Earliest description of NAO-type teleconnections in winter climates noted by seafaring Scandinavians Danish missionary Hans Egede Saabye (around 1745) described in his diary cold winters in Denmark were accompanied by mild winters in Greenland Records of 1709-1800 sea ice conditions were documented for Greenland (whale and seal hunting industry) and provided qualitative information about Greenland winter climate & variability
1780 first temperature measurements in Greenland Dove (1841) and Hann (1890) found anticorrelations in temperature records between Europe and Greenland De Bort (1883) made use of pressure observations and introduced the concept of centers of action: Icelandic Low Azores High (and Siberian High) Hildebrandsson (1897) inverse relation between Azores High pressure and Icelandic Low Source : Stephenson et al. The History of Scientific Research on the North Atlantic Oscillation (In the book The North Atlantic Oscillation: Climatic Significance and Environmental Impact)
The climatological average circulation Winter season (DJF) Sea level pressure Aleutian Low Siberian High Icelandic Low Azores High The climatological average circulation
Winter season (DJF) 500 hPa geopotential height (contours) Deviations from the zonal average: light shading higher than zonal average dark shading lower than zonal average One-point correlation map Winter season (DJF) 500 hPa geopotential height time series from
the Icelandic Low center with all time series in the gridded data set (years 1958-2001) Positive Cor. x Negative correlation Station-based NAO index http://research.jisao.washington.edu/data_sets/nao/ Station-based NAO index: Jones NAO index
SW Iceland (Reykjavik), Gibraltar and Ponta Delgada (Azores) Note that the reference period for standardizing the pressure anomalies has not been updated, and the long-term average of this index is not zero. (usually we prefer index time series centered to zero) http://www.cru.uea.ac.uk/~timo/datapages/naoi.htm Difference between positive and negative NAO phase Winter season (DJF) Sea level pressure difference positive NAO minus negative NAO phase (based on NCEP/NCAR reanalysis over 1958-2001) The arrows show wind anomalies: Stronger westerlies over the North Atlantic
(caused by the enhanced pressure gradient) Trade winds stronger than normal Negative SLP anomalies Regional Climate anomalies Positive NAO index phase: Stronger than usual subtropical high pressure center and a deeper than normal Icelandic low.
The increased pressure difference associated with more and stronger winter storms crossing the Atlantic Ocean on a more northerly track. More warm air advection towards Europe Scandinavia (Russia) and warm and wet winters in Europe cold and dry winters in northern Canada and Greenland due to more polar air masses The eastern US experiences mild and wet winter conditions Mediterranean rainfall reduced
Negative NAO index phase: Weaker than usual subtropical high pressure center and weaker than normal Icelandic low The decreased pressure difference associated with fewer winter storms crossing the Atlantic Ocean on a more southerly track. More cold air advection in Europe Scandinavia (Russia) and colder and dryer winters in Europe milder winters in northern Canada and Greenland due to less polar air masses colder winter conditions in eastern US (less precip but more snow?) Mediterranean rainfall increased First task : Creating a schematic map
showing the impacts of the NAO Put the described relationships on a map: Mark specific locations with plus sign + or with negative sign Rule: use the plus sign for the
NAO is in a positive phase and the temperature is warmer than normal, pressure is higher than normal, wind is stronger than normal (plus the wind direction) negative signs where you observe the opposite behavior. Extrapolate your positive/ negative correlations and draw lines or shadings for regions where you can expect similar signs in the correlations. Temperature Precipitation Sea level pressure Wind vectors Ocean SST* (sea ice) *use knowledge from what we
learned about the PDO Positive NAO phase Greenland Iceland NE US Scandinavi a Azore s A Atlantic Ocean Positive NAO vs negative NAO phase NAO variability and regional
climate response We see positive phases and negative phases produce similar patterns of regional climate impacts in winter temperature and precipitation but with opposite sign That indicates that the NAO impacts show a linear response: y'(t) = a*x'(t) with x'(t) an NAO index time series. regional impact y'(t)
(Note: the ' sign indicates anomalies from the climate average) Atmosphere-Ocean Interaction Negative phase of the NAO (weaker Iceland Low) Observation-based pattern in the SST anomalies (colors) and wind anomalies associated with a negative phase of the NAO Tripole SST anomaly pattern dominates the interannual variability of the NAO Warm SST anomalies caused by reduced wind speed
