EN... SO?

Emily Becker at Climate.gov: 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.

...As of the end of June, both the SOI and the EQSOI are at +0.2 (they have trended downward over the past few months), and the wind patterns are roughly average over the tropical Pacific, with some slight weakening of the trade winds toward the end of the month. There is increased convection in the central Pacific, but also some over Indonesia… all of which says we’re still waiting for the atmosphere to get dressed in its El Niño clothes and come out to play.

However, we think it’s likely that the atmosphere will get on board soon, and we’re still predicting El Niño, with about a 70% chance that conditions will be met in the next few months, and around an 80% chance by this fall. If you’re interested in how the ocean and atmospheric conditions are evolving, CPC has weekly updates available....

Figure 1. Average state of ocean temperatures, rainfall, pressure, and winds over the Pacific during ENSO-neutral conditions. Figure 2. Generalized state of the ocean and atmosphere during El Niño conditions. NOAA image created by David Stroud.