monitoring the atlantic overturning circulation

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Download the RAPID data on MOC overturning strength at 26°N

We have collected the last 10 years of RAPID data on AMOC overturning strength in a format that may be read by Excel or other spreadsheets:

What the data shows

This data estimate the strength of the overturning circulation in the North Atlantic. The overturning circulation is a system of currents that moves heat northwards to higher latitudes. It is called an overturning circulation as it comprises the warm, shallow (top 1000 m), northward flowing Gulf Stream being balanced by cold, deep (1000 m to 5000 m), southward flowing water. Hence the water "overturns", much like a conveyor belt. The AMOC numbers estimate the strength of the overturning by adding together all of this water going northwards in the top 1000 m.

Very occasionally you will see a negative AMOC. This means the flow is reversed, so the net water transport in the top 1000 m is southward instead of northward.

More detailed data sets, which include the component flows used to calculate the AMOC, can be downloaded from the RAPID-MOC data download page, which the team has provided for ocean and climate scientists wanting to use the data.

Some ways to visualise the data

The data contains the following fields:

A: the measurement year,

B: the measurement month,

C: the day of the month,

D: the measurement hour (two measurements per day)

E: the quarter (1 = Jan-Mar; 2 = Apr-Jun; 3 = Jul-Sep; 4 = Oct-Dec)

F: the number of days from the first measurment - days_since_start,

G: the overturning strength measured in Sverdrup (Sv), a unit commonly used for the flow of ocean currents.
1 Sv = 1 million cubic metres of water per second (106 m3 s-1).

The quickest way to see how the AMOC behaves is to plot the overturning strength (G) against the day since the start (F). This gave figure 1 below.

Figure 2 is a little more sophisticated; it plots the overturning strength against the decimal year, which has been calculated from the year, the month (12 in each year), the day (365.25 days in an average year, accounting for leap years) and the hour (365.25 * 24 hours in each year).

AMOC time-series plot

Figure 1. Overturning strength plotted against the number of days since the start.

AMOC time-series plot

Figure 2. AMOC overturning strength plotted against the year.

Variability at different time scales

Like weather and climate the Atlantic overturning circulation varies on different time-scales. You may want to consider these before you submit your estimate to the RAPID Challenge.

Seasonal variability is commonly studied by calculating at the average of all the Januaries, Februaries, etc. for the whole time-series and seeing how these change from month to month. Sometimes the averages of all the 1st, 2nd, 3rd and 4th quarters are used.

Interannual variability may be studied by looking at how the flow in a particular month differs from the seasonal average for that month. For example: how did Jaunary 2010 differ from the average of all the other Januaries? If we can find a pattern for the interannual variablity it may help to predict what the next January may show.

Long-term trends are sustained changes - a decrease or increase in average flow, which continues for a very long time. Such trends may have serious long-term impact on European and global climate. When a time-series is too short, it is not possible to distinguish between a long-term trend and decadal variablity - periodic fluctuations that occur over time scales of decades. The RAPID time-series of AMOC flow is not long enough yet to make this distinction.