NAO | R Documentation |

Compute the North Atlantic Oscillation (NAO) index based on the leading EOF of the sea level pressure (SLP) anomalies over the north Atlantic region (20N-80N, 80W-40E). The PCs are obtained by projecting the forecast and observed anomalies onto the observed EOF pattern or the forecast anomalies onto the EOF pattern of the other years of the forecast. By default (ftime_avg = 2:4), NAO() computes the NAO index for 1-month lead seasonal forecasts that can be plotted with PlotBoxWhisker(). It returns cross-validated PCs of the NAO index for forecast (exp) and observations (obs) based on the leading EOF pattern.

```
NAO(
exp = NULL,
obs = NULL,
lat,
lon,
time_dim = "sdate",
memb_dim = "member",
space_dim = c("lat", "lon"),
ftime_dim = "ftime",
ftime_avg = 2:4,
obsproj = TRUE,
ncores = NULL
)
```

`exp` |
A named numeric array of North Atlantic SLP (20N-80N, 80W-40E)
forecast anomalies from |

`obs` |
A named numeric array of North Atlantic SLP (20N-80N, 80W-40E)
observed anomalies from |

`lat` |
A vector of the latitudes of 'exp' and 'obs'. |

`lon` |
A vector of the longitudes of 'exp' and 'obs'. |

`time_dim` |
A character string indicating the name of the time dimension of 'exp' and 'obs'. The default value is 'sdate'. |

`memb_dim` |
A character string indicating the name of the member dimension of 'exp' (and 'obs', optional). If 'obs' has memb_dim, the length must be 1. The default value is 'member'. |

`space_dim` |
A vector of two character strings. The first is the dimension name of latitude of 'ano' and the second is the dimension name of longitude of 'ano'. The default value is c('lat', 'lon'). |

`ftime_dim` |
A character string indicating the name of the forecast time dimension of 'exp' and 'obs'. The default value is 'ftime'. |

`ftime_avg` |
A numeric vector of the forecast time steps to average across the target period. If average is not needed, set NULL. The default value is 2:4, i.e., from 2nd to 4th forecast time steps. |

`obsproj` |
A logical value indicating whether to compute the NAO index by projecting the forecast anomalies onto the leading EOF of observational reference (TRUE) or compute the NAO by first computing the leading EOF of the forecast anomalies (in cross-validation mode, i.e. leaving the year you are evaluating out), and then projecting forecast anomalies onto this EOF (FALSE). The default value is TRUE. |

`ncores` |
An integer indicating the number of cores to use for parallel computation. The default value is NULL. |

A list which contains:

`exp` |
A numeric array of forecast NAO index in verification format with the same dimensions as 'exp' except space_dim and ftime_dim. If ftime_avg is NULL, ftime_dim remains. |

`obs` |
A numeric array of observed NAO index in verification format with the same dimensions as 'obs' except space_dim and ftime_dim. If ftime_avg is NULL, ftime_dim remains. |

Doblas-Reyes, F.J., Pavan, V. and Stephenson, D. (2003). The skill of multi-model seasonal forecasts of the wintertime North Atlantic Oscillation. Climate Dynamics, 21, 501-514. DOI: 10.1007/s00382-003-0350-4

```
# Make up synthetic data
set.seed(1)
exp <- array(rnorm(1620), dim = c(member = 2, sdate = 3, ftime = 5, lat = 6, lon = 9))
set.seed(2)
obs <- array(rnorm(1620), dim = c(member = 1, sdate = 3, ftime = 5, lat = 6, lon = 9))
lat <- seq(20, 80, length.out = 6)
lon <- seq(-80, 40, length.out = 9)
nao <- NAO(exp = exp, obs = obs, lat = lat, lon = lon)
# plot the NAO index
## Not run:
nao$exp <- Reorder(nao$exp, c(2, 1))
nao$obs <- Reorder(nao$obs, c(2, 1))
PlotBoxWhisker(nao$exp, nao$obs, "NAO index, DJF", "NAO index (PC1) TOS",
monini = 12, yearini = 1985, freq = 1, "Exp. A", "Obs. X")
## End(Not run)
```

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