2004 LOM Workshop Tuesday 10:20 - 11:30 a.m. Poster Session
Modeling Gravity Currents in Isopycnal and Quasi-Isopycnal Ocean Circulation Models
Whit Anderson, Paul Schopf
George Mason University
datatrident@mac.com
ABSTRACT
This work investigates several modeling issues related to the representation of deep gravity currents in predominantly isopycnal ocean circulation models. We examine the role of various Richardson-number mixing schemes on the behavior of bottom currents. Previous studies have examined smoothly varying Ri-dependent schemes, and the present study re-affirms the basic results to date. But we extend these examples to consider the relative roles of bottom drag and entrainment mixing in setting the pathways and character of deep plumes. The study examines the implications of vertical resolution on the character of solutions. Richardson-number dependent mixing schemes rely fundamentally on the adequacy of vertical resolution, while global climate simulations place significant limits on the vertical resolution available for global ocean models. The implications of limited vertical resolution on such simulations is discussed. We compare the quasi-isopycnal approach to mixing and the recently developed implicit mixing techniques for isopycnal models. Both methods use the ALE technique for treating separately the processes of horizontal dynamics and vertical remapping of coordinates, but in the first case, vertical diapycnal mixing is solved after horizontal advection but before remapping, while in the latter, vertical remapping is constructed so as to include the effect of mixing.
2004 LOM Workshop Tuesday 9:40 - 10:00 a.m.
Development of HIM into a global isopyncal model for studies of ocean circulation, biogeochemistry, and climate
Brian K. Arbic, Anand Gnanadesikan, Robert Hallberg
Program in Atmospheric and Oceanic Sciences, Princeton University
arbic@splash.princeton.edu
ABSTRACT
We have developed the Hallberg Isopycnal Model (HIM) into a realistic one degree global model useful for studies of ocean circulation, biogeochemistry, and climate. Mixed layer depths in the bulk mixed layer scheme we use are sensitive to an included parameterization for the efficiency of wind-driven mixing. The overflows are sensitive to parameterizations of the bottom boundary layer. We will present preliminary results of CFC-11, CFC-12, and perturbation anthropogenic CO2 distributions.
2004 LOM Workshop Tuesday 9:00 - 9:20 a.m.
Alternative numerical scheme for the primitive equations in HYCOM
Mats Bentsen
Nansen Environmental and Remote Sensing Center
mats.bentsen@nersc.no
ABSTRACT
A numerical scheme for the inviscid, unforced primitive equations for an ocean model with isopycnic layers, will be presented. The proposed scheme uses a two-level time-stepping scheme based on a total variation diminishing second order Runge-Kutta method with modifications to the treatment of the Coriolis and Montgomery potential gradient terms for stability reasons and, in addition, a more accurate representation of the geostrophic balance. The scheme used for the transport terms in the layer equations, is based on a weighted essentially non-oscillatory scheme. A two-dimensional barotropic equation system is integrated with a time step dictated by the speed of the fast external gravity waves. These fast motions are filtered out of the layer equations which then may be integrated with a time step sufficient to resolve the much slower three-dimensional internal motions. The implementation of this split time stepping method is shown to be stable for a linearized and simplified system. Results from simulations of an idealized frontogenesis experiment and a simple wind-driven basin-scale configuration will be shown.
