By Richard Willoughby
May 2021
(The author estimates the availability of the NASA Earth observation satellite datasets used in this analysis.)
This is a three-part series that analyzes the role of atmospheric water in regulating the Earth’s thermal balance.
Part 1 is an analysis of the temperature of warm pools in the tropical ocean and the processes of temperature limitation.
Part 2 discusses the mechanism of deep convection and concludes with the persistence of clouds over the warm pools of the ocean.
part 3 examines the global energy balance of the oceans over an annual cycle from month to month to determine the role of atmospheric water in regulating the energy balance.
Part 1: Observed regulation of sea surface temperature – upper limit
Sea surface temperature observations show that less than 10% of open sea surface water exceeds an annual average surface temperature of 30 ° C. In addition, less than 1% of the sea surface exceeds 32 ° C for more than a few days. The thermostatic upper limit of 30 ° C for oceanic warm water pools has been scientifically observed since at least the 1970s and can now be easily observed worldwide using satellite measurements. Every year in April, warm pools reach their maximum extent of around 9% of the sea water surface, as both the tropical western Pacific and the Indian Ocean achieve the highest heat absorption. This is observed with reference to FIG.
Illustration 1: Global map highlighting tropical warm pools at or above 30 ° C, shown in red – April 2020.
The northern hemisphere continues to warm from April to August. The Pacific warm pools contract as they advance north and the warm pools of the Indian Ocean dissolve when the northern monsoons set in over the Arabian Sea and Bay of Bengal, while the Gulf of Mexico becomes a temperature regulating warm pool and the Persian Gulf becomes the warmest sea surface on the globe. Figure 2 highlights this transition compared to Figure 1.
Figure 2: Circumference of the warm pools in August 2019 – red over 30 ° C, yellow over 34 ° C.
By September, the solar zenith had moved south and the world’s oceans are in a cooling phase. The Gulf of Mexico and the Persian Gulf are cooling off quickly. Ocean warm pools are reduced to a minimum in September, with only 3% of the sea water surface being at or above 30 ° C (see Figure 3).
Figure 3: Minimum extension of the warm pool from September to September 2019
The upper limit of the surface temperature of 32 ° C becomes clearer when the proportion of the sea surface at a certain temperature is represented as shown in Figure 4.
Figure 4: Area of the sea surface at a certain temperature as a proportion of the total sea surface and the cumulative proportion of the area at or below the specified surface temperature.
More than 50% of the sea water surface exceeds 22 ° C, as shown in Figure 4 on the right, while the peak portion of the area is in the range of 28 ° C to 29 ° C, but drops sharply above 29 ° C, so that less than 1% of the The sea surface is warmer than 32 ° C, left plate.
Process control to limit the temperature of the warm pool
The reaction of the temperature limiting process to tropical warm pools can be observed at hourly intervals when the warm pools are present on the tropical moored buoys. The same temperature limitation process is observed in the three tropical oceans as shown in the series of temperature limitation tables for the sea surface in Figure 5.
Figure 5: Temperature limiting process in the tropical warm pools; Upper Pacific, Middle Atlantic, and Lower Indian (chart data from NOAA Pacific Marine Environment Laboratory).
Referring to the surface temperature control charts in Figure 5, the temperature limiting process begins when the surface temperature rises above 30 ° C on a clear sky before the local convection cycle begins. The cycle begins with a downpour, followed by a persistent cloud that reduces surface radiation to slow down the temperature rise. Moist air diverges from cooler adjacent zones to the warm pool, resulting in heavy rainfall that increases the rate of cooling in the warm pool. The extremely humid air was cooled over water at 28 ° C to 29 ° C, which absorbs heat at the surface due to the predominantly clear sky conditions above. The convergence of the moist air to the warm pool continues until it is no longer the warm pool. This can be seen particularly in the middle picture, where the Atlantic hot water pool cools below 30 ° C from day 22 and precipitation falls to zero if the hot water pool continues to move.
The annual cycle of warm pools
As mentioned earlier, the warmest regions of the tropical oceans can move noticeably all year round, but the equator usually observes the highest annual average surface temperature. Always close to 30 ° C in all warm pools, as observed in the series of south-north averages through warm pools shown in Figure 6.
Figure 6: Monthly and annual mean sea surface temperature from south to north – Upper West Pacific, Central Indian Ocean through the Arabian Sea, Lower Gulf of Mexico and East Pacific
The Persian Gulf becomes the warmest sea surface every August. It is the only sea surface that regularly exceeds 34 ° C in a year. This unique deviation from the temperature control function of the tropical oceans is highlighted in Figure 7.
Figure 7: Monthly and annual mean sea surface temperature from south to north divides 53.5 W by the Persian Gulf and Indian Ocean – August highlighted with red curve.
Performance of climate models in predicting sea surface temperature
The tropical warm pools pose a challenge to climate models as the models rely on the parameterization of the clouds rather than the actual physics of the convective instability that limits the maximum temperature of the sea surface. The U.S. National Oceanic and Atmospheric Administration provides a high quality data set for ocean surface temperature in the tropical Pacific that combines sea water temperature measurements observed at the tropical buoys interpolated between buoys using satellite imagery. Figure 8 shows the poor performance of the CISIRO and Bureau of Meteorology model, the Australian Community for Climate and Earth Systems (ACCESS) climate model in the Nino34 region in the tropical Pacific. The Nino34 region is between latitudes 5N and 5S from 120W to 170W. This region is considered to be an important indicator of global weather and, in particular, of the Pacific El Nino and La Nina modes, which affect weather conditions in the eastern states of Australia.
Figure 8: Comparison of the forecast and the background of the ACCESS climate model over five decades with the actually observed sea surface temperature over four decades.
It’s pretty obvious that the model has cooled the past extremely to maintain its warming trend. According to the ACCESS hindcast, the well-known 1998 El Nino in Australia may not have occurred as the temperature in this region is observed to exceed 28 ° C for El Nino conditions to form, as shown in Figure 8 in the measured data shown. The model output clearly shows the poor performance in this important weather-influencing region.
The uniqueness of the Persian Gulf provides another example of the poor performance of the ACCESS model in this region compared to the measured data in Figure 9.
Figure 9: Comparison of the measured and modeled surface temperature in the Persian Gulf for the month of August 1960 to 2020.
In August each year, the Persian Gulf becomes the warmest sea surface in the world. The unique, medium-level low humidity conditions prevent the formation of convective instability and the surface temperature regulation phenomena associated with a high-level convective cloud. Here the ACCESS model again delivers a bad result, but in this case up to four degrees Celsius cooler than the measured data.
Data sources
https://neo.sci.gsfc.nasa.gov/view.php?datasetId=CERES_LWFLUX_Mhttps://neo.sci.gsfc.nasa.gov/view.php?datasetId=CERES_SWFLUX_Mhttps://neo.sci.gs. nasa.gov/view.php?datasetId=MYD28Mhttps://neo.sci.gsfc.nasa.gov/view.php?datasetId=MYD28M
I’ve been using these sets for many months. All charts and images are created independently, that is, images not copied from these links.
There is also data from the moored buoys that I am referring to: https://www.pmel.noaa.gov/tao/drupal/disdel/
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