From ancient times, farmers have looked to the skies for rain in due season. They have also noted patterns in nature that portend changes in the weather. The way animal's coasts thicken or then, the peculiarities of flowering trees, wooly-worm characteristics, etc. A connection would be made between events seen in nature and the weather they experienced. They would become accustomed to them and expect repeated results in the future.
These were the first empirical agrarian weather connections. They may have been seen as verified statistical correlations or perhaps just coincidence. But from these primitive attempts at connecting several seemingly unrelated observances with expected natural results has come the modern sub-discipline of atmospheric teleconnections. Since 1950 detailed weather parameter archives have been accumulating. Regarding the El Nino Southern Oscillation, these observed quantities have led to one of the most illuminating expansions of long range weather understanding. A climatological foundation of sea surface temperatures, as well as sub surface oceanic temperatures, have been established with which to compare current conditions and then assign expected weather anomalies throughout the world. Equatorial Pacific sea conditions, blended together with other atmospheric metrics like low levels winds, upper level winds and outgoing long wave radiation, has a measurable effect on the global weather economy. But over the past 25 years, intrepid researchers have distilled a special collection of methods that look at only a few averaged weather parameters to make stunning connections with general weather regimes. These are the teleconnections in use today by medium and long range forecasters. Knowing what the temperature or pressure variations are in one specific region of the world it is now becoming possible to predict with some skill the expected anomalies of temperature and precipitation in distant regions.
It is like the telecommunications revolution. Anyone can be contracted by anyone else to and from nearly any location in the world. The technology has been in place for more than a decade now. In the same manner, nearly every part of the world is thermodynamically weaved together. Large ridges and troughs that drape themselves around the northern hemisphere all impinge a dynamic force on each other, in accordance with the famous Rossby Long Wave equation. A blocking ridge may develop at high latitudes when warm air is forced north. These affect not only downstream patterns but upstream patterns too, like the Great Red Spot on Jupiter. Transient weather systems obediently go around a block these ridges usually develop a symbiotic relationship with one or two lows and form a "Rex" or an "Omega" block. But the point is that there are many indirect relationships temporarily formed. And here is where teleconnections come in. How pressure patterns relate to climatological norms usually dictate general truths about the atmosphere that can be trusted. Long term droughts and long term stormy periods are connected with these anomalies as well.
So what do we have? The two most prominent teleconnections widely referenced today are the North Atlantic Oscillation (NAO) and the Pacific-North American Pattern (PNA). Both take smoothed averages of 500 Mb heights at specific spots over the ocean and land. And they both have proven track records of reasonable reliability. Climatologists use them routinely. In general, when the NAO is positive, the east coast is warm. When negative, the east is cool and stormy. At the other coast, in general, when the PNA is positive, the west is warm and dry. When it goes negative the west is likely to see wetter weather. There ar4e a variety of seasonal adjustments and forecast reliability. In addition to these two non-El Nino teleconnections, a number of other popular gauges have been introduced over the years including the Arctic Oscillation (AO) and Antarctic Oscillation (AAO). Just as more points in a triangulation increase precision, so too when all of these teleconnections agree on a consensus, it lends greater credibility to a medium, long range or seasonal forecast. New techniques that involve past oceanic indices and well known recurring cycles have made it possible for some meteorologists and climatologists to predict general weather trends in many regions even beyond one year into the future. Tropical seasons are becoming more predictable as well with increased understanding of how diverse parts of the atmosphere and oceans have a measurable effect on other parts. The daily status of these indices may be monitored at the CPC website http://www.cpc.ncep.noaa.gov/products/precip/CWlink/pna/pna.shtml
None of us are an island. We each have an effect on our loved ones, associates, and even total strangers we come in contact with. As it turns out, so do all the major and minor weather features around the globe. Many aren't aware of these powerful weather diagnostics and forecast tools. If that has been you, now you too have an additional forecast tool. Enjoy!