By Andy May
In previous posts, see here and here, I have tried to show that since the oceans cover 71% of the earth and contain 99% of the thermal energy stored on the earth's surface, they dominate the speed and extent of climate change. In all of my posts, the surface of the earth is defined as everything from the ocean floor to the top of the atmosphere. The details of the calculation of the heat content in the oceans and in the atmosphere are given in this table. The enormous heat capacity of the ocean prevents large temperature fluctuations and dampens and delays those that occur.
Trying to show the direction, speed and extent of climate change by measuring and averaging the atmospheric surface temperatures is pointless in my opinion. The record of atmospheric and ocean surface temperatures is too short and far too imprecise to provide useful trends on a climatic timescale (over 30 years). Furthermore, these recordings are sporadic measurements in a chaotic surface zone with large temperature fluctuations. For example, in Montana, USA, recent minimum / maximum temperatures have been as low as -57 ° C (-70 ° F) and as high as 47 ° C (117 ° F). These enormous fluctuations make it extremely difficult to measure the global mean differences of 0.1 ° C from year to year. However, this is the precision required to correctly characterize a climate that is only warming at a rate of about 1.4 ° C / century, i.e. 0.014 ° C per year and 0.14 ° C / decade.
The measurements are particularly useful for predicting the weather but are unsuitable for measuring changes of less than half a degree over climatic time periods. To do this, we need to measure something more stable and less chaotic. This and the next post show that the mixed layer in the ocean seems well suited for the task.
In my opinion, we need to deal with ocean temperature changes to properly control the direction and speed of global warming. Especially those parts of the ocean that are in constant contact with the atmosphere. Climate change over a period of more than a thousand years affects the entire ocean. But for periods of a hundred years or less we are mostly dealing with the top few hundred meters of the ocean.
The temperature profile of the upper ocean is very complex. This is made difficult by the poor quality of our sea surface temperature measurements, especially before the advent of Argo floats and modern measurements of ocean buoys like the Triton buoys in the last 20 years. Ships cover a limited area of the ocean, and the depth, consistency, and quality of their temperature measurements are uncertain. Satellite measurements of the top of the ocean are possible, but these measurements are made difficult by the so-called ocean skin effect.
The ocean skin
Temperatures change rapidly at the interface between ocean and air. The extent of the change and the thickness of the uppermost affected ocean are determined by the cloud cover, whether day or night, and the wind speed. This “skin” is thicker on calm, cloudless days and thinner at night and on windy, cloudy days. The temperature at the ocean-air interface ("SST") is measured using radiometers and satellites. Unfortunately, the relationship between this temperature and the more stable mixed bed temperature, or "foundation" temperature, is unknown. The relationship changes quickly and is complicated. Several models have been proposed (Horrocks, O'Carroll, Candy, Nightingale & Harris, 2003) but none have the required reliability and accuracy.
To make matters worse, there is a population of cyanobacteria directly on the surface, which changes the temperature and lowers the salinity of the surface water (Wurl et al., 2018). The sea surface temperature problem is best illustrated by the graph in Figure 1 from GHRSST or the High Resolution Sea Surface Temperature group. They strive to understand the skin layer of the ocean so that satellite measurements of sea surface temperature can be properly combined with measured sea temperatures.
Figure 1. The GHRSST plot of sea surface temperature. Theocean conditions, especially the wind speed and the time of day or night, can cause a temperature difference of 2.5 ° C or more in the temperature gradient from the surface to the base temperature (stable part of the mixed layer). The depth to the top of the stable portion of the mixed layer can vary from substantially zero to 10 meters. Source: GHRSST.
The temperature difference between the SST and the stable part of the mixed shift can be three to six degrees per day (Wick & Castro, 2020). As Gary Wick and Sandra Castro explain:
“The daily cycle of solar radiation leads to a periodic warming of the near-surface layer of the ocean. At low wind speeds, the turbulent mixing is reduced, and during the day a warm layer and a daily thermocline can form near the sea surface. Mixing typically erodes this layer at night. While the amplitude of daily warming is relatively low on average (0.5 K), the surface warming detected by satellites can be of great importance under conditions of very low wind speeds and sufficient solar radiation. In situ observations have shown warming greater than 5 K at depths of 0.3-0.6 m. Satellite observations from multiple sensors have observed extreme warming events of up to 7 K on the surface, and it has been suggested that events above 5 K are not uncommon. "(Wick & Castro, 2020)
The temperatures in the quotation are given in Kelvin (K) and correspond to degrees Celsius. The main point is that on calm (cloudless) days in calm conditions, there are exceptionally large differences in the ocean's SST. Figure 1 shows that the rise in temperature can affect water up to a depth of ten meters. However, differences of more than 0.5 ° C are almost always limited to the top meter of the ocean. As we will see in the next post on the mixed layer, these known skin anomalies are ignored in ocean temperature data sets. They often have a measurement labeled zero depth, but it is taken below the surface, usually at a depth of 8 inches or more. The mixed layer temperature is often defined as the temperature of the layer whose temperature is within 0.5 ° C of the surface temperature (Levitus, 1982). This isn't exactly what they mean, the temperature of the ocean is just below the surface, maybe 8 to 100 inches. Except on clear, windless days, this is the "basic temperature". At night and on cloudy or windy days, the temperature is always the "basic temperature".
