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Eli Tziperman

Abstract

The problem of determining the (eastern boundary) basic stratification and the buoyancy-driven circulation of the oceans is addressed. A global integral constraint relating the interior stratification and the air-sea heat fluxes is derived, based on the condition that the total mass of water of given density is constant in a steady state ocean. This constraint is then applied to two simple analytic models: The first is a continuous nonlinear diffusive model of the lower mid-depth and bottom circulation below the influence of the wind-driven circulation. It shows the tendency of the vertical density profile to look like an exponential profile in the presence of mixing. The integral constraint is used to relate the stratification and circulation of the bottom and mid-depth waters to the air-sea heat fluxes at the surface, where the deep densities outcrop. The second model is a layered one of the wind-driven circulation, mid-depth, and bottom water. The eastern boundary stratification of the model is determined from the air-sea heat fluxes, using the integral constraint and a parameterization of the mixing processes in layer models. A two gyre mid-depth circulation is found, driven by the cross-isopycnal diffusive velocities and affected by the variations in the depth of the main thermocline above it. The bottom circulation in both models is similar to that of the Stommel-Arons model.

Air-sea heat fluxes affect the deep buoyancy-driven flows not by direct cooling or heating, but through the formation of water masses that sink and spreak in the deep ocean. Thermal boundary conditions for the interior thermocline problem seem to require specification of the air-sea heat fluxes in addition to the specification of the density distribution at the base of the Ekman layer. Cross-isopycnal mixing processes are a crucial part of the dynamics, although numerically small. Together with the air-sea heat fluxes, they determine the basic vertical stratification of the wind-driven and deep circulation.

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Eli Tziperman

Abstract

The relation between the circulation calculated from averaged hydrographic data (such as the Levitus data), and the actual time average circulation is examined using a CTD dataset which provides both time and space coverage of a region of the Mediterranean Sea. The connection between eddy mixing coefficients calculated from hydrographic data and the eddy fluxes (uT) they are intended to parameterize is also considered.

An inverse model is used to calculate circulation and mixing coefficients from the time average data. Then, the actual time averaged circulation is estimated by averaging six realizations of the instantaneous velocity field, and mixing coefficients are calculated by directly parameterizing the eddy fluxes of heat and salt.

Comparing the results obtained by the different procedures, it is concluded that the horizontal time average circulation can be reliably estimated from averaged and smoothed climatological data, but that it is nearly impossible to obtain physically meaningful mixing coefficient from such data.

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Eli Tziperman

Abstract

The relation between the mean state of the thermohaline circulation (THC) and its stability is examined using a realistic-geometry primitive equation coupled ocean–atmosphere–ice global general circulation model. The main finding is that a thermohaline circulation that is 25% weaker and less dominated by thermal forcing than that of today’s ocean is unstable within this coupled GCM. Unstable initial ocean climates lead in the coupled model to an increase of the THC, to strong oscillations, or to a THC collapse.

The existence of an unstable range of weak states of the THC provides a natural explanation for large-amplitude THC variability seen in the paleo record prior to the past 10 000 years: A weakening of the THC due to an external forcing (e.g., ice melting and freshening of the North Atlantic) may push it into the unstable regime. Once in this regime, the THC strongly oscillates due to the inherent instability of a weak THC. Hence the strong THC variability in this scenario does not result from switches between two or more quasi-stable steady states.

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Eli Galanti and Eli Tziperman

