Ocean General Circulation Model Simulations Research Articles

Which surface atmospheric variable drives the seasonal cycle of sea surface temperature over the global ocean?

The impact of near‐surface atmospheric variables used in driving the seasonal cycle of climatological mean sea surface temperature (SST) is quantified over the global ocean. The six atmospheric variables are air temperature, vapor mixing ratio, wind speed, precipitation, net shortwave radiation, and net longwave radiation, the first (last) three just above (at) the sea surface. Atmospherically forced ocean general circulation model (OGCM) simulations with no data assimilation are performed using monthly and annual means of those variables under the assumption that variations in climatological monthly SSTs are driven by atmospheric variables. SSTs resulting from these simulations are compared with those from a satellite‐based field to determine the impact of each atmospheric variable. Large spatial variability is found in the order of impact (most to least) of six atmospheric variables. In general, the SST seasonal cycle is driven primarily by shortwave radiation at midlatitudes, but wind speed is the major controlling variable in the Indian Ocean. Precipitation has almost no significant influence on monthly SST. Overall, shortwave radiation is the most influential variable controlling the seasonal cycle of SST over 34.3% of the global ocean. Wind speed is the second most important variable (27.2%). In tropical regions and the Arabian Sea, sources other than the atmospheric thermal forcing are found to play a significant role in regulating the SST seasonal cycle.

Water mass transformation along the Indonesian throughflow in an OGCM

The oceanic pathways connecting the Pacific Ocean to the Indian Ocean are described using a quantitative Lagrangian method applied to Eulerian fields from an ocean general circulation model simulation of the Indonesian seas. The main routes diagnosed are in good agreement with those inferred from observations. The secondary routes and the Pacific recirculation are also quantified. The model reproduces the observed salt penetration of subtropical waters from the South Pacific, the homohaline stratification in the southern Indonesian basins, and the cold fresh tongue which exits into the Indian Ocean. These particular water mass characteristics, close to those observed, are obtained when a tidal mixing parameterization is introduced into the model. Trajectories are obtained which link the water masses at the entrance and at the exit of the Indonesian throughflow (ITF), and the mixing along each trajectory is quantified. Both the ITF and the Pacific recirculation are transformed, suggesting that the Indonesian transformation affects both the Indian and Pacific stratification. A recipe to form Indonesian water masses is proposed. We present three major features of the circulation that revisit the classical picture of the ITF and its associated water mass transformation, while still being in agreement with observations. Firstly, the homohaline layer is not a result of pure isopycnal mixing of the North Pacific Intermediate Water and South Pacific Subtropical Water (SPSW) within the Banda Sea, as previously thought. Instead, the observed homohaline layer is reproduced by the model, but it is caused by both isopycnal mixing with the SPSW and a dominant vertical mixing before the Banda Sea with the NPSW. This new mechanism could be real since the model reproduces the SPSW penetration as observed. Secondly, the model explains why the Banda Sea thermocline water is so fresh compared to the SPSW. Until now, the only explanation was a recirculation of the freshwater from the western route. The model does not reproduce this recirculation but instead shows strong mixing of the SPSW within the Halmahera and Seram Seas, which erodes the salinity maximum so that its signature is not longer perceptible. Finally, this work highlights the key role of the Java Sea freshwater. Even though its annual net mass contribution is small, its fresh salinity contribution is highly significant and represents the main reason why the Pacific salinity maxima are eroded.

Super Cyclone Induces a Mesoscale Phytoplankton Bloom in the Bay of Bengal

Tropical cyclones when on land create havoc, but over the oceans they can trigger a very strong biological response, giving rise to phytoplankton blooms. The Super Cyclone (TC) 05B that occurred during October 25-29, 1999, in the Bay of Bengal over the tropical Indian Ocean was one of the most significant tropical cyclones on record to affect India, with maximum winds of 240 km/h, and the worst since 1971. Using satellite data, it is found that this tropical super cyclone helped spawn a notable mesoscale phytoplankton bloom in the domain (17deg-20degN; 87deg-90degE), which persisted for over a month. The bloom spanned 440 km zonally and 330 km meridionally, enhanced the chlorophyll-a concentrations to a maximum of 10 mg/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> and the net primary productivity by 200%. Furthermore, a cyclonic eddy over the bloom region is revealed from an ocean general circulation model simulation, helping the bloom to last for over month.

