Global sea surface temperatures have apparently peaked at a new record high of 21.2 Celsius (70ºF). Cooler than normal water has started welling up in the eastern Pacific ocean likely indicating the beginning of a switch from warm El Niño to cool La Niña. The switch form El Niño to La Niña is good news for global temperatures because La Niña brings on increased upwelling of cold water stored below the warm surface layers of the Pacific ocean. It should bring some relief by late this year from the rapidly rising global temperatures that we have observed for the past year. However, climate forecast models and climatology show that it is not likely to bring relief to the tropical north Atlantic ocean which is experiencing midsummer sea surface temperatures in mid-March when they should be at or near annual lows.
Global sea surface temperatures peaked in the second week of March, 2024 at a record high of 21.2 C
2023 brought record shattering ocean heat and sea surface temperatures to the global oceans and the tropical Atlantic ocean was an extreme high point. New Record Ocean Temperatures and Related Climate Indicators in 2023
The global physical and biogeochemical environment has been substantially altered in response to increased atmospheric greenhouse gases from human activities. In 2023, the sea surface temperature (SST) and upper 2000 m ocean heat content (OHC) reached record highs. The 0–2000 m OHC in 2023 exceeded that of 2022 by 15 ± 10 ZJ (1 Zetta Joules = 1021 Joules) (updated IAP/CAS data); 9 ± 5 ZJ (NCEI/NOAA data). The Tropical Atlantic Ocean, the Mediterranean Sea, and southern oceans recorded their highest OHC observed since the 1950s. Associated with the onset of a strong El Niño, the global SST reached its record high in 2023 with an annual mean of ∼0.23°C higher than 2022 and an astounding > 0.3°C above 2022 values for the second half of 2023. The density stratification and spatial temperature inhomogeneity indexes reached their highest values in 2023.
3-dimensional fields of oceanic temperature changes in 2023 relative to 2022 in the North Atlantic Ocean
There is some very good news despite the rapid warming of the tropical Atlantic shown in the 3-D graphic above. Cold, relatively fresh water has flowed out of the subpolar north Atlantic into the oceanic region offshore of New England and mid-Atlantic. This cooling and freshening will likely give some relief to the ocean heat waves that have happened there over the past decade. Moreover, this rejuvenation of the Labrador current has greatly reduced the size of the cold pool of water between southern Greenland and the British Isles. This outflow of fresh water from the Labrador and Irminger sea regions where deep water formation takes place may lead to a stronger Atlantic Meridional Overturning Circulation (AMOC)over the next decade or so, but that remains to be seen.
![Fig. 6. Anomalies of upper 2000 m OHC from (a) 2003 to 2012 and (b) 2013 to 2022,
relative to a 1981–2010 baseline but with the global mean value removed for each map [data
updated from Cheng et al. (2017a)].](https://images.dailykos.com/images/1281440/large/Screenshot2024-03-13at6.05.57PM.png?1710368488 "Fig. 6. Anomalies of upper 2000 m OHC from (a) 2003 to 2012 and (b) 2013 to 2022,
relative to a 1981–2010 baseline but with the global mean value removed for each map [data
updated from Cheng et al. (2017a)].")
The Atlantic ocean has taken up a disproportionately high percentage of global ocean heat over the past decade. That heat uptake has amplified over the past year, focused on the tropical Atlantic.
However, a stunning increase in the uptake of heat by the northern hemisphere oceans since around 2015 is reason for concern. The North Atlantic ocean has transitioned from a region that releases heat to the atmosphere, warming Europe, to a region that is taking up heat, most likely from increasing levels of solar radiation warming the surface waters.
The northern hemisphere's oceans shifted from releasing heat from the atmosphere to storing heat from incoming solar radiation around 2015.
The stunning shift of the North Atlantic from a source of heat to a heat sink does not necessarily indicate that the AMOC has slowed down, although it may have slowed for a few months this mid-winter like it did in early 2010. (The cable monitoring the Florida current has not been functioning for the past few months so there is no ready source available to me of real time data on the speed of the Gulf Stream.) The emitted thermal radiation has jumped up but the adsorbed solar radiation jumped up even more.
Fig. 2."CERES EBAF (Loeb et al. 2018) net radiative imbalance (positive down, 12-month running means), March 2000–September 2023 for the whole Earth (Nglobal; black, solid) and the North Atlantic (NNAtl; black, dotted), and annual-mean effective radiative forcing (Fglobal, from Forster et al. 2023; red) up to 2022. For visualization purposes, an arbitrary offset was added to the North Atlantic net radiative imbalance such that the time series averages out to zero."
My examination of the data and figures shown here and a number of other sources has brought me to conclude that the huge increase in ocean heat uptake in the North Atlantic over the past year was caused by multiple positive factors combining to cause a spike in heat uptake. One of those factors is El Niño, which reduced global trade winds, causing the tropical and subtropical oceans to warm. The second factor is a positive phase of winds from an anomalously southwesterly direction across the equator in the Atlantic ocean called the Atlantic Meridional Mode (AMM).
The third factor is a reduction of reflected light over the Atlantic. One part of this increase in light warming the ocean surface was a reduction of Saharan dust particles associated with slower trades and more southwesterly winds. The other factor is a reduction in sulfate emissions from shipping that began in 2016 and stepped up in 2020. Sulfate shipping emissions increase the brightness of low clouds. Bright low clouds are very efficient at cooling the subtropical ocean because they reflect light back to space in the day but they radiate heat to space at night at temperatures just slightly below the sea surface temperature. Regions where stratus clouds tend to form, such as off the west coasts of north Africa and California are very sensitive to the effects of sulfate aerosols. Scientific papers and analyses that use global aerosol figures are unable to capture the powerful regional impacts of the reduction of local sulfate emissions in oceanic areas where low clouds proliferate.
If the aerosol emissions effect is as strong as Dr. James Hansen thinks it is (and he is a radiation physicist by training) there could be a profound effect on heat storage and tropical storm formation in the tropical and subtropical north Atlantic in the years ahead. These issues continue to be debated and investigated. Note that, there are positive impacts of reduced sulfate pollution over the Atlantic beyond the health and environmental benefits of cleaner air. The increased evaporation and salinity brought on by warming may contribute positively to deep water formation in the North Atlantic. Efforts to modify the climate by adding reflective aerosols over the tropical and subtropical north Atlantic could produce negative unintended consequences by reducing evaporation and the density of sea water, slowing the AMOC.
Obviously, the build up of record levels of heat in the main development region for North Atlantic hurricanes is a matter of concern for people from coastal north Africa, across the Caribbean and to coastal North America. Florida's media has taken notice of the record ocean temperatures in the main development region. In my next post I will get into the details of the factors affecting the coming 2024 hurricane season. It looks likely to be an extremely active one.