2004 LOM Workshop Monday 11:30 - 11:50 a.m.
Toward the Implementation of Ripa's Inhomogeneous Layer Model
F. J. Beron-Vera, M. J. Olascoaga and J. Zavala-Garay
RSMAS/AMP, UMiami, 4600 Rickenbacker Cwy., Miami, FL 33149 USA
fberon@rsmas.miami.edu
ABSTRACT
Inhomogeneous layer models in which the velocity and buoyancy fields are allowed to vary only in the horizontal position and time have been very extensively exploited in ocean modeling. One example is the widely used Miami Isopycnic-Coordinate Model, whose upper layer is chosen as an inhomogeneous layer of this kind. These so-called "slab" models have the ability to partially incorporate thermodynamic processes, which are of fundamental importance in the ocean. For instance, in addition to momentum fluxes, these models can accommodate nonuniform heat and freshwater fluxes through the ocean surface. However, the slab models are known to have several limitations and deficiencies. In particular: (i) they cannot represent explicitly the thermal-wind balance which dominates at low frequencies; (ii) they have a zero-frequency mode not present in the exact fully three-dimensional model; and, in close relation to this, (iii) they cannot prevent spurious instabilities from developing. To cure the slab model limitations and deficiencies, Ripa proposed an improved closure to incorporate thermodynamic processes in a one-layer model. In addition to allowing arbitrary velocity and buoyancy variations in horizontal position and time, Ripa's model allows the velocity and buoyancy fields to vary \emph{linearly} with depth. Ripa's model enjoys a number of properties which make it very promising: (i) it represents explicitly the thermal-wind balance at low frequencies; (ii) the free waves supported by the model (Poincaré, Rossby, midlatitude coastal Kelvin, equatorial, etc.) are a very good approximation to the first and second vertical modes in the fully three-dimensional model; and, very importantly, (iii) in the absence of dissipation and external forcing, Ripa's model has a general invariant, quadratic in the departure from a state of rest (or at most with a uniform current), which is positive definite. This property, which is present in the fully three-dimensional model, prevents the system to explode by itself, unlike the slab models for which this integral of motion is nonnegative definite. In this work we generalize Ripa's model to an arbitrary number of layers, including the possibility of a free surface and irregular bottom topography or the (mathematically equivalent) case in which the stack of layers floats on top of a quiescent infinitely deep layer. As a test we consider the problem of ageostrophic upper-ocean baroclinic instability, by allowing for the bottom boundary in the classical Stone's model to move freely.
2004 LOM Workshop Monday 9:55 - 10:15 a.m.
The T/S/rho conservation conundrum in isopycnic models
Rainer Bleck
Los Alamos National Laboratory
bleck@lanl.gov
ABSTRACT
Temperature and salinity fronts in the ocean tend to adjust so as to minimize their joint impact on the buoyancy field. This phenomenon, known as T/S compensation, renders fronts dynamically invisible and long-lived. Numerically imperfect T/S advection in a circulation model can interfere with this T/S compensation. In an isopycnic model, lateral T/S transport errors are particularly troubling because they lead to nonphysical perturbations in layer thickness as the model struggles to restore the target density in affected layers. In MICOM and HYCOM, T/S compensation has traditionally been "built in" by diagnosing T from S & rho, rather than solving a prognostic equation for T. Due to the nonlinearity of the equation of state, this approach implies a small heat gain whenever S diffuses laterally, be it for numerical or physical reasons. Thermodynamic variables in a circulation model are subject to both transport and mixing. While enforcing T/S compensation during transport has clear physical advantages, our past strategy to treat T as a diagnostic variable during lateral mixing was strictly a matter of computational convenience. Experiments to reduce the internal heat source by retaining S and rho as the variables being transported, but evaluating lateral mixing in terms of T and S, have been successful. The resulting blend of S/rho advection and T/S mixing greatly improves the performance of MICOM and HYCOM in long-term climate simulations.
2004 LOM Workshop Wednesday 4:40 - 5:00 p.m.
Simulation of oceanic carbon sequestration via iron fertilization or direct injection
Rainer Bleck
Los Alamos National Lab
bleck@lanl.gov
ABSTRACT
As part of the international effort to retard climate change due to fossil fuel burning, the U.S. Dept. of Energy is exploring ways to remove carbon dioxide from the atmosphere. Among the technologies proposed to date are two that involve the ocean. One proposal is to inject either liquefied or solidified CO2 into the deep ocean. The second is to increase the air-sea CO2 flux by encouraging phytoplankton growth in regions where the latter is limited by lack of iron. The long-term effectiveness and environmental ramifications of the two approaches are being studied by numerical simulation. Time-averaged isopycnic mass fluxes extracted from two circulation models available in Los Alamos, HYCOM and POP, are being used to simulate the dispersion of liquefied CO2 in the world ocean on 100- to 1000-year time scales. A biogeochemistry model developed to simulate iron fertilization in an eddy-resolving version of POP is being ported to HYCOM to assess the likelihood (based on experiments conducted in parallel in both models) that large-scale, sustained iron fertilization will have a long-term net "drawdown" effect on atmospheric CO2. The project is in its initial phase; hence, only preliminary results will be shown.
