El Niño, La Niña and ENSO La NiñaEl Niño Time mean

Correlation coefficient of annual-mean sea-level pressure with pressure at Darwin. Darwin Tahiti

ENSO time series and spectrum year period (years)

El Niño impacts: Global Surface temperature (ºC)Precipitation (mm/day) NCEP/NCAR Reanalysis

ENSO mechanism: the Bjerknes feedback

Bjerknes feedback - equatorial sections El Niño ocean longitude La Niña atmos longitude

Southern oscillation index (SOI) = p(Tahiti)-p(Darwin) Nino 3.4 index (N34) = SST averaged over 120°W-170°W and 5°S- 5°N. Year N34 ( o C) SOI (hPa) El Niño and Southern oscilation

Southern Oscillation index (SOI) vs. Nino 3.4 SST (N34) SOI (hPa) N34 (degC) Regression line N34 = 28.5 – 0.4 SOI Pearson’s corr. coeff c = – 0.83 Variance explained c 2 = 0.70

EOFs in 2 dimensions e1 e2 SOI (hPa) N34 (degC)

Teleconnection map correlation of 500 hPa height at base point (20N,160W) with all other points base point

Teleconnectivity map, 500 hPa height PNA NAO The map is constructed as follows: -for each point in the grid, build a teleconnection map using that point as base point; -assign to that point the maximum (absolute value of) anticorrelation found in the teleconnection map -draw contours of the resulting field, add arrows showing points connected by max anticorrelation

Pacific/North American pattern (PNA) 500mb height, 1-point correlation map, base point 20N,160W cool, wet warm base point warm

North Atlantic Oscillation (NAO) The “NAO index” is defined as the difference in surface pressure measured at Stykkisholmur (Iceland) and Lisbon (Portugal) or Ponta Delgada (Azores): NAO index = p Azores – p Iceland. High positive value of the index means pressure is very low over Iceland and very high over the Azores. The map shows the regression of the the NAO index onto the northern hemisphere surface pressure field.

NAO impacts TemperaturePrecipitation dry wet

A cartoon of the NAO High Low

NAO time series anthropogenic climate change?

NAO index time series Index according to Jones et al. 1997: p Gibraltar -p Stykkisholmur or natural variability?

AO or NAO? Leading EOF of northern hemisphere surface pressure Regression of NAO index on surface pressure The leading EOF of surface pressure shows a pattern which is similar to the NAO in the Atlantic, but is more zonal -- there is a second “center of action” in the Pacific. This pattern has been labeled the “Arctic Oscillation”, to emphasize that it represents a zonally-symmetric oscillation centered on the North Pole. In this view, the NAO is just a locally-enhanced manifestation of the global, zonally-symmetric “annular mode”. However, there is no significant correlation between points in the Atlantic and in the Pacific, I.e no zonal teleconnection between Atlantic and Pacific basins. no correlation!

Physics of the AO or “annular mode” Climatological zonal-mean windRegression of leading EOF’s amplitude onto zonal-mean zonal wind The idea behind the “annular modes” is that: 1. The AO is just the surface signature of a mode that actually fills the whole troposphere 2. Random fluctuations in baroclinic eddy activity in the midlatidude storm tracks lead to random changes in momentum convergence, shifting the jet axis north or south

Role of zonal asymmetries in creating the NAO (from recent review paper by Vallis and Gerber, 2007) Climatological DJF Eady growth rate, a measure of how baroclinically unstable the atmosphere is. High growth rate favours frequent development of baroclinic storms Climatological DJF eddy kinetic energy, a measure of how much eddy activity there actually is in the atmosphere The atmosphere is most unstable on the eastern seaboards of continents, but the biggest eddy activity is somewhat downstream in the middle of the ocean basins. This is because eddies propagate as they develop, reaching maturity further downstream along the storm track. Eddy momentum fluxes are strongest in roughly the same region. Climatological DJF eddy meridional momentum flux. Positive values mean momentum is transported northward. Note the strong momentum convergence over the center of the Atlantic basin. 1. Observational picture

2. Results from numerical model with zonally symmetric statistics Role of zonal asymmetries in creating the NAO (from recent review paper by Vallis and Gerber, 2007) This figure shows the “teleconnection map” for surface pressure in a numerical model where forcing and boundaries are zonally symmetric. The base point is chosen randomly. According to the “annular modes” viewpoint, we would expect a rather ring- like patter to emerge, but instead we see something that is quite localized and looks surprisingly like the NAO. The leading EOF of surface pressure in this model is zonally-symmetric, however. The conclusion is that eddies do transport momentum and cause “wobbles” in the jet position, but the eddy dynamics are quite local and so the jet wobbles are local rather than global. In this model, such wobbles occur randomly at all longitudes, and the EOF represents them as a zonally- uniform “annular mode”.

Role of zonal asymmetries in creating the NAO (from recent review paper by Vallis and Gerber, 2007) 3. Results from numerical model with zonally asymmetric statistics This shows results from the same model as before, except that: -a thermal anomaly representing land-ocean heating contrast has been inserted (magenta lines) -a meridionally elongated mountain range representing the Rockies has been inserted (cyan lines). These fixed surface asymmetries set up permanent asymmetries in the atmospheric fields, leading to a zonally-confined storm track. The meridional wobbles due to eddy momentum convergence fluctuations still occur at all longitudes, but are stronger in the region of maximum eddy kinetic energy. The leading EOF shows a corresponding region of enhanced variability. In conclusion, the NAO is due to the same dynamics hypothesized for the annular modes, but these dynamics occur in a region that is zonally confined because of the permanent asymmetries in the atmospheric forcing.