.ipynb .pdf

Contents

3. Ocean-atmosphere coupling

The El Niño Southern Oscillation (ENSO) is a coupled ocean-atmosphere phenomenon. This means that the oceanic signature of ENSO in the SST impacts the atmospheric circulation, which then feeds back onto the ocean, affecting the the state of the ocean. The ocean and atmosphere are linked in an ongoing feedback cycle, such that anomalies in one produce anomalies in the other, producing anomalies in the first, and so on.

3.1. Normal conditions in the tropical Pacific

Fig. 3.1 shows a schematic of normal conditions across the tropical Pacific. In the ocean we see warm SST in the west and cooler SST in the east. You can look back at Fig. 2.1 in the previous section to see this in the reanalysis data. In the atmosphere, we see rising air and precipitation in the west, falling air with no precipitation in the east, and surface winds blowing from east to west (i.e., the trade winds) in between. This circulation cell is called the Walker cell.

Fig. 3.1 \(~\)|\(~\) Normal conditions in the Equatorial Pacific (source).

Fig. 3.2 shows maps of climatological winter SST, wind, and precipitation from the reanalysis data. We can see the imprint of the Walker cell in these maps. The features are not as well defined as in the schematic, but the general features are present. Click through the various tabs in Fig. 3.2 and note the similarities to the schematic in Fig. 3.1: warmer SST in the west, more precipitation in the west, and winds blowing from east to west.

Another aspect of the schematic in Fig. 3.1 is the shape of the thermocline. The thermocline is a thin layer of water in the ocean where the temperature changes rapidly, and it serves as a boundary of sorts between deep cold water waters and warm surface waters in the ocean. The normal shape of the thermocline in the equatorial Pacific is a slope from 150–200 meters deep in the west to 30–50 meters deep in the east.

Questions

1. Considering the direction of the winds along the equator (i.e., from east to west), why do you think the thermocline is closer to the surface in the east compared to the west?

2. What is the difference in SST across the Pacific at the equator? Hover over the map to get values—a whole number estimate is fine. How is the SST related to the depth of the thermocline?

Fig. 3.2 \(~\)|\(~\) Tabs 1–3: Average winter (December–February) sea surface temperature (SST), surface winds, and precipitation. The source of all three fields is the ECMWF ERA5 reanalysis.

3.2. El Niño and La Niña conditions

Earlier in the lab, we found that a defining characteristic of El Niño and La Niña events is sea surface temperature anomalies (SSTAs)—warm and cool, respectively—in the central and eastern equatorial Pacific. These SSTAs are coincident with substantial changes in the Walker cell.

To understand why SSTAs are associated with changes in the Walker cell, first remember that warm air rises. This is an aspect of everyday physics with which most people are familiar. The reason is that a chunk of air expands as it warms, which means the same air mass is taking up more space, which in turn causes the air density to decrease relative to the air nearby. This less dense air rises for the same reason a block of Styrofoam held at the bottom of a pool will rise to the surface—the block of Styrofoam is less dense than the surrounding water. When SSTs warm or cool, it affects the temperature of air at the surface and the rate at which air is rising or falling. This affects the ascending and descending branches of the Walker cell (i.e., the far left and right sides of the schematic in Fig. 3.1).

During La Niña events, areas with cool SSTs get even cooler, and areas with warm SSTs get even warmer. The enhanced SSTs are associated with an amplified Walker cell and the following characteristics (Fig. 3.3):

• stronger than normal trade winds,

• enhanced precipitation in the western Pacific,

• and a steeper slope in the thermocline.

Fig. 3.3 \(~\)|\(~\) La Niña conditions in the Equatorial Pacific (source).

In contrast, El Niño events are associated with warm SSTAs in the east and cool SSTAs in the west. This corresponds to a weakened SST difference across the equatorial Pacific, a weakened Walker cell, and the following characteristics (Fig. 3.4):

• weaker than normal trade winds,

• a shift in the location of the ascending branch of the Walker cell (and its associated precipitation) to the middle of the Pacific,

• and a flattened thermocline.

Fig. 3.4 \(~\)|\(~\) El Niño conditions in the Equatorial Pacific (source).

Questions

1. Fig. 3.5 shows maps of surface wind anomalies and precipitation anomalies from the 1998 El Niño and the 2000 La Niña winters, as well as the ENSO-neutral winter of 2004. Each tab shows a different year, but the years are not labeled. Based on what we have learned about the impact of ENSO phase on wind and precipitation, examine each set of maps and determine the specific year corresponding to each.

2. Fig. 3.6 shows depth-longitude sections of ocean potential temperature and ocean potential temperature anomalies from the 1998 El Niño, 2000 La Niña, and 2004 ENSO-neutral winters. Note that the white lines in A and B mark the 20ºC isotherm, which is a proxy for the depth of the thermocline. Based on what we have learned about the impact of ENSO phase on the slope of the thermocline, examine each set of depth-longitude sections and and determine the specific year corresponding to each.

Fig. 3.5 \(~\)|\(~\) Winter (December–February) surface wind anomalies (top) and precipitation anomalies (bottom) in the equatorial Pacific for three unspecified years. The source of all fields is the ECMWF ERA5 reanalysis.

Fig. 3.6 \(~\)|\(~\) Winter (December–February) potential temperature (top) and potential temperature anomalies (bottom) over the upper 500 meters of the equatorial Pacific for three unspecified years. White lines mark the depth of the 20ºC isotherm, which is a proxy for depth of the thermocline. The source of all fields is the ECMWF ORAS5 reanalysis.

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2. ENSO and sea surface temperature (SST) 4. Onset of an El Niño