Sea Surface Forcing Research Articles

Modulation of observed sea surface temperature variation by the quasi‐biweekly oscillation in the tropical western Pacific during boreal summer

AbstractUsing 17‐year in situ buoy observations, this study investigates how sea surface temperature (SST) over the tropical western Pacific Ocean is modulated by the quasi‐biweekly oscillation (QBWO) during boreal summer. The 10‐buoy mean phase‐by‐phase composites of SST indicate that SST evolution lags behind the convection cycle by approximately one quarter, with the maximum SST occurring in Phase −90° (29.68°C) and the minimum SST occurring in Phase 90° (29.37°C). The diurnal cycle of SST (dSST) is in phase with the convection cycle, with the minimum dSST occurring during the convectively active phase (0.34°C) and the maximum dSST occurring during the convectively suppressed phase (0.61°C). The fluctuation in the dSST reaches 0.27°C, which is similar to that in the SST (0.31°C). This kind of oscillation can be well explained by the evolution of the sea surface forcing parameters, especially the shortwave radiation and wind speeds, observed from the buoys. In addition, the composite range of the dSST suggests an interesting asymmetry between Phase −90° and 90°. The interaction of the background summer mean state and the 10–30‐day intraseasonal anomalies causes the asymmetry in wind speed between Phase −90° and 90°, which in turn results in the asymmetry in the dSST between these two phases. Since QBWOs are more likely to occur during El Niño developing summers than during El Niño decaying summers, the 10–30‐day intraseasonal SST and dSST variances during El Niño developing summers are stronger than those during El Niño decaying summers.

Sensitivity of sea-level forecasting to the horizontal resolution and sea surface forcing for different configurations of an oceanographic model of the Adriatic Sea

Abstract. At the Hydro-meteo-climate service of the Regional environmental agency of Emilia-Romagna, Italy (Arpae-SIMC), the oceanographic numerical model AdriaROMS is used in the operational forecasting suite to compute sea level, temperature, salinity and 3-D current fields of the Adriatic Sea (northern Mediterranean Sea). In order to evaluate the performance of the sea-level forecast and to study different configurations of the ROMS model, two marine storms occurred on the Emilia Romagna coast during the winter 2015–2016 are investigated. The main focus of this study is to analyse the sensitivity of the model to the horizontal resolution and to the meteorological forcing. To this end, the model is run with two different configurations and with two horizontal grids at 1 and 2 km resolution. To study the influence of the meteorological forcing, the two storms have been reproduced by running ROMS in ensemble mode, forced by the 16-members of the meteorological ensemble COSMO-LEPS system. Possible optimizations of the model set-up are deduced by the comparison of the different run outputs.

Evaluating CMIP5 simulations of mixed layer depth during summer

The ability of CMIP5 models in simulating surface mixed layer depth (MLD) during summer is assessed using 45 climate models. Their ocean models differ greatly in terms of vertical mixing parameterizations and model configurations. In some models, effects of surface waves, Langmuir circulations, submesoscale eddies, as well as additional wind mixing are included to improve upper-ocean simulation. Similar to findings by previous studies, the summer MLDs are significantly underestimated in most of the models. Compared with the observation, only five of these models have deeper summer MLDs in the Southern Ocean, eight models have deeper summer MLDs in the central North Atlantic Ocean, and nine models have deeper summer MLDs in the central North Pacific Ocean. This underestimation of MLD is not caused by sea surface forcing, because most of the models tend to overestimate the surface wind stress, while they underestimate the net surface heat flux. Therefore, insufficient vertical mixing in the upper ocean may still be one of the potential reasons for this systematic underestimation of MLD in the climate models.

Impact of sea-surface dust radiative forcing on the oceanic primary production: A 1D modeling approach applied to the West African coastal waters

[1] The impact of the dust sea-surface forcing (DSSF) on the oceanic Primary Production (PP) is investigated here by using 1D modelling approach coupling an atmospheric radiative transfer model and a simple PP model. Simulations reveal that dust are able to induce a significant decrease of PP due to the attenuation of light by about 15–25% for dust optical depth (DOD) larger than 0.6–0.7 (at 550 nm). For DOD lower than ∼0.2–0.3, the influence of dust on PP is weak (∼5%). In addition to DOD, the important role played by dust single scattering albedo (DSSA) is also shown. Realistic applications over the Senegal coast are studied using SeaWiFS and AERONET observations. The analysis showed that PP could be reduced by about 15–20% during the spring period. This study highlights that dust/light interactions need to be parameterized in coupled ocean-atmosphere models used to estimate PP at regional scales.

