Tommy G. Jensen

Comparison of Radiative and Physiological Effects of Doubled Atmospheric CO2 on Climate

The physiological response of terrestrial vegetation when directly exposed to an increase in atmospheric carbon dioxide (CO2) concentration could result in warming over the continents in addition to that due to the conventional CO2 “greenhouse effect.” Results from a coupled biosphere-atmosphere model (SiB2-GCM) indicate that, for doubled CO2 conditions, evapotranspiration will drop and air temperature will increase over the tropical continents, amplifying the changes resulting from atmospheric radiative effects. The range of responses in surface air temperature and terrestrial carbon uptake due to increased CO2 are projected to be inversely related in the tropics year-round and inversely related during the growing season elsewhere.

Equatorial variability and resonance in a wind-driven Indian Ocean model

A numerical isopycnal ocean model has been designed and applied to model the Indian Ocean north of 25°S. Vertical normal modes are used in the open boundary conditions and for selections of initial layer depths. A 21-year integration with a reduced Hellerman-Rosenstein monthly averaged wind stress has been made with 3.5-layer and 1.5-layer versions of the model. Both solutions reproduce the main features of the observed wind-driven seasonal circulation in the Indian Ocean above the main thermocline. The transient semiannual equatorial surface jets are more intense, more coherent, and in better phase agreement with observations when three layers are active. The associated undercurrents below the main thermocline are also included in the 3.5-layer model solution. Second baroclinic-mode, reflecting, equatorial Kelvin and Rossby waves combine to give a semiannual, resonant basin mode. Experiments with an equatorial band of semiannual zonal winds suggest a very strong response of the Indian Ocean to wind forcing with this period. Further, the amplitudes of the 28–30 day oscillations in the western equatorial model region are found to be strongly damped with depth; they have upward phase propagation and downward energy propagation.

Interactions between Vegetation and Climate: Radiative and Physiological Effects of Doubled Atmospheric CO2

Abstract The radiative and physiological effects of doubled atmospheric carbon dioxide (CO2) on climate are investigated using a coupled biosphere–atmosphere model. Five 30-yr climate simulations, designed to assess the radiative and physiological effects of doubled CO2, were compared to a 30-yr control run. When the CO2 concentration was doubled for the vegetation physiological calculations only assuming no changes in vegetation biochemistry, the mean temperature increase over land was rather small (0.3 K) and was associated with a slight decrease in precipitation (−0.3%). In a second case, the vegetation was assumed to have adapted its biochemistry to a doubled CO2 (2 × CO2) atmosphere and this down regulation caused a 35% decrease in stomatal conductance and a 0.7-K increase in land surface temperature. The response of the terrestrial biosphere to radiative forcing alone—that is, a conventional greenhouse warming effect—revealed important interactions between the climate and the vegetation. Although th...

Arabian Sea and Bay of Bengal exchange of salt and tracers in an ocean model

Exchanges of mass and salt between the mixed layers in the Arabian Sea and the Bay of Bengal are examined in a model of the Indian Ocean using passive tracers as a tool to map the pathways. Inflow of high salinity water from the Arabian Sea into the Bay of Bengal is significant and occurs after the mature phase of the southwest monsoon. Freshwater transport out of the Bay of Bengal is southward throughout the year along the eastern boundary of the Indian Ocean. Low salinity water transport into the Arabian Sea occurs in the Somali Current during the southwest monsoon, closing a clockwise path of water mass transport. Only a small fraction of low salinity water is advected into the eastern Arabian Sea from the Bay of Bengal.

Modeling the seasonal undercurrents in the Somali Current system

Seasonal changes in the Somali Current system are studied using a four-layer model where the lowest layer is at rest. The results suggest that barotropic instability is likely to cause the generation of the Great Whirl in early June. We find a very good agreement between the observed undercurrents and the simulations in the model. Equatorial onshore flow below the thermocline in June is associated with the disappearance of the undercurrent below the Somali Current. The early return of the southward undercurrent in the fall is caused by baroclinic instability of the Great Whirl followed by a reversal to eastward undercurrents along the equator. Remote winds are suggested to control the intermediate depth flow along the Somali coast acting through equatorial waves and Rossby waves generated along the west coast of India. Experiments where the duration of the summer monsoon is extended show that the initial decrease in the magnitude of the Great Whirl is due to eastward and downward energy transfer rather than due to relaxation of the wind. The model solutions suggest that baroclinic instability plays an important role in the decay of the Great Whirl.

Cross-equatorial pathways of salt and tracers from the northern Indian Ocean: Modelling results

Abstract The pathways of cross-equatorial flows originating in the Arabian Sea and the Bay of Bengal are modelled using a 4.5-layer model of the Indian Ocean. Passive tracers and drifters are used to diagnose the transports. The model results show that relatively fresh Bay of Bengal water is transported southward across the equator throughout the year east of 90°E, but during the southwest monsoon as far west as 60°E. In the western part of the ocean, northward transport of low-salinity water across the equator takes place in a narrow region of positive relative vorticity flows in the Somali Current. Substantial southward cross-equatorial exchange of Arabian Sea water occurs as far east as 95°E, primarily from May to September. During the northeast monsoon the net transport is small, but large variability in the exchange of Arabian Sea water is associated with planetary equatorial waves. The cross-equatorial circulation emerge as a clockwise gyre, with southward flow in the mixed layer of the interior of the ocean, and a northward flow in the western boundary current region, including the mixed layer as well as subsurface layers. The path of the low-salinity or freshwater transport is associated with this circulation.

