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The Red Sea has several characteristics not seen in other oceans: extremely warm temperatures, high evaporation, and odd circulation, including currents that change in summer and winter. (Natalie Renier, Woods Hole Oceanographic Institution) [ Hide caption ]

Mysteries of the Red Sea

This largely unexplored sea has features found nowhere else

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When my friends and family ask me what I am doing in my research, I respond that “I am investigating the winds and currents of the Red Sea in the Middle East.” Scary faces pop up. All they see are the winds of wars—the ever-present terrorist attacks, fighting, and killings in the region. “Are you crazy?” they say.

I get this same question (and sometimes the same reaction) from my oceanography colleagues. Since I began my postdoctoral research at Woods Hole Oceanographic Institution, working with WHOI physical oceanographers Amy Bower and Tom Farrar, I have learned two things: first, that few people realize how beautiful the Middle East is, and second, that the seas there have fascinating and unusual characteristics and far-reaching impacts on life in and around them. These seas furnish moisture for the arid Middle Eastern atmosphere and allowed great civilizations to flourish thousands of years ago around these seas.      

For an oceanographer like myself, the Red Sea can be viewed as a mini-ocean, like a toy model ocean. Most of the oceanic features in a big ocean such as the Atlantic, we can also find there.

But the Red Sea also has its own curious characteristics that are not seen in other oceans. It is extremely warm—temperatures in its surface waters reach than 30° Celsius (86° Fahrenheit)—and water evaporates from it at a prodigious rate, making it extremely salty. Because of its narrow confines and constricted connection to the global ocean and because it is subject to seasonal flip-flopping wind patterns governed by the monsoons, it has odd circulation patterns. Its currents change in summer and winter.

The Red Sea is one of the few places on Earth that has what is known as a poleward-flowing eastern boundary current. Eastern boundary currents are so called because they hug the eastern coasts of continents. But all other such eastern boundary currents head south in the northern hemisphere. But the Red Sea Eastern Boundary Current, unlike all others, flows in the direction of the North Pole.

Unravelling the intricate tapestry that creates this rare eastern boundary current in the Red Sea was a goal of my postdoctoral research. But I have found that the Red Sea is far more mesmerizing and complex than I initially imagined. A variety of exotic threads are woven into the tapestry that produce the Red Sea’s unusual oceanographic phenomena: seasonal monsoons, desert sandstorms, wind jets through narrow mountain gaps, the Strait of Bab Al Mandeb that squeezes passage in and out of the sea—even locust swarms.

The politics of the nations surrounding the Red Sea are also complex and make it among the more difficult places to collect data. That explains why many Red Sea phenomena have remained unknown. But unexplored regions are the juiciest for scientists, because they are the ripest places to make new discoveries.

Gateway to the Red Sea

In the Red Sea, the water evaporates at one of the highest rates in the world. Like a bathtub in a steam room, you would have to add water from the tap to keep its water level stable.

The Red Sea compensates for the large water volume it loses each year through evaporation by importing water from the Gulf of Aden—through the narrow Strait of Bab Al Mandeb between Yemen on the Arabian Peninsula and Djibouti and Eritrea on the Horn of Africa.

The Strait of Bab Al Mandeb works as a gate. All waters in and out of the sea must pass through it. No other gates exist, making the Red Sea what is known as a semi-enclosed marginal sea.

In winter, incoming surface waters from the Gulf of Aden flow in a typical western boundary current, hugging the western side of the Red Sea along the coasts of Eritrea and Sudan. The current transports the waters northward. But in the central part of the Red Sea, this current veers sharply to the right. When it reaches the eastern side, it continues its convoluted journey to the north, but now it hugs the eastern side of the sea along coast of Saudi Arabia.

Here’s where the mystery deepens. The Red Sea Eastern Boundary Current exists only in winter. In summer, it’s not there. I wanted to find out how it forms, how it changes, and why it seasonally disappears.

Detectives and pirates 

To unravel the complex tapestry that makes the Red Sea Eastern Boundary Current, I am like a CSI (Crime Scene Investigator) agent, sifting through as much data as I can get and putting them together to solve a mystery.

But it’s hard to obtain data from the Red Sea. Its narrow confines mean that its waters are restricted by countries around it that are often in conflict. It’s hard for researchers to get permission to enter them.

In addition, many waters in and enroute to the Red Sea have been beset by piracy. In the spring of 2018, I was aboard of the NOAA ship Ronald H. Brown in the Arabian Sea. It was the first time in more than a decade that the U.S. Navy allowed an American research vessel to go to the Arabian Sea. We were allowed to go only on the eastern side, and we couldn’t go anywhere beyond 17.5° N, because it wasn’t safe. On board, we conducted many safety drills, learning how to hide from pirates.

The Red Sea is also hard for satellites. Its width is small compared with the spatial resolution of most oceanographic satellites. Altimeter and wind satellites have a spatial resolution around 30 to 50 kilometers. The maximum Red Sea width is only 355 kilometers. In addition, satellites can give us information about what happens at the sea surface, but they can’t reveal the mixing and other processes that go on beneath the surface.

