New Study Describes Link Between South Atlantic Ocean and Global Rainfall Variability
By Edward Pritchard, AOML
In a recent paper published in the Journal of Climate, scientists with NOAA and the University of Miami have identified how variability in ocean circulation in the South Atlantic Ocean may influence global rainfall and climate patterns. The study by researchers at NOAA’s Atlantic Oceanographic and Meteorological Laboratory (AOML) and the Cooperative Institute for Marine and Atmospheric Studies (CIMAS) suggests that the South Atlantic is a potential predictor of global rainfall variability with a lead-time of approximately 20 years. This link between the South Atlantic Ocean and weather and climate could provide significant long-term insight for water management on a global scale.
The majority of efforts to understand the dynamics of the Atlantic circulation system and its climate impact have focused on the North Atlantic. In recent years, the research community began investigating and assessing the South Atlantic and its role in climate. The circulation in this region is unique, in that it is the only major ocean basin that transports heat from the pole towards the equator, a dynamic ocean process that is important to the global distribution of energy.
The study illustrates that multi-decadal variability in South Atlantic circulation is an important moderator of global atmospheric circulation by influencing the transfer of momentum, heat, and moisture across hemispheres. This link is based on the movement of heat in the South Atlantic known as the South Atlantic Meridional Heat Transport. The study indicates that a weaker transport yields cooler sea surface temperatures in the South Atlantic about 20 years later. This impacts global rainfall patterns, including seasonal monsoons, by driving atmospheric heat from the Northern Hemisphere to the Southern Hemisphere and moisture from the Southern Hemisphere to the Northern Hemisphere.
An illustration of the role of a weaker-than-normal South Atlantic Meridional Heat Transport in the atmospheric circulation at 20 years lead-time. This results in a cooler than normal South Atlantic Ocean, which produces an anomalous Hadley circulation labeled by counterclockwise circulation. The lower branch of this circulation (red arrow) brings warm and moist air from the Southern Hemisphere to the Northern Hemisphere. The moist air brought in produces more precipitation in the Northern Hemisphere, thus enhancing monsoons. Image credit: NOAA
Changes in monsoonal rainfall have vital socioeconomic impacts as the majority of the world’s population lives in countries that rely on the regular return of the monsoon rains. These countries often have semi-arid climates that are highly vulnerable to climate variability and change, particularly in terms of water availability and food security. From the time researchers monitor changes in the South Atlantic, it will take 20 years, according to the study, to see a response in monsoonal precipitation. This 20-year predictability could provide vital information for governments seeking to allocate resources accordingly.
As the seasonal march of the monsoons dominates the global circulation, the influence of monsoons reaches far beyond their immediate geographical domain. The anomalous circulation pattern associated with variability in the South Atlantic also has great implications for long-term climate variability over the whole globe. For example, it could bring drier and warmer summer conditions over North America and Europe, exacerbating droughts. It is, therefore, imperative to predict future changes in the monsoon with as much skill as possible. Although some monsoon regions feature abundant rains, the strong seasonality in rainfall means that understanding the timing, duration and intensity of monsoons is vital. The results presented in this study also highlight the need and value of sustained ocean observational efforts, which are necessary to improve our knowledge of the complex interaction between the South Atlantic Ocean and global climate variability and monsoons.