Scientists Discover Alarming Deep Ocean Trends Under Global Warming: What It Means for Our Future

Changing Ocean Currents Under Global Warming

The ocean’s movements are complex, driven by heat, momentum, and freshwater exchanges at the surface. Recent decades have seen a rapid increase in greenhouse gases, leading to significant alterations in these exchanges. Understanding how ocean currents respond to global warming is crucial for predicting heat uptake, carbon sequestration, and future climate impacts.

Due to sparse ocean observations and coarse climate models, our knowledge of anthropogenic changes has been limited to large-scale circulations like gyres and equatorial currents. Recent studies, however, show a rise in kinetic energy (KE) of these circulations in the upper layers, despite debates about the causes.

Mesoscale eddies, smaller currents forming a major fraction of the ocean’s KE spectrum, have been overlooked. Satellite data over the past three decades shows increased surface eddy activity, particularly in regions like the Western Boundary Currents (WBC) and Antarctic Circumpolar Current.

However, it’s unclear if surface intensification of eddies indicates total KE change in the global ocean under warming. Climate models suggest future changes in total ocean KE, driven by large-scale circulations and mesoscale eddies, using high-resolution simulations with the Community Earth System Model (CESM-H).

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Mesoscale Eddies and Their Impact

Changes in the total ocean KE are influenced by both mean KE (MKE) and eddy KE (EKE). In high-KE regions, trends in MKE and EKE are generally in sync, as stronger large-scale circulations lead to more energetic eddies.

In low-KE regions, trends diverge. The MKE trend shows patches of positive and negative values, while the EKE trend is generally negative, contributing to a widespread decrease in KE in basin interiors.

The complex dynamics governing regional changes in ocean currents could mask global responses to greenhouse gas forcing. For instance, increased KE in the Southern Hemisphere’s subtropical WBCs may result from intensified westerlies.

• The slowdown of the Atlantic Meridional Overturning Circulation could decrease KE in the Atlantic, especially along the Gulf Stream.
• Weakening of the Indonesian Throughflow may reduce KE in the Indian Ocean.
• Shifts in the Antarctic Circumpolar Current may cause alternating zonal bands of KE trends.

Future KE Trends and Model Projections

Climate models project a significant positive trend in globally integrated KE in the upper 200 meters, driven by accelerated large-scale ocean circulations. However, the trend in surface EKE increases but attenuates rapidly with depth, remaining almost unchanged when integrated over the upper 200 meters.

Below 200 meters, a negative KE trend is primarily due to reduced EKE, overwhelming upper ocean increases and causing a decline in globally integrated KE. This highlights deep-ocean eddies’ critical role in global KE response to warming.

State-of-the-art climate models provide insights into future total ocean KE changes, suggesting high-resolution models like CESM-H are superior in projecting changes over low-resolution models, which often misrepresent total KE changes.

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Observations and reanalysis data affirm these models’ projections, but uncertainties remain due to the short observational records and the need for improved deep-ocean monitoring and adaptive strategies.

Mesoscale Eddies and Energy Dynamics

To understand the weakening of mesoscale eddies under global warming, an EKE budget analysis decomposes EKE changes into dynamics like barotropic and baroclinic energy conversions, surface wind power, and dissipation. For the mean state, EKE is balanced between energy sources and sinks.

Under warming, baroclinic energy conversion decreases significantly, driving the reduced EKE. This leads to a new quasi-equilibrium state with decreased dissipation. Other dynamics like barotropic conversion and wind power play minor roles.

Enhanced vertical stratification, a robust feature under warming, weakens the baroclinic pathway, reducing EKE by diminishing the mean available potential energy (MAPE). This is consistent across observations, reanalysis, and climate simulations.

The overall decrease in MAPE under warming aligns with low-resolution climate models, supporting the robustness of CESM-H results. However, local EKE trends can vary significantly, suggesting complex dynamics that warrant further investigation.

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