Guest post by Richard Willoughby
The solar input to Earth’s climate system is dominated by the daily rotation causing night and day. The daily temperature cycle can be significant but the thermal inertia of the atmosphere and oceans dampen the temperature swings. There is a significant annual cycle that is observed across the planet as seasons. In temperate regions there are four seasons identified as Spring, Summer, Autumn and Winter. In tropical zones the seasons are best described as Wet and Dry. These seasons are the result of the tilt of Earth’s rotational axis relative to the orbital plane around the sun. The elliptical orbit of the Earth around the Sun also varies the intensity of the insolation at the top of the atmosphere during the annual cycle. In the present era, the precession cycle results in perihelion occurring in early January and aphelion occurring in early July.
Insolation over Ocean and Land
The distribution of land and water surfaces over the globe has a significant bearing on the insolation that these different surfaces of the globe receive. Figure 1 depicts the December 2019 insolation over oceans and land.
It is important to observe the significant difference in the insolation arriving over the oceans compared with land. For the December 2019 the oceans were exposed to an average insolation of 397W/m2 while land was exposed to an average of 273W/m2.
Net Average Power Flux
The oceans and the atmosphere above them operate as Earth’s solar collectors. Not all insolation reaching the top of the atmosphere enters the climate system. Some is reflected and remains short wave radiation. Some insolation enters the atmosphere and is thermalised resulting in temperature rise at the surface and in the atmosphere as well as latent heat of evaporation from water or moist land surfaces. The surface and atmosphere radiate infrared radiation back to space. Over the long term, there is an energy balance between incoming insolation and the energy reflected or thermalised to be emitted as infrared radiation. However the climate system never achieves a state of equilibrium; over any given period there is usually a Net Flux imbalance. Figure 2 displays the net power input (+ve) or lost (-ve) from ocean and land including the atmosphere above.
The Oceans and the atmosphere above act as solar absorbers. In December 2019 the area average absorbed power flux was 47.6W/m2. The Land and atmosphere above the land were net energy radiators in December 2019, losing an area average power flux of 56.2W/m2.
Energy Transfer Ocean to Land
Land and the atmosphere above it have limited thermal capacity by comparison with water and the atmosphere over the oceans. However it is well known the water evaporated from the oceans can condense over land thereby transferring latent heat from oceans to land. Figure 3 is a pictorial representation of this process assuming all the heat lost by the land in December 2019 was entirely balanced by transfer of latent heat by water evaporation from the oceans to condense over land.
The area average net power flux of 47.6W/m2 into the ocean system during December 2019 equates to 43.9ZJ for the ice free oceans. For the same period, the land system lost 25.7ZJ. Assuming all the land system losses were directly balanced by latent heat transfer, the oceans lost 25.7ZJ in latent to give a net heat gain of 17.2ZJ.
[Aside: A Zetajoule is 1E21 joules. For context, annual human primary energy consumption is 0.57ZJ]
Taking the latent heat of condensation as 2.4MJ/kg, the net amount of water transferred from ocean to land to balance the heat loss from the land is determined to be 10,702Gt.
The transfer of ocean water to land to balance the energy deficiency of land during December has significance in terms of the global energy balance. Figure 4 shows the distribution of atmospheric water over the globe in December 2019.
The total mass of atmospheric water in December 2019 was 10600Gt. This compares with 10702Gt of water that would be transferred to land to supply the net radiation energy loss from the land during the same period. Effectively, the entire atmospheric water mass has been cycled to land during the single month. It equates to net rainfall of 64mm averaged over the entire land area. By observing the high atmospheric water level over tropical land masses, it becomes apparent why the Amazon rain forest, Northern Australia, Malaysia, PNG, Indonesia and the Congo experience December rainfall of typically 200mm. The atmospheric water column rarely exceeds 6cm (60mm) so the high precipitation zones turn over the atmospheric water three times in a month.
The Annual Cycle
In the annual cycle July 2019 to June 2020, December 2019 has the highest atmospheric water turnover. Table 1 sets out corresponding data for the annual cycle by month.
Key points of note:
- January has the highest ocean insolation of 398W/m2
- January has the lowest land insolation of 258W/m2
- May is the only month where land insolation is higher than ocean insolation
- Net flux over oceans is positive for every month
- Net flux over land is negative for every month
- If all heat loss from land was balanced by latent heat transfer then 63,643Gt of water would be transferred from ocean to land and returned as river runoff – equivalent to net land precipitation or 381mm or net ocean evaporation of 186mm.
- The mass of atmospheric water was cycled from ocean to atmosphere to land 5.67 times during the year.
- The minimum ice free ocean surface temperature of 19.64C occurred in January when net heat input was at its peak of 48.7W/m2
- The maximum ice free ocean surface temperature of 20.18C occurred in August, which was just after the July minimum net heat input 1.1W/m2
The Ocean-Atmosphere Water Cycle
The significance of the last two points is usefully observed when the ocean net heat uptake and sea surface temperature are plotted over the annual cycle as in Figure 5.
The net heat into the oceans and the atmosphere above ranges from 1.1W/m2 in July to 48.7W/m2 in January. This large variation in heat uptake results in a relatively small temperature change of just 0.54C, suggestive of the thermal inertia of the oceans. However the temperature range is completely out of phase with heat input, indicative of the surface cooling as the water cycle increases while surface heating when the water cycle slows down.
The calculated annual net ocean to land water transfer of 63,643Gt is somewhat higher than the estimated 1901-2016 average river runoff of 39,828Gt determined by the Global Runoff Data Centre (GRDC). However estimates of net land rainfall and river runoff vary depending on the available measured data and method of reanalysis. The general conclusion with the various studies of the global water cycle is consistent with GRDC findings that the water cycle has experienced a downward trend since 1950 but with substantial annual swings ranging from 35,000Gt up to 43,000Gt as well as statistically significant decreasing and increasing regional trends.
A downward trend or slowing of the water cycle is consistent with an upward trend in sea surface temperature as observed by various sea surface data sets.
The ocean averaged net evaporation of 186mm is the result of very high net evaporation from tropical and sub-tropical regions where surface temperature ranges between 24 and 29C to net precipitation zones in the mid-latitudes where the ocean surface temperature is less than 15C. Evaporation rates can reach 7mm/day in the high evaporation zones like the nino34 region of the Pacific Ocean. High evaporation rates result in shoaling of the thermocline and deep ocean cooling as the water cycle intensifies. In contrast, the oceans retain more heat when the water cycle slows down as the thermocline steepens.
Insolation over the oceans peaked for the current precession cycle in 1585, being the last year that perihelion occurred before the austral summer solstice. Over the last 400 years, perihelion has been occurring later than the austral summer solstice and now occurs in early January. The watery end of Earth still points toward the sun when Earth is closest to the sun but the peak solar input and peak net energy uptake in the oceans has now moved into January and is slightly less than 400 years ago when it peaked for the current precession cycle. The global oceans are receiving less sunlight while the land surface is receiving more as the precession cycle progresses for the next 12,000 years. The water cycle is in gradual decline due to reducing difference between ocean and land insolation. The oceans are retaining more heat due to a reduction in net ocean evaporation; synonymous with the declining water cycle.
The data used to produce the various images and charts were downloaded from NASA’s Earth Observation web site.
Global runoff data referenced in the report was sourced from the Global Runoff Data Centre (GRDC)
Richard Willoughby is a retired electrical engineer having thirty years of experience in the Australian mining and mineral processing industry with roles in large scale operations, corporate R&D and mine development. A further ten years was spent in the global insurance industry as an engineering risk consultant where he developed an enduring interest in natural catastrophes and changing climate.