Date of Award

2022

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Atmospheric and Environmental Sciences

First Advisor

Christopher Thorncroft

Abstract

In this dissertation we research two aspects of the mountain gap wind northerlies of the Isthmus of Tehuantepec in Southern Mexico. The causes of extreme gap wind northerlies, emphasizing the causes by tropical weather systems, like easterly waves troughs or hurricanes, and the impacts of an extreme gap wind events on tropical cyclogenesis.

Through the use of an index to diagnose the mountain gap wind strength, its evolution through the year and the mechanisms leading to extreme mountain gap winds was highlighted. The index revealed that the gap wind is strongly related to high surface pressure anomalies over the GoM throughout the year. We also observed that between June and August (summer), low surface pressure anomalies over the EPac also impact the gap wind, albeit to a lesser degree. Also, we detected that the gap wind has a significant diurnal cycle in summer.

The index diagnosing the mountain gap also allows the identification of the extreme gap northerlies. We found that extreme northerlies are strongest between November and January (winter) and weaken between June and August (summer). Nevertheless, throughout the year, most of the extreme gap northerlies were associated with fluctuations with periods between 2 and 10 days and a surface pressure gradient across the mountain gap established in association with high surface pressures over the Gulf of Mexico. The high pressures in the Gulf of Mexico were related to surface ridges of midlatitude origin.

In summer, when the midlatitude ridges retreat poleward and their pressure gradient diminishes over the Gulf of Mexico, other factors may trigger extreme gap northerlies. These factors consist of easterly wave troughs and tropical cyclones. When these tropical systems are located over the EPac, their low surface pressure establishes a pressure gradient that enhances the gap northerlies. When these systems are located over the GoM and their circulations are strong, their associated northerlies may enhance the gap northerlies as well. Additionally, we found that the gap outflow in the Eastern Pacific merges with the circulation of the easterly wave and the circulation intensifies.

The case study of hurricane Patricia (2015) is used to explore the potential impacts of the gap northerlies on tropical cyclogenesis. Hurricane Patricia was formed from a low-level vortex, at around 950 hPa, underneath an easterly wave trough over the Eastern Pacific on 0600 20th of October, 2015. Leading up to cyclogenesis, three distinctive features were involved: an EW trough, the gap outflow vorticity and the Intertropical Convergence Zone (ITCZ) vorticity anomalies. The latter two were more evident at low levels. The EW trough stalled over the Eastern Pacific and the Yucatan peninsula for two days from the 18th until cyclogenesis occurred on the 20th of October. While stalled, the trough southerlies advected ITCZ vorticity anomalies and moisture underneath the trough. At the same time the trough northerlies as well as a midlatitude ridge enhanced the gap wind. Eventually, the two low-level vorticity anomalies merged underneath the trough and formed a vortex which leads to cyclogenesis. Once the vortex was formed, the ITCZ vorticity anomalies kept merging with the vortex for about 48 hours, between 20th and 22nd. These vorticity anomalies were associated with convection in the trough, also released latent heat by condensation helping to intensify the vortex. The vortex achieved hurricane strength winds on the 22nd of October. and kept intensifying for the next two days.

Through a numerical experiment consisting of filling the mountain gap of the Isthmus of Tehuantepec the role of the gap outflow on the cyclogenesis of hurricane Patricia was studied. The experiment revealed that when the mountain gap is filled, the gap wind is suppressed, the vorticity underneath the EW trough is weaker, and cyclogenesis does not occur. We found that the gap outflow vorticity helps to form a low-level vortex underneath the trough. The main consequence of the absence of the gap wind and its outflow is that the vorticity in the EPac at low levels is decreased and only the ITCZ vorticity anomalies remain in the EPac. In this situation, the ITCZ vorticity anomalies remain scattered over a broad vortex underneath the trough failing to merge around a sole vorticity maximum characteristic of TC vortices. Thus, the additional vorticity associated with the gap outflow contributes to establish a vorticity maximum for the ITCZ vorticity anomalies to merge around, leading to a developing vortex. Therefore, the vorticity of the gap outflow acts as a catalyst to trigger cyclogenesis for hurricane Patricia.

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