Abstract

We discuss the upper ocean mesoscale temperature variability field as sampled by XBT's in the joint US-USSR POLYMODE Synoptic Experiment during July 1977 to July 1978. There is a “background” persistent, relatively weak and rather large-scale pattern of variability, of alternating warm and cold areas which propagate nearly westward. For the background, very little point correlation between surface temperatures and temperatures beneath the mixed layer is found, but sub-mixed-layer temperatures above and below the “18°C water” show negative correlation. We also find a number of smaller scale, apparently discrete, features which are somewhat more intense than the background. They exhibit little consistency in direction of propagation, and pattern propagation, rather than pattern evolution, as generally observed. The time-average depth field of the 15°C isotherm exhibits spatial variations comparable in magnitude to its pointwise standard deviation values. Removing the mean by linear fits in latitude and/or longitude produces horizontal auto-correlation functions substantially different from those based on removing the time-averaged field. The statistics of the horizontal correlation are not satisfactorily determined by this data set.

Abstract

We present results from three numerical model experiments designed to study the thermal and hydrodynamics changes associated with downwelling Kelvin wave passage and east coastal reflection along and near the equator. The model employs primitive equation dynamics in two active layers and a full thermodynamics equation, so that sea surface temperature, thermocline displacement and sea level are each independently predicted. Wind and thermal finding are used. The surface layer is a slab mixed layer using Kraus and Turner-style bulk physics. Kelvin waves are excited by introducing a westerly wind anomaly in the western part of the basin, and the temperature and current changes caused by the waves are studied as the wave fronts propagate through the circulation forced by three different mean wind fields: no mean winds, southerly men winds and easterly mean winds. The wave-induced changes depend strongly on the conditions that prevail when the waves are forced. Anomalous advection of the existing SST field is the primary SST change mechanism. The two internal Kelvin-wave modes allowed by the model sometimes induce comparable temperature changes near the east coast MA sometimes the effect of one mode substantially dominates that of the other. The shear mode wave does not always propagate to the east coast; it can be destroyed by nonlinear effects associated with the meridional circulation along the equator. Temperature changes near the east coast, similar in magnitude to those observed in the early stages of El Niño events, are caused in the mean southerly wind case, but no broad westward tongue appears latter on. The implications of these results on existing models of El Niño and for future model studies are examined.

Abstract

The dynamical balances of the mean flow of a numerical model ocean general circulation experiment are examined through evaluation of regional vorticity budgets. The instantaneous flow is strongly time dependent and the effect of eddy terms in the mean budgets is of primary interest. Budgets have been computed over volumes ranging in size from less than that of a typical model eddy up to an entire wind-driven gyre, using time series of 5 and 10 year durations. The statistical reliability of terms in the budgets varies significantly with the region size; over regions the size of an eddy or smaller the reliability is often poor, but over the selected larger regions it is satisfactory. The final analysis regions are selected by requiring that each be identified clearly with some part of the mean flow and that cancellation of the locally dominant terms within each region be minimized whenever possible.

The primary mechanism for balancing the wind-stress curl vorticity input in each half basin is found to be horizontal transport of relative vorticity by the eddies across the zero wind-stress curl latitude that separates the distinct flow systems of the two half basins. However, net meridional eddy vorticity transport is generally unimportant away from the half-basin boundary latitude. Eddy horizontal transports over the analysis regions, away from the western part of the zero wind-stress curl latitude, also tend to be small. The transport flow budgets and upper layer budgets tend to be similar. The deep-layer flow is qualitatively different from these flows, a separate set of analysis regions is needed to study it, and the deep budgets are different in several respects. Away from the boundary currents and internal jets the volume integral analog of the classical geostrophic balance—vortex stretching balancing advection of planetary vorticity—holds very well. In particular, over the interior of each gyre, the net input of vorticity by the, wind balances the loss by advection of planetary vorticity to better than 10%. This result is quite different from the conclusion that would he drawn from examination of the vorticity balance at a point over much of the interior, where the divergence of the eddy relative vorticity flux is often large (but of limited statistical reliability). The eddy heat-flux divergence plays an important role in establishing the interfacial vertical velocity contribution to vortex stretching in some of the regions, and appears essential in forcing one of the deep flow currents. No simple summary of the bound current and jet region budgets can be offered, except that mean nonlinear transport often dominates eddy horizontal transport and that frictional effects can be quite small. These results are compared with classical wind-driven ocean circulation ideas and the strengths and limitations of this type of analysis for studying eddy-mean flow interaction are discussed.

