New research by the Woods Hole Oceanographic Institution (WHOI) found that a key component of the Gulf Stream– a warm Atlantic ocean current stretching to the tip of Florida– has steadily slowed and is now weaker than at any other point in the past 110 years. The connection between coastal sea level and the strength of near-shore currents was analyzed to trace the evolution of the Florida Current and come up with the conclusion.
“In the ocean, almost everything is connected,” said research author Christopher Piecuch, an assistant scientist in the Physical Oceanography Department at WHOI.
“We can use those connections to look at things in the past or far from shore, giving us a more complete view of the ocean and how it changes across space and time.”
Piecuch specializes in coastal and regional sea-level change, and he used a connection between coastal ocean level and the strength of currents near the shore to track the evolution of the Florida Current.
The Gulf Stream flows north along the South Atlantic Coast of the U.S. and eventually east into the North Atlantic, carrying properties that affect the Earth’s climate such as heat, salt, and momentum.
Because almost constant records of water level date back to more than a century along Florida’s Atlantic Coast, Piecuch was able to use mathematical models and physics to widen the reach of direct measurements of the Gulf Stream.
This led him to his conclusion that the current has weakened steadily and is weaker now than any other time in the last 110 years.
Image credit: Carol Anne Clayson, Woods Hole Oceanographic Institution
However, one of the biggest uncertainties in climate models is the behavior of currents leading or responding to changes in the world’s climate. One of the most significant of these is the Atlantic Meridional Overturning Circulation (AMOC)– a large system or conveyor belt of ocean currents in the Atlantic that helps balance global climate.
Piecuch’s study agrees with relationships displayed in models between the deeper branches of the AMOC and the Gulf Stream. It verifies other studies suggesting that the deeper branches of AMOC have slowed in the past years. His method also provides the potential to monitor ocean currents like the Gulf Stream from the coast, complementing moored instruments and expensive research vessels.
“If we can monitor something over the horizon by making measurements from shore, then that’s a win for science and potentially for society,” the author said.
Reference
“Likely weakening of the Florida Current during the past century revealed by sea-level observations” – Piecuch, C. G. – Nature Communications – https://doi.org/10.1038/s41467-020-17761-w
Abstract
The Florida Current marks the beginning of the Gulf Stream at Florida Straits, and plays an important role in climate. Nearly continuous measurements of Florida Current transport are available at 27°N since 1982. These data are too short for assessing possible multidecadal or centennial trends. Here I reconstruct Florida Current transport during 1909–2018 using probabilistic methods and principles of ocean physics applied to the available transport data and longer coastal sea-level records. Florida Current transport likely declined steadily during the past century. Transport since 1982 has likely been weaker on average than during 1909–1981. The weakest decadal-mean transport in the last 110 y likely took place in the past two decades. Results corroborate hypotheses that the deep branch of the overturning circulation declined over the recent past, and support relationships observed in climate models between the overturning and surface western boundary current transports at multidecadal and longer timescales.
Featured image credit: Carol Anne Clayson, Woods Hole Oceanographic Institution
Based on the observations of ocean heat content in the North Atlantic Ocean, the climate in the northern hemisphere is on the verge of a change that could last for several decades. This change is associated with the Atlantic Multidecadal Oscillation (AMO)1 – a mode of natural variability occurring, with a period of 60 – 80 years, in the North Atlantic Ocean sea surface temperature (SST) field.
Observations made by Argo buoys2 have shown that the North Atlantic Ocean (60-0W, 30-65N) is rapidly cooling since 20073. This is associated with the natural variability in the North Atlantic Ocean sea surface temperatures – the Atlantic Multidecadal Oscillation (AMO). However, the observed cooling does not only apply to the sea surface, but to the uppermost 700 m (2 296 feet) of the ocean.
The AMO index appears to be correlated to air temperatures and rainfall over much of the northern hemisphere4. The association appears to be high for North Eastern Brazil, African Sahel rainfall and North American and European summer climate. The AMO index also appears to be associated with changes in the frequency of North American droughts and is reflected in the frequency of severe Atlantic hurricanes.
“As one example, the AMO index may be related to the past occurrence of major droughts in the US Midwest and the Southwest. When the AMO is high, these droughts tend to be more frequent or prolonged, and vice-versa for low values of AMO. Two of the most severe droughts of the 20th century in the US occurred during the peak AMO values between 1925 and 1965: The Dust Bowl of the 1930s and the 1950s drought. On the other hand, Florida and the Pacific Northwest tend to be the opposite; high AMO is associated with relatively high precipitation.”
Cooling of the Atlantic is likely to bring drier summers in Britain and Ireland, accelerated sea-level rise along the northeast coast of the United States, and drought in the developing countries of the African Sahel region, a press release for a study by scientists from the University of Southampton and National Oceanography Centre (NOC) published last year said5. “Since this new climatic phase could be half a degree cooler, it may well offer a brief reprise from the rise of global temperatures, as well as result in fewer hurricanes hitting the United States. The study proves that ocean circulation is the link between weather and decadal scale climatic change. It is based on observational evidence of the link between ocean circulation and the decadal variability of sea surface temperatures in the Atlantic Ocean.”
Lead author of this study, Dr. Gerard McCarthy from the NOC, said: “Sea-surface temperatures in the Atlantic vary between warm and cold over time-scales of many decades. These variations have been shown to influence temperature, rainfall, drought and even the frequency of hurricanes in many regions of the world. This decadal variability is a notable feature of the Atlantic Ocean and the climate of the regions it influences.”
