GPM In Position to Watch 2015’s El Niño
Since late in 2014, scientists in many different disciplines (including meteorologists, climate scientists, physical and biological oceanographers, hydrologists, and geologists) have been watching a slow-to-develop El Niño even in the tropical Pacific Ocean. After teasing observers with conditions that did not quite meet El Niño criteria1, the event finally reached official El Niño status in March and April, and is now expected to become a powerful event lasting into the next Northern Hemisphere winter. If these conditions, typified by warm sea surface temperatures (SST) in the tropical Pacific, persist, California might get some rain and snow to alleviate the fierce drought conditions the state has been under over the past four years.
But with an El Niño, which can wax and wane in intensity in a matter of weeks (and transform into a La Niña in a month-and-a-half, as it did in 1998), nothing is ever certain.
Accumulated rainfall data from the NASA Global Precipitation Measurement (GPM) constellation of satellites, which monitors precipitation around the world, are shown in Figure 1. This figure uses the Integrated Multi-Satellite Retrievals for GPM (IMERG) data set, which has a horizontal resolution of 0.1 degrees, and records precipitation data every half hour, to explore what the National Oceanic and Atmospheric Administration has called “the wettest month ever recorded [in the United States]" 2.
Figure 1 shows that an astonishing amount of rain - more than 30 inches – fell over the south-central United States during May 2015, especially over Texas, Oklahoma, Louisiana and Arkansas. In contrast, much of bone-dry California received very little to no rainfall during the same month of May, consistent with the current drought conditions. These high resolution IMERG data also show some other interesting rainfall patterns. In northwestern South America, for instance, there is an area of low rainfall coincident with the Andes Mountain range. This feature, caused by air drying as it descends the mountains, is often called a rain shadow.
Figure 2 displays the IMERG data for the continental United States in May 2015, showing where the heaviest amounts of rain fell in Texas and Oklahoma. As might be expected, this inundation saturated the ground with water, which contributed to severe flooding in many areas. Figure 3 is a map of total column soil moisture from the North American Land Data Assimilation System (NLDAS) for May 2015, plotted with Giovanni.
There is also a significant amount of rainfall over the tropical Pacific ocean, just north of the equator. This rainfall is the result of converging trade winds near the ocean surface, a well-known meteorological feature called the intertropical convergence zone (ITCZ). Convergence (when air masses flow toward each other horizontally) in the low levels of the atmosphere causes air to rise vertically, which subsequently causes it to cool and condense, often leading to rainfall. The ITCZ can be found year-round north of the equator in the east Pacific, and is commonly the location of strong thunderstorms, which are visible as a band of heavy cloud cover in geostationary satellite images of the hemisphere.
As noted earlier, the El Niño event has been developing over the eastern tropical Pacific during the past few months. Figure 4 shows surface skin temperature and 250 hPa geopotential height anomalies from NASA’s Modern-Era Retrospective analysis for Research and Applications (MERRA) dataset for April and May of this year. Surface skin temperature over the ocean is nearly identical to sea surface temperature (SST), which makes it particularly useful for studying the impacts of an El Niño. These anomalies were created by subtracting the 1979 - 2010 monthly mean values of this data variable.
When the Pacific is in an “El Niño – Southern Oscillation” (ENSO) neutral state, which means it is neither in an El Niño nor a La Niña state), the warmest waters are found in the Pacific warm pool which straddles the equator near Indonesia (see annotation in Figure 1). This warm water migrates eastward during an El Niño, which causes the surface water in the central and east Pacific to become anomalously warm. This process is evident in Figure 4, as the positive SST anomalies over the central Pacific during April migrated eastward through May and June.
Warm SSTs can have an important effect on the atmosphere, which is particularly evident in the April and May maps in Figure 4. Warm ocean temperatures lead to atmospheric convection, which often results in large clusters of thunderstorms. High within the clouds, water vapor condenses into liquid water and releases energy into the air, a process known aslatent heat release. (The same process provides hurricanes with their energy.) This leads to upper-level ridge formation, and can be perceived as the solid black contours near 210°E, marked by an “R” in the maps. This ridge is known as a Rossby wave source, as it generates a Rossby wave train that emanates from the convection and typically follows a great circle path, bending to the northeast. The wave trains have been marked on the maps using alternating Rs and Ts to show the ridges and troughs.
The relationship between the warm SSTs and the Rossby wave train is often called an “atmospheric teleconnection”. This is the primary mechanism by which the tropical oceans exchange energy with the atmosphere. These teleconnections are important, because they can affect weather around the world, and thus we can tie this particular teleconnection to the rainfall map shown in Figure 1.
The persistent trough over the southwestern United States and the ridge to its northeast are common features during the spring season with an El Niño present in the Pacific. The counterclockwise flow around the trough, paired with the clockwise flow around the ridge over the interior of the US, leads to a predominantly southerly flow that advects moisture from the humid tropics into the central US. This weather setup is likely at least partially responsible for the record-breaking rainfall in May over the central US, and is consistent with El Niño effects observed for past events.
It is impossible to attribute the excessive rainfall during May solely to El Niño without running controlled experiments using specialized weather models. We can, however, say that the atmospheric flow during May was consistent with other El Niño events, and this gives us confidence that the current El Niño was at least partially responsible for the record breaking rainfalls.
This article demonstrates how NASA’s GPM and MERRA datasets allow us to monitor weather and climate around the world. In particular, the high resolution IMERG dataset allows us to see a remarkably detailed view of global rainfall. As El Niño continues to influence the oceans and atmosphere in the coming months of 2015, these data will be particularly important to scientists seeking to better understand and characterize the diverse linkages of El Niño.
The same day this Featured Article was published on the GES DISC Web site, we learned about the paper linked below, which was published online August 7 in Geophysical Research Letters, a "fast response" scientific journal. The more detailed analysis and description in the paper is very similar to the discussion above.
Wang, S.-Y. S., W.-R. Huang, H.-H. Hsu, and R. Gillies (2015). Role of the strengthened El Niño teleconnection in the May 2015 floods over the southern Great Plains. Geophysical Research Letters, 42, doi:10.1002/2015GL065211.