Sea Surface Temperature Feedback Extends the Predictability of Tropical Intraseasonal Oscillation

Abstract The possible impacts of different sea surface temperature (SST) configurations on the predictability of the boreal summer tropical intraseasonal oscillation (TISO) are assessed with a series of ensemble forecasts. The five different lower boundary conditions examined in this study are, respectively, (i) the fully interactive ocean–atmosphere coupling, (ii) “smoothed” SST, which excludes the intraseasonal signal from sea surface forcing, (iii) damped persistent SST, (iv) coupling to a slab mixed-layer ocean, and (v) daily SST from the coupled forecast. The full atmosphere–ocean coupling generates an interactive SST that results in the highest TISO predictability of about 30 days over Southeast Asia. The atmosphere-only model is capable of reaching this predictability if the ensemble mean daily SST forecast by the coupled model is used as the lower boundary condition, which suggests that, in principle, the so-called tier-one and tier-two systems have the same predictability for the boreal summer TISO. The atmosphere-only model driven by either smoothed or damped persistent SSTs, however, has the lowest predictability (∼20 days). The atmospheric model coupled to a slab mixed-layer ocean achieves a predictability of 25 days. The positive SST anomalies in the northern Indo–western Pacific Oceans trigger convective disturbances by moistening and warming up the atmospheric boundary layer. The seasonal mean easterly shear intensifies the anomalous convection by enhancing the surface convergence. An overturning meridional circulation driven by the off-equatorial anomalous convection suppresses the near-equatorial convection and enhances the northward flows, which further intensify the off-equatorial surface convergence and the TISO-related convection. Thus, the boreal summer mean easterly shear and the overturning meridional circulation in the northern Indo–western Pacific sector act as “amplifiers” for the SST feedback to the convection of the TISO.

Correcting the errors in the initial conditions and wind stress in storm surge simulation using an adjoint optimal technique

An adjoint data assimilation methodology is applied to the Princeton Ocean Model and is evaluated by obtaining “optimal” initial conditions, sea surface forcing conditions, or both for coastal storm surge modelling. By prescribing different error sources and setting the corresponding control variables, we performed several sets of identical twin experiments by assimilating model-generated water levels. The experiment results show that, when the forecasting errors are caused by the initial (or surface boundary) conditions, adjusting initial (or surface boundary) conditions accordingly can significantly improve the storm surge simulation. However, when the forecasting errors are caused by surface boundary (or initial) conditions, data assimilation targeting improving the initial (or surface boundary) conditions is ineffective. When the forecasting errors are caused by both the initial and surface boundary conditions, adjusting both the initial and surface boundary conditions leads to the best results. In practice, we do not know whether the errors are caused by initial conditions or surface boundary conditions, therefore it is better to adjust both initial and surface boundary conditions in adjoint data assimilation.

Modeling the Global Thermohaline Circulation

This paper reviews recent progress and problems in modeling the thermohaline circulation of the world ocean by use of z-coordinate ocean general circulation models. Discussions focus on four issues: sea surface forcing, mixing in the deep ocean interior, eddy-induced tracer transport, and bottom boundary layer processes. Although some widely used techniques and parameterizations deal with these issues, some aspects are still overlooked and more sophistication is certainly required.

