Eli Galanti

ENSO’s Phase Locking to the Seasonal Cycle in the Fast-SST, Fast-Wave, and Mixed-Mode Regimes

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 fastSST 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.

The Equatorial Thermocline Outcropping—A Seasonal Control on the Tropical Pacific Ocean–Atmosphere Instability Strength

Abstract One of the major factors determining the strength and extent of ENSO events is the instability state of the equatorial Pacific coupled ocean–atmosphere system and its seasonal variations. This study analyzes the coupled instability in a hybrid coupled model of the Indo–Pacific region, using the adjoint method for sensitivity studies. It is found that the seasonal changes in the ocean–atmosphere instability strength in the model used here are related to the outcropping of the thermocline in the east equatorial Pacific. From July to December, when the thermocline outcrops over a wide area in the east Pacific, there is a strong surface–thermocline connection and anomalies that arrive as Kelvin waves from the west along the thermocline can reach the surface and affect the SST and thus the coupled system. Conversely, from February to June, when the thermocline outcropping is minimal, the surface decouples from the thermocline and temperature anomalies in the thermocline depth range do not affect the s...

A Midlatitude–ENSO Teleconnection Mechanism via Baroclinically Unstable Long Rossby Waves

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 (158N and 128S). 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 258N and 308S 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.

Comparing Jupiter interior structure models to Juno gravity measurements and the role of a dilute core

The Juno spacecraft has measured Jupiters low-order, even gravitational moments, J2–J8, to an unprecedented precision, providing important constraints on the density profile and core mass of the planet. Here we report on a selection of interior models based on ab initio computer simulations of hydrogen-helium mixtures. We demonstrate that a dilute core, expanded to a significant fraction of the planets radius, is helpful in reconciling the calculated Jn with Junos observations. Although model predictions are strongly affected by the chosen equation of state, the prediction of an enrichment of Z in the deep, metallic envelope over that in the shallow, molecular envelope holds. We estimate Jupiters core to contain a 7–25 Earth mass of heavy elements. We discuss the current difficulties in reconciling measured Jn with the equations of state and with theory for formation and evolution of the planet.

An Evaluation of Air-Sea Flux Products for ENSO Simulation and Prediction

This paper presents a quantitative methodology for evaluating air‐sea fluxes related to ENSO from different atmospheric products. A statistical model of the fluxes from each atmospheric product is coupled to an ocean general circulation model (GCM). Four different products are evaluated: reanalyses from the National Centers for Environmental Prediction (NCEP) and the European Centre for Medium-Range Weather Forecasts (ECMWF), satellite-derived data from the Special Sensor Microwave/Imaging (SSM/I) platform and the International Satellite Cloud Climatology Project (ISCCP), and an atmospheric GCM developed at the Geophysical Fluid Dynamics Laboratory (GFDL) as part of the Atmospheric Model Intercomparison Project (AMIP) II. For this study, comparisons between the datasets are restricted to the dominant air‐sea mode. The stability of a coupled model using only the dominant mode and the associated predictive skill of the model are strongly dependent on which atmospheric product is used. The model is unstable and oscillatory for the ECMWF product, damped and oscillatory for the NCEP and GFDL products, and unstable (nonoscillatory) for the satellite product. The ocean model is coupled with patterns of wind stress as well as heat fluxes. This distinguishes the present approach from the existing paradigm for ENSO models where surface heat fluxes are parameterized as a local damping term in the sea surface temperature (SST) equation.

A Study of ENSO Prediction Using a Hybrid Coupled Model and the Adjoint Method for Data Assimilation

An experimental ENSO prediction system is presented, based on an ocean general circulation model (GCM) coupled to a statistical atmosphere and the adjoint method of 4D variational data assimilation. The adjoint method is used to initialize the coupled model, and predictions are performed for the period 1980‐99. The coupled model is also initialized using two simpler assimilation techniques: forcing the ocean model with observed sea surface temperature and surface fluxes, and a 3D variational data assimilation (3DVAR) method, similar to that used by the National Centers for Environmental Prediction (NCEP) for operational ENSO prediction. The prediction skill of the coupled model initialized by the three assimilation methods is then analyzed and compared. The effect of the assimilation period used in the adjoint method is studied by using 3-, 6-, and 9-month assimilation periods. Finally, the possibility of assimilating only the anomalies with respect to observed climatology in order to circumvent systematic model biases is examined. It is found that the adjoint method does seem to have the potential for improving over simpler assimilation schemes. The improved skill is mainly at prediction intervals of more than 6 months, where the coupled model dynamics start to influence the model solution. At shorter prediction time intervals, the initialization using the forced ocean model or the 3DVAR may result in a better prediction skill. The assimilation of anomalies did not have a substantial effect on the prediction skill of the coupled model. This seems to indicate that in this model the climatology bias, which is compensated for by the anomaly assimilation, is less significant for the predictive skill than the bias in the model variability, which cannot be eliminated using the anomaly assimilation. Changing the optimization period from 6 to 3 to 9 months showed that the period of 6 months seems to be a near-optimal choice for this model.

