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  • Moetasim Ashfaq

A plausible explanation for UAE floods

The recent UAE floods on April 16, 2024, have sparked a flurry of discussions online. These unprecedented floods, not witnessed in the past 75 years of recorded history, have led to various explanations, including links to climate change, ENSO, urbanization, and cloud seeding. As researchers studying this region, focusing on understanding the drivers of precipitation variability and change, we can offer a detailed and alternative explanation of the processes that might have contributed to this episode and the overall wetter April in central Southwest Asia.  




  Animation 1: The accumulated daily precipitation anomaly between November 2023 and April 2024. The anomaly is with respect to the 1981-2009 period.


The background climate


The Arabian Peninsula is mostly an arid region with maximum annual precipitation ranging between 150 to 300 mm. The so-called Arabian Peninsula rainy season stretches between November and April. Western disturbances are the main rain-producing systems. The predominant moisture sources for the first few months are the Mediterranean/Caspian Sea and the western Indian Ocean. The Mediterranean/Caspian Sea contribution reduces in the late season (Mar-April), while the western Indian Ocean becomes the dominant source. A noticeable contribution also comes from the tropical Atlantic and precipitation recycling over tropical African land in the late season. These increasing contributions from the South are consistent with the start of the seasonal march of the intertropical convergence zone toward the Northern Hemisphere.

 


Figure 1. Moisture Sources over the Arabian Peninsula during the rainy season. Note the higher contributions from the western Indian Ocean, tropical Atlantic, and African continent. The figure is taken from Horan et al., 2023.


ENSO is one of the major contributors to rainfall variability over the region, but the arid climate warrants that no teleconnections are super strong.  Apart from that, ENSO’s relation with central southwest Asia, including the Arabian Peninsula, is complicated given the vicinity of the Indian Ocean.  Without getting too technical, I can note that inter-basin interaction plays a major role in ENSO’s teleconnection. These interactions (via atmospheric bridging) are stronger at the start of the rainy season but weaken as the season progresses, and so does the direct ENSO role.

 

Inter-basin interaction leads to a strong precipitation dipole in the Indian Ocean, the main mediator of ENSO’s influence over central Southwest Asia through atmospheric diabatic heating anomalies and Rossby wave response. However, this Indian Ocean precipitation dipole is also the leading mode of variability and exists throughout the year, even without ENSO. On average, the relationship between ENSO and the Indian Ocean precipitation dipole is weakest in spring.

 

Knowing this background information and prevailing conditions in the tropical oceans, we have a reasonable explanation for UAE floods and ongoing wetter-than-normal conditions throughout the Arabian Peninsula and Central Southwest Asia.  

 

Was UAE flooding an isolated event?


UAE flooding was not an isolated event. The neighboring Oman experienced a much bigger precipitation anomaly during the last week of March. The Arabian Peninsula and Pakistan generally received much higher precipitation anomalies in March and April. Interestingly, most of Central Southwest Asia experienced a severe precipitation deficit until the first few months of the winter season, but by the end of April, it had higher-than-normal accumulated precipitation anomalies. Therefore, UAE flooding was among the sequence of unusual weather events the region experienced during March-April. The animation of accumulated precipitation anomalies explains the region's transition from drier to wetter than average between November and April (Animation 1).


What are the potential causes?


This spring, we are confident that the Indian Ocean played (and continues to play) a key role in providing both dynamical and thermodynamical forcing for excessive rains in the Arabian Peninsula and other parts of Central southwest Asia, leading to record flooding in Oman and UAE during March and April and exceptionally wet April in Pakistan. Stronger precipitation anomalies in tropical Africa and record sea surface temperatures in the tropical Atlantic also have a potential role, as both are significant moisture sources over the Arabian Peninsula during spring (see Figure 1). Evidence of moisture transport from the Indian Ocean, Congo Basin, and tropical Atlantic is visible in the satellite imagery between March 23 and May 2 (Animation 2).




Animation 2: Terra/MODIS daily images between March 23 and May 02 showing moisture transport from Africa, the tropical Atlantic, and the western Indian Ocean.

