Global scheme of the thermohaline current
Reviewing the global maps of world precipitation we can observe most of the tropical rainfall they occur in the northern hemisphere. Palmyra Atoll, at a latitude of 6 degrees north, receives around 445 cm of rain per year, while another place, located at the same latitude south of the equator, receives only 114 cm.
Scientists believed it was a quirk of the geometry of planet earth, because the ocean basins tilted diagonally as the planet rotates, pushing bands of tropical rain north of the equator. But a new study from the University of Washington shows us that the reason has more to do with the ocean currents generated at the poles, thousands of kilometers away.
The article, published on October 20 in Nature, explains one of the main characteristics of the planetary climate, and shows that the icy waters of the poles affect the seasonal rains, crucial for the growth of cereals in places like the African Sahel region and southern India.
In general, the hottest areas are more humid because the hot air rises rapidly and the water it contains precipitates.
This rain occurs more in the northern hemisphere because it is warmer. The question is, what makes the Northern Hemisphere hotter? And it has been observed that it is due to oceanic circulation.
The directors of this study (Frierson among others) used detailed measurements of the satellites of the Radiant Energy System of the Earth and the Clouds of NASA (CERES), to observe that the sunlight supposes a greater contribution of heat to the southern hemisphere In this way, if we only took into account atmospheric radiation, the southern hemisphere should be the most humid.
Using observations to determine ocean heat transport and using computer models to show the key role of the great thermohaline current that sinks near Greenland, travels along the ocean floor to Antarctica, then rises to the surface and drifts northward. If we were to eliminate this current, the bands of tropical rain would remain in the southern hemisphere.
This is because when water circulates north for decades it gradually warms up, displacing around 400 billion Watts of power from the southern hemisphere to the north across the equator.
For many years, the slope of the ocean floor has been the accepted reason for asymmetry in tropical storms. But at the same time, many researchers never took this explanation as valid as it is quite a complicated argument and usually for global behaviors like this, there is a simpler explanation.
The current that they determined as responsible was made known to the general public in the film "The day after tomorrow", in which the premise was that this current called thermohaline circulation stops freezing New York. A total and sudden disappearance is not expected as in the film, but a gradual decrease is expected, reported by the United Nations and expected for the year 2100, this could change the tropical rains to the south, as it seems to be indicated by the geological record that happened in the past.
The slowdown of currents is predicted as follows: when there is an increase in rainfall, fresh water, and falling on the North Atlantic would decrease the density of oceanic water, this being less dense it would be less likely to sink.
This is just another of the vast amount of evidence that has appeared in the last 10-15 years showing how important high latitudes are to the rest of the world.
Frierson's previous works show how the change in the temperature balance between the hemispheres influences tropical rains. A recent study by him and his collaborators looks at how pollution from the Industrial Revolution blocked sunlight from the northern hemisphere during the 70s and 80s and reversed tropical rains to the south.
A lot of changes in the recent past have been due to pollution. The future will depend on air pollution and global warming, as well as changes in the circulation of the oceans. All of these factors make tropical rains very difficult to predict.
More information: The XNUMXth tropical depression of the season forms off the Gulf of Mexico, Has global warming stopped forever?