Search

Restoring boundary conditions, wherein the temperature and salinity are restored to surface target fields of temperature and salinity, are traditionally used for studies of the ocean circulation in ocean general circulation models. The canonical problem with these boundary conditions is that, when the target fields are chosen as the observed fields, accurate simulation of the surface fields of temperature and salinity would imply that the surface fluxes and therefore the ocean heat transports approach zero, a clearly unrealistic situation. It is clear that the target fields cannot be chosen as the observed fields. A simple but effective method of modifying conventional restoring boundary conditions is introduced, designed to keep the calculated values of surface temperature and salinity as close to observations as possible. The technique involves calculating the optimal target fields in the restoring boundary conditions by an iterative procedure. The method accounts for oceanic processes, such as advection and eddy mixing in the derivation of the new boundary conditions. A reduced version of this method is introduced that produces comparable results but offers greater simplicity in implementation. The simplicity of the method is particularly attractive in idealized studies, which often employ restoring surface boundary conditions. The success of the new method is, however, limited by several factors that cannot be easily compensated by the adjustment of the target profiles. These factors include inaccurate model dynamics, errors in the observations, and the too-simplified form of restoring surface boundary conditions themselves. The application of the method in this study with a coarse-resolution model leads to considerable improvements of the simulation of sea surface temperature (SST) and sea surface salinity (SSS). Both amplitude and phase of the annual cycle in SST greatly improve. The resulting magnitudes of surface heat and freshwater fluxes increase on average, and the meridional heat transport gets stronger. However, the fluxes in some regions remain unrealistic, notably the too-strong freshwater forcing of the western boundary currents in the Northern Hemisphere. Southern Ocean cooling and freshening are also likely to be too strong. The subsurface values of temperature improve greatly, proving that a large part of errors in the subsurface temperature distribution in our model can be corrected by reducing errors at the surface. In contrast, the reduction of errors in surface salinity fails to improve uniformly the simulated subsurface salinity values.

The authors identify and describe the important dynamical mechanisms that explain the significant sensitivity of the Atlantic thermohaline circulation to the parameterization of heat and salt transports by mesoscale eddies in numerical models. In particular, the effects of the Gent–McWilliams (GM) scheme, which has a strong flattening effect on isopycnals, and a simple horizontal diffusion scheme are considered and compared. Two control runs, one with each scheme, exhibit very different circulations and density structures. To analyze the dynamical reasons for the differences between the control runs, a number of numerical experiments with regionally varying diffusion coefficients are carried out, emphasizing the effects of different schemes in key regions. The main effect of eddies in the Southern Ocean in nature is to shoal the subsurface isopycnal surfaces, thus increasing the density of the northward inflow of relatively dense intermediate waters into the Atlantic—as will be seen, this is more...

Improved observations in the tropical Pacific during the Tropical Ocean-Global Atmosphere (TOGA) program have served to corroborate preexisting notions concerning the seasonally dependent relationships between sea surface temperature, sea level pressure, wind stress, rainfall, upper tropospheric circulation, and ocean thermal structure anomalies in the El Nino-Southern Oscillation (ENSO) phenomenon. However, the paradigm of a quasiperiodic “ENSO cycle,” phase locked with the annual march, does not capture the complexity of the evolution of the anomalies. The inadequacy of this model was particularly apparent during the second half of TOGA when the variability was highly aperiodic. Also, a single modal structure or empirical orthogonal function does not appear to be capable of representing the range of spatial patterns of ocean-atmosphere interaction in the tropical Pacific. These results suggest the need for a more inclusive phenomenological description of ENSO. Data collected during TOGA serve to confirm the influence of tropical Atlantic sea surface temperature anomalies upon rainfall in northeast Brazil.

