Gibraltar's waters today

Physical

• Sea Surface Temperature (SST): the "foundation" temperature just below the surface, without daytime warming, from a gap-filled satellite analysis.
• Marine heatwave: at least 5 days in a row warmer than the warmest 10% of days for that time of year over a 30-year reference period (Hobday method). Categories I to IV (moderate, strong, severe, extreme) show how far above normal the peak went.
• Light (PAR): photosynthetically available radiation, the sunlight plankton and seagrass can use, as moles of photons per square metre per day.
• Attenuation Coefficient (KD490): how quickly blue light (490 nm) fades with depth, in m⁻¹. Higher values mean murkier water. We also show roughly how deep 1% of surface blue light reaches (4.6 ÷ KD490).
• Surface current: speed of the top metre of water and the direction it flows towards. Through the Strait, Atlantic water usually flows east into the Mediterranean.
• Salinity: saltiness in practical salinity units. Fresher Atlantic water (about 36.5) meets saltier Mediterranean water (38 and above) around Gibraltar.
• Mixed layer depth: how deep the surface layer is stirred by wind and cooling. Deep winter mixing brings nutrients up; shallow summer layers trap plankton near the light.
• Below the surface: sea temperature at 10, 20, 30, 50 and 100 m, and salinity at 50, 100 and 200 m, from the Copernicus physics model. Marine heatwaves are also worked out at 10, 30 and 50 m, where seagrass, urchins and fish live.
• Atlantic–Mediterranean interface: fresher Atlantic water flows in over saltier Mediterranean water flowing out. We give the depth where salinity reaches 37.5, a common marker for the boundary between the two layers.
• Upwelling: westerly winds blowing along the Spanish Alboran coast push surface water offshore (Ekman transport), and cooler, nutrient-rich water rises to replace it. The index is the offshore transport per kilometre of coast, calculated from hourly wind at Gibraltar. An event is 2 or more days above 500 m³/s per km, about a steady 6 m/s westerly. A negative index means the opposite: an easterly Levanter pushing surface water towards the coast, which suppresses upwelling (downwelling). So the index swings either side of zero with the wind, and both signs are normal.
• Light on the seabed: daily light reaching 5, 10, 15 and 20 m, estimated from satellite PAR and KD490 using the Morel et al. (2007) relation between KD490 and the attenuation of PAR. It is an area average, so sheltered or turbid spots will differ.
• Tide: times and heights of high and low water, predicted from a harmonic analysis of the last 12 months at the Gibraltar tide gauge, in local time and above chart datum, the level printed tide tables measure from. We work chart datum out from the gauge record itself as the lowest astronomical tide, using a separate harmonic fit for every year so the 18.6-year lunar cycle is covered; it can also be set to the published figure. The surge is the measured sea level minus the predicted tide: the effect of wind and air pressure, with slow drift removed so a serviced gauge cannot look like weather. Days whose level sits more than half a metre from their neighbours are dropped as sensor faults. Daily sea level is shown against the average of the past year rather than as a raw gauge height, because the gauge's own zero has moved over the record; it shows weather and season, not long-term sea level change. The moon phase is calculated for each day. Spring tides, with the biggest range, come a day or two after new and full moon, when the sun's and moon's pulls line up; neap tides, with the smallest range, follow the first and last quarter moons. The two-week strip shows each day's predicted range (highest high water minus lowest low water).
• Forecasts: the next 5 days from the Copernicus Mediterranean forecast models (sea temperature, currents, waves) and Open-Meteo (wind). Sea surface temperature forecasts are adjusted by the average difference between the model and satellite over the last 14 days.
• Waves: significant wave height off Europa Point, roughly the average height of the highest third of waves.
• Wind: 10 m wind at the airport. Easterly winds are the Levanter, westerlies the Poniente.

Wildlife (NEMO)

• Sightings: records from NEMO, TNP's citizen science app. They show what people reported, which depends on who was out looking, so a quiet month can mean quiet seas or a quiet app. Weekends carry about three times a weekday's records.
• Jellyfish and gelatinous: jellyfish, salps, comb jellies and the wind-blown drifters like the Portuguese man o' war. A busy spell is flagged only when these make up most of what is reported that day, so a busy reporting day on its own does not count.
• Invasive watch: species on a watchlist TNP maintains, including some not yet recorded here. A species appears publicly only once a record has been verified from a photograph or video.
• Strandings: dead or stranded animals reported through the app. TNP keeps its own strandings log, which is the authoritative record; these public reports are a check against it.
• Observer names, user accounts and written notes never leave NEMO. Positions are published rounded to a 1 km square, and whales, dolphins, turtles and other sensitive species are published with no position at all.

