The Atlantic Multidecadal Oscillation (AMO)
The Atlantic Multidecadal Oscillation (AMO)Prioritize...
When you're finished with this page, you should be able to:
- define the AMO and its key characteristics and phases.
- explain the different hypotheses about what drives the AMO, including its potential links to the AMOC and external factors like aerosols and volcanic eruptions.
Read...
Let’s explore another major climate oscillation, this time in the Atlantic Ocean: the Atlantic Multidecadal Oscillation (AMO). The AMO represents a long-term pattern of sea surface temperature (SST) changes across the North Atlantic, with cycles that span 60-80 years. Like the PDO, the AMO has two phases: positive and negative. During the positive phase, SSTs across the North Atlantic—particularly in the subpolar regions near Greenland and the Labrador Sea—experience a noticeable warming. See all the red area I've circled in green in the figure below! In the negative phase, these same regions cool down -- like with the PDO, just imagine all of the colors "flipping" and you have a map of the negative phase.
Because the AMO covers such a large area and lasts for decades, it has significant impacts on both global and regional climate patterns. You can even spot the AMO’s influence in the global average temperature record. For example, during the early 1900s, a colder-than-usual period aligns with the AMO’s negative phase (below timeseries). Regionally, shifts in the AMO affect important features like the Atlantic Intertropical Convergence Zone, the North Atlantic jet stream, and the storm tracks that guide weather systems across North America and Western Europe. It also influences rainfall patterns during Africa’s monsoon season and plays a role in Atlantic hurricane activity. When the AMO is in its positive phase, the tropical Atlantic tends to be warmer, which is linked to more intense and frequent hurricanes.
The AMO index throughout the past century. The AMO index is calculated as SST anomalies averaged over the North Atlantic (northward of the equator). Red indicates the positive phase of the AMO, whereas blue represents the negative phase of the AMO.
Beyond its effects on weather and climate, the AMO also impacts marine life. Changes in ocean temperature associated with the AMO are thought to affect fish populations, such as the eel population in the Gulf of Maine, showing how far-reaching and interconnected these climate cycles can be.
Because observations of the North Atlantic Ocean have been quite limited[1], the available sea surface temperature (SST) records only capture about two full cycles of the Atlantic Multidecadal Oscillation (AMO). And even with this data, we still don’t fully understand the processes driving these temperature changes. In fact, there’s ongoing debate about the very nature of the AMO! Some scientists question whether the AMO is a true oscillation at all, or if it’s just a form of random low-frequency variability -- essentially just "static" like you'd get on an AM radio station. This uncertainty has led some to suggest using the term Atlantic Multidecadal Variability (AMV) instead, reflecting the idea that the SST changes we observe could simply be a response to factors like human-made aerosols or volcanic eruptions over the past century.
On the other hand, those who argue that the AMO is a physical oscillation often point to climate model-based results that show links between the AMO and the Atlantic Meridional Overturning Circulation (AMOC) we talked about in the last lesson. Remember, the AMOC is a vast system of ocean currents that transports heat from the equator toward the poles. When the AMOC speeds up, it carries more heat into the North Atlantic, leading to widespread warming across the basin. Since the AMOC moves slowly, its effects are felt over decadal to multidecadal timescales, aligning with the timing of the AMO phases.
However, the AMOC may not be the only factor driving the AMO. More recent research suggests that other components, like atmospheric circulation patterns (such as the North Atlantic Oscillation, which we’ll discuss next), as well as changes in the radiative properties of the atmosphere, freshwater fluxes, and even sea ice, could all play a role in shaping the AMO’s behavior.
Note [1]
It is noteworthy that the North Atlantic is already the most well sampled ocean basin. However, the observational records (back to the 1900s) of the North Atlantic, even for surface variables like SST, barely capture one or two cycles of the AMO/AMV. Without observational records, we can only infer the AMO mechanisms from numerical model simulation, which itself contains a range of sources of uncertainty. Thus, to date, the causes of AMO remain an open question without a definitive answer!
Read on.