You've heard by now that this is expected to be another active hurricane season.

Coming off two seasons that have included nine direct hurricane strikes and at least two near-misses on the U.S. coastline, the stakes are very high this season on many levels: politically, economically and psychologically.

In thinking about why we've seen such a rash of severe hurricanes hitting the United States in recent years, one phenomenon to consider is the Atlantic Multidecadal Oscillation.

The Atlantic Multidecadal Oscillation is a long-term pattern, spread over decades and even centuries, whereby the sea surface temperatures over the Atlantic show slight changes upward and downward, generally less than 1 degree Fahrenheit. These changes are believed to be linked to changes in ocean salinity, though the mechanism is not fully understood.

Since the mid-1990s, the AMO has been in a "warm phase." During that time, we have seen an increase in the number of major Atlantic hurricanes over the previous three decades.

Where it gets tricky on a scientific basis and even more so on a political basis is when the debate about the recent severe Atlantic hurricane seasons focus on this question: How much of the increased intensity is due to the natural AMO cycle, and how much is due to human-induced global warming?

I will not even attempt to describe all the facets of that debate. The arguments run the gamut from "there is no global warming," to "it's all global warming."

The Atlantic Oceanic and Meteorological Laboratory bridges that debate by noting that the AMO "alternately obscures and exaggerates the global increase in temperatures due to human-induced global warming."

The AMO is linked to more than just hurricanes. According to the Atlantic Oceanic and Meteorological Laboratory, the warm phase of the AMO is also linked to an increase in middle-United States drought. The major droughts of the 1930s and 1950s occurred during warm AMO phases, the laboratory points out. We've already talked quite a bit this year about the ongoing drought problems in the central U.S. currently, so it seems to fit the bill.

The ongoing warm phase of the AMO is only one factor in the forecasts of an active hurricane season. Sea-surface temperatures in the equatorial Pacific are also important.

That region oscillates on a shorter time frame than the Atlantic, the cool phase being known as "La Nina" and the warm phase as "El Nino." The various phases of what is known as the El Nino Southern Oscillation usually last a year or less.

When the equatorial Pacific waters warm to what is known as an El Nino, it often has a dramatic effect on the Atlantic hurricane season. Warm water in that region can blow up more storms in the Pacific, and in turn, increase westerly winds aloft. These westerly winds, blowing over the Atlantic, create strong shear that can literally rip the tops off of developing tropical systems. Therefore, El Nino patterns dramatically reduce the number and intensity of Atlantic storms.

This occurs even in a warm AMO phase. With a developing El Nino in the 1997 season, there were only eight named storms and three hurricanes in the Atlantic.

Currently, the equatorial Pacific is basically neutral in temperature, and has been weakly in a cool phase at times during the past three years. As a result, it is not expected to have a dampening effect on Atlantic hurricanes.

Of course, we really won't know what kind of Atlantic hurricane season this will be until it's well under way. An active season doesn't guarantee that there will be major U.S. strikes like the last two years. 2000 was similar to what is forecast this season, with 15 named storms and eight hurricanes, but not a single hurricane made landfall in the U.S. that season.

On the flip side, even an inactive season can cause lots of mayhem if one storm hits the right place: Andrew was one of only six named storms in 1992, but it was a Category 5 hit on a part of the Miami metropolitan area.

More on the AMO: www.aoml.noaa.gov/phod/amo_faq.php