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Weather forecasting and analysis, space and historic events, climate information

9:30 AM | A downward trend in global temperature anomalies and the role of the Pacific Ocean

Paul Dorian


Overview Solar and oceanic cycles are the most important drivers of all weather and climate on our planet and the most important ocean of all is the Pacific. This large body of water covers about a third of the planet’s surface and is bigger than all of the Earth’s land masses combined. Both the Pacific and Atlantic Oceans go through sea surface temperature phases that are characterized as cool (negative) and warm (positive). While there are other important factors that impact global temperatures (e.g., solar cycles, volcanic eruptions), the given temperature phase of the Pacific Ocean has historically been found to be a major factor in global temperature anomaly trends. Indeed, the Pacific Ocean seemingly flipped into a long-term cool phase during the previous decade and global temperature anomalies have generally been trending downward since that period of time.

Pacific Decadal Oscillation (PDO) The Pacific Decadal Oscillation (PDO) is a climate index based upon long-term patterns of variation in sea surface temperature of the north Pacific. The PDO is detected as warm or cool surface waters in the Pacific Ocean, north of about 20°N. During a warm (positive) phase, the west Pacific becomes cool and part of the eastern ocean warms; during a cool (negative) phase, the opposite occurs. These phases result from the direction of winter winds in the northern Pacific: winter winds blowing chiefly from the southwest result in warmer conditions in the northern California Current (CC); conversely, when winds blow primarily from the north, upwelling occurs both in the open ocean and at the coast, leading to cooler conditions in the northern CC.

The PDO temperature phase in the north Pacific tends to have an impact on the shorter-term sea surface temperature cycles of the tropical Pacific in the equatorial region. Specifically, warm phases of the PDO are generally associated with stronger and more numerous El Nino (warmer-than-normal) events in the tropical Pacific and weaker and fewer La Nina (colder-than-normal) episodes. During cold phases of the PDO, La Nina tends to dominate El Nino in the tropical Pacific. In the past several years, during the current cool phase of the PDO, La Nina conditions have indeed dominated the scene in the tropical Pacific region.

Warm and cool phases of the PDO can persist for decades, usually about 20 to 30 years. A warm phase occurred from 1925 to 1946, a cool phase from 1947 to 1976, and then another warm phase from 1977 to 1998 [PDO Index plot below]. Global temperature anomalies have tended to track these phases quite well with, for example, the very warm decade of the 1930’s having occurred during a warm phase, the colder-than-normal period of the 1950’s, 1960’s and first part of the 1970’s corresponding with a cool phase, and the warmer-than-normal decades of the 1980’s and 1990’s having occurred during a warm phase of the PDO. While the PDO trend has been somewhat jagged after the turn of the 21st century, it now appears that it slipped into a long-term cool (negative) phase during the middle part of last decade and that same sea surface temperature pattern continues today.

PDO PDO Index Plot courtesy NOAA: Time series of shifts in sign of the Pacific Decadal Oscillation (PDO), 1925 to present. Values are averaged over the months of May through September. Red bars indicate warm (positive) years; blue bars cool (negative) years. Note that 2008 and 2012 were the most negative PDO Index values recorded since 1956. For more info:

A downward trend in global temperature anomalies Global temperature anomalies have tended to trend downward, albeit in a jagged fashion, since around the time of the latest flip in the PDO from a warm-to-cool phase. The 2-meter global temperature anomaly plot (below) produced by WeatherBELL Analytics utilizes data from NOAA/NCEP’s Climate Forecast System (CFS). This dataset is an appropriate choice for historical global temperature comparisons and it is described in detail below.

temp_plot [2-meter global temperature anomaly plot since 2005; courtesy WeatherBELL Analytics at]

NOAA/NCEP Climate Forecast System “reanalysis” NOAA/NCEP’s Climate Forecast System is a model representing the global interaction between the oceans, land and atmosphere. This model offers hourly data with a horizontal resolution down to one-half degree (about 56km) on a global basis for a number of weather parameters. The CFS model uses the latest scientific approaches for taking-in, or assimilating, observations from many data sources: surface observations, upper air balloon observations, aircraft observations, and satellite observations. This approach has advantages over using “thermometer-based” surface data alone as that type of data is more limited in spatial resolution and often requires “adjustments” in order to reduce or eliminate the known problem of the “urban heat island” effect which impacts many official weather stations around the world. Indeed, the NOAA/NCEP “reanalysis” effort has produced a uniform, continuous, and best-estimate record of the state of the ocean-atmosphere for use in climate monitoring and diagnostics. The NOAA/NCEP method keeps the model’s software constant and runs the model retrospectively from 1979 (beginning of the satellite observation record-keeping era) to the present. [For more info on the NOAA/NCEP CFSR data:].