by Jim Johnstone and Judith Curry
With the Atlantic hurricane season underway, we’re keeping a close eye on particularly warm SSTs in the Atlantic. This post describes what happened and why.
SST Atlantic
Sea surface temperatures (SST) over much of the North Atlantic were particularly high in late June 2023, largely due to a period of rapid warming that began around March-April.
Figure 1. SCT anomalies for June 27, 2023 (top) and daily time series for the regions highlighted above.
The daily SST time series in Figure 1 shows recent SST development in several regions, highlighted in the map above, including the eastern Pacific Niño3 region, the North Atlantic Arc , the Caribbean Sea and the Tropical North Atlantic Major Development Area (MDR). The North Atlantic SSTs show evidence of unusual warming from March to April and the highest levels of warmth in early June. Warm conditions in the eastern equatorial Pacific reflect reflect the current El Niño phenomenon, which has steadily intensified since the beginning of 2023.
The local SST anomalies ≥1°C (baseline 1995-2019) appearing in the entire basin-scale ‘Arc’ sample represent the top natural structure of consistent SST variability across multiple time scale (Figure 1). Exceptional levels of recent warming in the North Atlantic can be seen in the average monthly SST irregularities of the Arc since 1980, illustrated in Figure 2. After a relative period of time little volatility since 2020, SST Arc has surged over the past few months, reaching an all-time high. peak in June.
Figure 2. Monthly Arc Average SST Anomalies from 1980 to June 2023.
Atmospheric circulation model
The June atmospheric conditions over the North Atlantic were also highly anomalous, as indicated by the anomalous North Atlantic Oscillation (NAO) index nearly 3 standard deviations below the monthly average. NAO is defined by the subtropical-subpolar difference in the atmospheric sea-level pressure (SLP) anomaly and approximates the intensity of low-level atmospheric circulation over the North Atlantic Basin. Positive. The extremely low NAO values in June are consistent with a very weak subtropical high (the Bermuda High) and relatively stagnant surface winds, associated with a weakening of the mid-latitude westerly currents and trade winds. tropical-subtropical.
The June SLP climate patterns and surface winds are shown in the left panel of Figure 3. The June 2023 conditions, in the center panel, show a significantly weak subtropical high and no surface westerly winds at mid-latitudes. June 2023 SLP and wind anomalies, shown in the right panel, include anomalous low pressure and cyclonic circulation over much of the basin, reflecting the weakness of the mean anticyclonic flow pattern .
Figure 3. June model of atmospheric SLP and surface wind. Left: June average (1995-2022). Center: Conditions in June 2023. Right: Outliers in June 2023 (except climatological observations). The + and – signs in the left panel show the active centers of the North Atlantic Oscillation, calculated as the subtropical SLP anomaly difference (+) minus the subpole (-).
The extremely negative June NAO was preceded by moderate negative anomalies in both March and April (Table 1), resulting in monthly warming of the North Atlantic Arc and its effects. extreme cumulative effects at the end of June. The arc warming observed from February to June (+0.85°C) exceeds the total warming of the Atlantic Ocean over the past century.
tmaybe 1. Monthly anomaly of the NAO, Arc SST (dSST) and SST trends. March, April, and June (red) are the months with negative NAO anomalies (low subtropical SLP and weak surface winds) and Arc warming.
Figure 4 illustrates the magnitude of the June NAO anomaly in the context of the historical record since 1980. Very low June 2023 NAO values are not unprecedented in monthly climate values; however, it is extremely low compared to previous June values and spring values (March-June).
Figure 4. The North Atlantic Oscillation Index (NAO), determined by the normalized subtropical SLP (Azores) anomaly difference (Azores) minus the subpole (Iceland). Top: Monthly anomalies (all months), with values from March to June 2023 highlighted in red. Middle: June value. Bottom: Average from March to June.
Unusual combination and change
NAO-related atmospheric anomalies are the main cause of Arc SST perturbation over short time periods (monthly to seasonal). Figure 5 illustrates the monthly anomalies of shortwave surface heat fluxes (solar), turbulent heat fluxes (latent/evaporative plus rational), total heat fluxes, and SST trends. A negative NAO anomaly corresponds to a negative subtropical SLP anomaly and weak southerly trade winds, including the MDR. Weak transactions warm the underlying sea surface primarily by hindering evaporative cooling, but affect tropical SSTs through a number of additional mechanisms, while also favoring warming. through stratum cloud reduction, Saharan dust advance, oceanic mixing and coastal upwelling of Northwest Africa. Overall, turbulent currents combined with weak winds prevailed over shortwave currents in warming the Atlantic.
Figure 5. Monthly downward surface heat flux anomalies and SST trends from March to June 2023. Arc warming in March, April and June can be mainly explained by surface winds. weak surface (due to anomaly – NAO) and low evaporative cooling rate (positive turbulent heat flow).
