On the recent destabilization of the Gulf Stream path downstream of Cape Hatteras

Author Posting. © American Geophysical Union, 2016. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 43 (2016): 9836–9842, doi:10.1002/2016GL069966.Mapped satellite altimetry reveals interannual variability in the position of initiation of Gulf Stream meanders downstream of Cape Hatteras. The longitude where the Gulf Stream begins meandering varies by 1500 km. There has been a general trend for the destabilization point to shift west, and 5 of the last 6 years had a Gulf Stream destabilization point upstream of the New England Seamounts. Independent in situ data suggest that this shift has increased both upper-ocean/deep-ocean interaction events at Line W and open-ocean/shelf interactions across the Middle Atlantic Bight (MAB) shelf break. Mooring data and along-track altimetry indicate a recent increase in the number of deep cyclones that stir Deep Western Boundary Current waters from the MAB slope into the deep interior. Temperature profiles from the Oleander Program suggest that recent enhanced warming of the MAB shelf may be related to shifts in the Gulf Stream's destabilization point.NSF Grant Numbers: OCE-1332834, OCE-15585212017-03-2

Moored observations of the Deep Western Boundary Current in the NW Atlantic: 2004–2014

Author Posting. © American Geophysical Union, 2017. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 122 (2017): 7488–7505, doi:10.1002/2017JC012984.A moored array spanning the continental slope southeast of Cape Cod sampled the equatorward-flowing Deep Western Boundary Current (DWBC) for a 10 year period: May 2004 to May 2014. Daily profiles of subinertial velocity, temperature, salinity, and neutral density are constructed for each mooring site and cross-line DWBC transport time series are derived for specified water mass layers. Time-averaged transports based on daily estimates of the flow and density fields in Stream coordinates are contrasted with those derived from the Eulerian-mean flow field, modes of DWBC transport variability are investigated through compositing, and comparisons are made to transport estimates for other latitudes. Integrating the daily velocity estimates over the neutral density range of 27.8–28.125 kg/m3 (encompassing Labrador Sea and Overflow Water layers), a mean equatorward DWBC transport of 22.8 × 106 ± 1.9 × 106 m3/s is obtained. Notably, a statistically significant trend of decreasing equatorward transport is observed in several of the DWBC components as well as the current as a whole. The largest linear change (a 4% decrease per year) is seen in the layer of Labrador Sea Water that was renewed by deep convection in the early 1990s whose transport fell from 9.0 × 106 m3/s at the beginning of the field program to 5.8 × 106 m3/s at its end. The corresponding linear fit to the combined Labrador Sea and Overflow Water DWBC transport decreases from 26.4 × 106 to 19.1 × 106 m3/s. In contrast, no long-term trend is observed in upper ocean Slope Water transport. These trends are discussed in the context of decadal observations of the North Atlantic circulation, and subpolar air-sea interaction/water mass transformation.G. Unger Vetlesen Foundation; Woods Hole Oceanographic Institution; US National Science Foundation2018-03-1