Abstract

The physical mechanism underlying ENSO’s phase locking to the seasonal cycle is examined in three parameter regimes: the fast-SST limit, the fast-wave limit, and the mixed SST–wave dynamics regime. The seasonal cycle is imposed on simple ordinary differential equation models for each physical regime either as a seasonal ocean–atmosphere coupling strength obtained from the model of Zebiak and Cane or as a climatological seasonal upwelling. In all three parameter regimes, the seasonal variations in the ocean–atmosphere coupling strength force the events to peak toward the end of the calendar year, whereas the effect of upwelling is shown to be less important. The phase locking mechanism in the mixed-mode and fast-SST regimes relies on the seasonal excitation of the Kelvin and the Rossby waves by wind stress anomalies in the central Pacific basin. The peak time of the events is set by the dynamics to allow a balance between the warming and cooling trends due to downwelling Kelvin and upwelling Rossby waves. This balance is obtained because the warming trend due to the large-amplitude Kelvin waves, amplified by a weak Northern Hemisphere wintertime ocean–atmosphere coupling strength, balances the cooling trend due to weak Rossby waves, amplified by a strong summertime coupling strength. The difference between the locking mechanisms in the mixed-mode regime and in the fast-SST regime is used to understand the effect of the SST adjustment time on the timing of the phase locking. Finally, in the less realistic fast-wave regime, a different physical mechanism for ENSO’s phase locking is revealed through the SST adjustment time and the interaction between the east Pacific region and the central Pacific region.

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Eli Galanti and Eli Tziperman

Abstract

The possibility of generating decadal ENSO variability via an ocean teleconnection to the midlatitude Pacific is studied. This is done by analyzing the sensitivity of the equatorial stratification to midlatitude processes using an ocean general circulation model, the adjoint method, and a quasigeostrophic normal-mode stability analysis. It is found that, on timescales of 2–15 yr, the equatorial Pacific is most sensitive to midlatitude planetary Rossby waves traveling from the midlatitudes toward the western boundary and then to the equator. Those waves that propagate through baroclinically unstable parts of the subtropical gyre are amplified by the baroclinic instability and therefore dominate the midlatitude signal arriving at the equator. This result implies that decadal variability in the midlatitude Pacific would be efficiently transmitted to the equatorial Pacific from specific areas of the midlatitude Pacific that are baroclinically unstable, such as the near-equatorial edges of the subtropical gyres (15°N and 12°S). The Rossby waves that propagate via the baroclinically unstable areas are of the advective mode type, which follow the gyre circulation to some degree and arrive from as far as 25°N and 30°S in the east Pacific. It is shown that the baroclinic instability amplifying these waves involves critical layers due to the vertical shear of the subtropical gyre circulation, at depths of 150–200 m.

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Eli Tziperman and Artur Hecht

Abstract

A finite difference linear inverse model is applied to hydrographic data from six summer and fall cruises in a small area (250 × 200 km) of the eastern Mediterranean sea. The temperature and salinity equations are used to form a linear set of equations for the reference geostrophic velocities and the mixing coefficients, which are then solved by singular value decomposition.

Advection by the horizontal velocities is the dominant process affecting the temperature and salinity fields in the region, and the model successfully resolves the horizontal velocities. Mixing and vertical advection are smaller by an order of magnitude, and the model cannot fully resolve the mixing coefficients and vertical velocities.

The six velocity fields calculated from the data indicate a very strong variability that makes it difficult to identify a repeating summer or fall circulation patterns on the scale of the region covered by the data.

An appendix contains the details of a new procedure for including linear inequalities in the solution of a rank deficient system of linear equations.

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Minmin Fu and Eli Tziperman

Abstract

Westerly wind bursts (WWBs) are anomalous surface wind gusts that play an important role in ENSO dynamics. Previous studies have identified several mechanisms that may be involved in the dynamics of WWBs. In particular, many have examined the importance of atmospheric deep convection to WWBs, including convection due to tropical cyclones, equatorial waves, and the Madden–Julian oscillation. Still, the WWB mechanism is not yet fully understood. In this study, we investigate the location of atmospheric convection which leads to WWBs and the role of positive feedbacks involving surface evaporation. We find that disabling surface flux feedbacks a few days before a WWB peaks does not weaken the event, arguing against local surface flux feedbacks serving as a WWB growth mechanism on individual events. On the other hand, directly suppressing convection by inhibiting latent heat release or eliminating surface evaporation rapidly weakens a WWB. By selectively suppressing convection near or farther away from the equator, we find that convection related to off-equatorial cyclonic vortices is most important to equatorial WWB winds, while on-equator convection is unimportant. Despite the strong resemblance of WWB wind patterns to the Gill response to equatorial heating, our findings indicate that equatorial convection is not necessary for WWBs to develop. Our conclusions are consistent with the idea that tropical cyclones, generally occurring more than 5° away from the equator, may be responsible for the majority of WWBs.

