Sea-ice melt water spills into Labrador Current

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) 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).

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).

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).

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).

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.

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.