amo documentation

amo computes a version of the Atlantic Multidecadal Oscillation index.

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Contents

Syntax

idx = amo(sst,t)
idx = amo(sst,t,lat,lon)

Description

idx = amo(sst,t) calculates AMO index from a time series of sea surface temperatures sst and their corresponding times t. sst can be a vector of sea surface temperatures that have been averaged over a region of interest, or sst can be a 3D matrix whose third dimension correponds to times t. If sst is a 3D matrix, a time series is automatically generated by averaging all the grid cells in sst for each time step. Usually, the AMO is calculated from the area-weighted average of ssts in the northern Atlantic from 0-70ºN.

idx = amo(sst,t,lat,lon) calculates the AMO index for 3D sst time series and corresponding grid coordinates lat,|lon|. Using this syntax, grid cells within the AMO region are automatically determined and the AMO index is calculated from the area-averaged time series of ssts within that region.

Example

In this example, we will calculate the AMO index using the monthly north_atlantic_sst.mat dataset that comes with CDT. Start by loading the data that comes with CDT. It contains monthly sea surface temperatures from 1870-2017 for the Northern Atlantic region (0-70N)

load north_atlantic_sst.mat

Here are the variables we're working with:

whos % displays variable names and size
  Name           Size                    Bytes  Class       Attributes

  lat           36x1                       144  single                
  lon           41x1                       164  single                
  sst_na        36x41x1776            10485504  single                
  t           1776x1                     28417  datetime              

The table above tells us that we have a lat array, a lon array, and a t array, which correspond to the dimensions of an sst_na data cube of SSTs in the North Atlantic.

We'll need to tell the amo function exactly what latitudes and longitudes correspond to each grid cell in the sst_na data, so convert the lat and lon arrays into grids like this:

[Lon,Lat] = meshgrid(lon,lat);

To get a little bit of context, let's take a look at the data we'll be working with. Make a pcolor plot of time-averaged mean SSTs (down dimension 3 of the sst_na data cube). Set the colormap to thermal via the cmocean function, and underlay with an earthimage to give a better sense of where we're looking at:

pcolor(Lon,Lat,mean(sst_na,3))
shading interp
cmocean thermal % sets the colormap
cb = colorbar;
ylabel(cb,'mean SST (\circC)')
hold on
earthimage('bottom') % underlay with a satellite image of Earth

The map above shows only the region from 0N to 70N and 75W to 5 E. That's the geoquad the amo function uses to calculate the AMO index, but if you're working with an SST dataset that covers a larger area, don't worry--the amo function uses the Lat,Lon grids you give it to make calculations based only on values in the North Atlantic.

Calculate the AMO Index

The amo function works by entering the sst data cube along with corresponding times and Lat,Lon grids:

idx = amo(sst_na,t,Lat,Lon);

Plot the raw AMO index:

figure
plot(t,idx)
ylabel 'AMO index'

The plot above shows North Atlantic sea surface temperature variability, and it seems to have a sort of oscillatory behavior, and the periods of the oscillations appears to be multiple decades long. You might say it's some sort of Atlantic Multidecadal Oscillation.

NOAA recommends taking a 10 year (121 month) moving average to smooth out the time series:

% Low-pass filter with a 121 month moving average:
idx_f = movmean(idx,121);

hold on
plot(t,idx_f,'k-','linewidth',2)
legend('AMO index','AMO (10 year moving mean)')

To show we're on the right track, we can use the anomaly function to mimic Figure 1a from Enfield et al., 2001:

figure
anomaly(datenum(t),idx_f,...
   'topcolor',rgb('orange'),'bottomcolor',rgb('yellow'))
axis tight
datetick('x')
xlim(datenum([1860 2000],1,1)) % sets x axis date limits
ylim([-0.3 0.3])               % sets y axis limits

Figure 1b of Enfield et al. shows the correlation between the AMO index and global sea surface temperatures. Unfortunately, our sample dataset only covers the North Atlantic, so for a comparison to other places around the globe we'll have to use the pacific_sst sample dataset:

P = load('pacific_sst.mat');

Enfield et al. correlated the lowpass-filtered AMO (which is inherently deseasoned) to deseasoned, lowpass-filtered global SSTs. So let's deseason and lowpass filter the Pacific Ocean SST dataset:

% Deseason and lowpass filter SSTs:
sstd = deseason(P.sst,P.t);
sstdf = movmean(sstd,121,3);

Before we can correlate our lowpass-filtered AMO index to the deseasoned, lowpass-filtered Pacific SSTs, we'll have to synchronize the two time series. An easy way to do that is to interpolate the idx_f array to the Pacific SST times, like this:

idx_fi = interp1(datenum(t),idx_f,P.t);

% Plot the interpolated values to ensure everything lines up:
hold on
plot(P.t,idx_fi,'b-','linewidth',4)

The fat blue line above indicates we've correctly interpolated the idx_f values to the times P.t, which correspond to the times of the Pacific Ocean SST time series.

Now we're able to correlate the lowpass-filtered, interpolated AMO index idx_fi with Pacific SSTs:

% Correlate ssts to AMO:
C = corr3(sstdf,idx_fi);

figure
imagescn(P.lon,P.lat,C)
caxis([-1 1])
cmocean balance

The map above does not exactly match figure 1b from Enfield et al., likely because we're only comparing the AMO to 802 months (66 years) of a multi-decadal phenomenon. A proper correlation would look at several complete cycles of the oscillation.

Reference

Enfield, D.B., A.M. Mestas-Nunez, and P.J. Trimble, 2001: The Atlantic Multidecadal Oscillation and its relationship to rainfall and river flows in the continental U.S., Geophys. Res. Lett., 28: 2077-2080. doi:0.1029/2000GL012745

Author Info

This function was written by Kaustubh Thirumalai of the University of Arizona, for the Climate Data Toolbox for Matlab. March 2019. http://www.kaustubh.info