Global-Scale Energy and Freshwater Balance in Glacial Climate: A Comparison of Three PMIP2 LGM Simulations

Abstract Three coupled atmosphere–ocean general circulation model (AOGCM) simulations of the Last Glacial Maximum (LGM: about 21 000 yr before present), conducted under the protocol of the second phase of the Paleoclimate Modelling Intercomparison Project (PMIP2), have been analyzed from a viewpoint of large-scale energy and freshwater balance. Atmospheric latent heat (LH) transport decreases at most latitudes due to reduced water vapor content in the lower troposphere, and dry static energy (DSE) transport in northern midlatitudes increases and changes the intensity contrast between the Pacific and Atlantic regions due to enhanced stationary waves over the North American ice sheets. In low latitudes, even with an intensified Hadley circulation in the Northern Hemisphere (NH), reduced DSE transport by the mean zonal circulation as well as a reduced equatorward LH transport is observed. The oceanic heat transport at NH midlatitudes increases owing to intensified subpolar gyres, and the Atlantic heat transport at low latitudes increases in all models whether or not meridional overturning circulation (MOC) intensifies. As a result, total poleward energy transport at the LGM increases in NH mid- and low latitudes in all models. Oceanic freshwater transport decreases, compensating for the response of the atmospheric water vapor transport. These responses in the atmosphere and ocean make the northern North Atlantic Ocean cold and relatively fresh, and the Southern Ocean relatively warm and saline. This is a common and robust feature in all models. The resultant ocean densities and ocean MOC response, however, show model dependency.

Three‐dimensional reconstruction of oceanic mesoscale currents from surface information

The ability to reconstruct the three‐dimensional (3D) dynamics of the ocean by an effective version of Surface Quasi‐Geostrophy (eSQG) is examined. Using the fact that surface density plays an analogous role as interior potential vorticity (PV), the eSQG method consists in inverting the QG PV generated by sea‐surface density only. We also make the extra assumption that sea‐surface temperature (SST) anomalies fully represent surface density anomalies. This approach requires a single snapshot of SST and the setup of two parameters: the mean Brunt‐Väisälä frequency and a parameter that determines the energy level at the ocean surface. The validity of this approach is tested using an Ocean General Circulation Model simulation representing the North Atlantic in winter. It is shown that the method is quite successful in reconstructing the velocity field at the ocean surface for mesoscales (between 30 and 300 km). The eSQG framework can also be applied to reconstruct subsurface fields using surface information. Results show that the reconstruction of velocities and vorticity from surface fields is reasonably good for the upper 500 m and that the success of the method mainly depends on the quality of the SST as a proxy of the density anomaly at the base of the mixed layer. This situation happens after a mixed‐layer deepening period. Therefore the ideal situation for the application of this method would be after strong wind events.

Vertical propagation of extratropical Rossby waves during the 1997–1998 El Niño off the west coast of South America in a medium‐resolution OGCM simulation

