The earth's climate is primarily driven by energy from the sun. Most of the sun's energy is reflected back into space, but some is trapped by gases in the atmosphere and retained by the sea, air and land.
Winds and ocean currents redistribute the heat and moisture around the globe, creating climate zones. Local oceanic, atmospheric, and temperature phenomena within these climate zones generate the weather that we experience on a localised basis across the globe.
Here in Australia, two of the key drivers of local weather and climate variability are the Indian Ocean Dipole (IOD) and the El Nino Southern Oscillation (ENSO).
The IOD, also known as the Indian Nino, is an irregular oscillation of sea-surface temperatures in which the tropical eastern Indian Ocean becomes alternately warmer and then colder than the western part of the ocean.
While the IOD looks at water temperatures in the Indian Ocean, the ENSO looks at water temperatures in the Pacific Ocean and influences the climate of much of the tropics and subtropics. The cooling phase of ENSO is known as La Nina, and the warming phase is known as El Nino.
So, what are the sea surface temperatures doing at the moment? The Pacific Ocean is currently near El Nino thresholds, and the IOD is in positive territory - both suggesting dryer than average conditions for the balance of winter and into spring.
The Bureau of Meteorology is calling the current ENSO status as El Nino watch. However, the BOM is forecasting it to shift to a more neutral outlook in coming months. The IOD is currently sitting in positive territory, and according to the Bureau, this is likely to persist and dominate the weather patterns across Australia into the spring.
Nevertheless, looking at the change in equatorial sea surface temperatures in June, the trend is encouraging. Cooling is evident in the eastern Pacific, and western Indian Oceans and the required warming is evident to the north of Australia in the western Pacific and eastern Indian Oceans.
The burning question here is, will it persist long enough to change the spring rainfall outlook?
Two or three weeks of encouraging data does not represent a sustained trend. It needs to persist for at least a couple of months before a wide-scale change in local weather patterns is likely.
Despite recent precipitation events, year to date rainfall registrations across almost all of Australia's cropping regions are still below average. In some parts of NSW and Queensland, the rain gauge has rarely been bothered in the last 18 months.
The crop may be in the ground through southern NSW, Victoria, South Australia and Western Australia but many areas will require above average winter and spring rainfall just to achieve average yields.
Continued warming off the Western Australian coast and cooling in the eastern Pacific Ocean are critical ingredients for a long-term change to more normal, or even wetter than normal, precipitation events in Australia for the balance of 2019.
Let's hope recent changes become a long-term trend that manifests itself in a spring conducive to above average winter crop yields, and enough early spring rain in the summer cropping regions to get a wide-scale sorghum plant.
