{"id":289,"date":"2023-05-08T06:43:56","date_gmt":"2023-05-08T06:43:56","guid":{"rendered":"https:\/\/tepukeweather.nz\/?page_id=289"},"modified":"2023-05-08T06:46:13","modified_gmt":"2023-05-08T06:46:13","slug":"what-is-el-nino-and-enso","status":"publish","type":"page","link":"https:\/\/tepukeweather.nz\/index.php\/about-weather\/what-is-el-nino-and-enso\/","title":{"rendered":"What is El Nino and ENSO"},"content":{"rendered":"\t\t
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What is El Nino and ENSO<\/h1><\/header>

Forecasters at the Climate Prediction Center haven\u2019t declared El Ni\u00f1o conditions, even though the Nino3.4 index is currently around 0.5\u00b0C above normal, and has been for the past two months. What\u2019s the hold up? In short, we\u2019re waiting for the atmosphere to respond to the warmer sea-surface temperatures, and give us the \u201cSO\u201d part of ENSO.<\/p>

SO what? The Southern Oscillation, that\u2019s 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\u2019s the atmospheric half of El Ni\u00f1o. Since ENSO is a coupled system, meaning the atmosphere and ocean influence each other, both need to meet the criteria for El Ni\u00f1o before we declare El Ni\u00f1o conditions.<\/p>

During average (non-El Ni\u00f1o) 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 Ni\u00f1o, 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.<\/p>

\"\"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 Ni\u00f1o conditions. NOAA image created by David Stroud.<\/p>

The pressure changes influence the wind patterns. The average (non-El Ni\u00f1o) 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.<\/p>

During El Ni\u00f1o, 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 Ni\u00f1o events, the trade winds along the equator even reverse, and we see low-level westerlies\u2026 but not every time. In fact, every El Ni\u00f1o is different, and both the ocean and atmospheric characteristics vary quite a lot from event to event\u2013but that\u2019s a topic for another post!<\/p>

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\u2019s pressure is higher than average and Tahiti\u2019s is lower than average: El Ni\u00f1o conditions. (I keep saying \u201chigher than average\u201d because we\u2019re not just comparing Darwin\u2019s pressure to Tahiti\u2019s, 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\u2019s the relative changes in the two prices that matter.)<\/p>

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\u2019s happening right along the equator.<\/p>

\"\"Figure 3. Two ways of measuring the Southern Oscillation: the SOI and the EQSOI. Both depend on comparing the strength of pressure anomalies in different parts of the Pacific basin. Map by NOAA Climate.gov.<\/em><\/p>

As of the end of June, both the\u00a0SOI<\/a>\u00a0and 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\u2026 all of which says we\u2019re still waiting for the atmosphere to get dressed in its El Ni\u00f1o clothes and come out to play.<\/p>

However, we think it\u2019s likely that the atmosphere will get on board soon, and we\u2019re still\u00a0predicting El Ni\u00f1o<\/a>, 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\u2019re interested in how the ocean and atmospheric conditions are evolving, CPC has\u00a0weekly updates<\/a>\u00a0available.<\/p>

By Emily Becker from Climate.gov, with thanks to David Stroud for his help with this post.<\/em><\/p>

To read more of Emily\u2019s work visit here:\u00a0http:\/\/www.climate.gov\/author\/emily-becker<\/a><\/em><\/p>

\u2013 NOAA<\/em><\/p><\/div><\/div><\/div><\/div><\/div><\/section><\/div><\/div>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/section>\n\t\t\t\t<\/div>\n\t\t","protected":false},"excerpt":{"rendered":"

What is El Nino and ENSO Forecasters at the Climate Prediction Center haven\u2019t declared El Ni\u00f1o conditions, even though the Nino3.4 index<\/p>\n

Read More<\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"parent":158,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"elementor_header_footer","meta":{"footnotes":""},"class_list":["post-289","page","type-page","status-publish","hentry"],"_links":{"self":[{"href":"https:\/\/tepukeweather.nz\/index.php\/wp-json\/wp\/v2\/pages\/289","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/tepukeweather.nz\/index.php\/wp-json\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/tepukeweather.nz\/index.php\/wp-json\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/tepukeweather.nz\/index.php\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/tepukeweather.nz\/index.php\/wp-json\/wp\/v2\/comments?post=289"}],"version-history":[{"count":4,"href":"https:\/\/tepukeweather.nz\/index.php\/wp-json\/wp\/v2\/pages\/289\/revisions"}],"predecessor-version":[{"id":295,"href":"https:\/\/tepukeweather.nz\/index.php\/wp-json\/wp\/v2\/pages\/289\/revisions\/295"}],"up":[{"embeddable":true,"href":"https:\/\/tepukeweather.nz\/index.php\/wp-json\/wp\/v2\/pages\/158"}],"wp:attachment":[{"href":"https:\/\/tepukeweather.nz\/index.php\/wp-json\/wp\/v2\/media?parent=289"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}