Use of Tracers in Classifying Climate-Relevant
North Atlantic Circulation Transients

Variability in the Meridional Overturning Circulation in the Atlantic Ocean has long played a key role in the conceptualization of climate transient scenarios, but the associated fluid dynamics problems are far from resolved. A strongly variable ventilation process will have distinct kinematic attributes, and hence produce characteristic "fingerprints" in various tracer distributions. Because they reflect a time history of Lagrangian particle displacements, such fingerprint patterns should provide valuable complements to observations of instantaneous circulation fields, in effort to describe and dynamically explain regional climate transient. Using archived circulation from a 40-year MICOM North Atlantic run and property data, the study will be divided into two phases. One will comprise a reference integration using the delivery history of chlorofluorocarbons (CFCs), and the other consist of multiple tracer release simulations, clustered in groups (ensembles) associated with distinct circulation anomalies in the North Atlantic basin. Special attention will be given to connections between tracer spreading patterns and surface anomalies in the NW subtropical Atlantic as an action center for closing the North Atlantic Oscillation (NAO) interaction loop.

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GOAL

A long term objective of this work is to help develop a basis for including transient tracer data as a quantitative tool in climate change assessments and for initializing coupled climate models. As first steps in this direction, our immediate goals are:

(i) to characterize the relation between ventilation and basin-scale tracer spreading patterns,

(ii) to test the proposition that the gross patterns ("fingerprints") in tracer distribution variability, by virtue of their association with potential vorticity anomaly, provide a useful portrait of large scale circulation anomalies.

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PARTICIPANTS

Rainer Bleck and Eric Chassignet will supervise the MICOM development, Rana Fine will supervise the tracer related aspects in terms of the data and analysis of results. Claes Rooth will be involved in the theoretical and interpretative aspects of the project. Robert Molinari will be involved in pattern recognition of upper ocean variability. Ge Peng will carry out the experiments in collaboration with aforementioned PIs.

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EQUATIONS FOR OFF-LINE CFC COMPUTATION

In order to carry out CFC tracer integration off-line, the continuity equation is used to derive vertical mass transport using archived files (monthly in our case). The vertical mass transport is then used to inject tracers into the interior layers from the mixed layer and buffer zone. Based on the tracer conservation equation, CFC concentration is integrated forward in time. Special treatments are needed in dealing with massless layers. The mixed layer CFC concentration is updated using prescribed time-dependent atmospheric concentration.

Equations especially formulated for CFC off-line computation are posted here. A postscript version can be downloaded if interested (click here.) Questions concerning the equations should be directed to Rainer Bleck or Ge Peng .