Of the dozens of oceanic oscillations, the Pacific Decadal Oscillation (PDO) is the most significant as it has the greatest impact upon our climates. The El Nino Southern Oscillation (ENSO) also has a significant influence but this switches phases within a year or two, and as such the positives and negatives soon cancel each other out.
On decadal time scales the PDO has a greater influence than all other oceanic oscillations combined.
To answer your question we need to look for a correlation between the PDO Index, the average global temperature and the total heat content of the oceans.
This first graph shows the PDO since 1900:
http://www.flickr.com/photos/trevorandclaire/12380654425/
And this second one shows the average global temperature over the same time period:
http://www.flickr.com/photos/trevorandclaire/12380810803/
What you’ll notice is that most pronounced periods of warming do coincide with the positive phases of the PDO, namely those of the early and late 20th century.
When the PDO entered its negative phase in the middle of the 20th century the average global temperature remained constant. The PDO is again in a negative phase as it has been since 1998, once more temperatures have remained constant.
If we combined the two graphs into one this is what we get:
http://www.flickr.com/photos/trevorandclaire/12381097814/
The PDO fluctuates between positive and negative phases and these cancel each other out, the average PDO Index for the period 1900 to 2013 is as good as zero (it’s actually 0.004009).
If the PDO was the cause of warming / cooling then there shouldn’t have been any overall change in the temperature, but as the second graph shows, the temperature has followed a long-term upward trend. Clearly there’s more to it than just the PDO, and this is where the greenhouse gases come in.
The warming signal is evident in that a negative PDO fails to cause cooling, all it really does is to mask the underlying warming. Once the PDO switches to positive then the warming trend bounces back.
With the PDO in a negative phase there is a greater upwelling of cold water from the oceans, this creates a greater temperature gradient between the atmosphere and the surface of the ocean and this allows more heat to transfer into the ocean.
The PDO switched to its current negative phase in 1998 and since that time the oceans have absorbed an additional 160,000 billion billion Joules of energy. On the face of it once could easily assume that the PDO was the sole cause of this additional uptake, but it’s not that simple.
In fact, the ocean heat content has been increasing since about 1970 and the reason for this is simply because the atmosphere has warmed up. The role that the PDO plays is to speed up or slow down the flow of heat into the oceans.
We’re presently close to the end of the negative phase of the PDO cycle. If we go back to c1955 – the last time the PDO was in this position – then we can look at ocean heat content across a full cycle. What we find is that there has been a more or less continual increase such that there’s now an extra 310,000 billion billion Joules of energy in the oceans.
TO SUMMARISE: Global warming is an ongoing trend, when the PDO is in its positive phase it amplifies the warming signal, in the negative phase it counters the warming signal and there’s a temporary pause in the warming. All the while, because the atmosphere has warmed, more heat energy enters the oceans; the PDO isn’t the cause of this but it does affect the rate at which energy is absorbed.
NOTE: The above is an over-simplification, there are some other key factors that I haven’t mentioned.