meridional Overturning Circulation (MOC) in the Atlantic(MOC) in the Atlantic

Greg PinkelGreg PinkelMentor: Dimitris MenemenlisMentor: Dimitris MenemenlisComentor: Michael SchodlokComentor: Michael Schodlok

Outline

● MOC: Meridional Overturning Circulation ● Observations: RAPID Array ● Model: ECCO2 Model ● Results: Comparisons

MOC: Meridional Overturning Circulation

● North Atlantic MOC:

– northward movement of near surface warm waters

– southward movement of deeper, colder waters

● Warm water emerges from Florida Strait as the Gulf Stream

● Water sinks down at higher latitudes

– begins to move southward at a depth of 2km

Surface winds and density gradients create northward flow ● Warmer, saltier, lighter at low latitudes

Colder, fresher, denser water and higher latitudes

- Max of 25% of the global heat flux (at 24.5 degrees N)

- Northwest parts of Europe to enjoy a mild climate despite the high latitude

- 2007: Intergovernmental Panel on Climate Change (IPCC) reported clear evidence that climate change is happening now

● Two things can slow this process

– Ocean surface warming

– Decreased Salinity

● Freshwater runoff from melting glaciers and the Greenland Ice Sheet ------>

● Controversially suggested that MOC has slowed by 30% since 1957

● 20% decrease in northward heat transport

ACIA (Arctic Climate Impact Assessment)

Determining The MOC● Surface: zonal (east-west) stress from winds cause Ekman transport

● Perpendicular to the direction of the wind

● Usually occurs within the top 50m

● Major component of the short-term variability in the MOC

● Wind measurements used to calculate the contribution of the Ekman transport to the MOC

● Measure the density of the water at different depths

● With densities, currents can be estimated at different depths

● Ekman transport and density transport gives a rough estimate of the meridional velocity

● Density-induced transport (light blue arrows)

RAPID/MOCHA Array● Natural Environment Research Council (NERC) in the UK

● Rapid Climate Change/Meridional Overturning Circulation and Heat Flux Array (RAPID/MOCHA)

● Deployed in March 2004 to monitor MOC at 26 degrees N

● 19 Moorings measure variations in pressure, temperature, and salinity

● Determine density at different depths

ECCO2: Estimating the Circulation and Climate of the Ocean, Phase II

● High resolution global-ocean and sea-ice data synthesis

● ECCO2 is a project from collaboration between JPL, MIT, and SIO (Scripps Institution of Oceanography)

● The estimates are derived from a least-squares fit (or regression) of the MIT-gcm model (MIT general circulation model) to observations

● Cubed-Sphere model

– ¼ degree grid

– 50 depth levels

– 1 cube per month

Rapid/ECCO2 Cube78 Comparisons

Summary/Outlook

● Model and In-situ Temperature data agree well for East and West Boundaries ● Model and In-situ Salinity data correspond well for Western Boundary ● Meridional velocities correlate to depths of ~1000m ● Zonal velocities don't agree but have similar trends

● Grid spacing, differences in depth spacing, interpolation, and large averages may account for some differences ● ECCO2 could be improved by using the RAPID data to further constrain the model

Acknowledgments

● Mentor: Dimitris Menemenlis ● Comentor: Michael Schodlok ● Robin McCandliss (BODC) ● The British Oceanographic Data Centre (BODC) ● JPL ● Caltech