That’s why my research was kidnapped and carried off into an unanticipated direction, and my focus shifted from sea to air.

A plague of locusts

In the Red Sea, evaporation is a critical factor driving how the sea operates, and to determine how much water evaporates, we need to know about the winds.   Why? Because evaporation rates depend on the winds. If the winds are stronger, the evaporation is stronger; if the winds are weaker, evaporation is weaker.

To complicate the situation a bit more, evaporation depends not only on the strength of the winds but where the winds are coming from. If the winds are coming over the sea, the air humidity in the winds will be higher, and evaporation will be lower; if the winds are coming from the desert, the air will be dry, and evaporation will be higher.

So, to unravel the Eastern Boundary Current, we needed to have a pretty good picture of how the winds blow in winter. When I started my postdoctoral research, I was really surprised to see that this important factor—the wind variability of the Red Sea—wasn’t well-known, even though interest on it goes way back!

Pioneering studies about winds in the Red Sea were motivated by a desire to determine the northward migration of desert locust swarms that invade areas and voraciously consume all the vegetation in it. This plague has been described in the Old Testament of the Bible and has tormented countries bordering the Red Sea since times immemorial.

The locusts breed along the shores of the Red Sea. Summer monsoon rains spur locust eggs to hatch. When enough rains fall to create plenty of water and vegetation for food, large numbers of locusts hatch and form swarms. Winds determine where the swarms will be carried off to infest neighboring regions.

Lining both sides of the Red Sea are tall mountains that create a kind of tunnel, so that winds blow predominantly along, not across, the Red Sea. In summer, the winds blow from north to south. In winter, however, the monsoon flips the wind direction in the southern part of the Red Sea, andtwo opposing airstreams meet at some point in the central Red Sea called the Red Sea Convergence Zone. It acts as a conduit for migrating locust swarms, and where it is positioned determines where the swarms go.

Mountain-gap wind jets

The mountains along Red Sea coasts affect the winds in another way. The mountains aren’t entirely compact; there are several gaps in them. The tunnel surrounding the Red Sea has a few holes in both sides. Sometimes the winds blow through one of these holes and cross the tunnel. These are the mountain-gap wind jets.

The mountain-gap winds in summer blow from Africa to Saudi Arabia through the Tokar Gap near the Sudanese coast. In winter, the mountain-gap winds blow in the opposite direction, from Saudi Arabia to Africa, through many nameless gaps in the northern part of the Red Sea.

These jets stir up frequent sandstorms carrying sand and dirt from surrounding deserts into the Red Sea. The sandstorms carry fertilizing nutrients that promote life in the Red Sea. The sands also block incoming sunlight and cool the sea surface.

But do these overlooked jets also affect the Red Sea in other ways?

Blasts of dry air 

We decided to put together lots of different data to find out the fundamental characteristics of these mountain-gap jet events. Our data came from satellites and from a heavily instrumented mooring that measured winds and humidity in the air and temperatures and salinity in the sea below. WHOI maintained the mooring for two-years in the Red Sea when it collaborated with King Abdullah University of Science and Technology.

The satellite images revealed that these events weren’t rare. We learned that in most winters, there are typically two to three events in December and January in which the winds blow west across the northern part of the Red Sea. In satellite images, they are impressive and beautiful.  

The mountain-gap events typically last three to eight days. We observed large year-to-year differences, with an increasing number of events in the last decade.  

­We discovered that the wintertime mountain-gap wind events blast the Red Sea with dry air. They are like the cold-air outbreaks that hit the U.S. East Coast in winter. Of course, for the Red Sea, it would be better to name them as dry-air outbreaks!

The dry-air blasts abruptly increase evaporation on the surface of the sea. This colossal evaporation removes a large amount of heat and water vapor out of the sea, leaving it much saltier. The mountain-gap winds also stir up deeper, cooler waters that mix with surface waters.

The waters become saltier and colder. This disrupts the Eastern Boundary Current. During most wind-jet events, it seems to fade away.    

Connecting the oceans

We are still looking for answers about how the Eastern Boundary Current forms and why it flows north. But we have learned much about the wind-jet events that cause it to disappear periodically in winter.

The large-scale evaporation from these wind-jet events may also drive waters in the northern Red Sea to become cooler, saltier, and dense enough to sink the depths and flow all the way south and back out of the Strait of Bab Al Mandeb.

These salty Red Sea waters escape to the Gulf Aden, where they start a long journey through the Indian Ocean. They cross the Equator. Some may travel into the Atlantic Ocean. Some may flow toward Western Australia.

How can these Red Sea waters travel so far away? I’m diving into that mystery now.  

This research was funded by National Science Foundation. The mooring data collected during the WHOI-KAUST collaboration was made possible by the KAUST in the Kingdom of Saudi Arabia.

 

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