The Effect Of Global Warming On North Pacific Decadal Variability And The Possible Mechanism

Pacific Decadal Oscillation (PDO) is the dominate mode of North Pacific decadal variability, with the periods of 15 years-25 years and 50 years-70 years. In PDO positive phase, the Aleutian Low strengthens, and the north pacific has the horseshoe-like SST anomalies, with negative SST anomaly in the central and western north Pacific and positive SST anomaly in the central and eastern equatorial Pacific, along the North America coast and in the Alaska Gulf. Since the second Industrial Revolution in 1870, human consumption of lots of fossil fuels and deforestation leads to elevating greenhouse gas concentrations, enhanced greenhouse effect and the global warming. Global warming not only modifies the climatology, but also affects the natural climate variability PDO. PDO influences the Asia and North America climate, the marine ecological environment and fishery, modulates the ENSO activities and its climate impact, so the effect of global warming on north pacific decadal variability (PDO) and the mechanism are significant to short and long term climate prediction and forecast.With the annual forcing of history solar constant and greenhouse gases, FOAM (Fast Ocean Atmosphere Model) is first used to simulate the 20th century climate, and captures the main characteristics of 20th century global warming, in consistent with the results of IPCC AR4 models experiments and observation. These main characteristics are as followings. (1) Global mean surface air temperature rises by 0.6℃～0.7℃, and global mean SST by 0.4℃～0.5℃; For temperature rise over different earth coverage, ice is larger than land, land larger than ocean, and the northern hemisphere larger than the southern hemisphere; The troposphere warms, while the stratosphere cools. (2) The mid-latitude westerly enhances, with the poleward migration of westerly belts and trade wind belts;The mid-latitude ocean warms less than the tropical ocean, and this might be cause or result of the stronger mid-latitude westerly. (3) Ocean absorbs huge heat, mitigating global warming; High latitudes become fresher, while low latitudes saltier, implying stronger hydrological cycle; The overall Pacific Ocean becomes fresher, while Indian Ocean and low and mid-latitude Atlantic Ocean become saltier; The Atlantic Meridional Overturning Circulation decreases.Global warming weakens North Pacific decadal variability (PDO). (1) Global warming comparison experiments of FOAM and IPCC AR4 models show that:in global warming, the decadal (>8 years) standard deviations of North Pacific SST and 500hPa geopotential height decrease respectively by 20%～40% and 5%～10%, most obviously in the Kuroshio Extension and the western Subpolar Ocean, indicating weakening of the North Pacific decadal variability. (2) PDO character Comparison in FOAM control experiment and the double CO2 experiment shows that:in global warming, the PDO mode weakens, shifting to higher frequency, and SST decadal amplitude decreases most obviously also in the Kuroshio Extension and the western Subpolar Ocean.Cause for PDO shifting to higher frequency points to ocean Rossby wave speed change in global warming. (1) Two ocean Partial Blocking experiments are done to detect the role of ocean Rossby wave in PDO; The result shows that the overall North Pacific ocean Rossby wave is vital to PDO, and the Subarctic Ocean Rossby wave is important to the periods of PDO (especially longer than 30 years). (2) In global warming, the upper ocean warms more than the deep ocean, leading to higher buoyancy frequency and faster Rossby wave. Additionally, larger zonal current vertical shear also speeds the first baroclinic Rossby wave in the North Pacific mid-latitude; North Pacific Rossby wave speeds by 10%-70% in global warming, most obviously in the Subarctic Ocean. (3) The faster Rossby wave shortens the north pacific transit time, speeds ocean adjustment to wind stress anomaly or buoyancy anomaly forcings, so PDO shifts to higher frequency (shorter period). It is also the first time to compare the atmosphere responses to mid-latitude SST anomaly in present climate and in global warming by doing initial value experiments. (1) In response to positive SST anomaly forcing in the Kuroshio Extension in present climate, the eddy feedback dominates and the atmosphere response is high with equivalent barotropic structure, leading to weaker westerly and slow decay rate of the initial positive SST anomaly. (2) In response to the same positive SST anomaly forcing in global warming, the upper troposphere baroclinity decreases in the mid-latitudes; the eddy activities and eddy feedback weakens, so the atmosphere response is low; As a result, the westerly enhances and the SST positive anomaly decays fast. In global warming, the faster decay rate of positive SST anomaly may contribute to the SST decadal standard deviation decrease in the Kuroshio Extension.