2004 LOM Workshop Tuesday 2:00 - 2:20 p.m.
Coupled Modes of variability in the South Atlantic. A comparison of model results with observations
Edmo Campos, Roberto DeAlmeida, Reindert Haarsma, Rainer Bleck and Carlos Lentini
Instituto Oceanografico da Universidade de Sao Paulo
edmo@io.usp.br
ABSTRACT
Coupled modes of variability in the South Atlantic were investigated with the output of the MICOM-CCM3 ocean-atmosphere model, run at the Los Alamos National Laboratory. The main objective was to compare the model patterns of variability, from interannual to multidecadal time-scales, with results of similar analyses with the NCEP data set. Using EOFs and Maximum Covariance Analysis (MCA) between SST and MSLP with zero lag, we found dipolar modes of SST variability very similar to those reported in the scientific literature in recent years. Spectral analysis of the dipole-like mode of SST variability in the model output shows two significative time-scales: one at 2.5 years and the othr at 27 years. The interannual time-scale is correlated with the ENSO signal while the multidecadal variability seems to be related to the Atlantic Meridional Overturning Circulation. Similarly to a previous study with NCEP data, lead-lag MCA between anomalous SST and atmospheric variables, with the model output, indicate the existence of two modes of variability of the atmospheric South Atlantic Convergence Zone (SACZ). One of these modes is a result of remote forcing while the other is a local response to the oceanic forcing. The differences between the two analyses are due to the different climatological states of the data and the model output: there is an excessive southward displacement of the Austral Summer ITCZ in the model result.
2004 LOM Workshop Tuesday 2:40 - 3:00 p.m.
A new model of buoyant plumes
VMCanuto, MSDubovikov and NLTausnev
NASA,- Columbia University, NY, NY
vcanuto@giss.nasa.gov
ABSTRACT
Buoyant Plumes play a key role in describing Deep Convection in the ocean. The canonical plume model of Taylor and Turner (TTM) cannot however be directly used since it was developed for a solitary plume whose cross section is much less than that of the environment. In the ocean case, stable stratification slows down the vertical plume velocity and since entrainment causes the mass flux to increase, the plume cross section increases toward unity thus making the TTM model inapplicable. A new plume model, free of this limitation, has been developed and tested against Deep Convectiion data of Schott et al.(1996). The theoretical basis of the model and the model predictions will be discussed.
2004 LOM Workshop Wednesday 9:40 - 10:00 a.m.
Latitude Dependent Vertical Mixing: OGCM results and the tropical thermocline
VMCanuto, AHoward, YCheng and RLMiller
NASA-GISS
vcanuto@giss.nasa.gov
ABSTRACT
Measurements by Gregg et al.(2003) have shown that the rate of dissipation of internal gravity waves is latitude dependent. The GISS-mixing model is used to transform this information into a latitude dependent momentum, heat and salt diffusivities. A 3D-OGCM is then run. Global avarage T/S profiles are closer to the Lavitus data than those without latitude dependence. The North Atlantic stream function is increased from 19 to 20Sv while the northtward heat transport is reduced from 1.36 to 1.24 Pw. The model also yields a much sharper equatorial thermocline in both Atlantic and Pacific than without the latitude dependence. A sharp equatorial thermocline is a necessary condition for a reliable descripion of the ENSO variations of SST.
2004 LOM Workshop Tuesday 4:00 - 4:20 p.m.
Micom as the ocean component of a regional coupled climate model
Jens Debernard
Norwegian Meteorological Institute
jens.debernard@met.no
ABSTRACT
Regional atmospheric climate models are popular tools used to refine climate change scenarios from coarse resolution global climate models to asses possible climate changes at regional scales. However, in some regions these models may experience unphysical behavior due to the lack of strong feedback mechanisms caused by sea ice. In the northernmost parts of Europe, this may be an important problem. Therefore, we develop a regional coupled climate model for the Arctic. The coupled system consist of a regional atmosphere model, an ice model that also acts at the coupler, and our version of the ocean model MICOM. The talk will focus on the strategy and methodology for the coupling together with results from coupled ice-ocean simulations for a domain covering the Atlantic and Arctic oceans.