A medium‐resolution ocean general circulation model (OGCM) simulation is used to investigate the subthermocline interannual variability in the eastern South Pacific. The focus is on isotherm vertical displacement variability associated with extratropical Rossby waves (ETRW) and their connection with equatorial Kelvin waves (EKW). The WKB theory is used to interpret the modeled subsurface variability. The analysis reveals vertical propagation of energy associated with the 1997–1998 El Niño near the coast at various latitudes. Consistent with theory, WKB raypaths are steeper southward and do not extend more than ∼600 km from the coast at ∼2000 m depth. A vertical mode decomposition of model variability showed that vertical propagation mostly involves the first three baroclinic modes. The vertical isotherm displacements along the raypaths, associated with the downwelling EKW of the 1997–1998 El Niño, are negative (rising isotherms) and peak (minimum isotherm) as El Niño reverses to La Niña conditions. The relationship between vertical propagation of ETRW and equatorial variability is interpreted in the light of EKW sequence. The evolution of the vertical isotherm displacements along the raypath at extratropical latitudes is controlled by the vertical structure of the EKW. As El Niño develops, high‐order baroclinic mode contributions to the EKW progressively increase, leading to the dominance of finer vertical scales of variability along the coast, triggering the vertical propagation of ETRW. This mechanism of connection between equatorial variability and subsurface variability at the extratropical latitudes illustrates how sensitive the ETRW characteristics in the eastern South Pacific are to equatorial forcing.

Oceanic Kelvin waves and tropical Atlantic intraseasonal variability: 1. Kelvin wave characterization

Oceanic Kelvin waves from the equator to the West African coast are investigated in the framework of tropical Atlantic intraseasonal variability. In order to better highlight the wave propagations, a 25–95 day band‐pass filter was applied to the sea surface height (SSH) product derived from the TOPEX/POSEIDON altimeter and an ocean general circulation model simulation for the 1993–2000 period. In addition to equatorial eastward propagations, our analysis reveals recurrent and continuous propagations distinguishable over thousands of kilometers poleward along the coasts as far as about 10°–15° latitude, a novel result with altimeter data. The variance of the filtered SSH signal goes from 1 cm at the equator to 2 cm at the African coast. Estimates of the phase speed range from 1.5 to 2.1 m/s along the equator and the West African coastline. Such values are very close to those of equatorial Kelvin wave propagations, likely dominated by the first two baroclinic modes, supporting the fact that the coastal propagations are coastally trapped Kelvin waves. In order to simplify the description of these Kelvin waves, we present an intraseasonal climatology which reveals regular boreal autumn–winter equator to coast propagations. An improved description is achieved thanks to the computation of an extended empirical orthogonal function for the boreal autumn–winter propagations. Lag correlation of SSH signals allows for a twofold quantification: the phase speed and the importance of remote forcing along the coast. The remote forcing effect of intraseasonal Kelvin waves is clearly evidenced over coastal upwelling regions as far as 10°–15° latitude. The physical mechanism associated with the forcing of the Kelvin waves and its impacts will be investigated in a future paper.

Arabian Sea response to monsoon variations

This study aims to quantify the impact of strong monsoons on the mixed layer heat budget in the Arabian Sea by contrasting forced ocean general circulation model simulations with composite strong and weak monsoon winds. Strong (weak) monsoons are defined as years with zonal component of the Somali Jet being greater (smaller) by more than a standard deviation of the long‐term mean of the National Centers for Environmental Prediction reanalysis winds. Coastal upwelling is shown to be demonstrably stronger for strong monsoons leading to significant surface cooling, shallower thermoclines, and deeper mixed layers. A coupled ecosystem model shows that surface chlorophyll, primary, and export production are indeed higher for strong monsoons compared to weak monsoons driven by the supply of colder, nutrient‐rich waters from greater than 100 m depths. The surprising result is that a strong monsoon results in stronger negative wind stress curl away from the coasts and drives Ekman pumping that results in a deeper thermocline. The weaker stratification and larger turbulent kinetic energy from the winds drive deeper mixed layers leading entrainment cooling with some contribution from the advection of colder upwelled waters from the coastal upwelling regions. Thus the strong monsoons, in fact, enhance oceanic heat uptake indicating that ocean dynamics are cooling the surface and driving the lower atmosphere which has implications for the interpretation of monsoon variability from paleorecords.