The effect of differential rotation on Jupiter's low‐degree even gravity moments

Israeli Ministry of Science; Minerva foundation; Federal German Ministry of Education and Research; Helen Kimmel Center for Planetary Science at the Weizmann Institute of Science; CNES; BSF; NSF; Juno project

AN ADJOINT-BASED METHOD FOR THE INVERSION OF THE JUNO AND CASSINI GRAVITY MEASUREMENTS INTO WIND FIELDS

During 2016-17 the Juno and Cassini spacecraft will both perform close eccentric orbits of Jupiter and Saturn, respectively, obtaining high-precision gravity measurements for these planets. This data will be used to estimate the depth of the observed surface flows on these planets. All models to date, relating the winds to the gravity field, have been in the forward direction, thus allowing only calculation of the gravity field from given wind models. However, there is a need to do the inverse problem since the new observations will be of the gravity field. Here, an inverse dynamical model, is developed to relate the expected measurable gravity field, to perturbations of the density and wind fields, and therefore to the observed cloud-level winds. In order to invert the gravity field into the 3D circulation, an adjoint model is constructed for the dynamical model, thus allowing backward integration. This tool is used for examination of various scenarios, simulating cases in which the depth of the wind depends on latitude. We show that it is possible to use the gravity measurements to derive the depth of the winds, both on Jupiter and Saturn, taking into account also measurement errors. Calculating the solution uncertainties, we show that the wind depth can be determined more precisely in the low-to-midlatitudes. In addition, the gravitational moments are found to be particularly sensitive to flows at the equatorial intermediate depths, therefore we expect that if deep winds exist on these planets they will have a measurable signature by Juno and Cassini.

On the spatial and temporal distribution of global thunderstorm cells

Estimates of global thunderstorm activity have been made predominately by direct measurements of lightning discharges around the globe, either by optical measurements from satellites, or using ground-based radio antennas. In this paper we propose a new methodology in which thunderstorm clusters are constructed based on the lightning strokes detected by the World Wide Lightning Location Network (WWLLN) in the very low frequency range. We find that even with low lightning detection efficiency on a global scale, the spatial and temporal distribution of global thunderstorm cells is well reproduced. This is validated by comparing the global diurnal variations of the thunderstorm cells, and the currents produced by these storms, with the well-known Carnegie Curve, which represents the mean diurnal variability of the global atmospheric electric circuit, driven by thunderstorm activity. While the Carnegie Curve agrees well with our diurnal thunderstorm cluster variations, there is little agreement between the Carnegie Curve and the diurnal variation in the number of lightning strokes detected by the WWLLN. When multiplying the number of clusters we detect by the mean thunderstorm conduction current for land and ocean thunderstorms (Mach et al 2011 J. Geophys. Res. 116 D05201) we get a total average current of about 760 A. Our results show that thunderstorms alone explain more than 90% in the variability of the global electric circuit. However, while it has been previously shown that 90% of the global lightning occurs over continental landmasses, we show that around 50% of the thunderstorms are over the oceans, and from 00-09UTC there are more thunderstorm cells globally over the oceans than over the continents. Since the detection efficiency of the WWLLN system has increased over time, we estimate that the lower bound of the mean number of global thunderstorm cells in 2012 was around 1050 per hour, varying from around 840 at 03UTC to 1150 storms at 19UTC.

A full, self-consistent treatment of thermal wind balance on oblate fluid planets

The nature of the flow below the cloud level on Jupiter and Saturn is still unknown. Relating the flow on these planets to perturbations in their density field is key to the analysis of the gravity measurements expected from both the Juno (Jupiter) and Cassini (Saturn) spacecrafts during 2016-17. Both missions will provide latitude-dependent gravity fields, which in principle could be inverted to calculate the vertical structure of the observed cloud-level zonal flow on these planets. Theories to date connecting the gravity field and the flow structure have been limited to potential theories under a barotropic assumption, or estimates based on thermal wind balance that allow analyzing baroclinic wind structures, but have made simplifying assumptions. Those include the effects of the deviations from spherical symmetry, the centrifugal force due to density perturbations, and self-gravitational effects of the density perturbations. Recent studies attempted to include some effects but not in a self-consistent manner. The present study introduces such a self-consistent perturbation approach to the thermal wind balance that incorporates all physical effects, and applies it to several example wind structures, both barotropic and baroclinic. The contribution of each term is analyzed, and the results are compared in the barotropic limit to those of potential theory. It is found that the dominant balance involves the original simplified thermal wind approach. This balance produces a good order-of-magnitude estimate of the gravitational moments, and is able, therefore, to address the order one question of how deep the flows are given measurements of gravitational moments. The additional terms are significantly smaller and none of these terms is dominant, so any approximation attempting to improve over the simplified thermal wind approach needs to include all other terms.