These climatic phases, referred to as positive or negative AMO’s, are the result of the movement of heat northwards by a system of ocean currents. This movement of heat changes the temperature of the sea surface, which has a profound impact on climate on timescales of 20 – 30 years. The strength of these currents is determined by the same atmospheric conditions that control the position of the jet stream. Negative AMO’s occur when the currents are weaker and so less heat is carried northwards towards Europe from the tropics. The strength of ocean currents has been measured by a network of sensors, called the RAPID array, which have been collecting data on the flow rate of the Atlantic Meridional Overturning Circulation (AMOC) for a decade.
The AMOC, part of which is known as the Gulf Stream, has been seen to weaken over the past 10 years, a study by Laura Jackson of the UK’s Met Office said6. Her study also suggests that this weakening trend is likely due to variability over decades. “The AMOC plays a vital role in our climate as it transports heat northwards in the Atlantic and keeps Europe relatively warm,” Jackson said. Any substantial weakening of a major North Atlantic ocean current system would have a profound impact on the climate of northwest Europe, including the UK. The research also showed a link between the weakening in the AMOC and decreases in density in the Labrador Sea (between Greenland and Canada) several years earlier.
In the diagrams below, courtesy of Ole Humlum4, only original (raw) AMO values are shown.
Humlum writes: “As is seen from the annual diagram, the AMO index has been increasing since the beginning of the record in 1856, although with a clear, about 60 yr long, variation superimposed. Often, AMO values are shown linearly detrended to remove the overall increase since 1856, to emphasize the apparent rhythmic 60 yr variation. This detrending is usually intended to remove the alleged influence of greenhouse gas-induced global warming from the analysis, believed to cause the overall increase. However, as is seen in the diagram below, the overall increase has taken place since at least 1856, long before the alleged strong influence of increasing atmospheric CO2 began around 1975 (IPCC 2007). Therefore, the overall increase is likely to have another explanation; it may simply represent a natural recovery since the end of the previous cold period (the Little Ice Age). If so, the general AMO increase since 1856 may well represent part of a longer natural variation, too long to be fully represented by the AMO data series since 1856. For the above reasons, only the original (not detrended) AMO values are shown in the two diagrams below:”
Annual Atlantic Multidecadal Oscillation (AMO) index values since 1856. The thin line indicates 3-month average values, and the thick line is the simple running 11-year average. Data source: Earth System Research Laboratory at NOAA. Last year shown: 2015. Last diagram update January 20, 2016.
Monthly Atlantic Multidecadal Oscillation (AMO) index values since January 1979. The thin line indicates 3-month average values, and the thick line is the simple running 11-year average. By choosing January 1979 as starting point, the diagram is easy to compare with other types of temperature diagrams covering the satellite period since 1979. Data source: Earth System Research Laboratory at NOAA. Last month shown: May 2016. Last diagram update: June 13, 2016.
The map below shows the North Atlantic area within 60-0W and 30-65N, for which the heat content within the uppermost 700 m is shown in the diagrams below it3.
North Atlantic area within 60-0W and 30-65N. Credit: Climate4you
Global monthly heat content anomaly (GJ/m2) in the uppermost 700 m of the North Atlantic (60-0W, 30-65N) ocean since January 1979. The thin line indicates monthly values, and the thick line represents the simple running 37 month (c. 3 year) average. The starting month (January 1979) is chosen to enable easy comparison with global air temperature estimates within the satellite period. Data source: National Oceanographic Data Center (NODC). Last period shown: January-March 2016. Last diagram update June 7, 2016.
Global monthly heat content anomaly (GJ/m2) in the uppermost 700 m of the North Atlantic (60-0W, 30-65N) ocean since January 1955. The thin line indicates monthly values, and the thick line represents the simple running 37 month (c. 3 year) average. Data source: National Oceanographic Data Center (NODC). Last period shown: January-March 2016. Last diagram update June 7, 2016.
Interestingly, in a study by Zhou et al.7, a significant correlation was found between the solar wind speed (SWS) and sea surface temperature (SST) in the region of the North Atlantic Ocean for the northern hemisphere winter from 1963 to 2010, based on 3-month seasonal averages. “The correlation is dependent on Bz (the interplanetary magnetic field component parallel to the Earth’s magnetic dipole) as well as the SWS, and somewhat stronger in the stratospheric quasi-biennial oscillation (QBO) west phase than in the east phase. The correlations with the SWS are stronger than those with the F10.7 parameter representing solar UV inputs to the stratosphere. SST responds to changes in tropospheric dynamics via wind stress, and to changes in cloud cover affecting the radiative balance. Suggested mechanisms for the solar influence on SST include changes in atmospheric ionization and cloud microphysics affecting cloud cover, storm invigoration, and tropospheric dynamics. Such changes modify upward wave propagation to the stratosphere, affecting the dynamics of the polar vortex. Also, direct solar inputs, including energetic particles and solar UV, produce stratospheric dynamical changes. Downward propagation of stratospheric dynamical changes eventually further perturbs tropospheric dynamics and SST.”
The solar-wind speeds peak about 3 or 4 years after the Total Solar Irradiance (TSI) and sunspots peak in each cycle8.
Sunspot number progression observed from 2000 – May 2016. Credit NOAA/SWPC
Based on the current sunspot observations, their number for this solar cycle has peaked in January 2015, and our star is now on a steady path toward its next Solar Minimum, expected to hit the base just after 2020.
Global sea surface temperature anomaly for June 13, 2016 – current deviation of the surface temperature of Earth’s oceans from normal. Credit: NCEP (link leads to the latest map)
North Atlantic Ocean sea surface anomaly for June 13, 2016 – current deviation from normal. Credit: NCEP (link leads to the latest map)
Argo – UCSanDiego – Argo is a major contributor to the WCRP ‘s Climate Variability and Predictability Experiment (CLIVAR) project and to the Global Ocean Data Assimilation Experiment (GODAE). The Argo array is part of the Global Climate Observing System/Global Ocean Observing System GCOS /GOOS
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