Climatic control on river discharge simulations, Zackenberg River drainage basin, northeast Greenland

AbstractA spatially distributed, physically based, hydrologic modeling system (MIKE SHE) was applied to quantify intra‐ and inter‐annual discharge from the snow and glacierized Zackenberg River drainage basin (512 km2; 20% glacier cover) in northeast Greenland. Evolution of snow accumulation, distribution by wind‐blown snow, blowing‐snow sublimation, and snow and ice surface melt were simulated by a spatially distributed, physically based, snow‐evolution modelling system (SnowModel) and used as input to MIKE SHE. Discharge simulations were performed for three periods 1997–2001 (calibration period), 2001–2005 (validation period), and 2071–2100 (scenario period). The combination of SnowModel and MIKE SHE shows promising results; the timing and magnitude of simulated discharge were generally in accordance with observations (R2 = 0·58); however, discrepancies between simulated and observed discharge hydrographs do occur (maximum daily difference up to 44·6 m3 s−1 and up to 9% difference between observed and simulated cumulative discharge). The model does not perform well when a sudden outburst of glacial dammed water occurs, like the 2005 extreme flood event. The modelling study showed that soil processes related to yearly change in active layer depth and glacial processes (such as changes in yearly glacier area, seasonal changes in the internal glacier drainage system, and the sudden release of glacial bulk water storage) need to be determined, for example, from field studies and incorporated in the models before basin runoff can be quantified more precisely. The SnowModel and MIKE SHE model only include first‐order effects of climate change. For the period 2071–2100, future IPCC A2 and B2 climate scenarios based on the HIRHAM regional climate model and HadCM3 atmosphere–ocean general circulation model simulations indicated a mean annual Zackenberg runoff about 1·5 orders of magnitude greater (around 650 mmWE year−1) than from today 1997–2005 (around 430 mmWE year−1), mainly based on changes in negative glacier net mass balance. Copyright © 2007 John Wiley &amp; Sons, Ltd.

What Drives the Seasonal Onset and Decay of the Western Hemisphere Warm Pool?

Abstract The annual heat budget of the Western Hemisphere warm pool (WHWP) is explored using the output of an ocean general circulation model (OGCM) simulation. According to the analysis, the WHWP cannot be considered as a monolithic whole with a single set of dominating processes that explain its behavior. The three regions considered, namely the eastern north Pacific (ENP), the Gulf of Mexico (GoM), and the Caribbean Sea (CBN), are each unique in terms of the atmospheric and oceanic processes that dominate the corresponding heat budgets. In the ENP region, clear-sky shortwave radiation flux is responsible for the growth of the warm pool in boreal spring, while increased cloud cover in boreal summer and associated reduction in solar radiation play a crucial role for the ENP warm pool’s demise. Ocean upwelling in the Costa Rica Dome connected to surrounding areas by horizontal advection offers a persistent yearlong cooling mechanism. Over the Atlantic, the clear-sky radiation flux that increases monotonically from December to May and decreases later is largely responsible for the onset and decay of the Atlantic-side warm pool in boreal summer and fall. The CBN region is affected by upwelling and horizontal advective cooling within and away from the coastal upwelling zone off northern South America during the onset and peak phases, thus slowing down the warm pool’s development, but no evidence was found that advective heat flux divergence is important in the GoM region. Turbulent mixing is also an important cooling mechanism in the annual cycle of the WHWP, and the vertical shear at the warm pool base helps to sustain the turbulent mixing. Common to all three WHWP regions is the reduction of wind speed at the peak phase, suggestive of a convection–evaporation feedback known to be important in the Indo-Pacific warm pool dynamics.

