Where is the ocean densest

Climate FAQ 3.1 | Ocean warming

Yes, the ocean is warming in many regions, depths, and periods of time, but neither everywhere nor continuously. The warming is most evident when looking at values ​​averaged globally or over individual ocean basins over periods of ten or more years.

The ocean temperature everywhere can vary considerably with the seasons. It can also vary significantly from year to year - or even from decade to decade - due to fluctuations in ocean currents and heat exchange between the ocean and the atmosphere.

Ocean temperatures have been measured for centuries, but it was not until around 1971 that measurements were sufficiently comprehensive that one can reliably estimate the mean global temperature of the top hundred meters of the ocean for a given year. In fact, before the international Argo array - consisting of drift buoys measuring temperature and salinity profiles - achieved worldwide coverage in 2005, the mean global temperature of the upper ocean in any given year was dependent on the method used for the calculation.

The mean global temperatures of the upper ocean rose every decade from 1971 to 2010. Despite great uncertainties in most of the annual mean values, this warming is a reliable result. In the upper 75 m of the ocean, the mean global warming trend for this period is 0.11 [0.09 to 0.13] ° C per decade. This trend generally decreases from the surface to the middle water depths. It drops to around 0.04 ° C per decade at a depth of 200 m and to less than 0.02 ° C per decade at a depth of 500 m.

Temperature anomalies reach layers below the sea surface not only through mixing from above, but also via certain paths (FAQ 3.1, Figure 1). Colder - and therefore denser - water from higher latitudes can sink from the surface and then flow below the warmer, lighter water in lower latitudes towards the equator. In some places - the northern North Atlantic and the Southern Ocean around Antarctica - the seawater cools down so much that it sinks to great depths, even to the bottom of the sea. This water then spreads and fills a large part of the rest of the deep sea. If the near-surface water of the ocean warms up, then these sinking water masses also warm up over time, and this means that the temperatures in the interior of the ocean rise significantly faster than would be the case if they were mixed from the surface alone.

In the North Atlantic, the temperature of this deep water fluctuates from decade to decade - sometimes it warms up, sometimes it cools down - depending on the atmospheric conditions prevailing in winter. Around Antarctica, groundwater has been shown to have warmed between 1992 and 2005, possibly due to the intensification of westerly winds over the Southern Ocean and their shift to the south over the past few decades. This warming signal is detectable in the deepest and coldest bottom water of the world ocean, even if it weakens to the north in the Indian, Atlantic and Pacific Oceans. The rates of warming in the deep sea are generally lower than those at the ocean surface (about 0.03 ° C per decade since the 1990s in deep and bottom water around Antarctica, and lower in many other places). However, the warming occurs in a large volume, which is why the warming of the deep ocean contributes significantly to the overall increase in heat content in the ocean.

Estimates of historical changes in global mean ocean temperature have improved in recent years, largely thanks to the detection and reduction of systematic measurement errors. By carefully comparing inaccurate measurements with few but more precise observations at neighboring locations and at similar times, scientists have reduced some instrumental errors in the historical records. These improvements made it clear that mean global ocean temperatures have increased much more steadily from year to year than had been assumed before 2008. However, the mean global warming rate may not be uniform over time. In some years the ocean appears to be warming faster than the long-term trend, in others the rate of warming appears to be decreasing.

Its large mass and heat capacity allow the ocean to store large amounts of energy - more than a thousand times what the atmosphere would store with an equivalent increase in temperature. The earth absorbs more heat than it emits back into space, and almost all of this excess heat goes into the oceans and is stored there. The ocean has absorbed over 93% of the heat stored between 1971 and 2010 from warming air, sea, and land, and from melting ice together.

The ocean's large heat capacity and slow circulation give it significant thermal inertia. Near-surface ocean temperatures take about a decade to adapt to a climatic forcing, such as changes in greenhouse gas concentrations (Section 12.5). Therefore, if greenhouse gas concentrations could be kept at today's levels in the future, the warming of the earth's surface would slow down within a decade. However, deep-sea temperatures would continue to rise over centuries to millennia (Section 12.5), and therefore the sea level would also continue to rise over centuries to millennia (Section 13.5).