Water plays a major role for our planet not only in its liquid form at the surface. In the atmosphere, it also affects our lives considerably in terms of weather and climate. Clouds and rainfall are only two examples, water vapour plays a prominent role on Earth as well: It's the most important greenhouse gas in the atmosphere, without it the Earth would be completely frozen. For climate variations, water vapour is particularly important in the stratosphere at altitudes between 15 and 50 kilometers. How much of the gas actually reaches the stratosphere mainly depends on the temperature at the transition between the lowest atmospheric layer, the troposphere, and the overlying stratosphere. This boundary layer is called the tropopause.
Scientists of the GEOMAR Helmholtz Centre for Ocean Research Kiel, together with a colleague from Bergen, were able to demonstrate that natural fluctuations in water temperatures of the Pacific - which occur on decadal timescales - are directly related to the temperature of the tropical tropopause. "It has long been thought that human influences already affected the tropopause. However, it seems that natural variability is still the dominating factor," says Dr. Wuke Wang from GEOMAR, lead author of the recent publication.
The model simulations show that the fluctuations in water temperatures also affect the wind systems over the tropical and subtropical Pacific. This in turn alters the air transport between the lower and upper layers of the troposphere, ultimately regulating the temperatures at the boundary to the stratosphere. "We were now able to demonstrate these relationships for the first time," said Dr. Wang.
So the current study contradicts earlier hypotheses about the temperature variability of the tropical tropopause. As early as in the late 20th century, scientists had seen a cooling trend which began in the 1970s. They traced this observation back to anthropogenic causes, in particular the increase in greenhouse gases. "However, this assumption was based on a rather patchy data base and simplified climate models. Our study shows that the cooling of the tropical tropopause does not have to be a one-way street but could also be part of a natural fluctuation which extends over several decades," Prof. Matthes emphasised.
This knowledge could be of paramount importance for climate research the general. The temperature of the tropopause decides on the input of water vapour into the stratosphere: The higher the water vapour content in the stratosphere, the higher the increase in surface temperatures. Anthropogenic climate change also has an effect on the temperature of the tropopause, and this effect could become more evident in the coming decades. "Only if we can clearly distinguish natural variability from anthropogenic influences, we can make reliable forecasts for the future development of our climate," Prof. Matthes summarises.
(© GEOMAR Helmholtz Centre for Ocean Research Kiel, AcademiaNet)