2004 LOM Workshop Monday 10:50 - 11:10 a.m.
Some Idealized Thermobaric Solutions
Roland de Szoeke and Scott Springer
Oregon State University
szoeke@coas.oregonstate.edu
ABSTRACT
We discretized the equations of motion and thermodynamics, written in terms of orthobaric specific volume (reciprocal of density) as vertical coordinate, following the principles of Hsu and Arakawa (1990), so that no spurious forces or energy sources are created. Because orthobaric sp. vol. is not materially conserved, even in the absence of diffusion and friction, there are unavoidable, but thermodynamically reversible, mass fluxes across orthobaric isopycnals (or discrete layer interfaces), because of the thermobaric character of seawater, coupled with geographic variation of the ocean's T-S relation. We will show some idealized test solutions designed to highlight thermobaric effects and T-S variation. First of all, we show rest states (stable, level in situ and orthobaric isopycnals) in which potential density isopycnals exhibit quite bizarre behavior. We show time-dependent, nonlinear solutions, such as solitary waves and hydraulic jumps, in which the essential nonlinearity is supplied by the thermobaricity of the equation of state. Variants of such behavior can be demonstrated in both the low-frequency planetary-wave realm, and in the ultra-Coriolis frequency band of internal gravity waves. The thermobaric behavior is evident even in simple two-layer idealizations. These tests should be useful benchmarks for numerical model codes that include thermobaric effects in their equations of state.
2004 LOM Workshop Tuesday 11:50 a.m. - 12:10 p.m.
The influence of internal variability on climate projections
Helge Drange, Asgeir Sorteberg, Nils Gunnar Kvamsto, Tore Furevik
Nansen Environmental and Remote Sensing Center
helge@nersc.no
ABSTRACT
With identical greenhouse forcing climate models shows a wide range of responses both globally and regionally. This divergence from a single solution may be partly due to different model formulations and partly due to unpredictability of the climate system due to internal variability within the climate system itself. The contribution to the total model spread from each of the two uncertainties is complex and dependent on type of climate variable, the strength of the greenhouse forcing as well as the spatial and temporal scales that are investigated. In order to estimate the contribution of the spread due to internal variability an ensemble of simulations using one coupled climate model is performed. Thus the influence of intermodel differences on the spread is cancelled and it is possible to make an estimate of the influence of internal variability on the climate projections. The ensemble was carried out with the coupled Bergen Climate Model (BCM) using the atmospheric model ARPEGE with a resolution of T63 and 31 levels in the vertical and the ocean model MICOM with a resolution of 2.4 degrees along the equator and 24 vertical levels. The ensemble members have all been integrated with a 1% increase per year in CO2 content for 80 years, but started in different initial ocean and atmosphere states (taken from a control run). Our findings suggest that climate change projections over a period of less than 40-50 years will be strongly influenced by chaotic (or unpredictable) internal climate variability. Thus multi-model spread on these time-scales may partly be influenced by real model differences and partly by internal chaotic variations.
2004 LOM Workshop Monday 1:20 - 1:40 p.m.
An improved vertical coordinate of the potential density type
John Dukowicz
Los Alamos National Laboratory
duk@lanl.gov
ABSTRACT
There are two types of quasi-neutral density functions suitable for use as vertical coordinates: Functions of density and pressure, such as orthobaric density, and functions of potential temperature and salinity, such as potential density. In this talk I will discuss two functions of the potential density type that improve on potential density as a materially-conserved, quasi-neutral vertical coordinate.
2004 LOM Workshop Tuesday 10:20 - 11:30 a.m. Poster Session
Comparison of HYCOM with terrain-following ocean models in two idealized experiments
Valerie Garnier, Mohamed Iskandarani, Eric Chassignet, Alan Wallcraft
Rosenstiel School of Marine and Atmospheric Science, University of Miami
vgarnier@rsmas.miami.edu
ABSTRACT
HYCOM simulations are compared with those of terrain-following ocean models on simple idealized standardized tests. The emphasis is on characterizing the hybrid coordinates system in various oceanographic situations. Two tests are considered here: a gravity-adjustment and a downwelling-upwelling experiment. The gravity adjustment problem tests the tracer advections in the model while the other stresses the terrain-following aspects. For the gravity adjustment, good behavior is seen with different advection scheme although there are additional issues with conservation. The downwelling-upwelling experiment shows sensitivity to the choice of vertical coordinates.
2004 LOM Workshop Wednesday 2:25 - 2:40 p.m.
North Atlantic simulations with HYCOM in sigma-2* coordinates.