Sensitivity of paleonutrient tracer distributions and deep‐sea circulation to glacial boundary conditions

We use a carbon cycle model coupled to an ocean general circulation model to explore the links between sea surface boundary conditions, the deep‐sea circulation, and the distribution of paleonutrient tracers (δ13C and Cd/Ca) from Last Glacial Maximum (21,000 B.P) sediments. A glacial flow field with a shallower and 50% reduced North Atlantic overturning circulation generally reproduces the tracer data but cannot explain the strong glacial‐interglacial shift in δ13C in the Southern Ocean. Sensitivity experiments with changes of ±1 in salinity in the glacial salinity boundary fields show circulation patterns ranging from even stronger than the present day one to nearly a shutdown of the Atlantic deep‐sea circulation. Our model results indicate that the overturning in the North Atlantic is linearly related to the zonal wind forcing in the Southern Ocean but with half of the sensitivity of Toggweiler and Samuels [1993]. Atmospheric pCO2 appears to be insensitive to changing circulation and sea surface forcing; a tropical cooling of 4°C can only explain 8% of the glacial‐interglacial pCO2 change documented in ice cores.

On the Black Sea water mass formation. Model sensitivity study to atmospheric forcing and parameterizations of physical processes

A numerical model of the Black Sea circulation is developed using the Bryan-Cox-Semtner modular ocean model. Some available climatic and atmospheric analysis data are used to force the model. The analysis of the simulations gives a strong motivation to construct new sea surface forcing functions for this basin. It is shown that they provide heat and water fluxes that are consistent with the observations and with some independent balance estimates. The improved model forcing results in simulating of a realistic circulation. The inflow through the Strait of Bosphorus is parameterized using physical concepts, based on theoretical models and observational data, relating the strait exchange to the net fresh water flux at sea surface. A new parameterization is included to simulate the Mediterranean plume. The model sensitivity to driving forces, physical processes, and different parameterizations is studied. The results are used to tune the model to simulate adequately the water mass formation. The most sensitive elements of the vertical stratification, as the cold intermediate layer and the deep temperature intrusions, are used in this tuning. The model circulation, ventilation of the deep and intermediate layers, some characteristic time and length scales show a good agreement with the observations. The cooling capacity of the intermediate water is analysed as dependent on the fresh water flux at sea surface. The model simulations give indications that the changes in thermal stratification in the intermediate depths are indicative not only for how severe the preceding winter was. These changes are also dependent on the variability in the fresh water. In a long run, they could illustrate climate changes in the catchment area of the Black Sea rivers.

Linear Equatorial Wave Mode Initialization in a Model of the Tropical Pacific Ocean: An Initialization Scheme for Tropical Ocean Models

Data assimilation in models of the tropical oceans can generate spurious equatorial wave modes which are potentially harmful to the model background fields. The amplitudes of these spurious wave modes can often be large and, in general, depend upon the nature and size of the imbalances introduced into the model by the data assimilation process. Spurious equatorial Kelvin waves are likely to give rise to the most detrimental effects since they are dynamically very important in the real ocean, and am responsible for a variety of phenomena in the equatorial and coastal regions of the ocean. In this paper, an initialization scheme based on the technique of normal mode initialization (used for many years by meteorologists) is developed, which can be used to suppress spurious equatorial wave modes during data assimilation experiments. The initialization scheme maps information from observational data onto the linear planetary wave solutions of the discretized model equations of motion. In addition, equatorial Kelvin waves and Yanai waves driven by sea surface forcing in the model are retained. Using this method, the rate of growth of the assimilation errors between one assimilation time and the next can be reduced considerably. It is shown that the model need only he initialized in a narrow channel spanning the equator, and that the initialization method also works well in a fully nonlinear model.

Aspects of Geostrophic Adjustment during Tropical Ocean Data Assimilation

Abstract Subsurface temperature data from the ship-of-opportunity network in the tropical Pacific Ocean was assimilated into a simple reduced-gravity model. A large initialization shock was found to occur in the model which takes the form of equatorially trapped waves. Observations in the western tropical Pacific are found to generate a more severe case of initialization shock than observations in the eastern half of the basin. In addition, the magnitude of the initialization shock was found to be dependent upon the strength of the sea surface forcing. Attempts to suppress the large amplitude equatorial Kelvin waves and Yanai waves excited as part of the initialization shock are partially successful, but the damage inflicted on the model first-guess fields by this procedure is greater than that which ensues if the Kelvin waves and Yanai waves are left unchecked. Despite the initialization shock, the model is able to predict the large-wale structure and variability of the major near surface currents in the...