Abstract

A hindcast of the 1982–1983 ENSO event using a primitive equation ocean circulation model forced by monthly mean wind stresses based on the SADLER pseudostress fields shows very good agreement with observations at 0°, 159°W between June 1982 and March 1983. The hindcast experiment is analyzed to explore the processes that caused the large accelerations, decelerations, and thermal changes observed during this time. Several hindcast experiments incorporating variations of the SADLER wind field and several idealized experiments incorporating a western Pacific westerly wind event are analyzed and compared with the 1982–1983 SADLER hindcast to explore the importance of local and remote forcing, the relative importance of zonal and meridional wind stress changes, and the dynamical signatures of the processes at work. Meridional wind stress changes have little effect on either the zonal velocity or temperature fields. Local zonal wind stress variations can account for the qualitative changes in the upper-ocean zonal flow, but cannot reproduce the observed thermal changes or the timing and quantitative evolution of the zonal flow. Remote forcing is needed to account for these latter aspects of the observations. Eastward-propagating Kelvin response appears to be quite important, but westward-propagating Rossby variance forced during 1982 from east of 160°W does not appear to play any significant role. The idealized remote-forcing experiments indicate that westerly events can account for the variability not explained by local forcing; the essential aspect is how the forcing projects onto the vertical modes defined by the stratification under the forcing at the time of the wind event. Modes higher than the first and second can be strongly forced and the sum over modes produces vertical structures in the near field of the forcing similar to those observed. Simple linear Kelvin mode ideas thus are useful for understanding the response to remotely forced variability. However, nonlinear processes affect the quantitative response, both by changing the stratification under the forcing region as the forcing event proceeds (and thereby altering the modal projection of the forcing) and through zonal advection and interaction between the response and the background mean flow. The dynamical balance of terms for zonal momentum in the SADLER hindcast is quite complex and the difficulty of identifying remote forcing from the balance of terms, even during periods when remote forcing is the primary agent of change, is discussed. This detailed study of a particularly interesting period of equatorial flow and thermal variability illustrates the many processes at work on the equator in the central Pacific during periods of substantial local and remote wind stress variability. It also illustrates some of the challenges that might be encountered in interpreting the results of an oceanic local dynamics experiment under conditions like these.

Abstract

Subsurface temperature trends in the better-sampled parts of the World Ocean are reported. Where there are sufficient observations for this analysis, there is large spatial variability of 51-yr trends in the upper ocean, with some regions showing cooling in excess of 3°C, and others warming of similar magnitude. Some 95% of the ocean area analyzed has both cooled and warmed over 20-yr subsets of this period. There is much space and time variability of 20-yr running trend estimates, indicating that trends over a decade or two may not be representative of longer-term trends. Results are based on sorting individual observations in World Ocean Database 2001 into 1° × 1° and 2° × 2° bins. Only bins with at least five observations per decade for four of the five decades since 1950 are used. Much of the World Ocean cannot be examined from this perspective. The 51-yr trends significant at the 90% level are given particular attention. Results are presented for depths of 100, 300, and 500 m. The patterns of the 90% significant trends are spatially coherent on scales resolved by the bin size. The vertical structure of the trends is coherent in some regions, but changes sign between the analysis depths in a number of others. It is suggested that additional attention should be given to uncertainty estimates for basin average and World Ocean average thermal trends.

Abstract

A simple linear model of the barotropic basin response to forcing imposed along the northern boundary is described. The dependence on latitude of the response may include oscillatory behavior or not, depending on whether the forcing frequency is smaller or greater than the fundamental free basin mode frequency. When oscillatory behavior is found, the forced solution may resemble oceanic mesoscale eddies. The relevance of this simple model to a description of the eddy fields of several mesoscale resolution general ocean circulation numerical experiments is examined. It is found that a single term of the analytical solution can very well describe the numerically produced eddy fields, away from the regions of strong currents. The possibility that this general mechanism might account for the existence of mesoscale eddies in the ocean is briefly discussed.

