Arabian Sea

In summer, strong winds blow from the south-west. In winter, the wind turns around and blows from the north-east.

Both winds mix the sea. They bring up food for tiny sea plants.

In summer, strong south-west monsoon winds blow across the Arabian Sea. Near the coasts of Yemen and Oman, these winds pull cold water full of nutrients up from the deep to the surface. This is called upwelling, and it makes tiny plant-like living things called phytoplankton grow fast.

In winter, the wind reverses and blows from the north-east. This winter monsoon also stirs up nutrients, so winter is the best season to see long swirls of phytoplankton twisting across the sea in satellite images.

NASA researchers describe two monsoon seasons of high productivity. In summer, intense south-west winds generate strong upwelling near the coasts of Yemen and Oman, bringing colder, nutrient-rich water to the surface; studies by Joaquim Goes and Helga do Rosario Gomes linked a stronger summer wind to warming over the Indian subcontinent as Himalayan snow cover shrank.

The winter monsoon reverses the wind, from south-westerly to north-easterly, and stirs up nutrients that help phytoplankton thrive. NASA's MODIS image of 14 February 2015 shows the resulting filaments twisting and curling near the coasts of Oman, Iran and Pakistan.

A tiny living thing called Noctiluca now grows in huge numbers in winter.

Sailors call it sea sparkle, because it glimmers at night.

In the winter of 2003, scientists sailing the Arabian Sea found the water slushy and green. It was full of Noctiluca scintillans, a tiny one-celled living thing that sailors call sea sparkle because it glimmers at night. Before then, it had been seen there only now and then.

Since the early 2000s, green Noctiluca blooms have appeared every year from January to March. Before 2000, tiny algae called diatoms ruled the winter blooms. Within about ten years, Noctiluca had replaced them.

Green Noctiluca scintillans, first detected in large winter blooms in the early 2000s, now blooms every year with predictable regularity from January to March, replacing the diatoms that dominated before 2000. Noctiluca cannot photosynthesize itself; it traps smaller phytoplankton with its flagellum and also gains energy from millions of green photosynthetic endosymbionts living inside its cell.

Research published in 2014 (Gomes et al., Nature Communications) tied the outbreak to an unprecedented amount of oxygen-deficient water near the surface. In shipboard experiments in low-oxygen seawater, Noctiluca's carbon-fixing rate rose sharply while the diatoms' fell. Why low oxygen favors it is still being studied; one hypothesis involves the endosymbiont's Rubisco enzyme, but other scientists caution that not enough is known yet.

Small sea animals used to eat the tiny plant-like algae. Then fish ate the small animals.

Noctiluca is too big for those small animals to eat. Scientists are watching what happens next.

In the past, the Arabian Sea's food web began with diatoms. Tiny animals called copepods ate the diatoms, and fish ate the copepods.

Noctiluca is too large for copepods to eat. Instead, it feeds animals such as jellyfish, salps and turtles. Scientists say this could change the food web, and nobody knows yet how it will affect fishing in the region.

NASA's Earth Observatory explains the stakes: fish once ate the copepods that grazed on plentiful diatoms, but N. scintillans appears too large for copepods and instead feeds creatures such as jellyfish and salps; the Earthdata feature adds turtles. How the disruption will affect regional fisheries remains to be seen.

The low-oxygen water itself is part of the story. The researchers link hypoxia to nutrients from sewage and farm runoff, which feed blooms whose decay by bacteria strips oxygen from the water. Circulation in the Arabian Sea is sluggish, so oxygen-poor water is replaced more slowly than in the Pacific or Atlantic.

At the beach, always swim with a grown-up close by.

If the water pulls you out, swim along the beach, not against it.

If a rip current, a powerful, narrow channel of fast-moving water, ever catches you while swimming, NOAA's advice is: don't fight it. Swim parallel to the shore, then swim back to land at an angle. Swimming straight back against it wears swimmers out.

At sea, blooms can be beautiful to see at night, but any blooming water is best enjoyed from a boat or the shore rather than by swimming in it. That is PlayOwl's tip, because some blooms can be harmful.

NOAA's National Ocean Service describes rip currents as powerful, narrow channels of fast-moving water that can move at up to about 2.4 m/s (8 feet per second), faster than an Olympic swimmer; the advice is not to fight one but to swim parallel to the shore and return to land at an angle.

NASA's Earthdata feature notes that harmful algal blooms produce toxins that can kill marine animals and poison humans, so watching a bloom from a boat or the shore, rather than swimming in it, is PlayOwl's sensible tip.