I was looking at this 3D map of satellites orbiting earth and noticed something interesting. Around the green ring of active geosynchronous satellites, there is a ribbon of what I assume are obsolete geosynchronous satellites. The strange thing is that this ring is skewed - above the Atlantic Ocean, most inactive satellites are positioned north of the active ones, while they are positioned to the south above the Pacific Ocean. Why is that? I'd expect the satellites to "average out" and form a band north and south of the equator regardless of longitude.
The current answers do not contain enough detail.
Although there are several answers, I still can't see exactly why the ribbon is skewed. There is some discussion of what that means, but to make it clearer I took one of the links and then added two skewed purple lines. Why exactly does this happen? Is it possible to link to a source with a detailed orbital mechanical explanation with equations etc.? i.stack.imgur.com/sUPpb.jpg
The apparent source of the data (as it is linked in the source linked in the description of the author of this map) is http://apps.agi.com/SatelliteViewer/
In this animated view you can see that these are the real orbits: Satellites that currently are in the northern hemisphere show up in the southern hemisphere half a day later.
As active station keeping is not possible for decommissioned satellites, their orbit will change more with every year they stay in orbit. One of the main influences is the sun: The GEO, aligned with the equator of Earth, is inclined by 23° with respect to the sun. Due to the gravitational pull of the sun, the inclination increases slowly until the orbit of the satellite is in the same plane as Earths' orbit around the Sun.
the effect of the lunar/solar gravitation [...] perturbs the orbit pole with typically 0.85 degrees per year
Here is a stab at explaining the "average out" part of the question, hopefully tie-ing together some of the points brought up in the other answers and comments.
Satellites that are not undergoing control of their orbital inclination will gradually adopt both an inclination that increases up to ~15 degrees (and after a long time, back down to 0 deg) and also a progressive change of Right Ascension of the Ascending Node, RAAN. This is driven by a combination of lunar and solar perturbations.
It doesn't matter whether they are at geosynchronous radius or in the normal graveyard, they are all affected. Operational satellites have to expend a lot of propellant to remain geostationary. As an aside, whilst re-orbiting (graveyarding) practices have improved over the years there are hundreds of abandoned satellites in the operational zone.
It is easier to visualise the combined behaviour of Inclination and RAAN by imagining that the satellite's orbit is a spinning top with an axis at the middle which we'll call the orbit pole. In a controlled geoSTATIONARY orbit the orbit pole is closely aligned with the Earth's spin axis. In a drifting orbit this pole traces out a circle whose own centre is ~7.5 degrees from the polar axis over a period of ~56 years. The Inclination and RAAN can be thought of as the magnitude and direction of the orbit pole (ok, but for an inconvenient 90 degrees which I will gloss over, its just a metaphor). Thus the maximum change in inclination is from 0 to about 15 degrees.
The key point of the question was, as I understand it, "why are all these satellites 'in phase'"? It is because they all adopt the same spinning top motion, i.e. all the orbit poles are in the process of tracing out the same big circle offset from the Earth's pole in the same direction in inertial space.
This means that at any instant all those satellites at similar longitudes will be at the same point in their approx daily cycle of latitude, North or South of the equator.
For all those that, at some time of day, are at their maximum latitude:
- 90 degrees of longitude around the equator there are others at 0deg latitude
- and another 90degrees further away there are more that are there greatest negative latitude.
The idea of it being related to the Atlantic and Pacific however is just a co-incidence of the snapshot. The orbit pole motion is relative to inertial space under which the Earth turns regardless.
You'll see those satellites drift over the equator every 12 hours. They are almost geosynchronous instead of geostationary, i.e. they are in an orbit that's 24 hours and a few minutes long, and inclined instead of right on the equator.
Those decommissioned satellites are in a graveyard orbit, 235+ km above the geostationary orbit. After decommissioning, they can no longer maneuver to stay in their orbit so they'll slowly drift away from the equator and to the ecliptic plane under the influence of the Sun's and Moon's gravity.