Oceanic interdecadal thermohaline oscillations are investigated with a coarse-resolution version of the Geophysical Fluid Dynamics Laboratory Modular Ocean Model. The geometry of the model is a box with a depth of 5000 m and a longitudinal width of 60°, spanning latitudes from 14.5° to 66.5°N. The model ocean is forced by a zonal wind stress, a heat flux parameterized by restoring the surface temperature toward a reference value, and a specified surface freshwater flux. Zonal wind stress, reference temperature, and freshwater flux are all longitudinally uniform, time-independent, and vary meridionally. It is shown that the ocean model can be in a state of interdecadal oscillations, and a physical mechanism is explained. For these oscillatory solutions, both surface mean heat flux and basin mean kinetic energy vary with interdecadal periods. Temperature and salinity budget analyses reveal that these oscillations depend primarily on advective and convective processes. Horizontal advective heat tran...

A 21/2-layer ocean model is developed to investigate the role of the first two baroclinic modes in determining the interannual variations of the sea surface temperature (SST) associated with the El Nino–Southern Oscillation (ENSO) phenomenon. Rather than simply adding an additional mode to the ocean component of the Zebiak–Cane coupled atmosphere–ocean model, it proved necessary to completely rethink all parts of the model. This allowed the external parameters to be specified more realistically. For example, the drag coefficient used in calculating the surface wind stress in the model is now consistent with that empirically derived, and the temperature of the water entrained in the surface layer that affects SST is now more carefully parameterized. When forced by observed wind stress anomalies for 1961–93, the ocean model reproduces the interannual variations of SST satisfactorily. The quantitative discrepancies between the model hindcast and observed SST anomalies are limited to an excessive coo...

Throughout the 1960's and 1970's, oceanographers referred to the large-scale warming of the equatorial eastern and central Pacific as El Nino. This anomalous warming was later shown to be associated with anomalies in the upper ocean thermal structure throughout the equatorial Pacific Ocean. At about the same time, scientists realized that the Southern Oscillation was intimately related to the large-scale changes in the tropical Pacific Ocean. In brief, the Southern Oscillation represents the variability in the Indo-Pacific Walker circulation in the tropics, and is manifest as a displacement of the convection over the maritime continent, and is associated with large-scale anomalies in the surface wind and sea level pressure throughout the tropical Pacific (and into the Indian Ocean) basin.

The implicit vertical diffusion (IVD) convective adjustment scheme in common use in ocean general circulation models (OGCMs) could have large residual static gravitational instability at each time step. An iterative and explicit scheme is devised, based on similar physical considerations as the ones for the IVD scheme. It guarantees a complete removal of static instability in a vertical water column and is more efficient than the IVD scheme in overall spinup of the model. The two convective schemes are compared in an ocean model that is in a state of interdecadal limit cycles. While the model solution with either of these two schemes is characterized by interdecadal oscillations, the variability is different in each scheme. The primary oscillation has a period of about 11 years, but the basin mean kinetic energy shows large differences. The 11-year cycle is modulated by a 33-year oscillation with the IVD scheme, while it is modulated by a 22-year cycle with the complete scheme. The amplitude of t...

A series of numerical experiments is conducted with a three-dimensional ocean general circulation model and a two-dimensional counterpart both designed for efficient integration over diffusive (millennial) time scales. With strong steady salinity fluxes (salting at low latitudes and freshening at high), basin mean temperature and several other diagnostics show a series of self-sustaining oscillations. The oscillations termed deep decoupling oscillations, exhibit halocline catastrophes at regular intervals, followed by warming deep decoupled phases (when the deep overturning is weak), cooling flushes, and in the lower range of salinity forcing, a coupled phase when the deep ocean advective/diffusive heat balance is almost, but not quite, met. It is suggested that oscillations arise when a steady overturning circulation encounters a contradiction: the poleward salt and heat transport needed to maintain convection in the polar ocean requires more overturning than is consistent with the reduced therm...

