Global warming, driven by rising atmospheric greenhouse gases, continues to occur and manifests itself in a variety of ways, with 2024 being the warmest year on record for global surface atmospheric temperatures. More than 90 % of the Earth’s energy imbalance has accumulated in the ocean over the past half century, leading to record highs in temperature and ocean heat content (OHC) year after year. These changes in OHC affect air-sea and ice-sea interactions, and thus have significant impacts on other components of the climate system.

Despite continued warming, it has been difficult to discern meaningful patterns so far. However, when looking at the ocean as zonal averages across latitude bands, striking patterns of change emerge. An analysis of the period from 2000 to 2023, when the availability of reliable data has improved, shows a clear picture. While the ocean is warming almost everywhere, the most significant increases in heat content are observed in mid-latitudes. Specifically, strong warming occurs in regions near 40°N and 40°–45°S. In contrast, only small warming is observed in subtropical regions near 20°N and 25°–30°S. These patterns are most evident in zonally averaged ocean heat content and are also noticeable in sea surface temperatures.

The strongest warming is recorded in the Southern Hemisphere, where aerosol effects are small. Nevertheless, sea surface temperatures (SST) have increased more in the Northern Hemisphere, especially after 2020. This contrast with the Southern Hemisphere results, among other things, from stronger winds, greater mixing, and a deeper mixed layer in the Southern Hemisphere, as well as the much larger extent of the ocean in the Southern Hemisphere.

The patterns of OHC changes are not directly linked to radiation at the top of the atmosphere (TOA). Rather, they appear in net surface energy fluxes and derived ocean heat transport, underscoring their connected (tied) origin. A key role in these changes is played by changes in atmospheric circulation, particularly through the poleward shift of ocean jet streams and storm tracks. These changes in the atmosphere are reflected in the surface wind-driven ocean Ekman transport.

Energy balance analyses show that the combination of TOA radiation and atmospheric energy transports leads to estimates of net surface energy fluxes (Fs). These surface fluxes have a pronounced meridional structure that resembles the pattern of OHC changes. Combining Fs with OHC changes yields the ocean heat divergence (OEDIV), which shows the divergence of heat away from the equator and some subtropics and the convergence of heat toward the midlatitudes.

The meridional ocean heat transport (MHT), derived from OEDIV, clearly shows this redistribution of heat. For example, in the Northern Hemisphere, the anomalous southern MHT peaks from 40° to 50°N, while the anomalous northern MHT from 10° to 35°N combine to contribute to convergence near 40°N. In the Southern Hemisphere, the anomalous southern MHT from the equator to 40°S and the slightly northern anomalous MHT from 40° to 65°S lead to huge convergence of heat near 40°SThis divergence of heat away from the subtropics towards the midlatitudes is evident in both hemispheres and provides a viable explanation for OHC patterns.

Changes in surface wind stress, a key driver of ocean currents, also play a role. Shifts in zonal wind stress lead to anomalous Ekman convergence near 40°S and 50°N, which contributes to changes in OHC and sea level changes. The observed poleward shift of the zonal mean jet stream in both hemispheres, particularly in the Southern Hemisphere, is associated with changes in atmospheric and oceanic circulation.

It is important to note that in addition to human-caused warming, internal natural variabilityFor example, ENSO (El Niño–Southern Oscillation) causes large interannual variability in the deep tropics, although it is less visible in extratropical zonal averages. The Pacific Decadal Oscillation (PDO) also modulates ENSO and may have played a role in SST anomalies in recent years.

In conclusion, the characteristic patterns of global warming, most clearly visible in the zonally averaged OHC, are primarily the result of systematic redistribution of heat from global warming through coupled changes in atmospheric circulation and ocean currentsThese changes have a profound impact on the local climate. Spring

Study published on AMS

Glossary of key terms:

• Ocean heat content (OHC): The amount of heat stored in a given volume of ocean, usually measured at depths of 0-2000 meters. An increase in OHC is a key indicator of global warming because the ocean absorbs most of the excess heat in the climate system.
• Earth Energy Imbalance (EEI): The difference between the amount of solar radiation that the Earth absorbs and the amount of thermal radiation that it radiates back into space. A positive EEI means that the Earth is gaining energy that is stored in the climate system, especially in the oceans.
• Top of Atmosphere (TOA): The upper boundary of the atmosphere (usually around 100 km) where the balance of incoming solar radiation and outgoing terrestrial radiation is measured. TOA radiation is important for understanding the Earth's overall energy balance.
• Sea Surface Temperature (SST): The temperature of the water in the uppermost layer of the ocean. SST is important for the exchange of energy and moisture between the ocean and the atmosphere and influences atmospheric circulation.
• Zonal average: The average of a meteorological or oceanographic variable calculated along a latitudinal band (zonal). Zonal averages help reveal global patterns that are consistent across the entire circumference of the globe.
• El Niño–Southern Oscillation (ENSO): A natural climatic phenomenon in the tropical Pacific Ocean that involves fluctuations in sea surface temperature (El Niño and La Niña) and associated changes in atmospheric pressure and precipitation. ENSO has a significant impact on global climate patterns.
• Meridional heat transport (MHT): North-south heat transport in the oceans (meridional direction). MHT is important for the redistribution of heat from the tropics to higher latitudes and influences regional climate conditions.
• Ekman transport: Water transport in the upper ocean layer driven by surface wind tension. Due to the Coriolis force, Ekman transport is directed perpendicular to the wind tension (to the right in the Northern Hemisphere, to the left in the Southern Hemisphere). Changes in wind tension can affect Ekman transport and consequently heat redistribution and sea level.
• Jet stream: A fast, narrow stream of air high in the atmosphere that influences storm tracks and regional weather. Changes in the position and intensity of a jet stream are associated with changes in atmospheric circulation and energy transport.
• Atmospheric reanalysis: The systematic processing of historical observations using a climate model to create a consistent and global record of the state of the atmosphere. ERA5 is an example of modern atmospheric reanalysis.
• Surface energy fluxes (Fs): The net transfer of energy between the atmosphere and the surface (ocean or land). Fs includes the transfer of radiation, sensible heat, and latent heat and is important for the energy balance of the surface.
• Ocean Heat Divergence/Convergence (OEDIV): A measure of whether heat is being accumulated (convergence) or lost (divergence) in a given ocean region due to ocean currents.
• Subduction: The process by which surface water in the ocean sinks to deeper layers, often occurring in areas of convergence of Ekman transport or in areas of formation of water masses. Subduction can transfer heat and other properties to deeper parts of the ocean.
• Pacific Decadal Oscillation (PDO) and Interdecadal Pacific Oscillation (IPO): Longer-term (decadal) variations in sea surface temperature and associated circulation in the Pacific Ocean, which can modulate the influence of ENSO and influence global climate patterns.