Last week, we discussed the science behind gap forecasting, namely why it’s possible to forecast time horizons beyond weather forecasts. We discussed why weather forecasts can only forecast day-to-day fluctuations up to 14 days into the future with any skill, and how fundamental characteristics of the ocean as a geophysical fluid enable longer term forecasts. But using the power of the ocean for forecasting means that there needs to be some way for the ocean to communicate with land regions, some of which are quite distant from the ocean (e.g. the interior of North America or Eurasia). How does this happen?
The answer is the Earth system’s version of ‘action from afar’, scientifically known as a teleconnection. So, what is a teleconnection? Simply put, a teleconnection links distant regions of the Earth system together through the propagation of energy. Energy propagation through the Earth system occurs as it does through any fluid: the movement of waves from the source of a disturbance, like ripples moving outwards after a stone is thrown into a pond. In this case, the disturbance in the Earth system is not a stone, but a persisting energetic disturbance. Since the ocean has so much more heat capacity than the atmosphere, a temperature anomaly in the upper ocean constitutes an enormous, persisting energetic source (or sink) that creates disturbances that propagate enormous distances over the planet.
How the Atmosphere and Ocean Interact
How do the atmosphere and ocean exchange information? This occurs through three primary means: the transfer of heat, the transfer of freshwater, and the transfer of momentum. Momentum is usually a one-way street with the atmosphere calling the shots: the wind blows on the ocean surface, momentum transfers to surface waters, and the ocean moves in response. But heat and freshwater are more complicated, with transfer moving in both directions at the atmosphere-ocean interface. The atmosphere can either heat or cool ocean surface temperatures, and can either freshen or salinize ocean surface salinity. But the ocean can also drive the atmosphere. In particular, warm ocean temperature anomalies are energy sources, and heat and humidify the atmosphere above; similarly, cold ocean temperature anomalies are energy sinks, and cool and dry the atmosphere above. Taken together, these processes alter the atmosphere’s energy: warm and moist air is much more energetic than cool and dry air. And energy anomalies like this create waves, which can propagate around the planet, producing teleconnections, action from afar.
Teleconnections and Wave Propagation
Ocean temperature anomalies can create local energetic sources, or sinks, in the atmosphere, which lead to waves that propagate away. The physics of these waves is governed by the equations that describe shallow fluid flow on a rotating planet, which were first recorded by Carl-Gustav Rossby in the 1920s. We often think of Rossby waves (AKA planetary waves) as moving midlatitude disturbances (meanders) in the jet stream, which create midlatitude weather, usually sequences of cold fronts, warm fronts, and rain through the winter months. But Rossby waves also propagate away from localized ocean temperature anomalies, following paths that roughly form great circles through the atmosphere, connecting the ocean to distant land regions.
Like all waves, Rossby (or planetary) waves transport energy, and like all waves, they require a restoring force. For waves on the surface of the water (following our example of the pebble splashing into the pond), the restoring force is gravity acting on the water; when the water surface is tilted, gravity acts to restore it to its flat ‘at rest’ position. For a Rossby wave moving through the atmosphere, angular momentum conservation provides a similar restoring effect – undulations (i.e. ups and downs) are created by changes in planetary angular momentum (due to the Earth’s rotation) that are counteracted by changes in the air’s own angular momentum. Conservation of angular momentum is evident at all scales in the physical world, from the spin of subatomic particles to the rotation of enormous galaxies. It is this same fundamental physical principle that is at the heart of teleconnections.
The most well-studied example of a Rossby wave excited by ocean temperature anomalies is that created by the El Nino phenomenon in the tropical Pacific. Interactions between the atmosphere and ocean produce El Nino, where an enormous swath of the tropical East Pacific Ocean becomes anomalously warm. This warm ocean temperature anomaly produces a Rossby wave train that creates a temperature dipole over North America – the west side of the continent is warmer than normal while the east side is colder than normal. These patterns are amplified by interactions with the upper atmospheric jet stream, which meanders more strongly because of interactions with this wave. This wave is why El Nino is associated with distinct temperature and precipitation patterns over North America and other parts of the world.
Teleconnections matter for gap forecasting because they allow temperature anomalies in the ocean to propagate to the atmosphere and impact the land surface. Since the ocean is predictable over much longer time horizons than the atmosphere, this communication between ocean, atmosphere, and distant land regions is essential for skilled seasonal, annual, and decadal environmental forecasting.