A Study of Cyclogenisis in the North of Western Australia

The region of interest in this study is the ocean area to the north of the Western Australian coast; that is, the Timor Sea. It is the tropical cyclones (TC) that generate in this area that most often affect the people and industries located in this region of Western Australia. Accordingly, it is the case that there is a continuing need to improve our understanding of these systems using both observations and numerical models. After an introduction to the problems caused by TCs in the north of Western Australia, a description is made of the study area. A review of the various meteorological systems that can be identified in the tropics is provided. This is followed by a history of research on cyclogenesis. A detailed discussion is undertaken on the current state of knowledge of tropical cyclogenesis. This theoretical understanding subsequently is applied to three case studies. Following a description of the data used and the analysis techniques, the three case studies are presented. In each case study, a system, which later becomes a tropical cyclone, is analysed during the genesis period. The three case studies examined in this thesis are, case 1 (TC Tim, 1994), case 2 (TC Elaine, 1999) and case 3 (TC Isobel, 1996). In each case, the system was studied for at least 10 days prior to it being named. This approach was adopted to ensure that any potential development was not overlooked. A system is named when it reaches sufficient intensity for gale force winds to exist in all quadrants around the centre of that system. For each case, the environment in the vicinity of the location where the system was initially identified was studied until an evolving system was identified. Monitoring of the system continued until it was named.Observations from the Geostationary Meteorological Satellite and the Defense Meteorological Satellite Program comprised the physical data set. In parallel with this data collection activity, meteorological products from a numerical model were catalogued over the same time interval. The thesis presents comparisons of the satellite products and the model output over the study period. In part, motivated by the outcomes of this comparison, it was determined to investigate further prospects for using the array of satellite-derived products that might be more appropriate for use as a forecasting support tool. Finally, as an example, a prototype index is proposed which has potential to demonstrate the degree of development of a system. In this work, for want of a name, this index is termed the Hamilton Index (HI). It uses meteorological products derived from the microwave DMSP series of satellites and provides a temporal sequence of values of the index that are applied to monitor the developing of the TC systems in the three case studies. The meteorological variables used in the index were selected because they were accepted indicators of tropical cyclogenesis identified in the research literature. When applied to the three case studies, the HI showed a significant improvement in sensitivity to the state of development of the systems, especially when compared to the computer model data examined for the case studies

Impact of resolution on the atmosphere–ocean coupling along the Gulf Stream in global high resolution models

We have investigated the horizontal resolution dependence of the ocean–atmosphere coupling along the Gulf Stream, of simulations made by six Global Climate Models according to the HighResMIP protocol, and compared it with reanalysis and remote sensing observations. Two ocean–atmosphere interaction mechanisms are explored in detail: The Vertical Mixing Mechanism (VMM) associated with the intensification of downward momentum transfer, and the Pressure Adjustment Mechanism (PAM) associated with secondary circulations driven by pressure gradients. Both VMM and PAM are found to be active even in the eddy-parameterized models. However, increasing ocean and/or atmosphere resolution leads to enhanced ocean–atmosphere coupling and improved agreement with reanalysis and observations. Our results indicate that while one part of the stronger air–sea coupling is attributable to the refinement of the oceanic component to eddy-permitting, optimal results are obtained only by further increase of the atmosphere resolution too. The use of the eddy-resolving model show weaker or same coupling strength over the eddy-permitting resolution. We conclude that at least eddy-permiting ocean resolution and comparable atmosphere resolution are required for a reliable ocean–atmosphere coupling along the Gulf Stream

A Radial Basis Function Partition of Unity Method for Transport on the Sphere

The transport phenomena dominates geophysical fluid motions on all scales making the numerical solution of the transport problem fundamentally important for the overall accuracy of any fluid solver. In this thesis, we describe a new high-order, computationally efficient method for numerically solving the transport equation on the sphere. This method combines radial basis functions (RBFs) and a partition of unity method (PUM). The method is mesh-free, allowing near optimal discretization of the surface of the sphere, and is free of any coordinate singularities. The basic idea of the method is to start with a set of nodes that are quasi-uniformly distributed on the sphere. Next, the surface of the sphere is partitioned into overlapping spherical caps so that each cap contains roughly the same number of nodes. All spatial derivatives of the PDE are approximated locally within the caps using RBFs. The approximations from each cap are then aggregated into one global approximation of the spatial derivatives using an appropriate weight function in the PUM. Finally, we use a method-of-lines approach to advance the system in time. We analyze the computational complexity of this method as compared to global methods based on RBFs and present results for several well-known test cases that probe the suitability of numerical methods for modeling transport in spherical geometries. We conclude with possible future directions of the work

On the Dynamical Mechanism of the Southern Annular Mode, Including Seasonality: Inter-Annual Variability: and Trends

