The Pacific Decadal Oscillation
The Pacific Decadal Oscillation index (PDO) is a recurring atmosphere-ocean climate pattern centered over the mid-latitude Pacific Ocean basin. It consists of a warm and cool phase, which alters upper level atmospheric winds. Shifts in the PDO phase can have significant implications for global climate, affecting Pacific and Atlantic hurricane activity, droughts and flooding around the Pacific basin, the productivity of marine ecosystems, and global land temperature patterns. This climate pattern can persist for just a few years to multiple decades.
During a “warm”, or “positive”, phase, the west Pacific becomes cooler and part of the eastern ocean warms; during a “cool” or “negative” phase, the opposite pattern occurs.
The PDO weather patterns modify surface temperature, humidity, wind, and the distribution of clouds over the North Pacific that alter surface heat, momentum and freshwater fluxes. This affects sea surface temperature, salinity and the mixed layer depth of the ocean, which in turn may affect species distributions and productivity.
Historic Influence of the PDO on World Weather
Studies have shown that during the positive “warm” phase of the PDO, the wintertime Aleutian low is deepened and shifts southward. During this time, warm/humid air is advected along the North American west coast. Temperatures are higher than usual from the Pacific Northwest to Alaska. However, they are below normal in Mexico and the Southeastern United States.
Winter precipitation is higher than usual in the Alaska Coast Range, Mexico and the Southwestern United States. Temperatures are reduced over Canada, Eastern Siberia and Australia.
Other scientists have shown that the PDO along with the Atlantic Multidecadal Oscillation strongly influence multidecadal droughts pattern in the United States. If the PDO is associated with a positive AMO, drought frequency is enhanced over much of the Northern United States during the positive PDO phase and over the Southwest United States during the negative PDO phase.
The Asian Monsoon is also affected. Increased rainfall and decreased summer temperature is observed over the Indian subcontinent during the negative phase of the PDO.
The North Pacific Gyre Oscillation (NPGO)
The North Pacific Gyre Oscillation (NPGO) is a climate pattern. It is used as an index to measure changes in ocean currents and circulation patterns and explains key physical-biological variables. It is significantly correlated with previously unexplained fluctuations of salinity, nutrients and chlorophyll-a measured in long-term observations in the California Current (CalCOFI) and Gulf of Alaska.
Fluctuations in the NPGO are driven by regional and basin-scale variations in wind-driven upwelling and horizontal advection. These are the fundamental processes controlling salinity and nutrient levels. Nutrient fluctuations drive changes in phytoplankton concentrations, and may force similar variability in higher trophic levels.
The NPGO thus provides a strong indicator of fluctuations in the mechanisms driving planktonic ecosystem dynamics.
The Aleutian Low
The Aleutain Low is a semi-permanent, subpolar area of low pressure located in the Gulf of Alaska near the Aleutian Islands. Storms often reach maximum intensity in this area. It is most active during the late fall to late spring.
During the summer, it is weaker, retreating towards the North Pole and becoming almost nonexistent. The North Pacific High pressure system dominates the area at this time.