Large-scale annual and decade fluctuations in climate and weather are caused by changes in patterns of ocean circulation and atmospheric pressures.
In the NWT, indices for two of these phenomena, the Arctic Oscillation (AO), and the Pacific Decadal Oscillation (PDO) - El Niño, are particularly important to track to understand large natural fluctuations and changes occurring in NWT weather from year to year and decade to decade. These large fluctuations in weather are natural and began long before human-caused climate change. Natural fluctuations in weather have direct impacts on drivers of ecosystem change such as drought, forest fire, flooding, permafrost melt, forest pest outbreaks, and timing of vegetation greening. Natural fluctuations need to be taken into account if we want to track the effects of human-caused climate change on the NWT ecosystems.
A pattern in variation in atmospheric pressure of the North Pacific, called the Pacific/North American teleconnection pattern (PNA), is also an important source of variability in weather north of the tropics. This pattern is strongly influenced by El Niño events. The PNA is known to be correlated with some weather patterns in the NWT and may be tracked using an additional indicator in the future.