By: Daniel Hodson
The Earth is a world of water – oceans spread out across much of the planet and they exert a profound influence over the climate. Ascending from the Earth, the churning waves and surf shrink away and the oceans relax into seemingly silent, passive bodies of water. But this seeming passivity belies a complex network of currents and flows hidden beneath the surface, driven by heat at the equator flowing to the colder poles, but being frustrated in doing so by the spin of the Earth.
In the Atlantic, an immense flow of water drives northwards towards Greenland and Iceland in the top kilometre of ocean, before plunging down kilometres and returning southwards at depth, towards Antarctica (Figure 1). This is the deep Atlantic Meridional Overturning Circulation (AMOC). This circulation involves such large flows of water that oceanographers had to invent a new unit of measurement to think about the volumes involved: the Sverdrup (Sv) is a million metres cubed per second – that’s a cube of water 100m on a side, flowing past every second. This northward flowing water carries heat with it, sometimes speeding up, sometimes slowing down – bringing more or less heat as it does so, leading to warming or a cooling of the surface of the ocean. This heat can then be carried away by the atmosphere leading to warmer air temperature, or perhaps driving changes in surface wind patterns.
If, whilst orbiting over the Pacific, you tuned your eyes away from the blue of the Pacific and into the infrared, you would see what the satellites see: a vast pattern of warm and cold spread out across the expanse of the Pacific Ocean. Over the years, you would see this pattern pulse warm and then cold; in the semi-regular cycle of El Nino: the heartbeat of the climate system which dominates the tropics.Figure 2: The Pacific Decadal Oscillation pattern
El Nino is driven by complex interactions between the winds blowing over the Pacific Ocean, and the waters sloshing between Asia and the Americas. It leads to a 3-6-year cycle of warming and cooling in the equatorial Pacific Ocean. In the warm phase, large pulses of heat are released from the ocean into the atmosphere, shifting climate patterns leading to droughts and deluges across the globe. Over many decades of watching, a more widespread pattern of warming and cooling emerges across the Pacific – a pattern known as the Pacific Decadal Oscillation (PDO) (Figure 2). The connection between the PDO and El Nino remains to be fully understood.
Both the AMOC and the PDO play a key role in storing and moving heat around; their variations over time, in turn, modulate our climate system, potentially in profound ways. The way these climate features respond to external factors like changing levels of greenhouse gases or industrial pollution may affect the medium-term trajectory of anthropogenic climate change.Figure 3: The Pacific and Atlantic Oceans
Figure 4: The Tropical Walker Circulation
For a long time, it was thought that these two siblings (AMOC and PDO) continued their existence in ignorance of the other; bounded by Africa and Eurasia but divided by the Americas (Figure 3). They may hear distant echoes of each other, mediated by the turbulent Southern Ocean around Antarctica, or the icy Arctic Ocean – but signals in the ocean are ponderous, slow and noisy. New simulations with modern complex climate models suggest that they hear and feel each other’s presence over, rather than around, the wall of the Americas; mediated by the atmosphere. The Walker circulation is the large-scale pattern of ascending and descending air one encounters when travelling around the equator (Figure 4). Air heated and pushed upwards by a warm ocean in one place, must be replaced by descending air elsewhere in the tropics. This circulation seems to allow the two oceans to talk to and influence each other. Climate model simulations  seem to show that, over many decades, a warmer Atlantic can nudge a cooler Pacific Ocean, whilst a Warmer Pacific ocean can lead to a warmer Atlantic.
Whilst we are seeing a clearer picture of how these two oceans coordinate their climate modulations, challenges remain. Many decades of observations are needed to understand the slow influences of these twin oceans – but whilst the 21st-century ocean is well observed, ocean observations before 1950 are much scarcer. Remarkable efforts are underway, however, to utilize the vast datasets buried in old ships logs. We also rely on climate models to tease apart the complex interactions in the climate system. Are the models we use accurate enough? Are we doing the right experiments with these models to understand how these features of climate interact? If we can begin to understand the conversation between these two oceans better, we may be better able to predict their future influences on climate and, in turn, on us.