Z. Garraffo, E. Chassignet, G. Halliwell, L. Smith, H. Hurlburt, A. Wallcraft, T. Townsend
U. Miami. RSMAS/MPO
zgarraffo@rsmas.miami.edu
ABSTRACT
Several 1/3 degree HYCOM North Atlantic sigma-2* simulations were performed, with the objective of selecting the best cases to be realized at 1/12 degree resolution. Differences among the 1/3 degree cases are in the z-level spacing and stretching factors, in hybgen parameters, and in the advection scheme. The first results of a 1/12 sigma-2* simulation will also be discussed.
2004 LOM Workshop Tuesday 10:20 - 11:30 a.m. Poster Session
Using Earth Rotation Data to Test Ocean Models
Richard Gross
Jet Propulsion Laboratory
Richard.Gross@jpl.nasa.gov
ABSTRACT
Angular momentum is a fundamental conserved property of dynamical systems. It is an integrated measure of mass motion and redistribution and as such can be used to diagnose the oceans' changing general circulation. Furthermore, the angular momentum of the oceans is exchanged with that of the solid Earth causing the Earth's rotation to change. In fact, a number of studies have recently shown the importance of oceanic processes in causing Earth rotation changes, particularly in exciting polar motion, which is the motion of the rotation pole with respect to the Earth's crust. Because of this demonstrated importance of oceanic processes in exciting polar motion, polar motion observations have the potential to be used as a novel means of testing ocean models. This potential is illustrated here by computing the angular momentum from runs of two different ocean models, the MOM and MICOM models. Comparing the oceanic angular momentum from these runs to polar motion excitation observations from which atmospheric effects have been removed shows that the results from MICOM are in much better agreement with the observations than are the results from MOM.
2004 LOM Workshop Wednesday 9:20 - 9:40 a.m.
Investigation of Tropical Atlantic and Pacific variability with SPEEDO: A flexible coupled model for climate studies.
Reindert Haarsma, Wilco Hazeleger, Camiel Severijns, Frank Selten, Andreas Sterl
Royal Netherlands Meteorological Institute (KNMI), P.O. Box 201, 3730 AE De Bilt, The Netherlands
haarsma@knmi.nl
ABSTRACT
A flexible coupled atmosphere/ocean/land model SPEEDO has been developed for climate variability studies. The atmosphere model (SPEEDY, Molteni 2003) is a primitive equation model of intermediate complexity. The horizontal resolution is T30 and it has 7 layers. A simplified physical package makes the model very efficient. It is coupled to a land bucket model. The ocean model is the Miami Isopycnal Coordinate Model (MICOM). For MICOM a user friendly shell has been developed which facilitates the choice and setup of different model configurations. After carefull tuning of cloud and mixing parameters in the ocean the biasses in the Tropical Atlantic and Pacific are strongly reduced. The zonal SST gradients are well represented and the seasonal cycle compares well to observations. First results of the variability in the Tropical Atlantic and Pacific simulated by SPEEDO will be presented. References: Molteni, F., 2003: Atmospheric simulations using a GCM with simplified physical parameterizations. I: model climatology and variability in multi-decadal experiments. Clim. Dyn., 20, 175-191.
2004 LOM Workshop Monday 11:50 a.m. - 12:10 p.m.
The Slippery Sacks Ocean Model
Patrick Haertel
University of North Dakota
haertel@aero.und.edu
ABSTRACT
Over the past several years I have been developing a new Lagrangian ocean model. The model is based on the slippery sacks (SS) numerical method, under which a fluid is represented as a pile of conforming sacks. During this talk I will introduce the SS method and present tests in which SS simulations are compared to analytic solutions and simulations carried out with other models. The simulated circulations will range in complexity from idealized waves to the three dimensional flow in a large lake. I will discuss the numerical accuracy, the computational efficiency (for both single- and multi-processor simulations), and the advantages and disadvantages of the SS model.
