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CGD 2009 Profiles in Science: Dr. Markus Jochum

Summary of achievements

Markus Jochum's focuses on identifing the ocean parts and processes that are important for climate, and then improving the representation of important but poorly represented processes. The results obtained over the last several years show that thermocline diffusivity is surprisingly relevant for tropical precipitation, whereas mesoscale eddies are not. Most of his research is driven by serendipity, and a nice overview of its effect can be found in 'Ocean Viscosity and Climate'.

Publications

Neale, R.B., J.H. Richter and M. Jochum. 2008: The Impact of Convection on ENSO: From a Delayed Oscillator to a Series of Events. Journal of Climate, 21, 5904-5924, doi:10.1175/2008JCLI2244.1.



Figure 1: High resolution figure

Abstract: The NCAR Community Climate System Model, version 3 (CCSM3) exhibits persistent errors in its simulation of the El Niño–Southern Oscillation (ENSO) mode of coupled variability. The amplitude of the oscillation is too strong, the dominant 2-yr period too regular, and the width of the sea surface temperature response in the Pacific too narrow, with positive anomalies extending too far into the western Pacific. Two changes in the parameterization of deep convection result in a significant improvement to many aspects of the ENSO simulation. The inclusion of convective momentum transport (CMT) and a dilution approximation for the calculation of convective available potential energy (CAPE) are used in development integrations, and a striking improvement in ENSO characteristics is seen. An increase in the periodicity of ENSO is achieved by a reduction in the strength of the existing “short-circuited” delayed-oscillator mode. The off-equatorial response is weaker and less tropically confined, largely as a result of the CMT and an associated redistribution of zonal momentum. The Pacific east–west structure is improved in response to the presence of convective dilution and cooling provided by increased surface fluxes. The initiation of El Niño events is fundamentally different. Enhanced intraseasonal surface stress variability leads to absolute surface westerlies and a cooling–warming dipole between the Philippine Sea and western Pacific. Lag-regression analysis shows that intraseasonal variability may play a significant role in event initiation and maintenance as opposed to being a benign response to increased SSTs. Recent observational evidence appears to support such a leading relationship.

Figure caption: Phase orbits of Niño-3 SST and zonal mean (5°S–5°N, 130°E–80°W) 20°C isotherm depth compared with the observations of Kessler (2002). See text for details.

Jochum, M. 2009: Simulated climate impacts of latitudinal variations in diapycnal diffusivity. J. Geophys. Res., 114, C01010, doi:10.1029/2008JC005030.



Figure 2: High resolution figure

Abstract: The currently available theoretical and observational evidence for a latitudinal structure of thermocline vertical diffusivity is synthesized and included in a state of the art coupled climate model. Compared to the standard background value of 0.1 cm² s1, the simulations with the latitudinal structure show only little change in the meridional overturning circulation or northward heat transport. However, two regions are identified which are sensitive to the value of vertical diffusivity: the equatorial band, where only small changes in sea surface temperature lead to precipitation responses with basin-wide teleconnections, and the North Atlantic, where diffusivity affects the spiciness of Labrador Sea water and subsequently the Gulf Stream path.

Figure caption: Hovmoeller diagram of thermocline depth (color, m) and zonal wind stress anomaly (contour interval: 0.02 N m-2) along the equator in C3NEW.

Jochum, M., B. Fox-Kemper, P. Molnar and C. Shields. 2009: Differences in the Indonesian Seaway in a coupled climate model and their relevance for Pliocene climate and El Nino. Paleoceanography, 24, PA1212, doi:10.1029/2008PA001678.



Figure 3: High resolution figure

Abstract: A fully coupled general circulation model is used to investigate the hypothesis that during Pliocene times tectonic changes in the Indonesian seas modified the Indo-Pacific heat transport and thus increased the zonal sea surface temperature gradient in the equatorial Pacific to its large, current magnitude. We find that widening the Indonesian seaway by moving the northern tip of New Guinea south of the equator leads to an increased inflow of South Pacific waters into the Indian Ocean, but because of potential vorticity constraints on cross-equatorial flow it also leads to reductions in both the inflow of North Pacific waters and the total Indonesian throughflow transport. The reduced throughflow is matched by increased eastward transport of warm and fresh North Pacific surface waters along the equator to the central equatorial Pacific. As a result, the Intertropical Convergence Zone lies closer to the equator and the western Pacific Warm Pool expands farther east than for present-day conditions. This reduces the delayed oscillator component of El Nin˜o–Southern Oscillation (ENSO) and enhances the role of stochastic perturbations. Thus, with a more open Indonesian seaway, ENSO becomes weaker and more irregular.