Improving SST Anomaly Simulations in a Layer Ocean Model with an Embedded Entrainment Temperature Submodel

Abstract In this study, an improved sea surface temperature (SST) anomaly (SSTA) solution for the tropical Pacific is presented by explicitly embedding into a layer ocean general circulation model (OGCM) a separate SSTA submodel with an empirical parameterization for the temperature of subsurface water entrained into the ocean mixed layer (Te). Instead of using subsurface temperature directly from the OGCM, Te anomalies for the embedded SSTA submodel are calculated from a historical data-based empirical procedure in terms of sea level (SL) anomalies simulated from the OGCM. An inverse modeling approach is first adopted to estimate Te anomalies from the SSTA equation using observed SST and simulated upper-ocean currents from the OGCM. A relationship between Te and SL anomalies is then obtained by utilizing an empirical orthogonal function (EOF) analysis technique. The empirical Te parameterization optimally leads to a better balanced depiction of the subsurface effect on SST variability by the mean upwelling of anomalous subsurface temperature and vertical mixing in the equatorial Pacific. As compared with a standard OGCM simulation, SSTA simulations from the embedded submodel exhibit more realistic variability, with significantly increased correlation and reduced SSTA errors due to the optimized empirical Te parameterization. In the Niño-3 region (5°S–5°N, 150°–90°W), the anomaly correlation and root-mean-square (RMS) error of the simulated SST anomalies for the period 1963–96 from the standard OGCM are 0.74° and 0.58°C, while from the embedded SSTA submodel they are 0.94° and 0.29°C in the Te-dependent experiment, and 0.86° and 0.41°C in the experiment with one-dependent-year data excluded, respectively. Cross validation and sensitivity experiments to training periods for building the Te parameterization are made to illustrate the robustness and effectiveness of the approach. Moreover, the impact on simulations of SST anomalies and El Niño are examined in hybrid coupled atmosphere–ocean models (HCMs) consisting of the OGCM and a statistical atmospheric wind stress anomaly model that is constructed from a singular value decomposition (SVD) analysis. Results from coupled runs with and without embedding the SSTA submodel are compared. It is demonstrated that incorporating the embedded SSTA submodel in the context of an OGCM has a significant impact on performance of the HCMs and the behavior of the coupled system, with more realistic simulations of interannual SST anomalies (e.g., the amplitude and structure) in the tropical Pacific.

Is a shutdown of the thermohaline circulation irreversible?

The Atlantic thermohaline circulation (THC) plays a vital role in explaining past abrupt climate changes and in maintaining the current climate. Its remarkable nonlinear dynamics, first demonstrated by H. M. Stommel, have been supported by various types of climate models. This has led to severe concerns that global warming may shut down the THC irreversibly, with consequent catastrophic climate changes, particularly for Europe. Here we use an uncoupled ocean general circulation model (OGCM) and a coupled atmosphere‐ocean general circulation model (AOGCM) to investigate the nonlinear response of the THC to freshwater perturbations in the northern North Atlantic. We find that the THC shuts down irreversibly in the uncoupled OGCM simulations but reversibly in the coupled AOGCM simulations. This occurs because of different feedback processes operating in the uncoupled OGCM and AOGCM. The reversal of the THC in the uncoupled OGCM tends to stabilize the “off” mode of the THC by decreasing the mean salinity of the Atlantic, whereas a crucial negative feedback in the AOGCM helps the THC recover. This negative feedback results from complex air‐sea interactions, and its operation needs the full participation of the atmosphere. Thus given the more realistic simulation by the AOGCM, the irreversible shutdown of the THC caused by freshwater addition appears to be an artifact of the uncoupled OGCM rather than a likely outcome of global warming.

Frequency distributions of transient regional climate change from perturbed physics ensembles of general circulation model simulations

