2. ENSO and sea surface temperature (SST)¶
The El Niño Southern Oscillation (ENSO) is a global climate phenomenon rooted in the equatorial Pacific Ocean. It is not a true oscillation with regular and predictable periods like the seasons or tides, but the ENSO state does fluctuate between El Niño (the warm phase) and La Niña (the cold phase) with a rough period of every 2–7 years. It would be hard to understate the importance of ENSO in global climate, as it is pervasive in variations across ocean and atmospheric science. Currents, winds, hurricanes, precipitation, and even biology—from plankton to monkeys—are affected by ENSO. We will start our exploration of ENSO with its relationship to sea surface temperature (SST) and build out to other impacts from there.
2.1. Climatology¶
Sea surface temperature is—as we will see in this lab—one of the primary ways to determine whether global climate is experiencing an El Niño, La Niña, or ENSO-neutral state. In fact, the reason El Niño and La Niña are referred to as the warm and cool phases of ENSO, respectively, is due to the relationship of these phases with equatorial Pacific SST. To understand how ENSO alters oceanographic conditions, let’s begin by examining the climatology (i.e., typical seasonal variation) of SST. Even in an average or “normal” year, conditions change with the seasons, so let’s begin by looking at the global SST field from the ERA5 reanalysis and note some of the changes that occur from winter to summer. Note that when we refer to winter and summer, we are referring to Northern Hemisphere seasons such that winter is December–February and summer is June–August.
Exploring data in this lab
The figures provided below are interactive. You can explore the data by hovering your mouse over the maps, which will display the value of the mapped field and latitude/longitude coordinates. You can also zoom and pan the maps using the tools to the right of each map. The symbol that looks like a recycle symbol will reset the plot to its original state. Most figures include multiple tabs that allow you to flip back and forth between views.
Take a minute to play with these options, as we will use these tools throughout the lab.
Fig. 2.1 below has two tabs. The first is a map of climatological winter SST, and the second is a map of climatological summer SST. Use these maps to answer the following questions related to SST climatology.
Questions¶
If you flip back and forth between the two tabs in Fig. 2.1, you can see the temperature pattern shift north and south with the seasons (e.g., focus on the ocean regions around the southern portions of North America). To get an idea of the typical SST range with the seasons at different latitudes, mouse over the following locations and record the average winter and summer temperatures at the each location, as well as the seasonal range (i.e., difference between the seasons). No need to get the exact location (just get close; zoom if you need) and round to the nearest tenth of a degree.
a. Bering Sea (215ºE, 50ºN)
b. Hawaii (202ºE, 21ºN)
c. Equatorial Pacific (230ºE, 0ºN)
Based on your answers above, do higher or lower latitudes tend to experience greater seasonal SST range?
Do changes in latitude (i.e., Hawaiʻi vs. Bering Sea) or changes in season (i.e., winter vs. summer at the same location) lead to larger changes in SST?
Fig. 2.1 \(~\)|\(~\) Climatological sea surface temperature (SST) during 1991–2020 from the ECMWF ERA5 reanalysis for winter (December-February, first tab) and summer (June–August, second tab).¶
2.2. SST and ENSO¶
Now that we have an idea of typical variations in SST by location and season, let’s investigate what happens to the global SST field during an El Niño event. The most recent large El Niño event occurred in 2015–2016, so let’s look at that event first. Also note that the most significant changes to global oceanographic conditions during an El Niño event occur during Northern Hemisphere winter, so let’s look at the 2016 winter season, where the year corresponds to the year of January. The first tab in Fig. 2.2 shows the average SST during the 2016 winter season, and the second tab shows the climatological winter SST (same as the first tab in Fig. 2.1).
Questions¶
Compare and contrast the winter 2016 SST map (Fig. 2.2, first tab) to the map of climatological winter SST (Fig. 2.2, second tab). What differences do you see?
Go back to the North Pacific locations from Question 1 and record the SST values for winter 2016. Are there any commonalities between the locations for winter 2016 compared to the climatological values?
a. Bering Sea (215ºE, 50ºN)
b. Hawaii (202ºE, 21ºN)
c. Equatorial Pacific (230ºE, 0ºN)
Why is it difficult to visually identify the details of differences between winter 2016 map and the climatological winter map? Think about your answers to the previous question versus your answers to the similar question in the section above.
Fig. 2.2 \(~\)|\(~\) Average winter (December-February) SST during the 2016 El Niño event (first tab) and climatological winter SST during 1991–2020 (second tab) from the ECMWF ERA5 reanalysis.¶
2.3. SST anomalies¶
In order to better isolate local changes due to variations in climate, scientists often work with anomalies rather than raw data values. When an oceanographer or climate scientist refers to an anomaly, they are most likely referring to the difference between some measured value and what is expected or “normal” for that location and time of year. In other words, an anomaly is the observed value minus the climatological value.
For example, if we want to calculate the SST anomalies (SSTA) for winter 2016, we can subtract the climatological or “normal” winter SST map (Fig. 2.2, second tab) from the observed winter 2016 map (Fig. 2.2, first tab). When we do that, we get the SSTA map in the first tab of Fig. 2.3 below. This map highlights the local departures of SST from normal during the 2016 El Niño winter. Let’s compare the SSTA maps from the 2016 El Niño winter to the 2008 La Niña winter (Fig. 2.3, second tab).
Interpreting the anomaly maps
Red colors mark areas where SSTAs are positive, which means these areas of the ocean surface are warmer than normal. Blue colors mark areas where SSTAs are negative, indicating that these areas are cooler than normal.
Questions¶
What region of the global ocean experiences the greatest difference in SST anomalies between the 2016 El Niño and 2008 La Niña years?
Are the differences in SST anomalies confined to a few isolated regions or are the differences global in nature? Name two additional ocean regions (excluding your answer to the question above) that experienced large differences in SSTA between the 2016 El Niño and the 2008 La Niña.
Based on the spatial patterns of SSTA during the 2016 El Niño and the 2008 La Niña winters, classify the winters in the remaining tabs (2000, 1998, 1994, and 1983) as either El Niño, La Niña, or ENSO-neutral. What key feature or features of the maps did you use to make your classifications?
Every ENSO event is unique
You may have noticed in answering the last question that not all El Niño and La Niña events are the same. By observing past events, we know the general features and impacts of typical ENSO phases, but the details of each event are unique and difficult to predict.