ENSO Wrap-Up
Current state of the Pacific and Indian Ocean

Sea surface temperatures (SSTs) are warmer than average along virtually all of the equator in the Pacific Ocean, across much of the tropics to the north of the equator, and much of the western Pacific both north and south of the equator. SSTs within both the NINO3 and NINO3.4 region have  been above the El Niño threshold for the past four weeks.

The latest values of the key NINO indices in the tropical Pacific for the week ending 18 November are: NINO3 +0.9 °C, NINO3.4 +0.8 °C and NINO4 +0.9 °C.

Persistent NINO3 or NINO3.4 values warmer than +0.8 °C are typical of El Niño, while persistent values cooler than −0.8 °C typically indicate La Niña.

Around Australia, SSTs are warmer than average along much of the east coast, with weak warm anomalies extending across much of the Bight and parts of the northern coastline. In the past fortnight, a large area of warm anomalies has developed over the far northwest of the country, adjacent to the Top End of the Northern Territory.

The tropical Pacific continues to meet some, but not all, El Niño criteria, while a positive Indian Ocean Dipole (IOD) persists in the Indian Ocean. The Bureau's ENSO Outlook remains at El Niño ALERT, meaning there is at least a 70% chance of El Niño fully forming in 2018.

Sea surface temperatures in the tropical Pacific Ocean now exceed El Niño thresholds. However, atmospheric indicators—such as trade winds, cloudiness, pressure patterns and the Southern Oscillation Index (SOI)—have yet to show consistent or sustained signs of El Niño. This clearly indicates that the tropical ocean and atmosphere are not currently reinforcing each other and remain 'uncoupled'. This coupling is required to not only fully develop and sustain an El Niño but is what drives widespread Australian and global weather and climate impacts.

International climate models predict sea surface temperatures to remain above El Niño levels in the coming months. By February, two of the eight surveyed models dip just below El Niño thresholds. El Niño effects in Australia over summer typically include higher fire risk, greater chance of heatwaves, and fewer tropical cyclones.

The positive IOD event, which began in early September, persists. However, model outlooks suggest it will decay by early summer, consistent with its natural decay cycle. A positive IOD during spring typically increases the chance of below-average rainfall for southern and central Australia. The IOD typically has little influence on Australian climate from December to April.

Cloudiness near the Date Line was mostly above average (negative OLR anomalies) during the last fortnight, with values hovering close to the long-term average in recent days. However, cloudiness has been below average since mid-September. While decreased cloudiness near the Date Line is typically a signal seen during La Niña, the broader pattern across the tropical Pacific is consistent with neutral ENSO. Again, this indicates that coupling of the ocean and atmosphere has yet to occur.

Equatorial cloudiness near the Date Line typically increases during El Niño (negative OLR anomalies) and decreases during La Niña (positive OLR anomalies).

Trade winds for the five days ending 18 November were close to average across the tropical Pacific. This also suggests that the atmosphere and ocean are not yet reinforcing each other, which is required for an event to become firmly established. Weaker-than-normal trade winds have appeared at times during the past month, but have been associated with transient events, such as pulses of the Madden–Julian Oscillation.

During El Niño there is a sustained weakening, or even reversal, of the trade winds across much of the tropical Pacific. Conversely, during La Niña, there is a sustained strengthening of the trade winds.

All surveyed climate models predict sea surface temperatures (SSTs) across the central tropical Pacific Ocean will be above El Niño thresholds during December,  and continue to exceed El Niño thresholds until at least January. Thereafter, several models indicate a gradual cooling trend will occur, while others maintain values consistent with El Niño until at least mid-autumn. All models predict central equatorial Pacific SSTs will be warmer than usual until at least April 2019. 

El Niño onset during December would be later than usual, although not unprecedented.

Sea surface temperatures (SSTs) for October were warmer than average across the equatorial Pacific Ocean, and much of the tropics north of the equator, and much of the western Pacific both north and south of the equator.

The October values for NINO3 were +0.8 °C, NINO3.4 +0.8 °C, and NINO4 +0.9 °C. NINO3 and NINO3.4 have both warmed by half a degree compared to September.

The 30-day Southern Oscillation Index (SOI) to 18 November was -1.3, and the 90-day SOI was −2.3. The SOI has remained within the neutral ENSO range since early September, when it briefly reached El Niño thresholds.

Sustained negative values of the SOI below −7 typically indicate El Niño while sustained positive values above +7 typically indicate La Niña. Values between +7 and −7 generally indicate neutral conditions.

The lack of a clear, sustained signal in the SOI is one of several indicators (see Trade Winds and Cloudiness) that show the atmospheric circulation required to signal the start of an El Niño event, to reinforce and sustain an El Niño, and to produce widespread Australian and global impacts, is not yet present.

A positive Indian Ocean Dipole (IOD) has been underway since early September. The latest weekly index value to 18 November was +0.43 °C, just above the threshold value of +0.40 °C. Given the time of the year, it is likely that this event is near its end, and that values will decline further over the coming weeks.

All but one of the six international climate models surveyed by the Bureau indicate that the positive IOD event will breakdown during December; this would be in line with the typical seasonal pattern of the IOD. Due to the movement of the monsoon trough in the Indian Ocean, the IOD typically has little influence on Australian climate from December to April. When the monsoon trough shifts southwards into the southern hemisphere, it changes the broadscale wind patterns, meaning that the IOD pattern is unable to form.

A positive IOD event typically reduces spring rainfall over much of northern, central and southern Australia, and can exacerbate any potential El Niño-driven rainfall deficiencies.

The four-month sequence of sub-surface temperature anomalies (to October) shows warm anomalies in the sub-surface have increased significantly compared to the preceding months. A large pool of warmer than average water extends across the sub-surface of the equatorial Pacific, between about 150°E to about 100°W. Parts of this region are more than 3 degrees warmer than average.

Temperatures for the five days ending 18 November show warmer than average waters in the top 150-200 m of the sub-surface of the equatorial Pacific. A large volume of the sub-surface is more than 3 degrees warmer than average, with waters in part of the shallow eastern Pacific sub-surface more than 4 degrees warmer than average.

The magnitude and volume of the warm anomalies has remained near-constant in the past fortnight. Since mid-September, eastwards movement of the warm anomalies has been apparent. The eastward shift of anomalously warm water is a typical precursor of El Niño.