Evaluation of Interior Ventilation Pathways & Timescales using an Off-Line Tracer Model
In collaboration with Matthew England (UNSW)

1. Introduction
2. Model & Experiments
3. CFCs Inter-Decadal Ventilation
4. Passive Tracer - Multi Century Ventilation
5. Conclusion
6. Animated sequence - *CFCs from 1935-2065 at 0-1000m; 1000-2000m; 2000m +
7. Animated sequence - *Passive tracer - NADW release - 300 yrs
8. Animated sequence - *Passive tracer - AABW release - 300 yrs
 *NB in animated sequences time does not necessarily move at a constant rate

1. Introduction
The ocean has an enormous capacity to absorb and store heat and a variety of anthropogenic greenhouse gases. As a result ventilation - the transport of surface waters into the ocean interior - is of critical importance to climate studies.

Diagnosing ventilation pathways and timescales over periods greater than a few years currently means that simulations must be run at course resolutions well below the scales that much of the important ocean dynamics is taking place.

One option for running a model at high resolution while using only relatively modest computational resources is by executing the model in an 'off-line' mode in which only tracer evolution is prognostic. This requires two computational tools. The first is the 'on-line' Ocean General Circulation Model (OGCM), which defines the grid resolution, the speed and direction of ocean currents and the temperature and salinity distributions. The second component is the 'off-line' tracer dispersion model, which computes the effects of advection and mixing on the tracer using a prescribed source function and the fields obtained from the on-line OGCM.
2. Model & Experiments
Our study investigates global-ocean water-mass ventilation pathways and timescales using this off-line methodology at an `eddy-permitting' 1/4° resolution. The model is forced by monthly averaged velocity, temperature and salinity fields derived from the outputs of a state-of-the-art Ocean General Circulation Model.

Any convective mixing that would have occurred as a result of an unstable water column in the prognostic model would be `mixed out' by the time the temperature and salinity fields are incorporated into the off-line tracer model. In addition, the monthly averaging of these fields mean that mixed layers would not reach the significant depths attained by sporadic and transient convective overturning. To remedy this, a mixed layer parameterization was implemented.

A representation of CFC-11 air-sea gas exchange is also incorporated and the simulated tracer distributions are compared with observations to investigate inter-decadal timescale ventilation.
Multi-century ventilation of the outflow of North Atlantic Deep Water into the Southern Ocean and Bottom Water from the Antarctic margin are investigated using passive tracer experiments with interior source regions, integrated over 1000 years. A comparison between model derived `tracer age' and 14C `advection age' provides a semi-quantitative assessment of model skill at these longer timescales.
3. CFCs Inter-Decadal Ventilation

Simulated CFC in the North Atlantic shows many of the observed features: (i) homogeneous layer of mode water; (ii) persistent front between North and South Atlantic Central Water masses at the southern rim of the subtropical gyre; (iii) southward spread of upper North Atlantic Deep Water (NADW); (iv) some of the equatorial dynamics associated with equatorial deep jets & extra equatorial jets. The model shows a spurious intrusion of CFC-poor water (v) at intermediate depths (~1000m) which corresponds to an intrusion of North Atlantic Current water. Notably absent is the lower core of NADW (vi), a consequence of the model domain which excludes the formation zone of denser Overflow Waters.

In the North Pacific fresher conditions prevents the formation of any deep water masses. The model captures the observed (i) shallow penetration of CFC in the subpolar region; (ii) the increasing penetration evident in the subtropical gyre with the intrusion of subsurface, tracer-rich mode water, and (iii) a weak front between the North Equatorial Current and Counter Currents across the north equatorial divergence.

Figures of CFC concentration on deep neutral density surfaces show the spread of Bottom Water in the Southern Ocean. The model reproduces the observed Bottom Water formation in the Weddell and Ross Sea areas and subsequent spreading aided by the polar gyres. The model fails to capture the extensive area of high tracer concentration between 1500E and 500E that flow east from the Adelie coast and the Amery Ice Shelf.
4. Passive Tracer - Multi Century Ventilation

Modeled (coloured contours) and observed (black contours) of water 'age' can be derived from simulated passive tracer and observed 14C concentrations, respectively. For bottom water (below) qualitative agreement is good, showing that the predominant northward penetration is in the Pacific as a western boundary current following elevated bathymetry north east of New Zealand. Oldest waters are found across the northeastern Pacific basin.

As overall agreement with observed 14C is good for these multi-century timescales we can seek to identify the detailed pathways into the Pacific. By looking at the time evolution of the model tracer field (in this case the 1% tracer concentration contour every 10 years) it is possible to identify the major bottom water ventilation pathways into the North Pacific. The implied pathways (superimposed) correspond well to a number of deep hydrographic sections.

For analysis of Deep Water 100% concentration passive tracer is maintained north of 50 N and below 985m and allowed to evolve for 1000 years.
A single-cored Deep Western Boundary Current (DWBC) travels southward at a depth centered around 2500m (somewhere between the depths of the observed double cores). Numerous branches break from the DWBC in line with observation. In particular a strong signal is carried across the basin at the equator, in a Equatorial Deep Jet. The DWBC finally separates from the continental margin near 45S and is advected eastward in the Antarctic Circumpolar Current where much of the tracer is either upwelled into the surface layers, or is mixed downwards to join the bottom waters that penetrate northwards.

The Southern Ocean plays a vital part in the conversion of water masses. In the Southern Ocean, both Deep Water from the North Atlantic and Bottom Water released around the Antarctic margin are effectively mixed throughout the watercolumn. Northward penetration is predominantly as Intermediate Water in all ocean basins or as Bottom Water in the deep Pacific.
5. Conclusion

The off-line methodology provides a powerful tool for investigating ventilation pathways and timescales. It has allowed us to integrate ventilation experiments for unprecedented lengths of time, for such high resolutions. Simulated tracer distributions due to inter-decadal and multi-century ventilation are in good agreement with observations.

The primary limitation of the off-line model is the lack of any possible feedback onto themodel dynamics, thus making, for example, simulations of climate change impossible. It can, however, be an ideal tool for understanding present-day ocean ventilation. It also provides the only practical means for assessing interior ocean circulation regimes, at multi century time-scales, in models of eddy permitting resolution.

For more information see
Alex Sen Gupta and Matthew H. England, 2003. Evaluation of interior circulation in a high resolution global ocean model; Part I: Deep and Bottom Waters. (See abstract) submitted to Journal of Physical Oceanography