\name{NAO}
\alias{NAO}
\title{
Compute the North Atlantic Oscillation (NAO) Index
}
\description{
Compute the North Atlantic Oscillation (NAO) index based on
the leading EOF of sea level pressure (SLP) anomalies. The
PCs are obtained by projecting the forecast and observed
anomalies onto the observed EOF pattern (Pobs) or the
forecast anomalies onto the EOF pattern of the other years
of the forecast (Pmod). By default (ftime_average = 2:4) NAO()
computes the NAO index for 1-month lead seasonal forecasts
that can be plotted with BoxPlot(). Returns cross-validated PCs 
of the NAO index for forecast (ano_exp) and observations (ano_obs) 
based on the leading EOF pattern.
}
\usage{
NAO(ano_exp, ano_obs, lon, lat, ftime_average = 2:4, obsproj = TRUE)
}
\arguments{
  \item{ano_exp}{
Array of North Atlantic SLP (0-60N, 80W-0) forecast anomalies from 
\code{Ano()} or \code{Ano_CrossValid()} with dimensions (n. of experimental
data sets, n. of ensemble members, n. of start dates, n. of forecast time 
steps, n. of latitudes, n. of longitudes).
  }
  \item{ano_obs}{
Array of North Atlantic SLP (0-60N, 80W-0) observed anomalies from 
\code{Ano()} or \code{Ano_CrossValid()} with dimensions (n. of observational
data sets, n. of obs. ensemble members, n. of start dates, n. of forecast time 
steps, n. of latitudes, n. of longitudes).
  }
  \item{lon}{
Vector with the longitudes of \code{ano_exp} and \code{ano_obs}.
  }
  \item{lat}{
Vector with the latitudes of \code{ano_exp} and \code{ano_obs}.
  }
  \item{ftime_average}{
A vector with the forecast time steps to average across defining the target
period. Takes by default 2:4, i.e. from 2nd to 4th forecast time steps.
  }
  \item{obsproj}{
Whether to project the forecast anomalies on observed EOF pattern (Pobs), 
which is the default (TRUE), or to project on the EOF pattern of other 
years of the forecast (FALSE).
  }
}
\value{
  \item{NAO_exp}{
Array of forecast NAO index in verification format (ensemble members, 
start dates).
  }
  \item{NAO_obs}{
Array of observed NAO index in verification format (1, number of start
dates).
  }
  \item{EOFs_obs}{
EOFs of the observational references.
  }
}
\examples{
# See examples on Load() to understand the first lines in this example
  \dontrun{
data_path <- system.file('sample_data', package = 's2dverification')
expA <- list(name = 'experiment', path = file.path(data_path,
             'model/$EXP_NAME$/$STORE_FREQ$_mean/$VAR_NAME$_3hourly',
             '$VAR_NAME$_$START_DATE$.nc'))
obsX <- list(name = 'observation', path = file.path(data_path,
             '$OBS_NAME$/$STORE_FREQ$_mean/$VAR_NAME$',
             '$VAR_NAME$_$YEAR$$MONTH$.nc'))

# Now we are ready to use Load().
startDates <- c('19851101', '19901101', '19951101', '20001101', '20051101')
sampleData <- Load('tos', list(expA), list(obsX), startDates,
                   leadtimemin = 1, leadtimemax = 4, output = 'lonlat',
                   latmin = 27, latmax = 48, lonmin = -12, lonmax = 40)
  }
  \dontshow{
startDates <- c('19851101', '19901101', '19951101', '20001101', '20051101')
sampleData <- s2dverification:::.LoadSampleData('tos', c('experiment'),
                                                c('observation'), startDates,
                                                leadtimemin = 1,
                                                leadtimemax = 4,
                                                output = 'lonlat',
                                                latmin = 27, latmax = 48,
                                                lonmin = -12, lonmax = 40)
  }
# Now ready to compute the EOFs and project on, for example, the first 
# variability mode.
ano <- Ano_CrossValid(sampleData$mod, sampleData$obs)
nao <- NAO(ano$ano_exp, ano$ano_obs, sampleData$lon, sampleData$lat)
# Finally plot the nao index
PlotBox(nao$NAO_exp, nao$NAO_obs, "NAO index, DJF", "NAO index (PC1) TOS",
        monini = 12, yearini = 1985, freq = 1, "Exp. A", "Obs. X")
}
\author{
History:\cr
0.1  -  2013-08  (F. Lienert, \email{flienert at ic3.cat})  -  Original code\cr
0.2  -  2014-03  (V. Guemas, \email{virginie.guemas at bsc.es})  -  Removing the 
rotation\cr
0.3  -  2014-05  (L. Batte, \email{lauriane.batte at ic3.cat})  -  Changes to
simplify function and add Pobs and Pmod options for NAO projection 
calculations\cr
0.4  -  2015-03  (L. Batte, \email{lauriane.batte at ic3.cat})  -  Polarity 
check and correction is wrong. Switched to have a negative NAO index when the
anomaly pattern corresponds to NAO-.
1.0  -  2016-03  (N. Manubens, \email{nicolau.manubens at bsc.es})  -  Formatted to CRAN
}
\keyword{datagen}