[1] The main goal of this study is to determine how well large-scale temperature, salinity, upper ocean heat content (UOHC), and surface mixed layer fields and their variability can be reconstructed from the Argo observing system. The approach is to sample and reconstruct these oceanic fields from a coarse-resolution ocean general circulation model (OGCM), quantify the errors in the reconstructed fields, and analyze the factors controlling these errors. In particular, this study analyzes the effects of float movements on the spatial coverage and reconstruction of temporal variability. Overall performance of the simulated Argo array is good, and the reconstructed climatological means of such key quantities as the temperature, salinity, UOHC, and mixed layer depth are very close to the actual OGCM-simulated values in most of the global ocean. However, the differences between the reconstructed and actual fields ("reconstruction errors") are more significant in several regions with strong currents, such as the Antarctic Circumpolar Current (ACC). The results also suggest that the detection of the year-to-year changes in UOHC in the ACC, in high-latitude North Atlantic, and near the coasts can be particularly problematic. As illustrated by sensitivity experiments, the main effect of float movements is to increase reconstruction errors. This adverse effect of float movements is the main cause of large errors in the UOHC interannual difference in the ACC. When the spatial sampling coverage is improved, for example, by increasing the number of floats, the accuracy of reconstruction improves substantially.

Several simple numerical experiments are conducted, using both single- and double-hemisphere ocean basins under symmetric steady forcing to study de ocean's thermohaline circulation. It is shown that a stable steady state obtained under a restoring surface boundary condition on salinity becomes unstable upon a switch to a flux boundary condition. The polar halocline catastrope of F. Bryan occurs. It is shown that further integration of this collapsed state ultimately yields a steady, stable one-cell circulation with the approach being essentially chaotic but with significant energy at decadal period. The two-hemisphere ocean passes through many stages in which violent overturning occurs O(80 × 101 m3 a−1). These flushes occurs in both hemispheres and are of one-cell structure. The time period between them Bushes varies from seveal hundred to about one thousand years. A single 12-vertical-level hemispheric basin, spun up from an initial state of rest under mixed boundary conditions (restoring boun...

A series of numerical experiments involving long‐time integrations are conducted using the Bryan‐Cox Ocean General Circulation Model under mixed surface boundary conditions (i.e. a Newtonian restoring surface boundary condition on temperature and a specified flux boundary condition on salinity). Under steady forcing the system oscillates with significant energy at decadal period. This oscillation is shown to be an advective phenomenon, associated with the propagation of salinity and temperature anomalies from the region between the subtropical and subpolar gyres, where they are generated, to the eastern boundary, where deep water is formed. Furthermore, the oscillation is characterized by the fluctuation of the thermohaline circulation between a state in which deep water is formed and a collapsed state in which no deep water is formed. Over the period of the oscillation the poleward heat transport changes by as much as a factor of 3 at certain latitudes. The anomalies are initially formed by the ...

In order to test certain aspects of the ENSO mechanism proposed by Suarez and Schopf and by Battisti and Hirst, we force a shallow water ocean with 28 years of observed (FSU) winds and decompose both the atmospheric forcing and ocean response into equatorial modes. The proposed mechanism was verified in the following sense. For each warm phase of the cycle, a downwelling Kelvin mode directly forced by the weakening of the trade winds in the central Pacific was found. The same wind anomaly forces an upwelling gravest Rossby mode which propagates freely westward and reflects as a freely propagating Kelvin mode at the western boundary. This Kelvin mode returns to the scene of the original warming and acts as a retarded forcing (of opposite sign) eventually switching the phase of the oscillation from warm to cold. The cold phase then proceeds by the same mechanism but with modes of opposite signs, to eventually switch to the warm phase. The contribution of the higher Rossby modes to this process is e...

In centered difference models of ocean circulation, two grid-point computational modes can be excited if grid Reynolds and Peclet numbers are greater than two. The Bryan-Cox General Circulation Model (GCM) is used to show the dramatic effect that this instability has on the equatorial thermohaline circulation. In many recent numerical calculations researchers have used 12 vertical levels. It is shown that this resolution produces an artificial cell at the equator when typical values of the vertical diffusivity and viscosity parameters are used. This artifical cell rotates counter to the primary cell driven by deep water formation at high latitudes, is driven by downwelling at the eastern boundary near the equator and is 40% the strength of the primary cell for the parameters used in the present study. When the vertical resolution is increased the cell vanishes. It is suggested therefore that higher vertical resolution should be used in Bryan-Cox GCM deep-ocean modeling studies when current values...