Large ensembles of coupled atmosphere–ocean general circulation model (AOGCM) simulations are required to explore modelling uncertainty and make probabilistic predictions of future transient climate change at regional scales. These are not yet computationally feasible so we have developed a technique to emulate the response of such an ensemble by scaling equilibrium patterns of climate change derived from much cheaper “slab” model ensembles in which the atmospheric component of an AOGCM is coupled to a mixed-layer ocean. Climate feedback parameters are diagnosed for each member of a slab model ensemble and used to drive an energy balance model (EBM) to predict the time-dependent response of global surface temperature expected for different combinations of uncertain AOGCM parameters affecting atmospheric, land and sea-ice processes. The EBM projections are then used to scale normalised patterns of change derived for each slab member, and hence emulate the response of the relevant atmospheric model version when coupled to a dynamic ocean, in response to a 1% per annum increase in CO2. The emulated responses are validated by comparison with predictions from a 17 member ensemble of AOGCM simulations, constructed from variants of HadCM3 using the same parameter combinations as 17 members of the slab model ensemble. Cross-validation permits estimation of the spatial and temporal dependence of emulation error, and also allows estimation of a correction field to correct discrepancies between the scaled equilibrium patterns and the transient response, reducing the emulation error. Emulated transient responses and their associated errors are obtained from the slab ensemble for 129 pseudo-HadCM3 versions containing multiple atmospheric parameter perturbations. These are combined to produce regional frequency distributions for the transient response of annual surface temperature change and boreal winter precipitation change. The technique can be extended to any surface climate variable demonstrating a scaleable, approximately linear response to forcing.

Ocean model predictions of chemistry changes from carbon dioxide emissions to the atmosphere and ocean

We present ocean chemistry calculations based on ocean general circulation model simulations of atmospheric CO2 emission, stabilization of atmospheric CO2 content, and stabilization of atmospheric CO2 achieved in total or in part by injection of CO2 to the deep ocean interior. Our goal is to provide first‐order results from various CO2 pathways, allowing correspondence with studies of marine biological effects of added CO2. Parts of the Southern Ocean become undersaturated with respect to aragonite under the Intergovernmental Panel on Climate Change Special Report on Emissions Scenarios (SRES) A1, A2, B1, and B2 emission pathways and the WRE pathways that stabilize CO2 at 650 ppm or above. Cumulative atmospheric emission of 5000 Pg C produces aragonite undersaturation in most of the surface ocean; 10,000 Pg C also produces calcite undersaturation in most of the surface ocean. Stabilization of atmospheric CO2 at 450 ppm produces both calcite and aragonite undersaturation in most of the deep ocean. The simulated SRES pathways produce global surface pH reductions of ∼0.3–0.5 units by year 2100. Approximately this same reduction is produced by WRE650 and WRE1000 stabilization scenarios and by the 1250 Pg C emission scenario by year 2300. Atmospheric emissions of 5000 Pg C and 20,000 Pg C produce global surface pH reductions of 0.8 and 1.4 units, respectively, by year 2300. Simulations of deep ocean CO2 injection as an alternative to atmospheric release show greater chemical impact on the deep ocean as the price for having less impact on the surface ocean and climate. Changes in ocean chemistry of the magnitude shown are likely to be biologically significant.

The influence of the Galápagos Islands on tropical temperatures, currents and the generation of tropical instability waves

The Galápagos Islands provide a topographic barrier for the Southern Equatorial Current (SEC) and the Equatorial Undercurrent (EUC). An island wake effect can be diagnosed from the difference of an ocean general circulation model simulation which includes the Galápagos Islands and one which ignores their presence. Cold thermocline water upwells on the western side of the islands, and only during boreal winter season these cold waters can linger around the Islands at a depth of about 80 m and affect the far eastern equatorial Pacific surface waters. This effect is partly offset by the westward transport of cold surface waters by the SEC which creates a wake on the western side of the Islands. It is furthermore shown that changes in horizontal current shear, induced by the presence of the Galápagos Islands modify the generation of tropical instability waves and lead to a basin scale SST anomaly pattern.