The mixed layer has homogeneous properties due to the turbulent mixing, and the use of a temperature difference limit of 0.5 ° C is a convenient definition, but it collapses near the poles in winter when a more complex definition is required. Numerous methods have been suggested, too many to list here, but the complex method described by James Holte and Lynne Talley (Holte & Talley, 2008) is currently preferred. Their technique is widely used today to choose a "mixed layer depth" that forms the bottom of the mixed layer. This is necessary because in the polar regions in winter, deep convection caused by surface heat loss can mix the water column to 2000 meters or even lower. As we will see in the next post, heat energy is transferred from the surface to the deep sea in these areas.
There are many data sets on sea temperature and we will discuss the data from several in the next post. Figure 2 is a graphic representation of the global average sea temperatures in December from the surface up to 140 meters from the datasets of the University of Hamburg. This diagram shows the temperature profile terms we discussed with real global data.
Figure 2. December global average temperature profile from surface to 140 meters. This graphic shows the "basic temperature" in the mixed layer, the temperature is almost constant from the surface up to about 20 meters and then begins to decrease. As soon as the temperature just below the surface differs by 0.5 ° C, the "mixed layer depth" is reached. Data source: University of Hamburg.
The temperatures reported by the University of Hamburg are average temperatures over more than 12 years and do not represent a specific year. The NOAA MIMOC temperatures, which we'll cover in the next post, are the same. Figure 3 shows the average measurement year and the standard deviation of the years.
Figure 3. The data used in Figure 2 is not from a single year, but from the average of the data over 12 years. The central year for each depth is shown in blue (left scale) and the standard deviation of all years used is shown in orange (right scale). Data source: University of Hamburg.
Both Universität Hamburg and NOAA acknowledge that the Argo data, which makes up most of their raw data, is sparse. There is one swimmer for every 3 ° latitude and longitude (~ 32,913 square miles at 40 ° north or south or 84,916 square kilometers). This swimmer sends us a full profile every ten days. The university and NOAA have decided to take monthly averages of all data found to combat the lack of data. As we'll see in the next post, the big changes happen in the mixed shift by month and by location, so this makes sense.
There are other zones identified in Figure 1 above the foundation or mixed layer. These are defined by GHRSST as follows. I've edited the GHRSST text for the sake of clarity. The original text can be viewed here.
The interface temperature (SSTint)
At the exact air-sea interface, a hypothetical temperature is defined, called the interface temperature (SSTint), although this has no practical use as it cannot be measured with current technology.
The skin sea surface temperature (SSTskin)
Skin temperature (SSTskin) is defined as the temperature measured with an infrared radiometer, typically operating at wavelengths from 3.7 to 12 µm (chosen for consistency with most infrared satellite measurements) that diffusion-dominated the temperature within the conductive Underlayer at represents a depth of ~ 10-20 microns. SSTskin measurements are subject to a large potential daily cycle, including cool skin layer effects (especially at night with clear skies and low wind speeds) and warm layer effects during the day.
The subcutaneous sea surface temperature (SST subcutaneous tissue)
The subskin temperature (SSTsubskin) represents the temperature at the base of the conductive laminar underlayer of the sea surface. For practical purposes, SSTsubskin can be approximated well to the measurement of surface temperature by a microwave radiometer operating in the frequency range of 6-11 GHz, but the relationship is neither direct nor immutable to changing physical conditions or the specific geometry of the microwave measurements.
The surface temperature in depth (SSTz or SSTdepth)
All measurements of the water temperature under the SST subcutis are known as deep temperatures (SST depth), which are measured with a variety of platforms and sensors such as floating buoys, vertical profile swimmers (like Argo) or deep thermistor chains at depths of 10 to 750 m ( like Triton and Tao). These temperature observations differ from those obtained with remote sensing techniques (SSTskin and SSTsubskin) and must be qualified by a measurement depth in meters.
The foundation temperature (SSTfnd)
The basis SST, SSTfnd, is the temperature that is free from daily (daily) temperature fluctuations. That is, the top of the stable part of the mixed layer. Only in-situ contact thermometry can measure SSTfnd.
In summary, it can be said that SST and atmospheric surface temperatures are too strongly influenced by weather and daily variability to be able to measure climate changes reliably or precisely. Total ocean or deep sea temperatures give us an indication of long climate changes of a thousand years or more, but they say little about changes in the range of a hundred years.
The mixed ocean layer is a zone that begins between one millimeter and about ten meters below the sea surface. Above this depth, temperatures are affected by the atmosphere and sunlight from minute to minute. At night, the top of the mixed layer moves closer to the surface, but it can be affected by wind speed, precipitation, and cloud cover. Below the mixed layer, the temperature is more stable than the atmosphere and the sea surface. The temperature, salinity and density of the layer are almost constant due to turbulence from top to bottom. It reflects surface temperatures but is a function of the average over the past few weeks. The thickness of the mixed layer varies seasonally from a few tens of meters to several hundred meters. We'll discuss the mixed layer in more detail in the next post.
None of that is in my new book Politics and Climate Change: A Story, But Buy It Anyway.
You can download the bibliography here.