Our oceans make up a staggering 71% of the planet’s surface. Despite how much we have learned about the oceans through exploration and advances in technology, scientists estimate that there is still up to 95% that is unexplored; we seem to know more about deep space than the deep sea. In this article, we explore which ocean is the warmest, what affects the ocean’s temperature, and why it has such an appeal to us divers!

Which Ocean is the Warmest?

If you take a look at a world map and trace your finger along the equator, it will come as no surprise that the warmest ocean is the Indian Ocean.

The majority of its waters lie within the tropics, and surface temperatures rarely fall below a balmy 22℃! Thanks to its temperate ocean climate, the Indian Ocean is home to some of the most spectacular and serene underwater scenes. Giant, colorful coral reefs provide shelter and sustenance to myriads of fish, anemones, crustaceans, and sharks; while migratory whales seek the warm ocean currents to breed and nurse their young.

The Indian Ocean is littered with thousands of islands, and this abundance of ocean life has sustained its inhabitants for thousands of years.

Find out why most people choose a rash guard when swimming in warm and tropical temperatures.

What is interesting though, is that since the Indian Ocean is the warmest, there is limited marine life in this region due to the higher water temperature.

How Does the Ocean Warm Up?

All of the oceans are warmed by the sun’s radiation. Water has a higher heat capacity than air, meaning that it takes longer to heat up but retains heat for much longer than air does.

The surface layers of the ocean capture the sun’s energy and store it. Due to the angle of the sun at the equator versus the poles, the oceans along the equator will receive much more of the sun’s energy than the Arctic or Southern oceans, making them warmer overall.

This temperature difference varies as the seasons do and is even more exaggerated in the winter – with each pole receiving even less of the sun’s energy in the cold, dark months.

Only the first few feet of the ocean’s surface absorb the sun’s energy. This gives rise to a temperature gradient as depth and pressure increase. But it is not only depth which defines ocean temperatures; currents also play a huge role in temperature variations and are largely responsible for sculpting our oceans’ ecosystems as they exist today.

How do Currents Affect Ocean Temperature?

There are many currents which transport warm and cold water alike, and many factors which determine these currents: the earth’s rotation, the moon’s gravity, wind, salinity, depth contours and coastlines to name a few. One of the most well-known and widely studied ocean currents is the Gulf Stream.

It begins in the warm Caribbean Sea and flows all the way past North America, across the North Atlantic Ocean to western Europe, terminating in the Arctic Ocean. The warm water current forces the colder, denser water at its destination to sink, and flow southwards – completing the giant loop of flowing warm and cold water in the North Atlantic. This is just one example of five gigantic ocean “gyres”.

The remaining four are located in the Indian Ocean, North and South Pacific Oceans and the South Atlantic Ocean. Each gyre is a powerful, circular flow which spans thousands of miles, pushing great volumes of water clockwise (northern hemisphere) or counter-clockwise (southern hemisphere). These essential currents keep our ecosystems in balance and are vital for all life on earth as we know it.

Wetsuits are perfect when the ocean temperature drops. Find out how they keep you warm even in a cold current!

What are Ocean Thermoclines?

A more local manifestation of currents is known as a thermocline. If you have ever been on a dive and noticed an immediate change of temperature below a certain depth, it is likely that you have experienced one. Thermoclines in the ocean are generally caused by an upwelling of colder water as it meets a warmer surface layer. The resulting temperature change can be quite drastic – a few degrees Celsius – and can often look like a wall of shimmering glass, as light is refracted differently at the boundary between the two contrasting currents.

Use this handy wetsuit temperature guide before you get into the ocean.

Which Ocean is the Coldest?

Contrary to what you might think, the Arctic Ocean is actually the coldest ocean; even though the Southern Ocean surrounds the frozen continent of Antarctica. The average surface temperature in the Arctic is an astonishing -1.8C (28.6F) – just above the freezing temperature for saltwater. Despite their uninviting temperatures, cold water environments are far from barren and desolate. Many whales, seals and fish thrive here – even cold water corals, too. Each is specially adapted to survive in extreme cold, just as warm water species are adapted for their climate. 

The different between ‘Ocean’ and ‘Sea’

“Ocean” and “sea” are often used interchangeably. Perhaps this is due to historic tales of intrepid explorers, roaming the “Seven Seas” in search of new lands and fabled riches. Today, however, there is an accepted distinction between the two.

Our one world Ocean is divided into five regional oceans: The Antarctic, Pacific, Indian, Arctic and Southern. Each one lies between at least two continents; each with its own distinct characteristics.

“Seas” are smaller bodies of water which lie within an ocean, bordering a land mass. For instance, the Bali Sea, Flores Sea and Savu Sea all lie within the Indian Ocean, but are separated by the various island coastlines of Indonesia. More SCUBA diving takes place in seas as opposed to the open ocean. Dive sites are more accessible, and their waters are better protected against storms and large swells; but still benefit from a huge diversity of marine life and underwater landscapes to explore.

There is an unending bounty of life and landscapes in our oceans. As we learn more of the wonders that lie beneath the surface, we learn more of the fragility and intricacy of our world’s ecosystems. Divers play a huge role in this unfolding discovery as citizen scientists, photographers and storytellers who inspire others about our oceans and all that they hold. Whether a faithful warm or cold water diver – or a mixture of the two – there is no doubt that you have an endless ocean of wonders to explore.

The Lure of the Ocean

Personally, I prefer knowing which ocean is the warmest to choose a more comfortable dive temperature. And while every portion of the ocean is different, each part attracts intrepid divers to take the plunge and discover what lies beneath. Whether warm water, cold water, coral reef, or shipwreck; there is a whole world of beauty under the ocean’s surface which we have only just begun to understand.