In January 2021, a gap in the clouds gave a clear view on the Gulf Stream east of Virginia, USA.
04 May 2023
07 June 2021
By Jochen Kerkmann (EUMETSAT)
Originating at the tip of Florida, the Gulf Stream is a warm and swift Atlantic Ocean current that follows the eastern coastline of the US and Canada before crossing the Atlantic Ocean towards Europe (Source: Met Office).
Along the east coast of Florida the current is fed by the Antilles Current, and the flow runs parallel to the coast until it reaches Cape Hatteras where it leaves the coast and enters deeper water. While flowing in deep water the Gulf Stream often forms large meanders or fluctuations in its path.
Today, scientists can study the Gulf Stream from above, using satellites. For example, Meteosat Second Generation and GOES-R series satellites, with their SEVIRI and ABI instruments, collect information about sea surface temperature (SST) in the Atlantic Ocean. The instruments aboard the Copernicus Sentinel-3 satellites are able to simultaneously monitor SST, ocean colour and sea-surface topography with the SLSTR, OLCI and SRAL instruments, as shown in this companion case study — Tracking the Gulf Stream with satellite data. Satellite images of SST can show the path of the warm Gulf Stream current with great precision. Knowing the sea surface temperature can give scientists information about what is happening in, and around, the ocean. Changes in this temperature can influence the behaviour of fish, cause the bleaching of corals, and affect weather along the coast.
A pre-requisite for deriving SST information from instruments like ABI is that the scene is absolutely cloud free. In particular, the presence of thin cirrus can lead to false results. For this, it is not sufficient to look at one channel, but a combined view of several channels is needed. In particular, the new NIR1.3 channel (called Cirrus band) and the, so-called, split window difference (IR12.3–IR10.3) contribute to the detection of thin cirrus clouds. The NIR1.3 channel is part of the new Cloud Type RGB product devised by CIMSS. It combines the following channels: NIR1.3 on red (range: 0–10%, Gamma 2.5) to detect thin cirrus, VIS0.6 or VIS0.8 on green (range: 0–100%) to detect low clouds and ice/snow, and NIR1.6 channel on blue (range: 0–60%) to distinguish between ice and water clouds.
Figure 1 shows the Cloud Type RGB for the eastern coast of the USA on 13 January 2021 at 18:00 UTC. Most of North Carolina and Virginia, including the adjacent Atlantic Ocean, are totally cloud free. A cirrus cloud band can be seen in the southern part of the image, and low-level clouds dominate the north-eastern part of the image.
During the night, the best RGB product to check the cloudiness is the Night Microphysics RGB. It detects both, thin high clouds (using the IR12.3–IR10.3 difference on red) and low level clouds (using the IR10.3–IR3.9 difference on green). Figure 2 shows the 06:00 UTC Night Microphysics RGB for the same area as in Figure 1. Note the cloud free area in Virginia and the low fog clouds in the coastal area of North Carolina (see the CIMSS Satellite Blog case Freezing fog in the Carolinas and Virginia). Also, because of the use of the IR10.3 channel on the blue colour beam, different SSTs appear with different shades of blue. The warm Gulf Stream current can be clearly seen.
The best satellite images to show the Gulf Stream are colour-enhanced IR images. Figure 3 shows the enhanced IR10.3 channel of ABI on 13 January at 12:00 UTC (end of the night). The IR10.3 channel is the 'cleanest' window channel, i.e. it has less water vapour absorption than the other infrared window channels. The values shown are brightness temperatures (BT), which, in the case of low atmospheric moisture (polar airmass), correspond roughly to the real temperature of the scene (ocean, land, thick cloud).
White colour in the image denotes temperatures below 0 °C (273K), like cold land and clouds. The ocean has temperatures ranging from 5°C (278K, coastal areas) to about 20 °C (293K) in the Gulf Stream. These are winter temperatures (13 January); of course, in the summer the values are higher.
As stated above, at Cape Hatteras the Gulf Stream leaves the coast and enters deeper water. Once in deeper water, the current starts to meander and form eddies. When looking at the hourly animation of the IR10.3 channel (Figure 4), one can see the movement of the Gulf Stream and the formation of two eddies on the northern side of the ocean current.
We tried to measure the speed of the Gulf Stream east of Virginia. However, it was difficult to identify good targets in the centre of the current. Instead, we took a target on the left side of the current (looking downstream), where there was a good contrast, and we measured a speed of 50 km/12 hours. As expected, this is lower than the average speed of the Gulf Stream, which is 6.4 km/h (Source: NOAA). This means that in 23 hours (timespan covered by the loop) an ocean parcel moves in average about 147 km, more than one might think.
GOES-16 Night Microphysics RGB image of the Gulf Stream on 25 February 2021 06:00 UTC
What is the Gulf Stream? (Met Office)
How fast is the Gulf Stream? (NOAA National Ocean Service)
Gulf Stream Imagery and Data (NOAA Ocean Prediction Center)
High-resolution (1 km) Terra MODIS IR image of the Gulf Stream, 2 May 2001