Photo credit: Jeff Schmaltz, NASA, Wikimedia Commons
At the peak of the 2015 hurricane season the largest ever, recorded category 5 Hurricane hit the Pacific coast of Mexico, Hurricane Patricia. Hurricanes are amongst some of the most catastrophic meteorological phenomena that occur and one of particular impact in recent history is that of Hurricane Katrina which devastated New Orleans, USA, in 2005.
Hurricane Patricia maxed out as a category 5 storm of the Saffir-Simpson wind scale, meaning the Hurricane had a maximum sustained wind speed surpassing 156mph. The highest recorded wind speed was 200mph, and it documented a record low in barometric pressure for hurricanes in the Western Hemisphere . The unprecedented records of Patricia raise the question of whether this unique storm was a combination of perfect timing and conditions for initiation and intensification or whether it is part of the longer trend of changing climate through anthropogenic warming.
The formation of hurricanes is referred to as cyclogenesis. Cyclogenesis is a heat driven process. Initially, warm waters are required to provide a sufficient convection. The general consensus is that sea surface temperatures (SSTs) of at least 26.5oC for a depth at least 50m from the ocean surface is necessary to maintain a tropical cyclone. In the case of Hurricane Patricia the SSTs were recorded to be 30oC . Evaporation from the ocean surface occurs and a rising warm air mass cools dependent on the atmospheric lapse rate, and condenses. On condensation, the latent heat produced acts as another form of energy transfer and sustains the system.
This convection produces a pressure drop near the surface, which in turn promotes air masses from surrounding high pressure areas to respond to the pressure drop by accelerating inward and rotating (Figure 1) . The inflow of air from the surrounding high-pressure areas warms and rises, which increases the lifting effect and intensifies the storm. This establishes a feedback loop driven by the supply of warm air.
Patricia originated as an area of low pressure near the Gulf of Tehuantepec on the 14th October. By October 17th a large area of convection had established itself and steadily the feedback loop developed a concentrated area of convection around the core of the system .
Instability in the atmosphere produces pressure gradients that in essence develop regions of low pressure at the location of genesis, causing upper level divergence (an air mass moving away from an area). Hence in cyclogenesis, upper level divergence needs to be greater than lower level convergence in order to create a fall in pressure to allow the development of a zone of ascending air, promoting convection and energy transfer. This most commonly occurs when upper level waves of the atmosphere are meridional and strong. Meridional refers to a wind pattern with characteristic troughs and ridges, whereby the waves cross latitude lines at an angle.
Low vertical wind shear is also important for the formation and maintenance of tropical storm systems. Vertical wind shear refers to the change of winds with height, in particular the wind speed and direction between ~5,000 to 35,000 feet above the ground. Strong vertical shear can breakdown hurricanes and prevent them from forming. High vertical wind shear for example can distort the shape of a hurricane. A slant in the central column of ascending air can weaken the heat and energy transfer of the system, thus reducing the hurricanes efficiency. Therefore, strong zonal wind shear can be highly detrimental for hurricane formation or sustainability. For Patricia, wind shear was exceptionally light, promoting the rapid intensification of the storm to category 5 .
Therefore it seems apparent that conditions in the Gulf of Tehuantepec were ideal for tropical cyclogenesis. The atmospheric-ocean coupling suited the formation of Patricia and the high SSTs in East Pacific region promoted strong convection to sustain the storm. The speed of storm intensification was rapid, further indicating the suitability of conditions. Hurricane Patricia intensified rapidly from the 22nd to 23rd October becoming classified as a category 5 storm. On the 23rd October Hurricane Patricia hit land near Cuixmala, Mexico, as a category 5 although considerably weakened .
Despite the unprecedented size of the storm the damage inflicted on human populations as Patricia made landfall was minimal in comparison with hurricane Katrina for example. Hurricane Katrina did reach category 5 status in the Gulf of Mexico but reduced intensity to a category 3 when it hit New Orleans.
Hurricane Patricia did cause infrastructure damage and agricultural losses of billions of dollars but nothing near the realm of Hurricane Katrina. Fortuitously, Patricia made landfall in a relatively low-populated region of the Mexican West Coast and the mountain relief close to the coast in this region rapidly dissipated the storms energy.
Furthermore, the speed of Patricia’s intensification over the East Pacific partly explains why no storm surge was able to develop. The absence of a strong storm surge reduced the impacts of severe flooding inland. On the contrary, Hurricane Katrina produced the largest storm surge recorded on the US coast in excess of 8m . This in combination with the tracking of Katrina which made landfall directly in the deltaic region of the Mississippi River, with particular focus on New Orleans which is predominantly below sea level and protected by a levee system. The generic instability of delta regions combined with defenses that were not strong enough to withstand the unprecedented storm surge led to the city of New Orleans becoming a drainage system for surplus water, with 80% of the city being underwater to depth of up to 6m.
Climate projections and climate models do indicate that storm frequency and intensity are likely to occur as a result of climate change. However, climate change is not the sole attributable reason to the strength of Hurricane Patricia.
Currently the Pacific Ocean is enduring an El Niño phase of the El Niño Southern Oscillation (ENSO). The ENSO phenomenon is an irregular periodical variation in Sea Surface Temperatures over the Tropical eastern Pacific Ocean. Put simply, itis an atmosphere-ocean coupled system, whereby the changes in SSTs changes pressure systems and has an overbearing influence on seasonal climate fluctuation in many areas of the globe.
The El Niño is an oscillation, so during El Niño years the tropical Eastern Pacific SSTs are much warmer, creating a moister climate in general that is favorable for cyclogenesis (Figure 2) . The counterpart to this, La Niña, is characteristic of cooler SSTs and drier conditions over the tropical Eastern Pacific.
Climatic implications of ENSO do impact the cyclogenesis across the Pacific and Atlantic Oceans. Observations suggest that during El Niño years, strong hurricane activity occurs in the eastern Pacific Ocean due to the favorable warm and wet conditions, whereas in the Atlantic Ocean, fewer hurricanes form due to greater atmospheric stability, stronger vertical wind shear, and stronger trade winds.
The effects of ENSO certainly had some expression in Hurricane Patricia as NOAA (National Oceanic and Atmospheric Administration) announced an El Niño period developed in the spring of 2014 and currently ranks amongst the strongest trends on record since 1950 (Figure 3) [7, 8].
The current El Niño can, to some extent, contribute to our understanding of the occurrence of Hurricane Patricia, as it provided almost optimal conditions for genesis. Therefore, the implications of climate change may be somewhat masked by naturally occurring multi-decadal climate signals, such as ENSO. The ways in which climate change is affecting global weather events are still poorly understood but anthropogenic warming is having some influence over the weather patterns that we see today.