This thesis considers the dynamics of the leading mode of extratropical atmospheric variability, the so-called annular modes, with a focus on the Southern Hemisphere (SH). Various aspects of the annular modes are addressed, from the underlying mechanism, to variability at progressively longer time-scales; ranging from the seasonality; to inter-annual variability; to the observed and predicted trends. The underlying mechanism of the annular modes is approached in the context of the recent theory that eddy-driven jets may be self-maintaining. We show that the leading mode of variability is associated with changes in the eddy source latitude, and that the latitude of the eddy source region is organised by the mean flow. This is consistent with the idea that the annular modes should be thought of as the meridional wandering of a self-maintaining jet, and that a positive baroclinic feedback prolongs these vacillations. Further, the degree to which the eddy-driven flow is self-maintaining determines the time-scale of the leading mode in a simplified general circulation model (GCM). Preliminary results indicate that the same dynamics are important in the real atmosphere. Secondly the seasonality of the southern annular mode (SAM) is investigated. As with previous studies, during summer the SAM is found to be largely zonally symmetric, whereas during winter it exhibits increased zonal wave number 2-3 variability. This is consistent with seasonal variations in the mean-state, and it is argued that the seasonal cycle of near-surface temperature over the Australian continent plays an important role, making the eddy driven jet, and hence the SAM, more zonally symmetric during summer than winter. During winter, the SAM exhibits little variability over the South Pacific and southeast of Australia. Dynamical reasons for this behaviour are discussed. This seasonality is discussed in the context of New Zealand climate, where it is shown that the variability in rainfall and temperature data are impacted by the large-scale seasonality of the SAM. Thirdly the zonally symmetric response of the SH to the El Nino Southern Oscillation (ENSO) is examined. Such a response is only observed in the mid-latitudes during austral summer and autumn, the same period when the climatological mean flow and storm-track is most zonally symmetric. During all seasons the ENSO stationary wave, or Pacific South American mode affects the baroclinicity at 850 hPa in the South Pacific region, so that during La Nina (El Nino) events the baroclinicity is increased (reduced). During summer La Nina events the anomalous transient eddy activity is increased over the entire meridional extent of the storm-track in the South Pacific region, whereas down-stream, over the Atlantic and Indian Oceans, the storm track moves poleward. It is suggested that during La Nina events, more vigorous eddy activity in the South Pacific leads to a poleward shift of the storm-track immediately down-stream, in the East Pacific. During summer and autumn the location of the storm-track in the Pacific region may be communicated around the hemisphere because there is a single climatological storm track, and so eddies can propagate from the Pacific region to the Atlantic region. There is some evidence of these dynamics in that the anomalous eddy activity associated with La Nina events begins in the South Pacific region and subsequently propagates zonally. Finally the cause of the poleward shift of the mid-latitude eddy-driven jet streams under global warming is considered. GCMs indicate that the recent poleward shift of the eddy-driven jet streams will continue throughout the 21st Century. Here it is shown that the shift is associated with an increase in the eddy length-scale. The cause of the increase in eddy length-scale is discussed. Larger eddies are shown to propagate preferentially poleward, and it is argued that this may induce a corresponding shift in the mean flow that they maintain. The mechanism is investigated using a simplified GCM

Ocean-atmosphere interactions on decadal timescales

In this thesis, different processes that might contribute to the generation of decadal climate variability were investigated using general circulation models (GCMs) of the atmosphere and the ocean. First, the sensitivity of the atmospheric circulation to decadal changes in the underlying sea surface temperatures (SSTs) was esti- mated from an ensemble of six integrations of the Hadley Centre atmospheric GCM HadAMl, all forced by observed SSTs and sea-ice extents for the period 1949-93. Using a novel approach to estimate the 'true' SST-forced atmospheric response in the presence of spatially correlated internal atmospheric variability, the decadal at- mospheric variability was studied over the North Atlantic and North Pacific regions. After filtering out the atmospheric circulation changes associated with the El Niño - Southern Oscillation (ENSO) phenomenon, the dominant mode of forced variability over the North Atlantic exhibits a meridional dipole in the mean sea level pressure (MSLP) field and is related to a tripole in the anomalous North Atlantic SSTs. Over large parts of the North Atlantic region, however, the atmospheric response is not consistent enough to provide feedbacks to the underlying ocean that could cause self-sustained decadal oscillations. Over the North Pacific the atmospheric response is dominated by ENSO. In addition to the ENSO-related response an independent decadal atmospheric signal was detected. It consistently involves iarge-scaie wind stress curl anomalies over the North Pacific region. The effect of such wind stress curl anomalies on the ocean was studied in the second part of this thesis using the Hamburg Ocean Primitive Equation model (HOPE). It is shown how the adjust- ment of the North Pacific gyre circulation to large-scale wind stress curl anomalies determines the decadal timescale and how it may be exploited for predictions of decadal upper-ocean temperature changes in the central North Pacific. The HOPE model was also used to investigate a mechanism for the generation of decadal cli- mate variability in the tropical Pacific which relies on subduction of midlatitudinal North Pacific SST anomalies and their equatorward propagation within the oceanic thermocline. It is demonstrated that such a mechanism is unlikely to cause decadal climate variability in the tropical Pacific