Ocean Mixed Layer Temperature Variations Induced by Intraseasonal Convective Perturbations over the Indian Ocean

In situ and satellite observations reveal that the tropical intraseasonal oscillation is occasionally associated with large variations in sea surface temperature (SST). The purpose of this paper is to find the physical origin of such strong SST perturbations (up to 3 K) over the Indian Ocean by examining two intraseasonal events in January and March 1999. Analysis of SST data from the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) and from drifting buoys reveals that these two intraseasonal events deeply modify the SST field between the equator and 108S, while the surface flux perturbation extends over a wide area of the tropical Indian Ocean. Forced ocean general circulation model (OGCM) simulations are successful in reproducing the spatial patterns of this intraseasonal SST variability albeit with a weaker amplitude. The weaker amplitude given by the OGCM is partly related to the absence of warm-layer formation in the model. The model simulation reveals that the background oceanic subsurface structure explains the observed latitudinal distribution of the

The dynamics of the Sicily Strait: a comprehensive study from observations and models

Abstract The dynamics of the Sicily Strait are investigated in a comprehensive study using recent in situ observations, Ocean General Circulation Model (OGCM) simulations and process model results. In particular, results of a very high-resolution Mediterranean model (MED16) developed in the context of the Mercator project are described. The circulation in the Sicily Strait can be described as a two-layer exchange of Atlantic Water (AW) and Eastern Mediterranean Outflow Water (EOW). The Algerian Current brings AW (the upper layer) eastwards and splits into two branches at the entrance to the Sicily Strait, one flowing to the Tyrrhenian Sea, the other into the Sicily Strait. The later is composed of two streams, referred to as the Atlantic Ionian Stream (AIS) and the Atlantic Tunisian Current (ATC). In winter, the ATC signature is more pronounced. In summer, the AIS is associated with a number of well-known semi-permanent features including the intermittent northward extension of the AIS (called NAIS) at the Ionian shelf break, which seems to be driven by the surface density contrast between waters of the Sicilian and the Ionian basins. New intermittent patterns are shown by the MED16 simulations, for example an anticyclonic eddy (called MCA) south of the Malta Channel and an anticyclonic gyre associated to the NAIS (called NAISA). Upwelling is well developed in August, with temperature lower than 24°C along the western and southern coasts of Sicily. The seasonal transport of the AW through the strait has a maximum value (1.4 Sv ) in winter, and a minimum value at the end of summer (0.8 Sv ) . In the lower layer, at the exit of the Sicily Strait, the EOW, mainly composed of Levantine Intermediate Water (LIW), outflows into the Tyrrhenian Sea and veers to the right. Then, as well known, the LIW flows along the northern coast of Sicily before reaching the Sardinia Channel. A new LIW path from the Sicily Strait towards the Sardinia Channel is shown by the MED16 model, and is due to intermediate eddy shedding at the exit of the Sicily Strait. The topography of the sill in the Sicily Strait plays a major role in the circulation. A combination of barotropic/baroclinic double Kelvin waves on both side of the sill provides a mechanism, for splitting the Algerian Current.

Second and first baroclinic Kelvin modes in the equatorial Pacific at intraseasonal timescales

TOPEX/Poseidon sea level and time series from the TAO (Tropical Atmosphere Ocean) array of moorings over 1992–1999 are used to investigate the spatiotemporal characteristics of equatorial Kelvin waves in the Pacific at period shorter than 180 days. Spectral analyses show high oceanic energy in two separate bands of period, around 70 days and 120 days. Signals are coherent in both bands all along the equator. Ocean General Circulation Model (OGCM) simulations forced by different wind stress fields are also analyzed and exhibit the same features. Observed and modeled sea level, dynamic height and 20°C isotherm depth, band‐pass filtered around 70 days, show an eastward propagation at phase speed of 2.4–2.9 m s−1, typical of the first baroclinic Kelvin mode. The meridional and vertical structures of the equatorial waves at the onset of the 1997 El Niño are also those expected for a first Kelvin mode. At 120‐day period, phase speeds, meridional and vertical structures are closer to those expected for a second baroclinic mode. Both modes are nevertheless present in the two frequency bands. A simple Kelvin wave model shows that the wind at 120‐day period seems to force the Kelvin waves at the same period. However, the predominance of second baroclinic mode at 120‐day period cannot be explained solely by the characteristics of the wind. OGCM simulations rather suggest the importance of internal oceanic processes such as vertical redistribution of energy from first to second baroclinic mode as the waves propagate eastward. Model outputs over 1984–1999 indicate that these equatorial Kelvin waves at 70‐ and 120‐day period may have an effect on El Niño since they seem to be stronger at its onset.