Large-Scale Oceanic Variability Associated with the Madden-Julian Oscillation during the CINDY/DYNAMO Field Campaign from Satellite Observations

During the CINDY/DYNAMO field campaign (fall/winter 2011), intensive measurements of the upper ocean, including an array of several surface moorings and ship observations for the area around 75°E–80°E, Equator-10°S, were conducted. In this study, large-scale upper ocean variations surrounding the intensive array during the field campaign are described based on the analysis of satellite-derived data. Surface currents, sea surface height (SSH), sea surface salinity (SSS), surface winds and sea surface temperature (SST) during the CINDY/DYNAMO field campaign derived from satellite observations are analyzed. During the intensive observation period, three active episodes of large-scale convection associated with the Madden-Julian Oscillation (MJO) propagated eastward across the tropical Indian Ocean. Surface westerly winds near the equator were particularly strong during the events in late November and late December, exceeding 10 m/s. These westerlies generated strong eastward jets (>1 m/s) on the equator. Significant remote ocean responses to the equatorial westerlies were observed in both Northern and Southern Hemispheres in the central and eastern Indian Oceans. The anomalous SSH associated with strong eastward jets propagated eastward as an equatorial Kelvin wave and generated intense downwelling near the eastern boundary. The anomalous positive SSH then partly propagated westward around 4°S as a reflected equatorial Rossby wave, and it significantly influenced the upper ocean structure in the Seychelles-Chagos thermocline ridge about two months after the last MJO event during the field campaign. For the first time, it is demonstrated that subseasonal SSS variations in the central Indian Ocean can be monitored by Aquarius measurements based on the comparison with in situ observations at three locations. Subseasonal SSS variability in the central Indian Ocean observed by RAMA buoys is explained by large-scale water exchanges between the Arabian Sea and Bay of Bengal through the zonal current variation near the equator.

Wind-Driven Response of the Northern Indian Ocean to Climate Extremes*

Abstract Composites of Florida State University winds (1970–99) for four different climate scenarios are used to force an Indian Ocean model. In addition to the mean climatology, the cases include La Nina, El Nino, and the Indian Ocean dipole (IOD). The differences in upper-ocean water mass exchanges between the Arabian Sea and the Bay of Bengal are investigated and show that, during El Nino and IOD years, the average clockwise Indian Ocean circulation is intensified, while it is weakened during La Nina years. As a consequence, high-salinity water export from the Arabian Sea into the Bay of Bengal is enhanced during El Nino and IOD years, while transport of low-salinity waters from the Bay of Bengal into the Arabian Sea is enhanced during La Nina years. This provides a venue for interannual salinity variations in the northern Indian Ocean.

A Study of CINDY/DYNAMO MJO Suppressed Phase

AbstractThe diurnal variability and the environmental conditions that support the moisture resurgence of MJO events observed during the Cooperative Indian Ocean Experiment on Intraseasonal Variability (CINDY)/DYNAMO campaign in October–December 2011 are investigated using in situ observations and the cloud-resolving fully air–ocean–wave Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS). Spectral density and wavelet analysis of the total precipitable water (TPW) constructed from the DYNAMO soundings and TRMM satellite precipitation reveal a deep layer of vapor resurgence during the observed Wheeler and Hendon real-time multivariate MJO index phases 5–8 (MJO suppressed phase), which include diurnal, quasi-2-, quasi-3–4-, quasi-6–8-, and quasi-16-day oscillations. A similar oscillatory pattern is found in the DYNAMO moorings sea surface temperature analysis, suggesting a tightly coupled atmosphere and ocean system during these periods. COAMPS hindcast focused on the 12–16 November 2011 event sugg...

Surface Wind and Upper-Ocean Variability Associated with the Madden–Julian Oscillation Simulated by the Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS)

AbstractSimulation of surface wind and upper-ocean variability associated with the Madden–Julian oscillation (MJO) by a regional coupled model, the Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS), is evaluated by the comparison with in situ and satellite observations. COAMPS is configured for the tropical Indian Ocean domain with the horizontal resolution of 27 km for the atmospheric component and ⅛° for the ocean component. A high-resolution nested grid (9 km) for the atmospheric component is used for the central Indian Ocean. While observational data are assimilated into the atmospheric component, no data are assimilated into the ocean component. The model was integrated during 1 March–30 April 2009 when an active episode of large-scale convection associated with the MJO passed eastward across the Indian Ocean. During this MJO event, strong surface westerly winds (~8 m s−1) were observed in the central equatorial Indian Ocean, and they generated a strong eastward jet (~1 m s−1) on the equa...