For decades, it has been thought that the tropics and the extratropics were relatively independent connected, thought incorrectly, just through some form of a zonally symmetric Hadley Circulation and the instability of the cell that producing extratropical Rossby waves. In that, that the direction of the interaction was “tropics ⇒ extratropics” with the system responding to radiational imbalances.  Rather, we argue that there is a deeper dynamic synergy between the tropics and extratropics and there is a mutual dependency such that  “tropics ⇔ extratropics”. Through this process potential vorticity (PV) is sent poleward by divergent circulations that produce extratropical jets that spawn waves. These in turn break recursively in the westerly ducts (which they form) returning PV to the tropics. This conduit between the extratropics appears as a regional turbulent chaotic field (referred to as “washing machine-like” by Christian Jacob) producing both integrated impacts on the tropics, by an equatorward flux of PV, as well as important and discernible transients.

The synergetic system (tropics ⇒ extratropics & extratropics ⇒ tropics) is explored in terms of conservation of potential vorticity substance that is conserved even in the presence of diabatic heating and dissipation (as distinct from potential vorticity that is not). In essence, the synergy is an expression of the Haynes and McIntyre (1987) “impermeability theorem”.

Besides describing the interaction between the tropics and the extratropics, it is quite likely that interhemispheric interaction is explainable with the same physics. For example, despite the great differences in the geography of each hemisphere the annually averaged TOA albedo is the same as is the total rainfall, even though spread differently in each hemisphere between and ocean.  Both balance to ±2%. Also, it may help explain why the response is the same in both hemispheres even when orbital forcing is asymmetric between hemispheres.

Could it be that the overall constraint on climate, both between the tropics and extratropics and between hemispheres is constrained by the conservation of potential vorticity substance?