Tracing the footprints of an unusual wet spell

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Climate research experts at Abu Dhabi centre notice many global weather anomalies this year, including the recent rainfall in the UAE

Last week, the heaviest rainfall in more than a decade left many residents of the UAE and other Gulf countries soggy. While some played in puddles, or broke out umbrellas, those at New York University Abu Dhabi’s Centre for Prototype Climate Modelling tried to determine just what caused this unusual weather event.

Weather and climate enthusiasts often ask if a particular whether event is due to climate change. This question cannot be answered with certainty. It is too early to attribute last week’s rains — or any recent isolated weather event — to climate change. That kind of determination must come from a detailed comparison of the present climate to the past and future (projected) climates.

Scientists at our centre are in the process of such an investigation. In our recent work, we have found that 2013 is marked by many global weather anomalies, such as unusual jet stream patterns, strong monsoons, heat waves, intense snowfall over US and Europe — and last week’s rain here in the UAE.

The National Centre of Meteorology and Seismology in the UAE correctly forecasted and attributed the cause of the recent rain pattern to an atmospheric low pressure system that moved across the Middle East. However, we wanted to learn more about the origins of this weather event.

In mid-November, Cyclone Hilde formed off the southern coast of Greenland and brought high winds and rain to Norway, Sweden and Finland. Around the same time, another cyclone, Cleopatra, formed over the Mediterranean Sea and brought destructive rain and flooding to the Italian island of Sardinia. Cyclones occur rarely over the Mediterranean. The wave-like atmospheric flow controls the winter weather of mid-latitudes. These are basically a chain of cyclone-anticyclone circulation patterns propagating from west to east. An anticyclone, which is characterised by a high pressure centre around 55°N and clockwise rotating winds, followed Cleopatra. Over the next few days, when the rain over the Middle East occurred, this high pressure system moved south eastwards and was positioned close to the Middle East. Eventually, the cold dry air of the anticyclone met the warm moist air from the Arabian Sea and Africa.

When dry cold air meets warm moist air, a phenomenon called “occlusion” occurs. This results in the formation of a low pressure area (low), followed by clouds and, finally, rain. The amount of rain is determined by the amount of moisture carried by the warm air. This year, the unusually strong Indian monsoon left a great deal of moisture over the Arabian Sea. (The level of moisture over Arabian Sea in October and November has been 15-20 per cent higher than normal.) Last week, the meeting of the dry cold air from the anticyclone and the warm moist air from the tropics triggered a low over the upper Arabian Gulf, which moved south-east down the Gulf after dumping rain on Kuwait, Bahrain, Eastern Saudi Arabia, Qatar, the UAE and Oman.

Scientists at our centre conducted several “hindcasts” with the Weather Research and Forecasting (WRF) models, using the assimilated data obtained from various weather agencies like National Centre

for Environmental Prediction Global Forecast System. With these hindcasts, we were able to simulate the extreme rainfall events over the Middle East, which is an example of the ability of the contemporary weather and climate models in simulating extreme weather events. Note that these simulations are made with out in-situ data from the local weather stations from the Gulf region. Some of the mismatch with the simulated and observed rainfall patterns like relatively weak amplitude and the three-hour time lag in the simulations are due to this. We believe that model skill can be considerably improved, if more in-situ data is used for the assimilation.

We hope that work like this will help improve our understanding of the vagaries of weather and ability to predict future weather events.

R.S. Ajayamohan, S. Sandeep and V. Praveen work at the Centre for Prototype Climate Modelling, New York University, Abu Dhabi.