Sunday, March 29, 2009

The Atlantic Multidecadal Oscillation - Part I

One of the interesting features of the Earth Trends Modeler is the ability to compute Empirical Orthogonal Teleconnections (EOT). The intention of the technique (described in more detail at the end of the end of this post) is to uncover the major underlying patterns of variability in the analyzed series over space and time.

The first EOT in monthly anomalies in sea surface temperature from 1982-2007 (above) is the familiar El Nino / La Nina (ENSO) phenomenon -- not surprising, since it is unquestionably the dominant pattern of interannual variability in sea surface temperature (SST). However this post concerns the second EOT (below) which presents a less familiar pattern.

The second EOT is the largest pattern of space-time variability in SST anomalies that the technique can find in the residuals from ENSO (i.e., after the effects of ENSO have been removed). The image above shows the spatial pattern and the graph below shows the temporal pattern. Areas with high positive values in the image are strongly associated with the temporal pattern (and vice versa).

The space/time portrait of EOT2 is dramatic. First, most of the world's oceans show warming (i.e., a positive association with increasing anomalies in temperature after the mid 1990's). Second, the warming is most pronounced in the Atlantic. The image below shows the temporal graph of EOT2 along with an index to the Atlantic Multidecadal Oscillation (AMO) superimposed (in red) -- a low frequency Atlantic SST oscillation with what is thought to be a 65-70 year cycle associated with the thermohaline circulation.

Clearly the temporal evolution of EOT2 is a VERY close match to the AMO index (r = 0.71). However, the AMO is also the subject of significant controversy. The issue is the extent to which the pattern of Atlantic warming in recent years is attributable to a natural climate cycle (the AMO) or to global warming. Look for more on this issue in the next posting.

About the technique:

The Empirical Orthogonal Teleconnection technique (as implemented in the Earth Trends Modeler) searches for the single location in space (a pixel in this case) whose temporal profile can best describe the temporal evolution of all other locations. The profile at that location becomes the first EOT. A residual series is then created such that the effects of the first EOT are removed from the original image series. The process is then repeated to find the next EOT, and so on. In our implementation, after all the designated number of EOT’s have been found, a multiple regression is run with the original image series as the dependent variable and the EOT’s as independent variables to create partial R images as a spatial portrait of the EOT.

EOT's are similar to obliquely rotated Principal Components. They are independent in time, but not necessarily in space. They have the advantage that they are easily understood and are associated with a specific location.

Saturday, March 14, 2009

Sea Ice Concentration 1982 - 2007

The image below shows the median trend in monthly sea ice concentation from 1982 - 2007 as calculated by the Earth Trends Modeler. The image series comes from the National Snow and Ice Data Center. This analysis is based on anomalies in sea ice concentration. Thus it automatically accounts for seasonal variation. Most images of changes in sea ice concentration focus on the summer extent. However, sea ice is an important factor for Arctic ecology throughout the year. Thus this portrait provides a broader perspective. The image has been contrast stretched to a range from +/- 0.042% per month which translates to a rate for approximately 5% per decade. However, in some locations the rate exceeds 10% per decade.

Lower Tropospheric Temperatures 1982 - 2007

The image below shows the median trend of monthly lower tropospheric temperatures from 1982-2007 as determined by the Earth Trend Modeler from the Microwave Sounding Unit (MSU) image series processed by Remote Sensing Systems. These data are spatially coarse, but they represent a critical resource in understanding atmospheric dynamics. In the image below we can see that the strongest trends are in the Arctic. The image has been contrast stretched to a range between +/- 0.008 degrees Celsius per month, which translates to almost 2.5 degrees over the period of this series. It is interesting to note that the highest increases in temperature are in the northern hemisphere.

The second image (below) shows the monotonic trend analysis for MSU lower troposphere temperatures. Trend monotonicity measures the degree to which a trend is consistently increasing or decreasing. In the Earth Trends Modeler, trend monotonicity is measured using the Mann-Kendall statistic. Here the picture is quite different – the region with the most consistent increase in lower tropospheric temperatures is the tropics. Although the eastern Pacific is less pronounced, this is probably only a result of persistent ENSO events in that region that reduce the measure of monotonicity.

Sea Surface Temperature 1982 - 2007

The oceans are a critical component of the dynamic earth system. They are a major forcing agent on the atmosphere and possess significant inertia. The image below shows the median trend of anomalies in monthly sea surface temperatures from 1982 to 2007 inclusive as determined by the Earth Trends Modeler. The data are from the Optimally Interpolated Version 2 (OIV2) series by Smith and Reynolds. The median trend is notable because it is resistant to outliers and expresses trends that have endured for at least 29% of the length of the series. Thus the trends evident in this image have persisted for at least seven and a half years.

The most notable features of this image include:
  • Intense warming in the sub-polar gyre near Greenland, the North Sea and along the sea ice edge towards Svalbard.
  • A strongly higher frequency of warming sea surface temperatures than cooling temperatures.
  • Generally, a higher warming in the north Atlantic.
The units represent the rate of change per month in degrees Celsius. The image has been contrast stretched to a range between +/- 0.0045 degrees Celsius which represents a change of over 1.4 degrees Celsius over the length of the series. However, the change is even higher in the Labrador Sea in the top loop of the sub-polar gyre. Here is a temporal profile of anomalies over this period created using the Earth Trends Modeler.

The trend line is also a robust Theil-Sen median trend.