Response of Southwest Pacific storminess to changing climate January 14, 2019 Nilima Aditya Natoo Universität Bremen This page has been intentionally left blank Page 2 of 243 Response of Southwest Pacific storminess to changing climate Dissertation zur Erlangung des Doktorgrades der Naturwissenschaften submitted in partial fulfilment of the requirements for the doctoral degree in natural sciences Dr. rer. nat. im Fachbereich 5 (Geowissenschaften) der Universität Bremen in the Faculty of Geosciences of the University of Bremen vorgelegt von by Nilima Aditya Natoo 14th January 2019 Tag des kolloquiums Date of colloquium 5th July 2019 Gutachter Reviewers Prof. Dr. Michael Schulz (University of Bremen, Germany) Prof. Dr. Christian Winter (University of Kiel, Germany) Supervisors Dr. André Paul (University of Bremen, Germany) Dr. Mark Hadfield (National Institute of Water and Atmospheric Research, New Zealand) Prüfungsausschusses Defence Examining Committee Prof. Michael Schulz, Prof. Dr. Christian Winter, Prof. Dr. Katrin Huhn-Frehers, Prof. Dr. Dierk Hebbeln, Prof. Dr. Heiko Pälike, Dr. Ute Merkel, Dr. Heather Johnstone, Ms. Charlotte Decker, Ms. Sophie Möllenbeck Page 3 of 243 This page has been intentionally left blank Page 4 of 243 Versicherung an Eides Statt / Affirmation in lieu of an oath gem. § 5 Abs. 5 der Promotionsordnung vom 18.06.2018 / according to § 5 (5) of the Doctoral Degree Rules and Regulations of 18 June, 2018 Ich / I, Nilima Aditya Natoo, Bremen, 2458809 (Vorname / First Name, Name / Name, Anschrift / Address, ggf. 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Bremen, 14.01.2019 ___________________ Ort / Place, Datum / Date Unterschrift / Signature Page 5 of 243 This page has been intentionally left blank Page 6 of 243 Gutachter Prof. Dr. Michael Schulz Prof. Dr. Christian Winter Page 7 of 243 This page has been intentionally left blank Page 8 of 243 Abstract The midlatitude countries of Southwest Pacific (SWP) region, like New Zealand (NZ), receive at least one- third of the total tropical storms that develop during summer and autumn (Sinclair 2002), apart from the winter midlatitude storms – this rare combination makes it a unique as well as an interesting area of study. These weather disturbances bring along substantial damages to the lives and properties and therefore are an important subject of study. The process of formation of tropical and midlatitude winter storms; as well as extratropical transitioning storms are examples of tightly coupled air-sea interaction. In this study, storminess or storm activity of an area is defined based on the observational records of total storm counts and its intensity. Similarly, change in storminess is simply defined by the changes in these storm indices. Therefore, to study the future change in storminess, a tool that allows frequent interaction between atmosphere and ocean models is a prerequisite to resolve various oceanic and atmospheric processes. As of now, resolving them using a high resolution global model is computationally expensive. Further, the presence of complex chain of islands, reefs and ridges in the Southwest Pacific (SWP) region modulate the path of South Pacific subtropical gyre. This requires finer resolution ocean model to accurately simulate the turbulent eddies and currents of the study domain. Therefore, in addition to these global models, a regional climate model may be adopted to downscale and simulate additional circulation features at finer scale. In the absence of such a tool for the middle latitudes of the SWP region, a coupled atmosphere-ocean model (hereafter referred as SWP14 model) is developed and evaluated. The atmospheric and oceanic components are represented by WRF and ROMS, respectively. Both the ocean and atmosphere models uses exactly the same grid structure (mean horizontal resolution of 0.25°x0.25°) to avoid complications while exchanging information between the two. The model is integrated over five years period each for historical (1960-64) and RCP8.5 scenario (2095-99) using sequential multiple restart method. Its open boundaries are enforced by CMIP5 IPSL-CM5A-LR global model output. The SWP14 model is evaluated based on two criteria: first of all, model validation is performed by comparing the model output to climatology/reanalysis data; secondly, model performance is tested by comparing the model output to its own forcing data. It is observed that the regional scale circulation features, like, the eastward travelling Tasman Front whose path is modulated by the complicated bathymetry of that region seems to be very well resolved by the SWP14 ocean model and is distinctly visible. Although, the resulting regional circulation pattern is well resolved at chosen scale, however, smaller eddies including few quasi-permanent eddies usually present across the northwest to southeast of the North Island are missing and perhaps demand finer grid resolution. The modelled wind gives best match with that of its forcing IPSL global model, with slight overestimation. At higher latitudes, however, the wind data are incorrectly modelled by the IPSL model and hence carried over to the SWP14 model. Overall, the model performance is good in terms of the input forcing data, however, discrepancies are noted for some sampling stations while validating with the respective regional climatology. This implies that this coupled SWP14 model projection can further be improved by applying a better quality forcing data. The current study is limited to austral summer-autumn storm season. The second research objective is to elucidate and communicate recent changes in storminess in the study area keeping in mind the ongoing global change in climate. The analysis shows that in spite of an overall reduction in the storminess over the past four decades, the number of extreme storms (category 3 and above), corresponding to the mean maximum sustained storm wind speed, has increased. At the same time, a slight reduction in the corresponding mean minimum central pressure is observed. The transition time from category 1 (weakest) to 3 has reduced substantially (by 12 hours). This indicates possibility of higher availability of favourable conditions supporting quick intensification of storms. Apart from this, in recent decades, perhaps under the Page 9 of 243 influence of changing climate, such storms over SWP region are observed to travel longer distances towards pole. The meridional variation of depiction of changing storm counts across past four decades demonstrate clearly the presence of increasing number of tropical storms, 1990s onward, which continues even in the current decade of 2010s. The third and fourth objectives illustrate the importance and role of various large scale environmental (LSE) variables in influencing change in storminess. In the absence of storm observational records, the storm activity as well as its changes are estimated based on the changes in various large scale environmental conditions. Multiple linear regression analysis is used to establish relationship between recent change in storminess and respective change in large scale climate indices, which further is used to construct scenario of likely change in future storminess using simulated (RCP8.5 minus historical) differences. The analysis asserts that the latent heat flux and specific humidity are found to be the most crucial environmental indices influencing the Southwest Pacific change in storminess. They are followed by air-sea temperature difference, air and sea surface temperature, gradient SST, PBLH, local wind and precipitation rate. Any change in the gradient SST has a greater role compared to the change in SST in storm activity. Yet, the role of change in SST