Irving Berlin wrote the song "Heat Wave", and much of the United States joined in the chorus during the tropical heat wave month of July 2011, a month that brought record high temperatures to many places in the United States. In particular, Texas and Oklahoma experienced record high temperatures, with Oklahoma recording the highest-ever statewide temperature in the National Oceanic and Atmospheric Administration's records dating back to 1895. As data from the Atmospheric Infrared Sounder (AIRS) on the NASA Aqua satellite show, temperatures in the surface air in large areas of Texas and Oklahoma were persistently above 100° F during the day, for at least a month (Figure 1). At night, not much relief was provided, with low temperatures persistently close to 90° F. The positive temperature deviations in these places measured close to 20° F, day or night, as compared to the average July temperatures for the past eight years of AIRS observations (Figure 2).
Figure 1. AIRS surface air temperatures, for night (upper images) and day (lower images), for the month of July, 2011.
Figure 2. Anomalies of surface air temperatures from Figure 1, with respect to the AIRS climatological base period of 2003-2011. Click on either figure to see a much larger full-size version.
It is normal to expect heat waves that come and go in midsummer – but what caused a heat wave that was so persistent?
What immediately attracts attention is anomalously warm surface air above the north subtropical Atlantic and Pacific (Figure 2). Anomalies of this sort normally lead data users to look at atmospheric pressures in the lower troposphere. Geopotential heights are available in AIRS retrievals, and they reveal major domes of high atmospheric pressure at the surface (Figure 3). Such domes of high atmospheric surface pressure are not unusual: they are associated with the Hadley Cell circulation, and normally intensify in July in these regions (Pidwirny, 2006). However, what was different this summer is that these domes were stronger than normal, and apparently the areas of the maximum pressure also expanded poleward (Figure 3b). Such expansion of the Hadley Cell circulation has been reported before in the literature (Hu and Fu 2007) and this year may fit well into that picture
Figure 3. (a) Geopotential height (GPH) at 925 mb from AIRS, and (b) the anomaly of the GPH. Click on the image to see the full-size version.
It is worth noting that even though the pressure anomalies were small (Figure 3b), they could drive changes in the atmospheric circulation sufficient to break heat records at the surface. The configuration of the North Atlantic high pressure in July 2011 has been consistently driving surface winds from the warm and humid tropics into much of the continental United States (Figure 4). The result is apparent – as AIRS data show, much of the U.S.A. and Mexico were under distinctively higher humidity, and this humid air contributed greatly to keeping nighttime temperatures at anomalously high levels.
Figure 4. Winds from the Goddard Earth Observing System Model (GEOS-5) data assimilation, and water vapor mixing ratio from AIRS, all at 850 mb. Click on the image to see the full-size version.
Winds and humidity at the lower troposphere (Figure 4) are normally considered in conjunction with the winds aloft. Figure 5 indicates that the jet stream was also taking an unfavorable poleward path over the U.S.A. In this figure, the jet stream is clearly bulging over the high pressure systems aloft. The high-pressure dome over the U.S.A. is the strongest in the box outlined by the figure, and has been persistently present during the month of July, preventing the jet stream from producing low-pressure systems that would normally bring both cooler air and precipitation at the surface.
Figure 5. Geopotential Heights from AIRS, and winds from GEOS-5, all at 300 mb. The jet stream was forced northward, such that cooler air to the north could not move south. Click on the image to see the full-size version.
One irony of this situation was that Texas, Oklahoma, and the southern parts of Kansas had abundant water vapor in the air (Figure 4), as a result of southerly winds and evaporation. But this moisture didn't have the right environment to rise, condense, and precipitate (Figure 6). The moisture was likely advected away from the Midwest, and the states benefiting from this moisture in terms of precipitation were mostly states in the north and east (Figure 6). Note that in the lower troposphere over some parts of Texas, in spite of the very high humidity, the TRMM precipitation algorithm did not indicate any rain for the entire month of July – a picture similar to the dry Sahara!
Figure 6. Tropical Rainfall Measuring Mission (TRMM) accumulated precipitation for the month of July 2011. Click on the image to see the full-size version.
Hu, Y. and Fu, Q. (2007) Observed poleward expansion of the Hadley circulation since 1979. Atmos. Chem. Phys., 7, 5229-5236, doi:10.5194/acp-7-5229-2007.
Pidwirny, M. (2006). "Global Scale Circulation of the Atmosphere". Chapter 7p, Fundamentals of Physical Geography, 2nd Edition. http://www.physicalgeography.net/fundamentals/7p.html
As this article was "in press", we decided to also glance at the most recent global precipitation, retrieved by TRMM, in particular the first ten days of August. The accumulated precipitation data from the TMPA-RT retrieval are imported into a KMZ file (GoogleEarth, snapshot on the left) that can be downloaded.
Readers may find it interesting to see almost exactly Texas in a precipitation "hole". Even though the real-time TMPA-RT has less accuracy than the standard precipitation products that come later, this picture is likely to be close to the reality on the ground.
- Text and images by Dr. Andrey K. Savtchenko; editorial assistance by Dr. James Acker.
- AIRS data are distributed by GES DISC at NASA Goddard Space Flight Center. AIRS is managed by NASA's Jet Propulsion Laboratory, Pasadena, Calif., under contract to NASA. JPL is a division of the California Institute of Technology in Pasadena.
- The TRMM data used in this article are part of the Tropical Rainfall Measuring Mission (TRMM). The algorithms were developed by the TRMM Science Team. The data were processed by the TRMM Science Data and Information System (TSDIS) and the TRMM office; they are archived and distributed by the GES DISC. TRMM is an international project jointly sponsored by the Japan National Space Development Agency (NASDA) and the US National Aeronautics and Space Administration (NASA) Office of Earth Sciences.
- The GEOS-5 data used in this article have been provided by the Global Modeling and Assimilation Office (GMAO) at NASA Goddard Space Flight Center.