The recurrence and development of extreme NAO conditions and Arc warming in recent months may be partly due to the positive SST-atmospheric responses. The atmospheric/upper-ocean NAO/Arc models are closely related to that of the Atlantic Meridian Mode (AMM), in which warm SSTs and weak trade winds can be sustained or amplified through mutual reinforcement.
Another contributor to the remarkable spring changes in the North Atlantic may be the broader tropical warming associated with the transition to El Niño conditions. Surface warming in the tropics typically results in nearly uniform elevations of tropospheric temperatures and topographic elevations throughout the tropics from ~20°N to 20°S. Spring increase at topographic elevation of 250 hPa (Z250) over the tropics was accompanied by an immediate decrease towards the poles over the subtropical NE Pacific and North Atlantic, while the SLP anomaly decreased strong in the combined tropics from the central Pacific Ocean to the eastern Atlantic Ocean.
Figure 6. Topographic elevation anomalies of 250 hPa (Z250) and SLP in JFM and AMJ 2023. An increase in tropical Z250, decline in subtropical Z250, and decline in tropical SLP occur simultaneously over E Thai Binh Duong and North Atlantic Ocean in early 2023.
African dust, pollution, wildfire smoke and Hunga-Tonga
As shown in Figure 5, the extremely warm SSTs in the Atlantic during this season are partly due to anomalous radiant warming of the surface. Twitterati has blamed multiple factors: CO2 emissions, lower levels of sulfate particles from clearer carrier fuels, the Hunga-Tonga eruption and low levels of dust in Africa. And now the wildfires in Canada are a factor. These are all relatively small factors compared to the predominant effect of variations in clouds.
Atmospheric dust particles have been reduced globally from cleaner fuels destined for mandatory transportation in 2020, fuels that produce fewer sulfate particles. The sulfate particles have a cooling effect by reflecting solar radiation. Cleaner air means less reflected solar radiation, which contributes to surface warming. Indirect effects include darkening of subtropical clouds, through cloud microphysics effects that reduce their reflectivity. The ocean warming effect of this phenomenon will be greatest in the Northern Hemisphere, and subtropical clouds are most susceptible to this type of variation. In the image below you can see the train tracks from the brighter clouds, which reflect more solar radiation (these train lines are now greatly reduced) [link]
Particles from wildfire smoke have a similar effect in reducing the sun’s warming on the surface. My contacts in New York estimate that solar power production was cut in half during the period of a thick layer of smoke from the Canadian wildfires. For the smoke trajectory into the North Atlantic, there will be different surface cooling, depending on the smoke trajectory and optical depth.
African dust storms have a similar effect in terms of reflecting solar radiation and thus cooling the surface. Dust storms are unusually weak so far (from Michael Lowry):
But the bottom line is that a large dust outbreak in the Sahara does not have a major impact on sea surface temperatures: “Saharan dust outbreaks can reduce surface shortwave radiation by up to 190 W/m2and an analysis of the corresponding excise taxskin The changes using a thermal skin model show a dust-induced cooling effect as large as −0.24 K during the day and a warming effect of up to 0.06 K during the day and night respectively. ” [link]
Another radiation effect is from the Hunga-Tonga eruption in 2022. Normally, volcanoes spew sulfate particles in the stratosphere, reflecting sunlight and thus having a cooling effect on the surface. . However, the Hunga-Tonga eruption was associated with little sulfur dioxide (conversion to sulfate particles, with an estimated cooling effect of 0.004oC in 2022. The main climate impact of the Hunga-Tonga eruption was a large amount of water vapor pushed into the stratosphere from the Hunga-Tonga volcanic eruption, which is estimated to have increased the water content in the stratosphere. save up 10-15% with 0.034 oC global warming in the next 5 years. [link]
This is a screenshot from Zoom Earth on June 24, showing smoke from the Canadian wildfires and African dust. The whiteness of the smoke in the satellite image, compared with the Sahara Dust, indicates a large optical depth of the smoke (proportional to the surface cooling effect).
Summary
The extreme North Atlantic conditions that have developed in recent months are likely due to a combination of dynamic factors, including random weather anomalies, positive regional feedback and change Globally. The high rate of recent Arc warming is particularly notable due to the SST anomaly it produces; however, comparable warming over a period of ~4 to 6 months occurred earlier in late winter to spring 1983, 1987, 1989 and 2010 (Figure 2), preceded by a series of anomalous storms in late summer.
The main cause of warm SSTs is the dynamics (atmospheric circulation) that change the speed of surface winds (evaporation; obviously the biggest factor) and clouds (solar radiation). Small non-cloud radiation effects: Hunga-Tonga is a global effect, the sulphate train path is a local oceanic effect mainly in the northern hemisphere, the effects of wildfires on the oceans are comparable relatively rare and eventful, and African dust is a regular feature of summer characterized by sporadic events. Cloud variability in most locations will prevail over aerosol forces in terms of effects on sea surface temperatures. Some slight cooling of the Atlantic Ocean with Canadian smoke and Sahara dust can be expected.
