CGD's 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.

Figure caption: Regression of rain and wind that is sign. correlated with TIW induced SST variability in box.

Support: National Science Foundation.

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Jochum, M., M.F. Cronin, W.S. Kessler, and D. Shea, 2007: Observed temperature advection by Tropical Instability Waves. Geophys. Res. Lett., 34, L09604, doi:10.1029/2007GL029416.

Abstract

Velocity data from moored current meters is combined with satellite sea surface temperature (SST) to compute oceanic mixed layer temperature advection by Tropical Instability Waves (TIWs). For the years 2002 to 2005 it is found that this process heats the equatorial mixed layer at an annual mean rate of + 0.8 C/month at 0 N, 140 W and + 2.8 C/month at 0 N, 110 W. At 0 N, 110 W, approximately 25 % of the heating is contributed by zonal temperature advection, a process that has often been assumed to be negligible. From a nine month segment of data (May 2004 - February 2005), the zonal temperature advection at 2 N, 140 W has been estimated to be approximately 0.7 C/month, much larger than the equatorial value for the same time period. Thus, the data supports a recent hypothesis that tropical instability waves contribute a significant mean zonal temperature advection with off-equatorial maxima to the equatorial mixed-layer heat budget. Comparisons with numerical model results suggest that current ocean general circulation models can realistically simulate important aspects of tropical eddy-mixed layer interactions.

Figure caption: SST from TRMM satellite data (colors) is combined with velocity data from moorings (boxes) to show that TIWs produce a significant zonal heat flux. This finding was anticipated by earlier theoretical studies and overturns common wisdom about TIW dynamics.

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Seo, H., M. Jochum, R. Murtugudde, A.J. Miller and J.O. Roads, 2007: Feedback of Tropical Instability Wave induced atmospheric variability onto the ocean. J. Climate, accepted.

Abstract

The effects of atmospheric wind response to tropical instability waves (TIWs) in the equatorial Atlantic Ocean are examined using a regional high-resolution coupled climate model. It is demonstrated that the negative correlation between TIW induced wind perturbation and TIW induced ocean currents leads to a damping of the TIWs. Moreover, the modification of wind stress curl by TIWs also leads to a weakening of TIWs, but this term is negligible compared to the contribution to TIW growth by baroclinic instability. It is also shown that TIWs may significantly impact wind stress estimations from scatterometers especially at high frequencies. Lastly, examination of the rectification effect from the atmospheric response to TIWs in terms of latent heat flux shows that zonally averaged perturbation latent heat flux due to TIWs is small compared to the mean latent heat flux. This implies that rectification of perturbation heat flux to heat budget in the equatorial ocean is small and the low-frequency rectification by the TIWs is related to their contribution to the large-scale SST gradients and not directly to the high-frequency atmospheric response to TIWs.

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Seo, H., M. Jochum, R. Murtugudde, A.J. Miller and J.O. Roads, 2007: Precipitation from African Easterly Waves in a Coupled Model of the Tropical Atlantic Ocean, J. Climate, accepted.

Abstract

A regional coupled climate model is configured in the tropical Atlantic to explore the role of the synoptic-scale African Easterly Waves (AEWs) on the simulation of mean precipitation in the marine Inter-Tropical Convergence Zone (ITCZ). Sensitivity tests of varying atmospheric resolution in the coupled model show that these easterly waves are well represented on both fine and coarse grids of the atmospheric model, with the waves on finer simulation being roughly 20% stronger. The resultant wind shear associated with strong phase of the AEWs is comparable. The significant differences in the model simulations is found in the precipitation fields, where the extreme rainfall events take place over strong shear of the easterly waves only on the higher resolution grid, which is indicative to the strong coupling of waves and rainfalls. This is because the low-level convergence due to the waves is much larger and more realistic in finer simulation, which causes strong precipitation events. The variability in rainfall on this time scale accounts for significant fraction of the total variability. As a result, the simulation of mean rainfall in the ITCZ and its seasonal migration become more realistic. It is shown that this is primarily due to improved representation of atmospheric waves and convergent fields on a finer grid. This suggests that capturing these transient waves and the resultant low-level convergence is the key to improve precipitation simulation in coupled general circulation models.

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Zhou, R. Murtugudde and M. Jochum, 2007: Dynamics of the intraseasonal oscillations of the Indian South Equatorial Current. J. Phys. Oceanogr., accepted.

Abstract

The spatial and temporal features of intraseasonal oscillations in the southwestern Indian Ocean are studied by analyzing model simulations for the Indo-Pacific region. The intraseasonal oscillations have periods of 40-to-80 days, with a wave length of ~650 km. They propagate westward as Rossby waves, with a phase speed of ~25 cm s-1 in boreal winter and spring. The baroclinic instability is the main driver for these intraseasonal oscillations. The first baroclinic mode dominates during most of the year but during boreal winter and spring, the second mode contributes significantly and often equally. Consequently, the intraseasonal oscillations are relatively strong in boreal winter and spring. Whether the atmospheric intraseasonal oscillations are also important for forcing the oceanic intraseasonal oscillations in the southwestern Indian Ocean needs further investigation.