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

Sea surface temperatures (SSTs) for the week ending 27 October remain warmer than average across the western to central equatorial Pacific Ocean, and slightly cooler than average in parts of the east, but overall patterns are consistent with a neutral ENSO state. Most of the northern half of the Pacific Ocean is warmer than average, as well as areas south of 30°S, and much of the western tropics.

Cool anomalies in the eastern tropical Pacific have decreased compared to two weeks ago, but persist in small areas close to the coastline of South America. Weak cool anomalies also remain across waters between Australia and Papua New Guinea, the Arafura Sea, with stronger cool anomalies on the southern side of the Indonesian archipelago. SSTs are more than 2 degrees cooler than average in some areas close to Sumatra, with the general area slightly cooler compared to two weeks ago. Cooler waters in the eastern Indian Ocean typically occur during a positive IOD. Warm anomalies also persist off the Horn of Africa, but have weakened slightly compared to two weeks ago, now reaching between 1 and 2 degrees warmer than average. Consequently, very strong positive values of the IOD index also continue.

The latest values of the three key NINO indices in the tropical Pacific for the week ending 27 October are: NINO3 +0.4 °C, NINO3.4 +0.7 °C and NINO4 +1.0 °C. All three NINO indices have warmed compared to two weeks ago.

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.

The strong positive Indian Ocean Dipole (IOD) event continues, while the El Niño–Southern Oscillation (ENSO) remains neutral.

The Indian Ocean Dipole (IOD) index remains strongly positive, with the latest weekly value at +2.06 °C, well above the positive IOD threshold of +0.4 °C. Strong easterly trade winds across the tropical Indian Ocean have maintained cooler than average waters in the eastern Indian Ocean and warmer than average waters off the Horn of Africa.

Forecasts show an easing in the easterly trade winds in the coming weeks, suggesting the positive IOD may be near its peak. However, international climate models surveyed by the Bureau indicate the positive IOD is so strong that it is likely to take several weeks to decline and could persist into mid-summer. The slow decline is also due to the strong IOD slowing the movement of the monsoon into the southern hemisphere. Typically, it is the movement of the monsoon over Indonesia during December that ends IOD events.

A positive IOD typically brings below average spring rainfall to southern and central Australia, with warmer days for the southern two-thirds of the country. Positive IOD events are often associated with a more severe fire season for southeast Australia.

In the tropical Pacific Ocean, waters have warmed over the past month but have caused little change in the atmosphere, and hence the El Niño–Southern Oscillation (ENSO) remains neutral. The latest 30-day Southern Oscillation Index (SOI) is –5.2, returning it to more neutral levels.

The majority of climate models forecast neutral ENSO for the remainder of 2019 and into the first quarter of 2020. When ENSO is neutral, it has little effect on Australian and global climate, meaning other influences are more likely to dominate.

Cloudiness near the Date Line has fluctuated around average values since late April, consistent with neutral ENSO. However, cloudiness near the Date Line has generally remained below average since early to mid-September.

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

Cloudiness in the eastern tropical Indian Ocean has been well below average, while cloudiness is well above average in the western Indian Ocean. This indicates a mature and strong positive Indian Ocean Dipole.

Trade winds for the 5 days ending 27 October were close to average across the tropical Pacific.

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.

In the Indian Ocean, trade winds have been much stronger than average, typical of a positive IOD phase.

All eight surveyed international climate models indicate central tropical Pacific sea surface temperatures in the NINO3.4 region will remain at ENSO-neutral levels into early 2020.

One model briefly reaches El Niño thresholds for November, before dropping back to neutral values. One other model indicates that values may move towards La Niña thresholds over summer, but does not reach the threshold until March. The remaining six models are all maintain NINO3.4 values within the neutral range throughout the outlook period.

Sea surface temperatures (SSTs) for September were warmer than average across the western tropical Pacific Ocean, near-average for the central tropical Pacific, and cooler than average in some parts of the eastern equatorial Pacific, extending into the South Pacific close to South America.

SSTs were warmer than average in areas around eastern Australia and far southwestern Australia. SSTs were mostly close to average around the rest of Australia, though there were small areas of cool anomalies in some areas to the north of Australia.

The September values for NINO3 were −0.1 °C, NINO3.4 0.0 °C, and NINO4 +0.7 °C. NINO3 and NINO3.4 cooled compared to August, while NINO4 held steady.

The 30-day Southern Oscillation Index (SOI) for the 30 days ending 27 October was −5.2. The 90-day value was −7.8. Atmospheric pressure remains high over Darwin, but has lessened compared to recent weeks, resulting a return to SOI values within the ENSO-neutral range.

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 Indian Ocean Dipole (IOD) remains strongly positive, with the latest weekly index value to 27 October +2.06 °C. This is the second highest weekly value observed in the Bureau's dataset, behind +2.15 °C observed two weeks ago. The Bureau's dataset extends from 2001 to present. Before the current event, the previous highest value was +1.48 °C for the week ending 5 November 2006. Monthly values from a different dataset suggest the current event is the strongest positive IOD since 1997.

The very strong positive values of the IOD are due to the strong temperature gradient across the Indian Ocean. SSTs to the south of the Indonesian islands of Sumatra remain more than 2 degrees cooler than average for the week ending 27 October, while warm anomalies close to the Horn of Africa reach between 1 and 2 degrees warmer than average.

The overall pattern of sea surface temperatures has remained generally consistent with a positive IOD pattern since late May.

All six international climate models surveyed by the Bureau indicate the IOD will remain positive into December.

The retreat of the Southwest Indian Monsoon was very slow this year, and models are indicating a slower break down of the IOD than usual. However, it remains unlikely that the positive IOD will persist into the second half of summer. IOD events dissipate as the monsoon trough moves into the southern hemisphere, which changes the broadscale wind patterns over the IOD region and returns sea surface temperatures to near average.

By January four of six models are close to or well below the threshold value, indicating this return to neutral.

A positive IOD often results in below average winter–spring rainfall over southern and central Australia. It also typically means warmer than average winter–spring days for the southern two-thirds of Australia.

The four-month sequence of equatorial sub-surface temperature anomalies (to 24 October) shows cool anomalies in the western equatorial Pacific, at a depth of around 100 to 200 m, and in the shallow sub-surface east of 110°W between 0 to 100 m depth. Weak warm anomalies extend across most of the top 200 m in most of the remainder of the equatoiral Pacific.

Warm anomalies in the central Pacific have intensified in October, compared to September and August, while cool anomalies in the east and west have weakened. Warm anomalies reach up to 2.5 degrees warmer than average in small regions.

For the five days ending 27 October water temperatures were close to average across much of the sub-surface of the equatorial Pacific. Anomalies are generally slightly warmer than average across the top 100 m, and slightly cooler than average between 100 and 300 m depth.