2004 LOM Workshop Monday 9:15 - 9:35 a.m.
HIM algorithmic developments required to make GFDL's global isopycnal ocean model work
Robert Hallberg
NOAA/GFDL
Robert.Hallberg@noaa.gov
ABSTRACT
GFDL is in the process of building an IPCC-class coupled model using a global version of HIM at 1-degree, 48 layer resolution. In the course of this exercise, a number of algorithmic improvements have proven necessary. This talk will briefly cover several of these; some are brief updates on topics discussed in past LOM workshops. The pressure gradient formulation has been changed to avoid the thermobaric instability that arises in global isopycnal models with high resolution in density space and the full nonlinear equation state. This is the resolution to the problem discussed at the 2003 LOM workshop. An additional source of bottom mixing from the energy extracted by the bottom drag has proven necessary to avoid having thin, viscous plumes of overflow water sink to the bottom of the ocean. The use of a 2-layer refined bulk mixed layer enables shear-driven restratification and Ekman restratifation and destabilization, and may reduce the differences in the mixed layer dynamics between traditional bulk mixed layers and pressure-space hybrid coordinate mixed layers. Changes in the reconsilation of the internal and external estimates of the free surface height may also be described. The simulations generated by this model will be described in a separate talk by B. Arbic.
2004 LOM Workshop Monday 2:00 - 2:45 p.m.
HOME overview, plans, and best-practice studies
Robert Hallberg, R. Bleck, E. Chassignet, R. deSzoeke, S. Griffies, P. Schopf, A. Wallcraft
NOAA/GFDL
Robert.Hallberg@noaa.gov
ABSTRACT
This talk will be an overview of the Hybrid Ocean Modeling Environment (HOME). HOME will provide a versatile community open-source, ocean modeling environment using a predominantly Lagrangian vertical coordinate. The HOME development effort will also identify and refine best practices or describe trade-offs between alternatives for simulating a range of important ocean processes. Many of the principle developers of HYCOM, HIM, Poseidon, POSUM are actively engaged in the HOME effort. While we will maintain much of the existing algorithmic diversity within HOME, it is anticipated that new development and new model applications from each of these groups will be based primarily upon the HOME code within the next 3 years. HOME is intended to be a community model in the fullest sense, and we will strive to facilitate community participation in and contributions to the HOME development effort. This talk will provide an overview of HOME, the rationale behind it and the synergies that will emerge from it. Both the near term plans for HOME and the long-term prospects for HOME to act as a catalyst of broader collaborations with the users and developers of other classes of ocean models will be discussed. A particular emphasis will be placed on plans to conduct a broad series of "best-practice" studies for Lagrangian vertical coordinate ocean models.
2004 LOM Workshop Wednesday 4:00 - 4:20 p.m.
Initial Results From West Florida Shelf Simulations
George Halliwell
MPO/RSMAS, University of Miami
ghalliwell@rsmas.miami.edu
ABSTRACT
Simulatons of the West Florida Shelf at a horizontal resolution of 1/24 degrees and driven by high-frequency atmospheric forcing are performed to test the sensitivity of HYCOM coastal ocean simulations to vertical mixing and vertical coordinate choices, and also to evaluate the HYCOM nesting algorithm. The vertical mixing choices evaluated include the KPP, NASA/GISS, and Mellor-Yamada submodels. The vertical coordinate choices evaluated include pure sigma coordinates over the entire shelf versus p and isopycnic coordinates over the middle and outer shelf.
2004 LOM Workshop Tuesday 1:40 - 2:00 p.m.
Origins and dynamics of the 90-day and 30-60 day variations in the equatorial Indian Ocean
Weiqing Han
University of Colorado
whan@enso.colorado.edu
ABSTRACT
A thorough investigation of the origins and dynamics for the surface and subsurface zonal currents at 20--90 day period in the equatorial Indian Ocean is conducted using an ocean general circulation model (OGCM). To help understand the wind-driven equatorial wave dynamics that occur in the OGCM, a linear continuously stratified ocean model (LM) is also used. Solutions are found in a realistic tropical Indian Ocean basin, and they are forced by NCEP 3-day and monthly forcing fields, together with CMAP pentad and monthly precipitation, for the period of 1988--2001. At both the surface and subsurface, the OGCM solution forced by the 3-day mean fields shows a dominant spectral peak of zonal currents at 90-day period and secondary peaks at 30--60 days, with the 90-day maxima shifting westward as depth increases. These features also appear in the LM solution, suggesting that the wind-driven equatorial wave dynamics play a deterministic role in causing the intraseasonal currents. The dominant 90-day peak is observed by the TOPEX/POSEIDON altimetry data throughout the equatorial Indian Ocean, and is detected by current measurement near the Indonesian seas. The 90-day TOPEX/POSEIDON sea level anomaly and the OGCM surface current evidently show the equatorial Kelvin and the first meridional mode Rossby wave structure, demonstrating that the 