Sea-ice melt resulted in cold water spill over the Labrador Current

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Ice melt along the Labrador coast resulted in freshwater water spill over the Labrador Current, showing some very steep gradients in temperature and chlorophyll concentration fields while meeting the warm Gulf current around Newfoundland.

Sea-ice melt resulted in cold water spill over the Labrador Current
Date & Time
25 April 2018 00:00–23:00 UTC
Satellites
Meteosat-11, Sentinel-3A, GOES-16, Suomi-NPP
Instruments
SEVIRI, OLCI, SLSTR, ABI, VIIRS
Channels/Products
Airmass RGB, Natural Color RGB, True Color RGB, Dust RGB, Infrared

By Ivan Smiljanic (SCISYS), Hans-Peter Roesli (Switzerland), Vesa Nietosvaara and Jose Prieto (EUMETSAT), Hayley Evers-King and Ben Loveday (Plymouth Marine Laboratory), Debbie Richards (Serco)

Some very steep changes in the ocean surface temperatures were observed off the shores of Newfoundland, reaching estimated gradients of 15 °C/20 km in places. These temperatures were sensed by different instruments on board both low earth orbiting and geostationary satellites.

A wide overview of the surface temperatures in the area of interest was provided by the ABI imager instrument on board the geostationary GOES-16 satellite (Figure 1, top right, click to expand) where the blue shades show the temperatures were below 0 °C. Low temperatures of the ocean surfaces were also observed from the ship measurements in the area.

Confirmation of the shape and the extent of the sea currents can be seen on the animation (Figure 2), where the much faster-moving clouds can be distinguished from the slow moving currents (ca. 5 km/h), even at the same temperature (pixel colour).


Figure 2: GOES-16 infrared animation, 25 April 00:00–23:00 UTC

More evidence on the presence of very cold currents is given by GOES-16 RGB products (top two panels in Figure 3). Airmass RGB and Dust RGB products are giving very strong temperature signals of the cold currents, whilst the Natural Color RGB and Cloud Top Height product (as proxy for cloud mask) give confirmation on cloud-free area over these currents (bottom two panels in Figure 3).

Figure 3
 
Figure 3: Comparison of GOES-16 ABI products, Dust RGB (top left), Airmass RGB (top right), Natural Color RGB (bottom left), Cloud Top Height (bottom right), 25 April 12:00 UTC
 

The high resolution temperature field is observed through SLSTR and VIIRS polar-orbiting instruments. These instruments have spatial resolution of infrared channels at 1 km and 375 m, respectively (Figure 4).

Figure 4
 
Figure 4: Comparison of Sentinel-3A SLSTR (left) and Suomi-NPP VIIRS infrared imagery, 25 April 13:07 UTC
 

It is likely that the shape of the cold area in the temperature field is from the contours of the Labrador Current that meets the warm the Gulf Stream in the north west Atlantic. Temperatures close to 0 °C would be result of the upstream sea-ice melt, i.e. the cold freshwater spilled over the ocean current.

The possible source of the cold freshwater was observed along the north east coast of Newfoundland, and more upstream of the Labrador Current, where the patches of sea-ice detached from the big ice sheets and melted in the open ocean.

Using visible imagery, it is easy to distinguish the reflective signal from ice surfaces against the water background, which absorbs the majority of incoming photons. However, this is only the case for the cloud-free scene, because the assessment of sea-ice extent is biased by cloud presence, very often the case for the observed area (frequent presence of stratus clouds).

The True Color RGB product from the polar-orbiting VIIRS instrument showed, at very high resolution, detaching ice patterns thinning and disappearing in the open ocean, almost taking the shape of a horse's mane (Figure 5).

Figure 5
 
Figure 5: Suomi-NPP True Color RGB, 25 April 16:18 UTCC
 

A strong phytoplankton signal is sign of a freshwater presence in the ocean's upper layers, it follows the contours of the cold temperature field. Melted icebergs are rich in nutrients necessary for biological growth. Less dense freshwater layered on top of salty ocean water receives most of the sunlight necessary for photosynthesis. The synoptic-scale weather was relatively calm during the observed period, with little wind forcing and, therefore, less mixing of the ocean's upper layer.

Figure 6: Sentinel-3A, 25 April
OLCI Chlorophyll concentrations
Figure 7: Sentinel-3A, 25 April
SLSTR Sea Surface Temperature

Chlorophyll concentrations are presented through OLCI CHL product which relays on the chlorophyll refection in the visible electromagnetic spectrum (Figure 6). Sea Surface Temperature product from SLSTR instrument is presented over the same domain (Figure 7). Both instruments are on the Sentinel-3A satellite.

Image comparison
GOES-16 Natural Colour RGB GOES-16 infrared
Figure 8: Comparison of GOES-16 Natural Color and infrared images, 25 April 08:30 UTC

The GOES-16 images shown in Figure 8 are centred on one very peculiar shape in the ocean current (ca. 48 °N, 42 °W). It look like a very regular ‘L’ shape stretched over few hundred kilometres, with strong gradients in temperature and chlorophyll concentrations. From bathymetry in the region it is reasonable to believe that the shape was only at the surface. Also, the presence of the clouds that would potentially contribute to the shape (at least in the temperature field) is not confirmed from visible imagery — the early morning view over the area shows only traces of clouds in the temperature gradient region, which quickly disappeared.

The real cause for the formation of such a shape is not confirmed in the imagery. It could be a ‘simple’ deformation zone in the ocean current field, or caused by wind forcing. But none of these hypothesis can be confirmed looking at the imagery, so additional investigation is needed.

The temperature field was also detected with the SEVIRI instrument, through the Dust RGB. The overlap between two satellite views over the area of interest, GOES-16 and Meteosat-11, is considerable, and, therefore, was very useful for comparison (Figure 9).

Image comparison
Meteosat-11 Dust RGB GOES-16 Dust RGB
Figure 9: Comparison of Meteosat-11 and GOES-16 Dust RGB images, 25 April 09:00 UTC

Consistency in showing the cold current shape between two satellites is high. In general differences in imagery are arising from different factors: different viewing angles (e.g. pixel distortion, atmospheric cooling effect, parallax shift of higher clouds in particular), characteristic of the channels used for the same RGB product (e.g. resolution, selection of channels, Spectral Response Functions), scanning mechanisms/schemes (e.g. exact scanning time for particular pixels), etc.

 
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