 

The substantially warmer-than-usual western Indian Ocean—the key moisture provider in the late rainy season over the Arabian Peninsula—has sustained the excessive moisture supply in rain-producing weather systems throughout March and April, supplemented by moisture from overly warm tropical Atlantic (Figures 2, 3). Additionally, The OLR anomalies in the Indian Ocean in April are indicative of a diabatic heating dipole in the atmosphere (Figure 4), which, according to our published research work, contributes to maintaining an anomalous subtropical westerly jet in the vicinity of warm waters potentially through an atmospheric Rossby wave response.   

 


Figure 3. Current state of sea surface temperature anomalies. Note the very warm western Indian Ocean and the tropical Atlantic.



Figure 4. OLR anomalies in April 2024, taken from https://psl.noaa.gov/map/clim/olr.shtml


Did ENSO have a role?


For several reasons, we are not confident of ENSO’s direct role in causing these anomalies. First, ENSO’s strongest teleconnection with this region is in the early part of the rainy season (November and December). Even that connection is primarily mediated through the Indian Ocean when inter-basin interaction is strongest. ENSO’s teleconnection varies intra-seasonally and loses most significance during the rest of the season. Second, a weakening of ocean-atmospheric coupling was reported in early April, consistent with the ENSO’s characteristics during its transitioning phase. Therefore, one can reasonably assume that the current diabatic heating anomalies in the Indian Ocean are independent of ENSO. Third, based on our published work, we know through data analyses and model experiments that atmospheric diabatic heating anomaly in the Indian Ocean, independent of ENSO forcing, can significantly increase the chances of above-normal rains in Oman, UAE region in April (Figure 5).  

 


Figure 5. Indian Ocean forcing teleconnection with precipitation variability in April, independent of ENSO. Note excessive rainfall over the Oman/UAE region. The Indian Ocean forcing is defined as the leading mode of precipitation variability that causes anomalies in atmospheric diabatic heating and generates Rossby wave response. The contours represent geopotential height anomalies at 200hPa. Note the negative geopotential height anomaly that would favor the southward extension of the subtropical westerly jet. The figure is taken from Abid et al., 2020.


It is also worth noting that this year’s ENSO teleconnection has been quite anomalous over this region. The strong positive phase of ENSO means above-normal precipitation in Central Southwest Asia, particularly during the earlier half of the season. However, ironically, the first two months were the driest throughout the region (See Animation 1), even with very strong ENSO. I explained the drivers of unusual ENSO teleconnection in the CSWA region in a recent blog, relating it to strong out-of-phase atmospheric variability. While we have not investigated the role of atmospheric variability in causing the wettest April over UAE, Oman, and the rest of Central Southwest Asia, it may have a potential role worth investigating.


Nevertheless, it is important to note that extremes in the Earth system result from the compounding of multiple factors. Warm tropical oceans and anomalous dynamical forcing in the Indian Ocean are likely among the leading causes of unusual patterns in Central Southwest Asia and the Arabian Peninsula, but they may not be the only causes.


Did climate change, urbanization, and cloud seeding play a role?


As noted, UAE flooding was not an isolated event. The neighboring Oman experienced a much stronger precipitation anomaly around the same period. However, Oman's extreme precipitation and flooding did not receive as much media attention as the UAE flooding, likely due to their occurrence in a sparsely populated region. Other regions, such as northwestern Pakistan and parts of Kashmir, also received record precipitation in April and are only sparsely urbanized. Therefore, this evidence suggests that while urbanization may have exacerbated flood conditions in the UAE, it was not the primary driver of these extremes. Similar reasoning also supports the point that cloud seeding is not a factor in UAE floods. Cloud seeding is overly hyped - it, in no way, is capable of matching the power of nature. 


We cannot conclusively identify climate change as the direct cause of these unusual patterns throughout the region. The unprecedented and ongoing warming record since the latter half of 2023 is remarkable. It likely has a role in causing unusual global teleconnections, atmospheric variability patterns, and evaporative moisture sources, leading to the unfolding of unseen states in the Earth system. However, what has caused this unprecedented run of record temperatures still has to be identified. Other factors besides anthropogenic factors may contribute to this continuous warming in global oceans and land regions, but scientific understanding is needed to determine this. In any case, attributions are often more reliable when based on long-term statistics.

 

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