For the last few years, observers have been speculating about when the North Atlantic will start the next phase shift from warm to cold.The way 2018 went and 2019 followed,suggested this may be the onset. However, 2020 started out against that trend, now backing off a bit. First some background.
An example is this report in May 2015 The Atlantic is entering a cool phase that will change the world’s weather by Gerald McCarthy and Evan Haigh of the RAPID Atlantic monitoring project. Excerpts in italics with my bolds.
This is known as the Atlantic Multidecadal Oscillation (AMO), and the transition between its positive and negative phases can be very rapid. For example, Atlantic temperatures declined by 0.1ºC per decade from the 1940s to the 1970s. By comparison, global surface warming is estimated at 0.5ºC per century – a rate twice as slow.
In many parts of the world, the AMO has been linked with decade-long temperature and rainfall trends. Certainly – and perhaps obviously – the mean temperature of islands downwind of the Atlantic such as Britain and Ireland show almost exactly the same temperature fluctuations as the AMO.
Atlantic oscillations are associated with the frequency of hurricanes and droughts. When the AMO is in the warm phase, there are more hurricanes in the Atlantic and droughts in the US Midwest tend to be more frequent and prolonged. In the Pacific Northwest, a positive AMO leads to more rainfall.
A negative AMO (cooler ocean) is associated with reduced rainfall in the vulnerable Sahel region of Africa. The prolonged negative AMO was associated with the infamous Ethiopian famine in the mid-1980s. In the UK it tends to mean reduced summer rainfall – the mythical “barbeque summer”.Our results show that ocean circulation responds to the first mode of Atlantic atmospheric forcing, the North Atlantic Oscillation, through circulation changes between the subtropical and subpolar gyres – the intergyre region. This a major influence on the wind patterns and the heat transferred between the atmosphere and ocean.
The observations that we do have of the Atlantic overturning circulation over the past ten years show that it is declining. As a result, we expect the AMO is moving to a negative (colder surface waters) phase. This is consistent with observations of temperature in the North Atlantic.
Cold “blobs” in North Atlantic have been reported, but they are usually winter phenomena. For example in April 2016, the sst anomalies looked like this
But by September, the picture changed to this
And we know from Kaplan AMO dataset, that 2016 summer SSTs were right up there with 1998 and 2010 as the highest recorded.
As the graph above suggests, this body of water is also important for tropical cyclones, since warmer water provides more energy. But those are annual averages, and I am interested in the summer pulses of warm water into the Arctic. As I have noted in my monthly HadSST3 reports, most summers since 2003 there have been warm pulses in the north atlantic, and 2019 was one of them.
The AMO Index is from from Kaplan SST v2, the unaltered and not detrended dataset. By definition, the data are monthly average SSTs interpolated to a 5×5 grid over the North Atlantic basically 0 to 70N. The graph shows the warmest month August beginning to rise after 1993 up to 1998, with a series of matching years since. December 2017 set a record at 20.6C, but note the plunge down to 20.2C for December 2018, matching 2011 as the coldest years since 2000. December 2019 shows an uptick but still lower than 2016-2017.
December 2019 confirmed the summer pulse weakening, along with 2018 well below other recent peak years since 1998. Then came a surprise in 2020. Because McCarthy refers to hints of cooling to come in the N. Atlantic, let’s take a closer look at some AMO years in the last 2 decades.
This graph shows monthly AMO temps for some important years. The Peak years were 1998, 2010 and 2016, with the latter emphasized as the most recent. The other years show lesser warming, with 2007 emphasized as the coolest in the last 20 years. Note the red 2018 line was at the bottom of all these tracks. The black line shows that 2019 began slightly cooler than January 2018, then tracked closely before rising in the summer months, though still lower than the peak years. Through December 2019 is again tracking warmer than 2018 but cooler than other recent years in the North Atlantic.
In 2020 following a warm January, N. Atlantic temps in February, March and April were the highest in the record. Now May 2020 temps are still warm but lower than May 2016 and 2017. That is a concern for the upcoming hurricane season, along with the lack of a Pacific El Nino providing wind shear against developing tropical storms.
More recently, temps in higher Atlantic latitudes (45N to 65N) have cooled, as shown in this graph and map from Tropical Tidbits (Levi Cowan)
Footnote: Levi Cowan’s Tropical Tidbits is an excellent source of information regarding tropical storm activity, even before disturbances are assigned names, as well as ones like the current Christobal.
via Science Matters
June 10, 2020 at 02:07PM