Strange El Niño

Fig. 1. SST anomalies in August 2009.

As of mid-September 2009, SSTs are above-average across much of the equatorial Pacific indicating weak-to-moderate El Niño conditions (Fig. 1). Due to a high level of persistence (and therefore predictability) in ENSO during the second half of the year, one can expect that the current El Niño event will likely persist through the end of the year and perhaps through the forthcoming winter. Indeed, both statistical and dynamical model forecasts for the Niño-3.4 SST index (Fig. 2) suggest El Niño will reach at least moderate strength during the Northern Hemisphere fall. Many model forecasts even suggest a strong El Niño (3-month Niño-3.4 SST index in excess of +1.5°C) during the fall and winter.

Fig. 2. Model forecasts of ENSO from August 2009.

Although every El Niño is different, the current episode seems to be more unusual than one can expect from normal Niño-to-Niño variations. Here is a list of most noticeable abnormalities:

• The SOI is neutral and does not show an El Niño trend. The latest 30-day SOI value is −1. The SOI is usually consistently below −7 during El Niño events.
• While cloudiness near the date-line has shown an El Niño trend, the trend has been a relatively weak one compared with other El Niño events.

Fig. 3. SLP anomalies in August 2009.

• Wind anomalies along the equator in the western Pacific are currently westerly, but in the central and eastern portions of the basin winds remain more random. In addition, Trade Wind patterns do not exhibit a consistent El Niño signature.
• Below the surface of the tropical Pacific, the ocean is significantly warmer than the long-term mean. However, the ocean has been cooling steadily over the past 10 weeks and is not as warm as it was in June.
• Unlike previous El Niño events, conditions are warmer than average in the Coral Sea, off Australia's northern coasts and in the far western Tropical Pacific (Fig. 1). These regions are typically average to cooler than average during an El Niño event.

• In August, there was an unprecedented drop in sea level pressure (SLP) in the area south of Australia (Fig. 3). As shown in the time series for that area (Fig. 4), the SLP dropped to a record low value of 984 hPa, which was 2.8 std below the 1971-2000 average. There are some lagged relationships in the ECliPS’s knowledge base that link events like this in the vicinity of Australia with winter climatic conditions in the Northern Hemisphere. In particular, this drop in SLP, as well as other abnormalities of the current El Niño event, may have some interesting consequences for the forthcoming winter in North America.

Fig. 4. August SLP in the center south of Australia, 1948-2009.