Mesoscale analysis by numerical modeling coupled with satellite-based sounding

November 1988.Principal investigators: Thomas H. Vonder Haar, James F.W. Purdom.Includes bibliographical references.This dissertation deals with the development of a system for time-continuous mesoscale analysis and its use in studying the mesoscale distribution of summertime convective cloud development in the Northeastern Colorado region. There were two basic components of the system — a version of the CSU Regional Atmospheric Modeling System (RAMS) and an algorithm for retrieving temperatures and water vapor concentrations from VISSR Atmospheric Sounder (VAS) data. The system was designed to avoid some of the problems that researchers have encountered when satellite-retrieved parameters have been input to models. The primary distinguishing feature of the new method is that there is an intimate coupling of the retrieval and modeling processes. Water vapor concentrations and ground surface temperatures were the foci of the analyses. In preparation for analysis experiments we tested the sensitivity of a two-dimensional version of the model to various controls on the behavior of water vapor concentrations and surface temperatures. For water vapor mixing ratios, variations that might be caused by analysis errors had very little impact on the dynamics of circulations in the pre-convective stage. In contrast, ground surface temperature variations were shown to have a large impact on circulations, so analysis errors are very relevant to pre-convective dynamics. The first comparisons of the coupled analysis method with other, related, methods was by means of two-dimensional simulations. Analyses in which surface temperatures were derived from satellite-retrievals were compared with the alternative of relying on energy balance computations. The energy balance computations were so sensitive to soil characteristics, which were simulated as unknown, that the satellite retrieval method gave better results even with cloud contamination. In water vapor analysis comparisons no single method was superior in every respect, but the coupled method performed relatively well. Vertical gradients and horizontal gradients were well represented, and the method was relatively insensitive to a common problem in pre-convective analysis — contamination of satellite data by increasing amounts of small convective clouds. Analysis methods were further compared in a three-dimensional case study for 21 August 1983. The horizontal and time variations of satellite-retrieved surface temperatures closely corresponded to the conventional shelter temperature observations, but had much greater detail. In contrast, the energy balance-based temperatures tended to increase too quickly during the morning and lacked some of the observed gradients. According to the retrievals, there can be very large mesoscale gradients in temperatures at the ground surface even on the relatively flat plains. In the case study water vapor analyses there were substantial differences among the results of the several methods that were intercompared. The study demonstrated that, when the first set of satellite data is less reliable than the later sets, some of the contamination lingers throughout the time-continuous coupled analysis results. However, the coupled method generally appeared to be the most valuable of the methods considered in this study because it exploited the major strengths of the numerical model and the satellite data while making it relatively easy to recognize any impacts of their weaknesses. The results of this dissertation support the hypothesis that both ground surface temperatures and terrain variations can play important roles in pre-convective water vapor kinematics through their influences on vertical and horizontal winds. The development of convective clouds corresponded largely, but not exclusively, with convergence and deepening of low-level water vapor. The analysis system proved to be valuable for forecasting through the close correspondence between derived stability indices and later convective development. The new method is a step in the expanding capability of meteorologists to combine tools and sources of data for understanding and forecasting mesoscale phenomena.Research supported by National Oceanic and Atmospheric Administration Grant NA-85-RAH-05045 (53-1209) and in part by Army Research Office Center for Geosciences, Grant DALL-03-86-K-9175