Large Scale Air Sea Interactions In The Tropical Western Pacific On Interannual And Intraseasonal Time Scales

Improving the reliability of long-range forecasts of natural disasters, such as severe weather, droughts and floods, in North America, South America, Africa and the Asian/Australasian monsoon regions is of vital importance to the livelihood of millions of people who are affected by these events. In recent years the significance of major short-term climatic variability, and events such as the El Nino/Southern Oscillation in the Pacific, with its worldwide effect on rainfall patterns, has been all to clearly demonstrated. Understanding and predicting the intra-seasonal variability (ISV) of the ocean and atmosphere is crucial to improving long range environmental forecasts and the reliability of climate change projects through climate models. In the second edition of this classic book on the subject, the authors have updated the original chapters, where appropriate, and added a new chapter that includes short subjects representing substantial new development in ISV research since the publication of the first edition.

Tropical and Extratropical Air-Sea Interactions: Modes of Climate Variations provides a thorough introduction to global atmospheric and oceanic processes, as well as tropical, subtropical and mid-latitude ocean-atmosphere interactions. Written by leading experts in the field, each chapter is dedicated to a specific topic of air-sea interactions (such as ENSO, IOD, Atlantic Nino, ENSO Modoki, and newly discovered coastal Niños/Niñas) and their teleconnections. As the first book to cover all topics of tropical and extra-tropical air-sea interactions and new modes of climate variations, this book is an excellent resource for researchers and students of ocean, atmospheric and climate sciences. Presents case studies on the ocean-atmosphere phenomena, including El Nino Southern Oscillation (ENSO), Indian Ocean Dipole and different Nino/Nina phenomena Provides a clear description of air-sea relationships across the world’s ocean with an analysis of air-sea relations in different time scales and a focus on climate change Includes prospects for air-sea interaction research, thus benefiting young researchers and students

The Madden-Julian oscillation (MJO) is the leading mode of tropical intraseasonal variability affecting the global weather and climate. The MJO is characterized by large-scale organized convection and its associated circulation that develops over the tropical Indian Ocean (IO) and propagates into the West Pacific Ocean (WP) across the Maritime Continent (MC). The MJO has direct impacts on extreme rainfall and flooding over the MC, Southeast Asia and north Australia and tropical cyclones over the IO. The MJO’s downstream influences include tropical cyclones, atmospheric rivers, heat waves, and episodes of drought and flooding. Though the MJO has been the subject of many observational and modeling studies, it remains a challenging phenomenon for both theoretical understanding and accurate prediction in global weather and climate models. The overarching goal of this dissertation is to better understand the physical processes affecting the eastward propagation of the MJO from the IO to the WP and improve MJO modeling and prediction. In contrast to many existing MJO studies, we use a novel approach of identifying and tracking MJO events through their large-scale precipitation. Identifying individual MJO events as a physical phenomenon enables us to study the MJO and its in- teraction with the atmosphere, ocean, and land in its environment. We begin by using high- resolution coupled atmosphere-ocean model simulations and satellite- and field campaignobservations to study the physical processes that impact the MJO’s eastward propagation over the IO (Chapters 2 and 3). We follow up by conducting coupled model sensitivity experiments to better understand the MC barrier effects on the MJO (Chapter 4). Finally, we use 20 years of satellite-derived precipitation observations to investigate the seasonal and interannual variability of the MJO eastward propagation and its zonal and meridional variability (Chapter 5). Our findings confirm that the eastward propagation of MJO convection/precipitation is affected by how it interacts with its local ocean and land environments, and is modulated by seasonal and interannual variability. We identify some critical pathways that can help improve MJO modeling and prediction through a better representation of:Chapter 2: The multi-scale convective structure of the MJO. Chapter 3: Air-sea interaction of the MJO and its effect on the upper ocean. Chapter 4: Air-sea-land interactions over the MC. We find that:• Cloud-permitting resolution is better able to represent the various scales on which pre- cipitation occurs within the MJO (convective, mesoscale, and large-scale organization), and how convection interacts with the marine boundary layer. • Strong surface winds and intense precipitation associated with the MJO induce sea surface temperature and upper ocean cooling. Reduced air-sea fluxes create an envi- ronment unfavorable for sustaining intense precipitation, contributing to the MJO’s eastward propagation. • Mesoscale convective systems (MCSs) forming over islands suppress convection over surrounding waters, over which the MJO prefers to propagate through the region. The land-based MCSs grow larger and more intense when MC topography is flattened, which enhances the MC barrier effect. We also show that on a broader scale, MJO convection/precipitation and eastward prop- agation are modulated by modes of seasonal and interannual variability (Chapter 5). The seasonal cycle significantly affects both the zonal and meridional structure of the MJO, including its initiation, termination, and eastward propagation. On longer time scales, cli- mate variability associated with sea surface temperature patterns over the Indian and Pacific oceans (ENSO, Indian ocean dipole) can shift low-level zonal wind convergence regions to- ward or away from the MC. Low-level zonal wind convergence can provide background ascent that amplifies MJO activity, and can strongly affect the zonal variability of the MJO. Unlike the direct link between SST variability and the MJO, upper-tropospheric zonal wind vari- ability associated with the QBO shows a strong seasonal dependence and generally much weaker control over MJO propagation. The strength of the MC barrier on MJO propagation also shows seasonal and interannual variability and is linked to a combination of the back- ground state and the location of MJO convection. The MC barrier effect is weakest during the peak monsoon seasons (Dec-Mar and June-Aug) and during La Niña months, and it is strongest during the monsoon transition seasons (Apr-Jun and Sep-Nov) and ENSO-neutral conditions.

