The reason, says Wallace, is that it is a puzzle made up of many diverse parts. All involve patterns of atmospheric pressure and winds that reverse direction every few decades.
Reliable data are the key to medium and long-term predictions.
Graphic courtesy Candace Gudmunson, UW Dept. of Atmospheric Sciences. Between 1925 and 1945, for example, the winters in the Pacific Northwest were mild and the winds over the Gulf of Alaska were strong, conditions that UW Fisheries Professor Robert Francis believes were responsible for the abundant catches of Alaska salmon during those years. The following 30 years had exactly the opposite conditions. But starting around the late 1970s, winters turned milder again, winds over the Gulf strengthened and the Alaska salmon harvests improved dramatically, even surpassing those back in the '20s through the '40s. The difficulty for scientists is recognizing such long shifts while they are happening and not five to 10 years after they have occurred.