Figure caption: Correlation between NINO3 SST anomalies and zonal wind stress anomalies lagging 3 months for (top) CONT and (bottom) PLIO. The weaker correlation in the eastern equatorial Pacific, a key region for the delayed oscillator [Neale et al., 2008], indicates a weaker coupling between equatorial SST and off-equatorial wind stress.

Eden, C., M. Jochum and G. Danabasoglu. 2009: Effects of different closures for thickness diffusivity. Journal of Climate, 26, 47-59, doi:doi:10.1016/j.ocemod.2008.08.004.



Figure 4: High resolution figure

Abstract: The effects of spatial variations of the thickness diffusivity (K) appropriate to the parameterisation of [Gent, P.R. and McWilliams, J.C., 1990. Isopycnal mixing in ocean circulation models. J. Phys. Oceanogr., 20, 150–155.] are assessed in a coarse resolution global ocean general circulation model. Simulations using three closures yielding different lateral and/or vertical variations in K are compared with a simulation using a constant value. Although the effects of changing K are in general small and all simulations remain biased compared to observations, we find systematic local sensitivities of the simulated circulation on K. In particular, increasing K near the surface in the tropical ocean lifts the depth of the equatorial thermocline, the strength of the Antarctic Circumpolar Current decreases while the subpolar and subtropical gyre transports in the North Atlantic increase by increasing K locally. We also find that the lateral and vertical structure of K given by a recently proposed closure reduces the negative temperature biases in the western North Atlantic by adjusting the pathways of the Gulf Stream and the North Atlantic Current to a more realistic position.

Figure Caption: Annual mean temperature difference between simulation and the climatology of Levitus and Boyer (1994) in experiment CONST (a), VMHS (b), NSQR (c) and EG (d) after 500 years integration at 200 m depth. Also shown are contour lines of temperature (2 °C contour spacing). Note that the data have been interpolated from the model grid to a regular rectangular grid of similar resolution prior to plotting. The land mask in the figure (taken from Smith and Sandwell (1997)) differs therefore slightly from the model’s land mask.

Seo, H., R. Murtugudde, M. Jochum and A.J. Miller. 2008: Modeling of mesoscale coupled ocean–atmosphere interaction and its feedback to ocean in the western Arabian Sea. Ocean Modelling, 25, 120-131, doi:doi:10.1016/j.ocemod.2008.07.003.



Figure 5: High resolution figure

Abstract: Observations of the western Arabian Sea over the last decade have revealed a rich filamentary eddy structure, with large horizontal SST gradients in the ocean, developing in response to the southwest monsoon winds. This summertime oceanic condition triggers an intense mesoscale coupled interaction, whose overall influence on the longer-term properties of this ocean remains uncertain. In this study, a high-resolution regional coupled model is employed to explore this feedback effect on the long-term dynamical and thermodynamical structure of the ocean.

The observed relationship between the near-surface winds and mesoscale SSTs generate Ekman pumping velocities at the scale of the cold filaments, whose magnitude is the order of 1 m/day in both the model and observations. This additional Ekman-driven velocity, induced by the wind-eddy interaction, accounts for approximately 10–20% of oceanic vertical velocity of the cold filaments. This implies that Ekman pumping arising from the mesoscale coupled feedback makes a non-trivial contribution to the vertical structure of the upper ocean and the evolution of mesoscale eddies, with obvious implications for marine ecosystem and biogeochemical variability.

Furthermore, SST features associated with cold filaments substantially reduce the latent heat loss. The long-term latent heat flux change due to eddies in the model is approximately 10–15 W/m2 over the cold filaments, which is consistent with previous estimates based on short-term in situ measurements. Given the shallow mixed layer, this additional surface heat flux warms the cold filament at the rate of 0.3–0.4 °C/month over a season with strong eddy activity, and 0.1–0.2 °C/month over the 12-year mean, rendering overall low-frequency modulation of SST feasible. This long-term mixed layer heating by the surface flux is approximately ±10% of the lateral heat flux by the eddies, yet it can be comparable to the vertical heat flux. Potential dynamic and thermodynamic impacts of this observed air–sea interaction on the monsoons and regional climate are yet to be quantified given the strong correlation between the Somalia upwelling SST and the Indian summer monsoons.

