El Nino Southern Oscillation, usually referred to as ENSO is one of the main determinants factors in climate variability. ENSO influences winters around the globe, bringing wetter or drier conditions depending on location and warmer or cooler conditions depending on that same location as well. I have read an interesting paper by Meteorologists: Aiguo Dai and T. M. L. Wigley (2000) on how ENSO influences global patterns of precipitation around the globe. In this summary I will, however only focus on the results over the western hemisphere. Here’s this paper’s summary by me. Want to contact me? Send me an email to saravanessacrodriguesdasilva@pm.me. Want to advertise your business on Starlight Connect? Click the page that reads Advertise and get your business ad on this website for a week, month or year at a very accessible price.

In this paper, both meteorologists explain their hypothesis, methodology, research, and then conclusions. Their paper is divided in different sections: Introduction, Data analysis and methods, results and discussion. Next I will summarize all of these sections one by one in my own words.

Introduction

There has been studies over time of a possible correlation between ENSO and precipitation patterns around the globe. The data that meteorologists use is usually obtained in weather stations placed on land around the globe. However, such collection of data proves to make of the possibility for this research incomplete as there is not as much coverage over oceans and at times there are discontinuities over land. One way that meteorologists follow to mend this lack of data over certain regions / locations around the globe is to use satellite data and reanalysis computer models. For this study, both meteorologists used a combined data set for seasonal and annual precipitation through the application of EOF methods (mathematical).

Data and Analysis Method

For this study, Aiguo Dai and T. M. L. Wigley used the following data as described in their paper:

  • monthly precipitation anomaly data on a 2.5 x 2.5 (degrees) grid for 1900 – 1978 (obtained through records over more than 5300 weather stations located over land around the globe)
  • monthly anomalies 1978 – 1998 from X-A anomalies (X-A anomalies are with respect to the date set from 1979 through 1998)

Both meteorologists found that these data sets provided homogeneity for the covered areas, and they mention they did not find any points of discontinuity for these data sets through EOF analysis.

Then normalized annual precipitation anomalies by standard deviation (statistics).

The analysis covers the following latitudes range: 60 degrees South to 75 degrees North.

Results

They include in their paper seven graphs showing the results from their study. In these graphs and plots we can see EOF amplitudes variation in time (one plot), EOF pattern normalized to the considered grid box (1 figure) showing the variation of amplitude against annual precipitation values, annual precipitation anomalies for the same grid box (1 figure), and then four different images that show annual precipitation variation for the four different seasons = seasonal variation of annual precipitation.

Through observation and analysis, we can infer from these images / plots / graphs that the ENSO signal is much larger in the tropics and this is related to the large precipitation variability characteristic for those latitudes. These patterns can be divided in Northern branch and Southern branch and their variability depends on the season. The northern branch is observed to occur Dec through February and March through May, while the southern branch is observed to happen June through August and from March through May.

The images also show that during El Nino winters, drier than normal winter conditions are observed over the tropics, while wetter than normal conditions – please note that by “normal” I mean climatology value – extend on the sub tropical regions from North America through Europe. Precipitation is said to vary in around 30% during cold ans warm ENSO episodes.

It is also observed that during cold ENSO episodes usually called as La Nina years, the signs for these anomalies when contrasted with climatology values change sign. The ENSO EOF analysis results in showing that 4.5 – 7.0 % of total variance on precipitation global patterns are related to the sign of the ENSO episode that one is studying.

Discussion

Both meteorologists in this paper, reserve a section to discuss the results. This section includes conclusions. Following this section a section for references is attached. Dai and Wigley conclusions after this study point to:

  • Precipitation anomaly patterns in the tropics E – W result from variations in the Wallker circulation during ENSO episodes
  • During El Nino years – also referred to as warm ENSO episodes – ascending air motion weakens over South America while subsidence of air over the Atlantic strengthens
  • During the cold ENSO phase, also known as La Nina episodes, the signs of the anomalies change, implying that regions with wetter winter conditions will have drier conditions and vice versa.

This study shows very clearly that global precipitation patterns are correlated to ENSO episodes (warm and cold = El Nino and La Nina). These global patterns for precipitation during winter months are directly impacted by ENSO. ENSO is still the main precipitation climate variability atmospheric phenomenon.

Sources: I have read and analyzed this paper written by these two meteorologists: Aiguo Dai and T. M. L. Wigley, with the following title:

“Global Patterns of ENSO – induced precipitation” (2000)

National Center for Atmospheric Research – Boulder – Colorado – USA

Commentary 

This paper was very interesting to read, not only it was fairly straight forward in the way it has been written, it also has shown what we learned in Meteorology graduate classes is true and proven. ENSO is one of the main climate variability driving forces for precipitation patterns around the globe and winter temperature anomalies. The graphs on this paper prove as right and correct their hypothesis that stated that a possible relationship (correlation) between ENSO episodes (warm and cold) and global precipitation patterns would be observed.

Sara Vanessa C. Rodrigues da Silva

Meteorologist (MSc), Geophysicist (BSc), Author, Starlight Connect founder

Link to the scientific paper by Dai and Wigley: https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/1999GL011140

 

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