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English: Forecasters at the Climate Prediction Center haven’t declared El Niño conditions, even though the Niño3.4 index is currently around 0.5°C above normal, and has been for the past two months. What’s the hold up? In short, we’re waiting for the atmosphere to respond to the warmer sea-surface temperatures, and give us the “SO” part of ENSO.

SO what? The Southern Oscillation, that’s what. The Southern Oscillation is a seesaw in surface pressure between a large area surrounding Indonesia and another in the central-to-eastern tropical Pacific; it’s the atmospheric half of El Niño. Since ENSO is a coupled system, meaning the atmosphere and ocean influence each other, both need to meet the criteria for El Niño before we declare El Niño conditions.

During average (non-El Niño) times, the waters of the western tropical Pacific are much warmer than in the east/central area (Figure 1). As warmer water extends out to the east during an El Niño, it warms the air, causing it to rise (lower pressure) (Figure 2). In turn, there is less rising motion (higher pressure) near Indonesia, due to the relatively cooler waters and overlying air.

The pressure changes influence the wind patterns. The average (non-El Niño) state of the atmosphere over the tropical Pacific features convection and rainfall over Indonesia, low-level easterly winds (the trade winds that blow from east to west), and upper-level westerly winds (Figure 1). These are the basic components of the Pacific Walker Circulation.

During El Niño, the system shifts: we see weaker trade winds over the Pacific, less rain than usual over Indonesia, and more rain than usual over the central or eastern Pacific. During some El Niño events, the trade winds along the equator even reverse, and we see low-level westerlies… but not every time. In fact, every El Niño is different, and both the ocean and atmospheric characteristics vary quite a lot from event to event–but that’s a topic for another post!

This difference from average air pressure patterns across the Pacific is measured a few different ways. One is the Southern Oscillation Index (SOI), which is based on a long record of pressure measured by two stations: one in Darwin, Australia (south of Indonesia) and the other in Tahiti (east-central tropical Pacific) (Figure 3). A negative SOI indicates Darwin’s pressure is higher than average and Tahiti’s is lower than average: El Niño conditions. (I keep saying “higher than average” because we’re not just comparing Darwin’s pressure to Tahiti’s, but rather comparing the anomalies at each. Imagine comparing the price of a gallon of water to that of a gallon of gas. A negative index is if the price of the water goes up, and the gas goes on sale. The gas may still cost more than the water, but it’s the relative changes in the two prices that matter.)

A second way we describe the air pressure anomalies over the tropical Pacific is the Equatorial Southern Oscillation Index (EQSOI). The EQSOI is based on pressure differences between two regions located on the equator (Figure 3). The SOI is monitored because it has a very long record available, stretching back to the 19th century; the EQSOI depends on satellite observations, which means it is a shorter record, but it gives a better picture of what’s happening right along the equator.