Over the past few decades, the Earth’s tropical belt has expanded slowly but steadily – an average of around 60 kilometers per decade. The mechanism behind this development has so far remained unclear. Researchers have now found out that the warming of the oceans plays a decisive role in the extent of the tropics: wherever vortices and temperature gradients in the oceans have shifted a bit towards the poles, the tropical belts also expanded more. This relationship was shown in a comparison of measured values, but could also be reconstructed in a climate-ocean model.
The tropics are the warmest region on earth and stretch like a wide belt along the equator. In the center of this zone, known as the intertropical convergence, warm air rises and forms cumulus clouds, which empty in tropical showers and thunderstorms. This ensures the typical warm and humid tropical weather. At the top, the air that has risen flows north and south and then sinks in a strip on both sides of the equator as dry, cooler air. It thus shapes the dry subtropics. “In the last 15 years, numerous data have shown that the boundaries of the tropics and the adjacent subtropics are shifting towards higher latitudes with climate change,” report Hu Yang from the Alfred Wegener Institute and Helmholz Center for Polar and Marine Research in Bremerhaven his colleagues. According to satellite data, the tropical belt is expanding by 0.25 to 0.5 degrees of latitude per decade, depending on the region.
Ocean as a driving force?
But what drives this tropical expansion is still a matter of dispute. A connection with climate change is obvious, but whether and how global warming affects atmospheric circulation is less easy to determine. The shift could be due to changes in radiation levels or direct effects of greenhouse gases, but also to changes in aerosols or stratospheric ozone concentrations, as the researchers explain. However, if these factors are included in climate models, they can only partially reproduce the measured shifts in the climate zones. These models also do not capture the regional differences in the extent of the tropical belt. To create more clarity, Yang and his colleagues have now taken a closer look at the oceans. For their study, based on measured data on sea temperatures, they investigated whether and how the position of oceanic currents and temperature zones has changed since 1982. They put these data in relation to the regional shifts in the tropical belt.
The evaluations showed that the extent of the tropical belt extension is closely linked to the processes in the sea. Wherever the water of the subtropical seas has warmed up more strongly, currents and ocean eddies have shifted further towards the poles. As a result, the air pressure conditions and winds changed in these regions – and with them the outer boundary of the intertropical convergence zone. “The polar shift of the oceanic meridional temperature gradient corresponds to widespread wind anomalies in the area between the 20th and 45th parallel,” report Yang and his colleagues. “This pattern suggests that the shift in the oceanic gradients is closely linked to a shift towards the pole of the winds and thus also to the expansion of the tropical belt.”
Independent of natural fluctuations
According to the scientists, the warming of the oceans is one of the driving forces behind the shift in climate zones. This could also explain why the outer border of the tropics has shifted more towards the poles in the southern hemisphere than in the northern hemisphere: In the south, the proportion and thus the influence of the oceans is significantly greater. The question remains, however, whether these changes in the oceans are of natural origin or are due to anthropogenic climate change. It is known that climatic phenomena such as El Nino or the Pacific Decadal Oscillation (PDO) also significantly shift water temperatures, currents and winds. “However, the negative phase of PDO manifests itself only in the Pacific, while subtropical warming occurs in all sea basins on both hemispheres,” say the researchers. Measurement data and models also show that the changes in the oceanic temperature gradients persisted even in the phases in which these natural climatic fluctuations worked in the opposite direction.
To support their conclusions, Yang and his colleagues performed additional model simulations. In these ocean-climate models, they only changed the sea temperatures and the position of the gradients, but lowered the global air temperatures by 0.1 Kelvin instead of depicting their warming. “The expansion of the tropics continues despite this global cooling,” the researchers report. “This shows how strongly the oceanic temperature gradient influences the atmospheric circulation.” Overall, they conclude from their results that, in addition to the natural fluctuations in sea conditions due to phenomena such as El Nino or the PDO, there is a continuous trend, caused by climate change, towards the warming of the oceans and a shift in the gradients. “The increased subtropical ocean warming is independent of the natural climate oscillations,” says Yang. “It is the result of global warming.” And the warmer sea water of the subtropics, in turn, is the driving force behind the expansion of the tropical belt.
Source: Hu Yang (Alfred Wegener Institute, Helmholz Center for Polar and Marine Research, Bremerhaven) et al., Journal of Geophysical Research Atmospheres, doi: 10.1029 / 2020JD033158