90-day oscillation in the equatorial Indian Ocean results from the equatorial waves in responding to the basin scale, 90-day wind forcing. Although zonal currents peak at 90 days, zonal winds peak at 30-60 day period. The skew of frequency between forcing and response results primarily from two causes, consistent with the author's 4.5-layer model study. First, the lower frequency, larger scale Kelvin and Rossby waves at the 90-day period are more efficiently excited by the large scale winds than the higher frequency, shorter wavelength 30--60 day waves. Second, Rossby waves reflected from the eastern ocean boundary significantly enhance the interior response, because the second baroclinic mode resonates with the 90-day wind. The Maldives Islands in the central equatorial basin weaken the resonance somewhat. Although most energy is surface trapped due to mixing, pycnocline reflection, and critical layer absorbtion, there is a fair amount of energy penetrates through the pycnocline down to the deep ocean. A clear upward phase propagation presents in the 90-day zonal flow, indicating that energy is propagating down. Energy of the 90-day Rossby waves excited by the wind maxima near 70E--80E propagates down along the WKB ray path, which appears to explain the westward migration of the 90-day variance maxima. These maxima are significantly enhanced by the reflected Rossby waves at all depths owing to the resonance effect. At 30--60 day period, zonal currents also result largely from intraseasonal wind forcing. At the surface and in the basin interior, the 30--60 day zonal currents are caused by directly forced Rossby and Kelvin waves, and effects of reflected Rossby waves are negligible. As depth increases, however, reflected Rossby waves become more important. This is because the WKB ray paths for the 30--60 day Rossby waves are two to three times steeper than the 90-day ones, energy associated with the reflected Rossby waves penetrate down to the deep ocean efficiently and thus has little effects at the surface. Although intraseasonal wind forcing is important, oceanic instabilities are not negligible near the western boundary and in the central basin south of Sri Lanka near 5N.
2004 LOM Workshop Tuesday 2:20 - 2:40 p.m.
Impact of atmospheric intraseasonal variability in the Indian Ocean: low-frequency rectification in equatorial surface current and transport
Weiqing Han, Peter Webster, Roger Lukas, Peter Hacker, and Aixue Hu
Univeristy of Colorado at Boulder
whan@enso.colorado.edu
ABSTRACT
An ocean general circulation model, the HYbrid Coordinate Ocean Model (HYCOM), is used to investigate the low-frequency (period longer than 90 days) rectification of atmospheric intraseasonal variability (10--90 day periods) in zonal surface current and transport of the equatorial Indian Ocean. A hierarchy of HYCOM solutions are found in an actual tropical Indian Ocean basin for the period of 1988--2001. To help identify nonlinear processes, a linear continuously stratified model is also used. To isolate the contribution from various nonlinear processes, a 4.5-layer intermediate ocean model is applied. Results from HYCOM solutions suggest that intraseasonal atmospheric forcing acts to weaken the equatorial zonal surface currents. Amplitudes of the spring and fall eastward surface jets, the Wyrtki Jets (WJs), and the westward surface current during January--March are reduced by as much as 15-25 cm/s by intraseasonal atmospheric forcing. Important processes that cause the rectification are: asymmetric response of mixed layer depth to easterly and westerly winds, entrainment, and upwelling. During an intraseasonal event, westerly wind deepens whereas easterly wind shoals the surface mixed layer. A net, westward current is generated over an event mean because easterly wind acts onto a thinner surface mixed layer whereas westerly wind acts onto a thicker one. Meanwhile, the strong, episodical westerly winds enhance the entrainment of the slower subsurface water into the surface mixed layer, also tend to weaken the WJs. In contrast, during January--March when the seasonal winds are equatorial easterlies, surface currents are westward, equatorial undercurrents (EUC) develop, and mixed layers are thin. The rectified surface currents are eastward, which reduce the westward surface flow. This eastward rectification results largely from the vertical advection and entrainment of the EUC. In the layers above the thermocline, the rectified zonal transports are westward when the WJs are significantly weakened. The total rectified zonal transports over the entire water column, however, are eastward with an amplitude of 3-15 Sv. This is because westerly winds generate equatorial downwelling, advecting the surface eastward momentum downward and giving an eastward subsurface current. Easterly winds cause equatorial upwelling and produce an eastward pressure gradient force that drives an eastward subsurface current. This eastward subsurface current is advected upward due to upwelling. The mean effect over an intraseasonal event is to create an eastward transport in the water column. Results from this paper have important implication for understanding climate variability because modification of WJs strength and transport can affect the SST and heat storage in the eastern Indian Ocean warm pool region.
