Ocean surface transport plays a critical role in marine ecosystems, influencing the complex spatiotemporal patterns of both marine species and pollutants. The theory of Lagrangian coherent structures (LCSs) aims to identify fundamental patterns within time-dependent, nonlinear fluid flows. LCSs are material surfaces that act as dividing lines which fluid does not cross for a relevant period of time. LCS theory is still under active development, and there are multiple proposed ways to mathematically determine an LCS. Each proposed mathematical definition aims to capture the same physical properties, and some capture those properties more successfully and consistently than others. Here we examine two proposed definitions from the founder of the LCS field: finite time Lyapunov exponents (FTLEs) and geodesic detection. While geodesic detection was developed as an improvement on FTLEs, FTLEs remain the most popular method for using LCSs as an analytical tool. We apply both methods to a novel application. We analyze ocean surface current data in an area off the coast of central California slated for wind energy development, comparing their relative strengths and weaknesses both in theory and in practice.

Previous studies have suggested that ocean bottom pressure (OBP) can be used to measure the transport variability of the Antarctic Circumpolar Current (ACC). The OBP observations from the Gravity Recovery and Climate Experiment (GRACE) are used to calculate transport along the choke point between Antarctica and Australia. Statistical analysis will be conducted to determine the uncertainty of the GRACE observations using a simulated data set. There has been some evidence to suggest that Southern Hemisphere winds and the Southern Annular Mode (SAM) or the Antarctic Oscillation (AAO) play a significant role in accelerating/decelerating ACC transport, along with some contribution from buoyancy forcing. We will examine whether average zonal wind stress, wind stress curl, local zonal winds, or the SAM are representative of the low frequency zonal mass transport variability. Preliminary studies suggest that seasonal variation in transport across the Australia-Antarctica choke point is driven by winds along and north of the northern front of the ACC, the Sub Tropical front (STF). It also appears that interannual variations in transport are related to wind variations centered south of the Sub Antarctic Front (SAF). We have observed a strong negative correlation/positive correlation across the STF of the ACC in the Indian Ocean, which suggests wind stress curl may also be responsible for transport variations.

The advective motion of seawater governs the transport of almost everything, animate or inanimate, present in the ocean and those lacking the ability to outswim the currents have to follow the flow. This makes modelling of advective ocean transports a powerful tool in various fields of science where a displacement of something over time is studied. The present thesis comprises four different applications of ocean-transport modelling, ranging from large-scale heat transports to the dispersion of juvenile marine organisms. The aim has been to adapt the method not only to the object of study, but also to the available model-data sets and in situ-observations. The first application in the thesis is a study of the oceanic heat transport. It illustrates the importance of wind forcing for not only the heat transport from the Indian to the Atlantic Ocean, but also for the net northward transport of heat in the Atlantic. In the next study focus is on the particle-transport differences between an open and a semi-enclosed coastal area on the Swedish coast of the Baltic Sea. The modelled patterns of sedimentation and residence times in the two basins are examined after particles having been released from a number of prescribed point sources. In the two final studies the transport-modelling framework is applied within a marine-ecology context and the transported entities are larvae of some Scandinavian sessile and sedentary species and non-commercial fishes (e.g. the bay barnacle, the blue mussel, the shore crab and the gobies). The effects of depth distribution of dispersing larvae on the efficiency of the Marine Protected Areas in the Baltic Sea are examined. Further, the diversity in dispersal and connectivity depending on vertical behaviour is modelled for regions with different tidal regimes in the North Sea, the Skagerrak and the Kattegat. The spatial scales dealt with in the studies varied from global to a highly resolved 182-metres grid. The model results, excepting those from the global study, are based on or compared with in situ-data. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Submitted. Paper 3: Submitted. 4: Manuscript.</p>