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Jeffrey Shaman and Eli Tziperman

Abstract

Rossby waves are a principal form of atmospheric communication between disparate parts of the climate system. These planetary waves are typically excited by diabatic or orographic forcing and can be subject to considerable downstream modification. Because of differences in wave properties, including vertical structure, phase speed, and group velocity, Rossby waves exhibit a wide range of behaviors. This study demonstrates the combined effects of eastward-propagating stationary barotropic Rossby waves and westward-propagating very-low-zonal-wavenumber stationary barotropic Rossby waves on the atmospheric response to wintertime El Niño convective forcing over the tropical Pacific. Experiments are conducted using the Community Atmosphere Model, version 4.0, in which both diabatic forcing over the Pacific and localized relaxation outside the forcing region are applied. The localized relaxation is used to dampen Rossby wave propagation to either the west or east of the forcing region and isolate the alternate direction signal. The experiments reveal that El Niño forcing produces both eastward- and westward-propagating stationary waves in the upper troposphere. Over North Africa and Asia the aggregate undamped upper-tropospheric response is due to the superposition and interaction of these oppositely directed planetary waves that emanate from the forcing region and encircle the planet.

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Jeffrey Shaman and Eli Tziperman

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Numerous studies have demonstrated statistical associations between the El Niño–Southern Oscillation (ENSO) and precipitation in the Mediterranean basin. The dynamical bases for these teleconnections have yet to be fully identified. Here, observational analyses and model simulations are used to show how ENSO variability affects rainfall over southwestern Europe (Iberia, Southern France, and Italy). A precipitation index for the region, named southwestern European Precipitation (SWEP), is used. The observational analyses show that ENSO modulates SWEP during the September–December wet season. These precipitation anomalies are associated with changes in large-scale atmospheric fields to the west of Iberia that alter low-level westerly winds and onshore moisture advection from the Atlantic.

The vorticity anomalies associated with SWEP variability are linked to ENSO through a stationary barotropic Rossby wave train that emanates from the eastern equatorial Pacific and propagates eastward to the Atlantic and Mediterranean. Solutions of the linearized barotropic vorticity equation produce such eastward-propagating Rossby waves with trajectories that traverse the region of observed ENSO-related anomalies. In addition, these linearized barotropic vorticity equation solutions produce a dipole of positive and negative vorticity anomalies to the west of Iberia that matches observations and is consistent with the onshore advection of moisture. Thus, interannual variability of fall and early winter precipitation over southwestern Europe is linked to ENSO variability in the eastern Pacific via an eastward-propagating atmospheric stationary barotropic Rossby wave train.

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Eli Tziperman and Lisan Yu

Abstract

The correlation between parameters characterizing observed westerly wind bursts (WWBs) in the equatorial Pacific and the large-scale SST is analyzed using singular value decomposition. The WWB parameters include the amplitude, location, scale, and probability of occurrence for a given SST distribution rather than the wind stress itself. This approach therefore allows for a nonlinear relationship between the SST and the wind signal of the WWBs. It is found that about half of the variance of the WWB parameters is explained by only two large-scale SST modes. The first mode represents a developed El Niño event, while the second mode represents the seasonal cycle. More specifically, the central longitude of WWBs, their longitudinal extent, and their probability seem to be determined to a significant degree by the ENSO-driven signal. The amplitude of the WWBs is found to be strongly influenced by the phase of the seasonal cycle. It is concluded that the WWBs, while partially stochastic, seem an inherent part of the large-scale deterministic ENSO dynamics. Implications for ENSO predictability and prediction are discussed.

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