Coastal regions affected by cold air advection In addition wind-driven Ekman transport anomalies cause SSTA http://www.cru.uea.ac.uk/~timo/datapages/naoi.htm Atmospheric forcing of the ocean A number of model and observational studies have shown that the North Atlantic ocean responds to the sea level pressure anomalies In the 1970s the stochastic climate theory was formulated:
Temperature change = + ( ) Atmosphere random process (white noise) Memory effect (large heat capacity of ocean mixed layer exponential decay) Oceanic forcing of the NAO The ocean feedback:
SST anomalies change the temperature gradient between atmosphere and ocean Change in heat flux: Heat flux anomaly can affect atmosphere (weak response) On decadal and multidecadal time scales the ocean feedback becomes important The weak feedback enhances the atmospheric memory and lowfrequency variability (decadal and multidecadal variability) The extended stochastic climate model proposed by Barsugli and Battisti (1998) Note: The first equation is for the atmospheric temperature (Ta), the second is for the ocean temperature. (weak) feedback from ocean to atmosphere (strong) forcing from atmosphere to ocean
Seasonal prediction Prediction of upcoming winter season NAO index phases: Using Atlantic SST pattern information: statistical models Coupled ocean-atmosphere general circulation models (GCMs) Using the strength of the polar vortex in the stratosphere: a strong polar vortex in the stratosphere induces a strengthening of the westerly winds and a positive NAO phase
Seasonal predictions with climate models A global climate model (with Ocean and Atmosphere) is initialized with observationbased best estimates for the atmospheric and oceanic state (and land conditions) Start date of the simulations is around Nov. 1st of each year. Then the model forecasts the next season (DJF) Using small perturbations in the beginning, 24 different simulations are run (This method is called ensemble simulations; used also in weather or Hurricane forecasts) Met Office Global Seasonal forecast System 5 (GloSea5) using Scaife et al, Seasonal predictions with climate models Brand new research making news: UK scientists find a way to simulate one year in advance the phase of the NAO! (to the newspaper article)
(Nature Geoscience article, Oct 2016) Seasonal prediction using stratospheric polar vortex strength Seasonal prediction using stratospheric polar vortex strength Northern Annular Mode (NAM) is mode that measures the polar vortex strength through the stratosphere and troposphere. Downward signal propagation It is very strongly linked both physically as statistically with the North Atlantic Oscillation
(There is also a dynamical counterpart in the Southern Hemisphere, the Southern Annular Mode (SAM).) Response of terrestrial ecosystems to NAO-related climate anomalies In many parts of Europe warm winters are followed by early spring events in the ecosystems: Arrival of migratory birds Blossom of flowers Leaf unfolding on trees Long-term phenological* observations at specific sites can be compared with NAO index * See also https://www.nwf.org/Wildlife/Wildlife-Conservation/Phenology.aspx Effects of NAO variability on European birds Example White-throated dipper (Cinclus cinclus) Southern Norway 1978-1997
Habitat regions in Europe Southern Norway White-throated dipper feeds on insects under water In winter it leaves higher lands when rivers freeze in search for open waters Images source: Wikipedia (retrieved Oct 18th 2015) Effects of NAO variability on European birds Example White-throated dipper (Cinclus cinclus) Southern Norway 1978-1997 After Saether et al., 2000 NAO positive phase: More winter storms in Atlantic Milder temperatures More open waters and better food supply during winter
Population growth is higher following positive NAO winters Negative NAO phase: Population decline after severe winter Summary: North Atlantic Oscillation North Atlantic Oscillation: NAO acts like a pressure seesaw Defined by the sea level pressure systems over the North Atlantic with centers of action near Icelandic Low and the Azores High + NAO most active (strongest) during winter season Sea level pressure anomalies show a dipole mode (anti-correlation between Icelandic Low and Azores High) Large-scale effects on winter temperatures precipitation in Europe, Greenland, eastern
North America Atmosphere-Ocean interaction: Atmosphere acts as a random (white noise) forcing on the ocean, => ocean SST time series more low frequency variability ocean SST anomalies have a weak feedback on the atmosphere Seesaw concept: Icelandic low pressure Azores high pressure +
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