2004 LOM Workshop Wednesday 4:20 - 4:40 p.m.
Study of the Slopewater current from MICOM high resolution simulation outputs.
Angelique Haza, Arthur Mariano, Mike Chin, Donald Olson
RSMAS/ University of Miami
ahaza@rsmas.miami.edu
ABSTRACT
The Gulf Stream-Slopewater system is analyzed from four year-outputs of MICOM high-resolution simulations of the North Atlantic, forced by the ECMWF 6 hourly winds. The code was able to reproduce an eastward surface current in the Slope water region, with characteristics similar to the Slopewater Jet measured from hydrographic surveys. The mean path of the Slope current coincides with the sharp temperature and salinity surface fronts separating the Shelf waters from the Slope waters. Its mean transport doubles from 7-9 Sv downstream of the NESC, to 19 Sv south of the Grand Banks. A Taylor column effect of the NESC appears to be the main mechanism captured by the numerical model to account for the strong upper ocean-topographic coupling, resulting in a Gulf Stream bifurcation downstream of the chain and transport increase of the Slopewater current. The coherence and path of this current are maintained by the combination of the western intensification, and the vertical velocity shear of the DWBC, which acts as a barrier. Statistical analysis of the variability shows that the Slope Jet's path and transport are influenced by lateral shift and feeding of the Gulf Stream shortly downstream of the NESC, with 9 month and 1 year periodicities, while near the Grand Banks, the currents are regrouped and accelerated in this zone of convergence, and the SJ only varies in intensity. Mean flow and EOF analyses show good agreement with the latest observations on the Slopewater current, including a coupling in intensity of the Slope Jet with the DWBC and Labrador current.
2004 LOM Workshop Wednesday 2:40 - 3:00 p.m.
Sensitivity studies using HYCOM nested regional models
Patrick Hogan, Luis Zamudio, Alan Wallcraft
Naval Research Laboratory
hogan@nrlssc.navy.mil
ABSTRACT
HYCOM has been configured for several regional areas to investigate sensitivity to nesting parameters and ocean dynamics. Boundary conditions for the regional models are provided by high resolution (1/12 degree) basin-scale Pacific and Atlantic HYCOM simulations. An exact boundary condition is used for the barotropic mode and relaxation to temperature, salinity, pressure, and velocity (optionally) are used for the baroclinic mode. The nested regions include the Gulf of California, the Gulf of Mexico, the Mississippi Bight region, and the East Asian Seas region. Each region is characterized by unique processes and dynamics which are impacted by the accuracy of the open boundary conditions. Several 1/12 to 1/12 degree Gulf of California simulations have been used to investigate the sensitivity to several nesting parameters, such as updating frequency, the width of the buffer zone, the e-folding time within the buffer zone, and relative importance of the baroclinic and barotropic modes. A 0.08 to 0.027 degree (3x) nested Gulf of Mexico simulation is used to investigate the coastal circulation in the northern Gulf of Mexico, with emphasis on deep ocean - coastal exchange processes.
2004 LOM Workshop Tuesday 4:40 - 5:00 p.m.
Ice Shelf - Ocean Model Intercomparison Project
David Holland, Miguel Angel Morales Maqueda
New York University
holland@cims.nyu.edu
ABSTRACT
The Ice Shelf - Ocean Model Intercomparison Project (ISOMIP) is currently being proposed as an open, international effort to identify systematic errors in sub-ice shelf cavity ocean models. In this presentation, the first intercomparison results from the project are presented and discussed, with a focus on the results from the isopycnic model (i.e., MICOM) used in the study. The presentation also includes a discussion of modifications to the surface boundary conditions for salt flux in isopycnic models, whereby the usual virtual salt flux is replaced by a natural boundary condition, involving mass flux arising from the melting of the overlying ice shelf. The sensitivity of the modeled ocean circulation to this change in surface boundary condition is presented. Further information on the overall project may be found at URL http://fish.cims.nyu.edu/project_oisi/isomip/overview.html.