Abstract

We examine the variability of monthly mean winds at 850 mb and the surface from five island stations in the tropical western Pacific Ocean. Climatological winds and (850 mb-surface) wind shear are evaluated and used to construct time series of monthly mean wind and shear anomalies. Wind variance at 850 mb tends to be substantially greater than at the surface, and large temporal variations in shear are found. Prominent anomalies are associated with El Niño–Southern Oscillation periods. Composite El Niño event anomalies are examined; it is found that the westerly wind anomalies associated with warm central Pacific sea surface temperatures are stronger at 850 mb than at the surface, and that the anomalous (850 mb-surface) shears are as large as the surface wind anomalies themselves.

Several simple techniques are described to investigate the feasibility of estimating surface wind anomalies from 850 mb wind anomalies. Because strong correlations exist between the zonal winds at these levels, zonal estimate errors can be reduced to ≈0.5 m s⁻¹ if known shear statistics are included in the estimate algorithm. Estimates which extrapolate cloud level wind anomalies to the surface using only climatological shear are shown to produce much greater surface wind errors. If these results are representative and if accurate monthly mean winds at 850 mb can be obtained from cloud motion vectors, then very useful low-frequency surface wind fields can be derived from cloud motion data.

Abstract

Diabatic experiments with a multi-level primitive equation model were performed using three latent heating parameterization schemes. The concept and method of application of the schemes is discussed, as well as physical interpretations. Initial experiments were made with a two-dimensional shearline case. Five-day forecasts show realistic simulation of convergence zones to the north and south of the shearline. Real data experiments in three dimensions were performed using data during the early stages of hurricane Betsy, 29 August 1965. Selected forecast fields are shown for each of the parameterization schemes, and compared to adiabatic prognoses. Heat and energy balances are shown for both two- and three-dimensional models.

Abstract

A 10-level primitive-equation model designed for prediction of circulations within a limited region of the Tropics is described. The model is initialized from wind and temperature information. During the course of the integration two types of oscillations arose from the imposed horizontal boundary conditions—a short-period height oscillation and a longer period kinetic energy oscillation. The character of the two oscillations and the boundary conditions needed to remove the oscillations are described.

Abstract

Four different datasets of monthly mean new-equatorial Pacific sea surface temperature for 1982–83 are compared, and the space-time regions for which there was consensus that cooling or warming took place, are determined. There was consensus that warming took place east of the date line, averaged over the period July-December 1982, and that the warming progressed eastward from the central Pacific. There was also consensus that weak cooling took place in April 1983, and that substantial cooling occurred in June-July 1983, generally over the central and eastern Pacific. However, the analyses tend to agree on the sign of SST change only in periods of cooling or warming in excess of 1°C/month; quantitative agreement at the level of 0.5°C/month or better is almost never found.

SST changes in five ocean-circulation model hindcasts of the 1982–83 period (differing only in that each used a different analyzed monthly mean surface wind stress field to drive the ocean), are compared with the observations and with each other. There is agreement that net warming occurred in the July-December 1982 period and cooling in mid-1983. The heat budgets of these experiments indicate that the major model central Pacific warmings occurred primarily from anomalous eastward surface advection of warm water. Further, east zonal advection remains significant but a diminished cooling tendency from meridional advection can also be important; different hindcasts differ on the relative importance of these terms. Surface heat flux changes do not contribute to the warmings. The reduced cooling tendency from meridional advection is consistent with diminished surface Ekman divergence, suggesting that southward transport of warm north equatorial counter current water was not a major factor in the model warmings. The hindcasts do not agree on the relative importance of local or remote forcing of the eastward surface currents; while there is clear evidence of remote forcing in some hindcasts in particular regions, local forcing is also often significant. The main 1983 midocean cooling began because of increased vertical advection of cool water; but once cooling began horizontal advection often contributed. Further east, where the easterlies generally return later than they do in midocean, upwelling and horizontal advection all can be important. Again no model consensus exists concerning the details of SST evolution.

Because the observations do not agree on the sign of SST change during much of the 1982–83 period, improved SST data is needed in order to document the behavior of the ocean through future ENSO periods. Better forcing data will be needed to carry out improved ocean-model validation studies, and to explore the mechanisms likely responsible for SST change through entire ENSO cycles.