Coastal processes around islands encompass dynamics on a range of scales with physics that can differ from typical continental shelves due to steeper bathymetry, potentially allowing the surrounding basin waters to 'communicate' quickly to the shoreline. This thesis work advances our understanding about these interactions, using a unique set of nearshore oceanographic observations from around the island group encompassed by the Republic of Palau in the tropical western Pacific. The influence of large scale and climatic variability on nearshore island environments can be seen through an empirical model of fore reef temperature structure based on SST and sea level anomaly (SLA) made from nearly two decades of temperature observations (2-90m depth) from three stations around Palau. SLA complements SST by providing a proxy for vertical isotherm displacements driven by local and remote winds on intraseasonal to interannual time scales. Thermal stress on coral ecosystems can now be forecast into the mesophotic zone using this means of predicting subsurface temperatures which are easily accessible for the tropical Pacific. Baroclinic variability around islands has multiple drivers on a range of time scales. Observations of temperature and currents from around the main island group of Palau exhibit a persistent presence of baroclinic coastally trapped waves and internal tides. The largest amplitude signals of coastally trapped waves in fore reef temperature were concurrent with the passage of Typhoon Haiyan, which crossed the northern most Palauan islands in November of 2013. The sub-inertial signals present after Typhoon Haiyan were tracked propagating around the island group for upwards of a week after the typhoon passed. Internal tides were also deemed to be present, but with varying amplitude and phase modulating in and out of phase with the local surface tide. Surface currents impinging upon Palau have a direct impact on the local sea level field around the island group. An array of nearshore pressure gauges, in depths of 20-28 m, encircling the island group and a high resolution (1/120° x 1/120°) regional circulation model are used to examine the space-time characteristics of the flow in a channel in the southern extent of Palau in comparison to the large-scale currents near the island group. A balance between the along-channel pressure difference and bottom friction in the channel was inferred based on the current and pressure observations and the high-resolution model simulations. A drag coefficient for the channel, computed using in situ observations, is O(10-3-10-4). Variations in large-scale zonal currents correlate with the pressure difference across the channel as well as the along-channel flow. The model simulations indicate that as the large-scale flow impinges on the island group, topographic blocking results in a pressure difference on either side of the island which causes a pressure gradient along the channel. The fore reef waters of Palau are shown to be influenced by a range of dynamics across all spatial and temporal scales of our observations. There is an apparent omnipresence of both internal tides and coastally trapped waves throughout the observational window which provide a regular cycling of temperature at depth. These waves have their largest effects at the thermocline, the depth of which can be estimated in this region using only surface variables, as described above. Together, our assessments of these dynamics provide an enhanced perspective on the potential for thermal conditioning of benthic communities living on the outer reef slopes and an advanced perspective of how the large-scale oceanographic field translates to the fore reef environment.