Figure Caption: Mean (a) SST (°C), (b) Ekman pumping velocities (We, m/day), (c) thermocline depth (Z20, m) and (d) mixed layer depth (MLD, m) for August 2002. The contour in (b–d) denotes isotherms of 27.25 °C, which represents the cold filaments.

Jochum, M. 2009: Sensitivity of Tropical Rainfall to Banda Sea Diffusivity in the Community Climate System Model. Journal of Climate, 21, 6445-6454, doi:10.1175/2008JCLI2230.1.



Figure 6: High resolution figure

Abstract: Several observational studies suggest that the vertical diffusivity in the Indonesian marginal seas is an order of magnitude larger than in the open ocean and what is used in most ocean general circulation models. The experiments described in this paper show that increasing the background diffusivity in the Banda Sea from the commonly used value of 0.1 cm2 s-1 to the observed value of 1 cm2 s-1 improves the watermass properties there by reproducing the observed thick layer of Banda Sea Water. The resulting reduced sea surface temperatures lead to weaker convection and a redistribution of precipitation, away from the Indonesian seas toward the equatorial Indian and Pacific Oceans. In particular, the boreal summer precipitation maximum of the Indonesian seas shifts northward from the Banda Sea toward Borneo, which reduces a longstanding bias in the simulation of the Austral–Asian Monsoon in the Community Climate System Model. Because of the positive feedback mechanisms inherent in tropical atmosphere dynamics, a reduction in Banda Sea heat loss of only 5% leads locally to a reduction in convection of 20%.

Figure caption: Correlation of SST anomalies (relative to the climatology of the seasonal cycle) in the Java, Flores, and Banda Seas (region outlined with box) to precipitation anomalies 4 months later (based on NCEP; Kalnay et al. 1996). The correlation for the 95% confidence interval (=0.15) is marked with a black contour line.

Jochum, M. 2009: Impact of latitudinal variations in vertical diffusivity on climate simulators. Journal of Geophysical Research - Oceans, 114, C01010, doi:10.1029/2008JC005030.



Figure 7: High resolution figure

Abstract: The currently available theoretical and observational evidence for a latitudinal structure of thermocline vertical diffusivity is synthesized and included in a state of the art coupled climate model. Compared to the standard background value of 0.1 cm2 s-1, the simulations with the latitudinal structure show only little change in the meridional overturning circulation or northward heat transport. However, two regions are identified which are sensitive to the value of vertical diffusivity: the equatorial band, where only small changes in sea surface temperature lead to precipitation responses with basin-wide teleconnections, and the North Atlantic, where diffusivity affects the spiciness of Labrador Sea water and subsequently the Gulf Stream path.

Figure caption: Difference in (left) temperature and (right) salinity on the σ28 isopycnal between LEPSI and CONT and velocity (maximum velocities between 30 and 35 cm/s around Greenland) on this surface in LEPSI.

Jochum, M. and S. Yeager. 2009: The connection between Labrador Sea buoyancy loss, deep western boundary current strength, and Gulf Stream path in an ocean circulation model. Ocean Modelling, 30, 207-224. [article]



Figure 8: High resolution figure

Abstract: The sensitivity of the North Atlantic gyre circulation to high latitude buoyancy forcing is explored in a global, non-eddy resolving ocean general circulation model. Increased buoyancy forcing strengthens the deep western boundary current, the northern recirculation gyre, and the North Atlantic Current, which leads to a more realistic Gulf Stream path. High latitude density fluxes and surface water mass transformation are strongly dependent on the choice of sea ice and salinity restoring boundary conditions. Coupling the ocean model to a prognostic sea ice model results in much greater buoyancy loss in the Labrador Sea compared to simulations in which the ocean is forced by prescribed sea ice boundary conditions. A comparison of bulk flux forced hindcast simulations which differ only in their sea ice and salinity restoring forcings reveals the effects of a mixed thermohaline boundary condition transport feedback whereby small, positive temperature and salinity anomalies in subpolar regions are amplified when the gyre spins up as a result of increased buoyancy loss and convection. The primary buoyancy flux effects of the sea ice which cause the simulations to diverge are ice melt, which is less physical in the diagnostic sea ice model, and insulation of the ocean, which is less physical with the prognostic sea ice model. Increased salinity restoring ensures a more realistic net winter buoyancy loss in the Labrador Sea, but it is found that improvements in the Gulf Stream simulation can only be achieved with the excessive buoyancy loss associated with weak salinity restoring.

Figure caption: Ocean bathymetry (km) from the POP 1° model with geographical features mentioned in the text.