The Pacific’s “western boundary currents” convey huge quantities of warm water containing salt and nutrients and have a considerable influence on the climate. Although they were discovered long ago, still little is known about these ocean systems. Significant research efforts have been undertaken in the last seven years. An IRD team with Chinese, Australian and American partners recently published a report in Nature magazine on the state of knowledge on these Pacific currents, their variations, their impact and their response to climate change.
The Pacific’s western boundary currents were among the first dynamic systems studied by pioneering oceanographers. However, until now, little was known about their complex structure. Given their significance in ocean-atmosphere interaction, scientists have been redoubling their efforts for the last seven years. Following the SPICE programme, focused on the south-western Pacific, IRD researchers and their Chinese, Australian and American partners are continuing this work within the CLIVAR programme across the whole Pacific. Their objective is to understand exchanges of heat and water masses, possible changes in them as a result of increased greenhouse gas effects, and their impact on climate. They have recently published a report on the current state of knowledge in Nature magazine.
The scientists show that this ocean system functions as a single entity. For example, when a warm El Niño episode occurs, the entire western boundary currents system shifts to higher latitudes. It is also affected by other factors, such as the seasons, local monsoon winds, etc. It consequently shows great variability within seasons, from one year and decade to the next, and indeed over the longer term. In places, such as the Solomon Sea, the amounts of warm, saline water conveyed can double between a cold and a warm El Niño phase.
In return, this system of currents affects the climate in many ways. First of all, flows towards the Indian Ocean, via the Indonesian archipelago, contribute to the world’s thermohaline circulation, the “conveyor belt” spreading heat and regulating the climate across the globe. In addition, it exchanges heat and water masses with the equatorial zone and its famous “warm pool”, a huge reservoir of warm water in the middle of the Pacific. This is the planet’s main “heat pump”, supplying flows of heat and humidity for the majority of the Earth’s atmosphere. Lastly, one branch of the current peels off towards the South Pole. Depending on the intensity of the currents and their spatio-temporal variation in the inter-tropical zone, this system can disrupt the world’s weather quite considerably.
The consequences of these phenomena – cyclone formation, changes to precipitation patterns and the functioning of natural and agricultural ecosystems – strongly affect societies around the world.
With climate change, western boundary currents in the Pacific, as in other oceans, have intensified over the last century. They have extended towards the poles, where they have heated up two to three times more than in the rest of the world’s oceans. How will the climate respond to these fundamental changes? The challenge for researchers now is to find out…