The steadily increasing global greenhouse gas emissions are causing the Earth to warm gradually. Most of this extra heat is stored in our seas and oceans. Water has a high heat capacity, meaning that a relatively large amount of extra heat is needed to raise the seawater temperature. However, this is something we have observed happening over the past decades. Warmer seawater itself will cause few problems for operations at sea. But the increasingly warm seas and oceans are also causing a worldwide change in weather patterns. These changes can have significant consequences for offshore operations. In this blog, you will read all about it.
More than 70% of the Earth's surface is made up of seas or oceans. The sun is the primary source of heat for our seas and oceans. Water has a relatively high heat capacity. As a result, a large amount of heat is required for a relatively small temperature increase. Due to increasing greenhouse gas emissions, the Earth is warming, adding an extra input of heat to the oceans in addition to sunlight. Waves and currents constantly mix the ocean, distributing the additional heat to deeper layers in the ocean or to cooler latitudes.
But the extra heat in the ocean does not disappear with the mixing. The aforementioned high heat capacity of water also makes it difficult for seas and oceans to cool down. Once warmed, water retains its extra energy poorly. As a result, oceans remain warm for a long time, and very extended periods of globally cooler weather are needed to cool the oceans. However, due to a warming climate, these longer periods of cooler weather are becoming increasingly rare.
The largest input of extra heat comes from the atmosphere. As a result, the upper layer of the ocean warms much faster than the deeper layers (see Figure 1). Figure 1 shows three layers in the ocean: 1. Surface (0m) - 2300 ft (700m) deep, 2. 2300 ft (700m) - 6560 ft (2000m) deep, and 3. deeper than 6560 ft (2000m). The dataset covers the period from 1993 to 2019. The figure shows the ocean heat content plotted against the number of years. In the upper layer of the ocean, a heat increase of 0.38 to 0.44 W/m2 can be seen over the period from 1993 to 2019. For the layer between 2300 and 6560 ft, this increase is 0.17 to 0.32 W/m2, and for the deepest layer, it is 0.07 W/m2. Over the entire depth of the ocean, this results in a heat gain ranging from 0.64 W/m2 to 0.83 W/m2. Naturally, these figures are averages and vary slightly for each ocean, but they provide a good picture of what is currently happening in our oceans. Less than 1 W/m2 of extra heat input seems very little, but if you multiply this by the number of km2 of ocean on our Earth, it amounts to a gigantic amount of extra energy being pumped into our oceans.
Figure 1: Ocean heat content increase over time for three different ocean layers.
The effect of warming seas and oceans on the weather varies greatly by location. In general, warmer seas provide more energy for systems such as hurricanes. However, it is not entirely clear that hurricanes become stronger as the seas warm. Multiple factors play a role here, such as wind shear. A hurricane can only form in areas with low wind shear. If there is high wind shear, hurricanes will not develop even with warmer oceans. If the wind shear temporarily decreases, the extra energy available in a warmer ocean could potentially contribute to the development of severe hurricanes.
In the temperate latitudes (where the North Sea is also located), there are different effects. Tropical systems do not occur here. Nevertheless, even in these regions, a warmer sea means more energy is available for 'bad weather systems' to develop. In winter, a large temperature difference between the upper atmosphere (air at 5 km altitude) and the sea surface can lead to severe showers. The greater this temperature difference, the heavier the potential showers can be. As the seawater warms, more energy is available for these showers to develop, making them potentially more severe. The development of low-pressure systems also depends significantly on the temperature gradient. A larger temperature gradient can lead to deeper low-pressure areas and, consequently, more wind.
In general, we can conclude that as the sea becomes warmer, it also contains more energy to generate severe weather. This increases the likelihood of heavy showers. Additionally, the temperature difference between the upper atmosphere and the water surface is also increasing. As a result, especially in the autumn and winter, heavier showers can occur above and around the North Sea. Moreover, a larger temperature gradient also has the potential to create deeper low-pressure systems. The relationship between warming seawater and deeper low-pressure systems is not a direct one-to-one correlation, as many other factors play significant roles. However, the increasing temperature gradient does potentially provide more fuel for the formation of deep storm depressions.
How much these developments will impact offshore operations remains difficult to say for now. While the sea is warming at an incredibly fast pace in climatological terms, it takes decades for the sea or ocean to warm by an average of one degree. By that time, our offshore operations might look very different from today. Nevertheless, it remains important to monitor these developments to continue ensuring our workable windows in the future.
References:
Johnson, G., et all., 2023. Ocean heat content [in “State of the Climate in 2022”]. Bull. Amer. Meteor. Soc., 104 (9), S145-S148, https://doi.org/10.1175/BAMS-D-23-0076.2.
Rhein, M., et all., Wang,2013: Observations: Ocean. In: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. doi:10.1017/CBO9781107415324.010.
Levitus, J. I. Antonov, T. P. Boyer, R. A. Locarnini, H. E. Garcia, and A. V. Mishonov. 2009. “Global Ocean heat content 1955–2008 in light of recently revealed instrumentation problems” Geophysical Research Letters, 36, L07608, doi:10.1029/2008GL037155.
P. Boyer, et all., 2009. World Ocean Database 2009. S. Levitus, Ed., NOAA Atlas NESDIS 66, U.S. Gov. Printing Office, Wash., D.C., 216 pp., DVDs.
Simpson, R. H. The hurricane disaster potential scale. Weather Wise 27, 169–186 (1974).
Bhatia, K.T., Vecchi, G.A., Knutson, T.R. et al. Recent increases in tropical cyclone intensification rates. Nat Commun 10, 635 (2019). https://doi.org/10.1038/s41467-019-08471-z
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