Tropical Pacific baroclinic mode contribution and associated long waves for the 1994–1999 period from an assimilation experiment with altimetric data

An Ocean General Circulation Model (OGCM) of the tropical Pacific in which combined TOPEX/Poseidon and ERS sea level anomalies are assimilated over January 1994 through July 1999, is used to investigate equatorial wave characteristics during the intense 1997–1999 El Niño‐La Niña event. Near the equator, the linear vertical modes are estimated at each grid point of the OGCM simulation with and without assimilation. Consistently with an increase of the vertical gradient within the thermocline and a rise of the thermocline depth in the eastern basin, the assimilation results in an increased contribution of the higher‐order baroclinic modes in the eastern basin and a decreased contribution of the first baroclinic mode in the western Pacific for the zonal current variability. For pressure, the first baroclinic mode contribution is reduced whereas the higher‐order modes contribution is weakly impacted. Kelvin and first‐meridional Rossby waves are then derived for the first two more energetic baroclinic modes in the simulation with assimilation. Kelvin waves of both modes constructively contribute to the strong warming observed in 1997, with the first (second) baroclinic mode being more energetic than the second (first) baroclinic mode in the early (mature) stage of the warming. Kelvin waves of both modes reflect as first meridional Rossby waves at the eastern boundary (reflection efficiency of ∼95%) and contribute to push back the warm pool westward. The reversal of the warming is apparently initiated by the second baroclinic mode contribution which controls the position of the 28°C isotherm in the surface layer in the far eastern Pacific from January 1998. At the western boundary, reflections of Rossby waves take place for both modes with an estimated total efficiency of ∼50% at 165°E. This suggests that, in our model, the delayed oscillator theory is not applicable for explaining the reversal from warm to cold conditions in 1998 while the zonal advective feedback was at work. More generally, the study suggests that it is necessary to take into account the vertical structure of the ocean when interpreting altimetric data, which can be done through an assimilation experiment.

Sea surface salinity changes in the East China Sea during 1997–2001: Influence of the Yangtze River

The role of the Yangtze River discharge in changing sea‐surface salinity (SSS) in the East China Sea during the 1997–2001 periods is investigated by using (1) in situ SSS measurements along a merchant ship track running from Taiwan strait to Korea strait, (2) ocean general circulation model (OGCM) simulations and specific sensitivity studies, and (3) in land precipitation data to infer estimates of the river discharge in absence of gauged station data. The river effects on 1997–2001 sea surface temperature (SST) and surface currents are also examined. Both in situ SSS‐SST measurements and OGCM simulations clearly indicate that the integration of the 1997 drought signal and 1998–1999 flooding signal over the catchment area of the Yangtze River are manifest in the ocean where local precipitation played a rather modest role. Sensitivity studies with the OGCM with and without the river discharge further reveal that these signals can extend over several hundred kilometers off the river mouth, consistent with the along track in situ measurements. In particular, SSS along the ship track can drop from about 34 psu in August 1997 to less than 26 psu in July 1998 (almost the same for 1999) at the latitude of the river mouth although located 500 km to the west of the ship track. Model results indicate that the dynamic and thermodynamic effects of the freshwater input from the river may be phase lagged by up to a season with maximum surface warming occurring in the summer months when the river discharge is maximum whereas the largest differences in mixed layers with and without the river input occur during later winter and late autumn. The possible positive feedback between the increase of river runoff yielding warm SST anomaly off the river mouth and so enhanced atmospheric convection and precipitation is discussed.