Climate variability and change are considerably important for a wide range of human activities and natural ecosystems. Climate science has made major advances during the last two decades, yet climate information is neither routinely useful for nor used in planning. What is needed is a mechanism, a national climate service (NCS), to connect climate science to decision-relevant questions and support building capacity to anticipate, plan for, and adapt to climate fluctuations. This article contributes to the national debate for an NCS by describing the rationale for building an NCS, the functions and services it would provide, and how it should be designed and evaluated. The NCS is most effectively achieved as a federal interagency partnership with critically important participation by regional climate centers, state climatologists, the emerging National Integrated Drought Information System, and the National Oceanic and Atmospheric Administration (NOAA) Regional Integrated Sciences Assessment (RISA) teams in a sustained relationship with a wide variety of stakeholders. Because the NCS is a service, and because evidence indicates that the regional spatial scale is most important for delivering climate services, given subnational geographical/geophysical complexity, attention is focused on lessons learned from the University of Washington Climate Impacts Group's 10 years of experience, the first of the NOAA RISA teams.

Earth's present climate is intrinsically affected by the ocean—the climate without the ocean would be different in many essential ways: Without the evaporation of water from the sea surface, the hydrological cycle would be different; without ocean heat transport and uptake, the temperature distribution of the globe would be different; and without the biota in the ocean, the total amount of carbon in the atmosphere would be many time its current value. Yet, while we may appreciate the role of the ocean in climate, the difficulty and expense of making measurements below the ocean's surface has rendered the vast volume of the ocean a sort of mare incognita. Why is the ocean so important in Earth's climate, and which of its properties are of special significance for climate? How have we learned about the ocean and its role in climate, and what more do we need to know? Keywords: ocean density; thermohaline circulation; sea ice; World Ocean Circulation Experiment; Tropical Ocean–Global Atmosphere; El Nino–Southern Oscillation

THE ocean's thermohaline circulation, driven by fluxes of freshwater and heat through the ocean's surface, is important in the transport of heat from low to high latitudes. Changes in the intensity of this circulation, and hence the poleward heat transport, would have a significant effect on global climate. Numerous observations of the air–sea–ice climate system in and around the North Atlantic show significant variability on decadal and inter-decadal timescales1–11; these timescales suggest that the source of the variability lies in the ocean itself. Here we present the results of three numerical experiments which show the importance of freshwater flux forcing (the difference between precipitation and evaporation, or 'P–E') in exciting decadal and interdecadal oceanic variability. If a sufficiently strong local minimum exists in this P–E forcing field (as is observed over the Greenland Sea), self-sustained oscillations of the ocean circulation may be excited. We propose that such variability may be important in interpreting observations of decadal/interdecadal variability in the air‐sea–ice climate system.

It is shown that the (cumulus) convective processes in the tropics may be described by a one-dimensional cloud model, while the near-surface ocean may similarly be described by a one-dimensional mixed-layer model. The coupling is achieved through a sea surface flux budget combined with the flux parameterizations implied by Monin-Obukhov similarity theory. The coupled one-dimensional atmosphere-ocean model is applied to the equilibrium situation in which all temperatures reach a steady state. For the ocean, the fluxes must vanish in equilibrium, but the atmosphere maintains a stable lapse rate by balancing cumulonimbus heating against net radiative cooling. All water precipitating from cumulonimbus clouds must have evaporated from the sea. It is shown that this equilibrium system is closed and determinable solely in terms of the solar constant.

Seasonal heat transport is examined in a simple, linear, shallow-water model on the equatorial beta plane. It is found in this model that meridional transport by the seasonally varying western boundary current is of the same magnitude but opposite phase to the seasonally varying interior transport and therefore tends to cancel.