• Sea level: how high the sea stands against its 1993 to 2012 average, measured by satellite altimeters. This is the slow number underneath the tide. The tide card tells you when to walk to Sandy Bay; this one tells you what the sea is doing over decades.
• Surface current (measured): hourly surface currents across the Strait from a shore-based radar network, averaged as vectors over the day. Everything else we show for currents is a model, so this is the one that can disagree with it. A day where the two part company is interesting rather than wrong.
• UV index: strength of burning ultraviolet at the surface. 8 and above is where shade in the middle of the day stops being optional.

Chemical

• Saharan dust: modelled dust concentration near the ground. We flag a dust day above 50 µg m⁻³.
• Nitrate, phosphate and ammonium: the main nutrients phytoplankton need to grow, in millimoles per cubic metre.
• Dissolved oxygen: oxygen in surface water. Cold water holds more; plankton growth adds it.
• pH, pCO₂, dissolved inorganic carbon and alkalinity: the seawater carbonate system. Falling pH and rising pCO₂ over the years are signs of ocean acidification.
• Dissolved organic matter (CDOM): light absorbed at 443 nm by coloured dissolved material and detritus, often from rivers, runoff and plankton breakdown. From satellite.

• PM10 and PM2.5: particles small enough to breathe in, in micrograms per cubic metre. In Gibraltar these rise with Saharan dust, with shipping and with still weather that lets everything sit in the bay. PM2.5 is the finer fraction that reaches deepest into the lungs.
• Nitrogen dioxide and ozone: NO₂ comes mostly from engines, so it tracks traffic and shipping. Ozone forms in sunlight from other pollutants, so it peaks on hot bright afternoons rather than in rush hour.

Biological

• Chlorophyll-a: the green pigment in phytoplankton, the microscopic plants at the base of the marine food web. Higher values mean more plankton.
• Plankton groups and sizes: satellite estimates of how the chlorophyll is split between diatoms, dinoflagellates and other groups, and between micro (>20 µm), nano (2–20 µm) and pico (<2 µm) phytoplankton. Bigger cells tend to dominate in nutrient-rich water.
• Primary production: how much carbon phytoplankton fix through the whole water column each day, estimated from satellite chlorophyll, light and temperature.
• Phytoplankton and zooplankton carbon: plankton biomass as carbon, from the biogeochemistry model.
• Bloom watch: whether chlorophyll is in the top 10% for the time of year, how many times the normal level it is, and what happened in the days before each bloom.

Marine traffic

• Fishing activity: apparent fishing hours in the Strait, worked out by Global Fishing Watch from the AIS signals vessels broadcast. "Apparent" matters: fishing is inferred from how a boat moves, not from a logbook, and a vessel with AIS switched off or too small to carry it is invisible here. Read it as industrial activity visible from space, not as a catch record.
• Countries fishing: how many different flag states were fishing here that day. A rise can mean more boats or simply a wider mix of fleets.

Cards marked Model come from the Copernicus Mediterranean physics and biogeochemistry models for the top metre of water (4.2 km grid). Models are a best estimate that combines physics, biology and observations; they are not direct measurements, and in small areas such as the Bay they rest on only a few grid cells.

For every day of the year we gather all values within 7 days either side, across every year in the record. From those we calculate the average, the standard deviation, and the lowest and highest values. A 15-day window smooths out gaps where cloud hides the sea from the satellites.

"About normal" means within half a standard deviation of the average. "Slightly" is up to one standard deviation away, "much" is more than two.

"Combined" uses the best available source for each day: reprocessed satellite data before near-real-time data, and ERA5 before the airport station for weather. Where a measurement has more than one source, the Data source menu shows one source on its own, with its own average and limits.

The marine heatwave level uses its own method: an 11-day window, the 90th percentile over 1991–2020 (or every year available if the record is shorter), smoothed over 31 days. Heatwaves are worked out from the combined SST series.

For satellite ocean colour (chlorophyll, KD490, plankton groups, CDOM and primary production) each day's value is the median of the cloud-free pixels in the area, so a few bad coastal pixels can't skew it. Values outside a physically possible range are discarded. Chlorophyll is naturally skewed, so its lower band edge is held at zero.