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CGD Research Catalog
Dr. Markus Jochum
Abstract: Atmospheric general circulation model experiments are conducted to quantify the contribution of internal oceanic variability in the form of tropical instability waves (TIWs) to interannual wind and rainfall variability in the tropical Pacific. It is found that in the tropical Pacific, along the equator and near 25N/S, TIWs force a significant increase in wind and rainfall variability from interseasonal to interannual time scales. Because of the stochastic nature of TIWs this means that climate models that do not take them into account will underestimate the strength and number of extreme events, and may overestimate forecast capability. Support: National Science Foundation. Figure. (High resolution figure.) Regression of rain and wind that is sign. correlated with TIW induced SST variability in box.
Abstract: A numerical model of the tropical Pacific ocean is used to investigate the processes that cause the horizontal temperature advection of Tropical Instability Waves (TIWs). It is found that their temperature advection can not be explained by the processes on which the mixing length paradigm is based. Horizontal mixing of temperature across the equatorial SST front does happen, but it is small compared to the 'oscillatory' temperature advection of TIWs. The basic mechanism is that TIWs move water back and forth across a patch of large vertical entrainment. Outside this patch the atmosphere heats the water and this heat is then transferred into the thermocline inside the patch. These patches of strong localized entrainment are due to equatorial Ekman divergence and due to thinning of the mixed layer in the TIW cyclones. The latter process is responsible for the zonal temperature advection which is as large as the meridional temperature advection but has not yet been observed. Thus, in the previous observational literature the TIW contribution to the mixed layer heat budget may have been underestimated significantly. Support: National Science Foundation and NOAA. Figure. (High resolution figure.) Annual mean heat budget for the mixed layer along 140W.
Abstract: A regional coupled ocean-atmospheric model is used to investigate the effect of oceanic mesoscale features on the mean climate of the tropical Atlantic. It is shown that, compared to a non-eddy resolving ocean model, resolving oceanic mesoscale features leads to a cooler mean equatorial cold tongue and a cooler coastal upwelling zone. This changes the meridional SST gradient, and due to the resulting smaller low-level convergence this reduces the mean of rainfall in the marine Inter-Tropical Convergence Zone (ITCZ). The reduced rainfall and the cooler coastal upwelling regions represent a clear improvement of the model solution. Support: National Science Foundation. Figure. (High resolution figure.)
Abstract: The North Equatorial Countercurrent (NECC) is investigated at 35 W through a combination of theory, high resolution Ocean General Circulation Model outputs, and observations. Transport from ADCP measurements during four WOCE cruises and from 5-year SeaWinds scatterometer wind data (QuikSCAT satellite) were used to support model results. It is found that the NECC in the annual mean is approximately in Sverdrup balance, but that seasonal changes in the wind stress curl lead the transport expected from the Sverdrup balance by 1 month, the propagation time of the seasonal Rossby waves from the African coast to 35 W. An investigation of the vertical structure of the NECC shows that there is an eastward core throughout the year, but in spring it is below the westward Ekman flow. Only 60% of the total transport are above the thermocline. The most interesting result of the present study is that model as well as observations describe a distinct second, northern core of the NECC which has not yet found much attention in the literature. It is shown here that the two cores of the NECC are the direct result of the finite width of the Inter-tropical Convergence Zone (ITCZ). This suggests that observational programs that try to determine the NECC transport have to cover the area up to 15N. Support: National Science Foundation. Figure. (High resolution figure.) Schematic diagram of (a) the zonal wind stress in two different situations: when the ITCZ is narrow and the inflection of taux is a sharp peak (red curve) and when the ITCZ is a broad area of constant Taux generating a plateau shape (blue curve). (b) corresponds to the second y-derivative of each Taux in (a), which according to the Sverdrup theory, is proportional to the zonal transport (U). The ITCZ is shown in gray. |
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Jochum, M., C. Deser,
and A. Phillips, 2006: Tropical atmospheric variability forced by oceanic
internal variability. Journal of Climate, accepted.
Jochum, M., and R.
Murtugudde, 2006: Temperature advection by tropical instability waves.
Journal of Physical Oceanography, 36, 592-605.
Seo, H., M. Jochum,
R. Murtugudde, and A. Miller, 2006: Effect of ocean mesoscale variability
on the mean state of the tropical Atlantic climate. Geophysical Research
Letters, 33, L09606, doi:10.1029/2005GL025651).
Urbano, D., M. Jochum,
and I. Da Silveira, 2006: Rediscovering the second core of the Atlantic
NECC. Ocean Modelling, 12/1, 1-15.
