Ocean Sciences Meeting 2014

This post is mostly for people who are also at the Ocean Sciences Meeting, but if you’ve stumbled across this from somewhere else, welcome!

There were lots of details about my work that I wanted to share, but I couldn’t squeeze everything onto my poster. Even though the following details didn’t make the cut, I think that they’re really interesting and worth sharing. The rest of this post will make a lot more sense if you’ve read the poster first.

An electronic copy of my poster can be found here.

Model with one active layer

In the poster I stated that the model with one active layer was unable to reproduce the correlation pattern found by Zhang (2010), but I didn’t show the pattern. The following figure is the correlation pattern from the model with one active layer.

The black dot represents the reference latitude against which all of the correlaitons were computed. The colour represents the correlation coefficient between the geostrophic transport in the upper layer at the reference latitude and all other latitudes for a range of time lags. Figure 3 (a) is from the model version with two active layers while figure 3 (b) is from the version with three active layers. The cyan line represents the mean trajectory of a fluid parcel on the western boundary in the lowest active layer.
The black dot represents the reference latitude against which all of the correlations were computed. The colours represent the correlation coefficient between the geostrophic transport in the active layer at the reference latitude and all other latitudes for a range of time lags. The cyan line represents the mean trajectory of a fluid parcel on the western boundary in the active layer. This model, which has only one active layer, is not able to reproduce the multiple propagation regimes found by Zhang (2010).

This shows very clearly that the model with one active layer is unable to represent the different propagation regimes.

Vertical structure

Each propagation regime is confined to one of the gyres. In the northern gyre the Atlantic Meridional Overturning Circulation (AMOC) anomalies propagate at advective speeds, while in the southern gyre they move at the speed of a Kelvin wave.

The difference in propagation speed cannot be due to Doppler shifting of the wave. The direction of the flow in the northern gyre would speed up the wave, while the flow in the southern gyre would retard its propagation. Apart from the direction of the flow, the major difference between the two gyres is the vertical structure of the flow. In the northern gyre all of the interfaces are displaced from their at rest location, whereas in the southern gyre only the interface between the top two layers is substantially displaced. This can be seen in the following figure.

mean interface displacement
Interface displacement on the western boundary averaged in time for the model with three active layers. The interfaces are numbered for the layer above. That is, interface 1 is between layer 1 (the upper layer) and layer 2. The interface displacements in the northern gyre show that the flow penetrates down into all of the active layers. In the southern gyre the flow is mostly contained in the upper layer.

This leads us to conclude that the mean flows in the two gyres have different vertical structures. Combining this conclusion with the fact that the model requires multiple active layers to reproduce the propagation pattern reported by Zhang (2010) indicates that resolving the multiple propagation regimes and the transformation from one regime to another depends on the model being able to resolve multiple baroclinic modes. The role played by these multiple modes is not yet clear, but there are some hints.

Advective mode planetary waves

The fact that the anomalies travel at a speed very similar to the velocity of the fluid in the lowest active layer brings to mind the advective mode planetary waves identified by Liu (1999a, 1999b).

A possible propagation mechanism might be that the transport anomaly travels as an advective mode planetary wave in the northern gyre, until it reaches the intergyre boundary. At this point the wave is unable to propagate, perhaps due to the steep gradients in layer thickness, potential vorticity or velocity. The energy from these advective mode planetary waves is then radiated away as a Kelvin wave. The mechanism proposed above is, at the moment, conjecture, but it fits the available evidence.

Future work will focus on finding evidence to support or discredit it.

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