Sea Surface Boundary Conditions Research Articles

Sensitivity of results of Black Sea level modeling to the choice of boundary conditions on the free surface

This work is devoted to an analysis of the influence of sea surface boundary conditions on results of Black Sea level modeling. Two numerical experiments are carried out by using a z-coordinate model of the Marine Hydrophysical Institute, RAS. A linear kinematic condition on the free surface is used for calculating a level rise in the first experiment and a nonlinear one – in the second experiment. The analysis shows that both experiments reconstruct the Black Sea surface dynamics in accordance with altimetry data. It is found that the sea level near the western boundary of the basin decreases under the nonlinear kinematic condition in cases of storm winds. The greatest differences in the structure of the sea level fields are observed during weakening of the RIM Current in the evolution zones of Sevastopol and Batumi anticyclones. The minimal and maximal values of the level are almost identical in both experiments, but the locations do not match. The discrepancies in the spatial positions of raising/lowering levels are due to a mismatch in the motion phases of anticyclones and mesoscale eddies along the periphery of the RIM Current.

Effect of empirical correction of sea-surface temperature biases on the CRCM5-simulated climate and projected climate changes over North America

Dynamical downscaling (DD) consists in using archives of Coupled Global Climate Models (CGCM) simulations as the atmospheric and sea-surface boundary conditions (BC) to drive nested, Regional Climate Model (RCM) simulations. Biases in the CGCM-generated driving BC, however, can have detrimental impacts on RCM performance. It is well documented for the historical period that CGCM-simulated sea-surface temperatures (SST) suffer substantial biases, especially important near coastal regions. Assuming that these SST biases are time-invariant, they could in principle be subtracted from century-long CGCM projections before being used to drive RCMs. This paper investigates the performance of a 3-step DD approach as follows. The CGCM-simulated sea-surface temperatures (SST) are first empirically corrected by subtracting their systematic biases; the corrected SST are then used as ocean surface BC for an atmosphere-only GCM (AGCM) simulation; finally this AGCM simulation provides the atmospheric lateral BC to drive an RCM simulation. This is what we refer to as the 3-step approach CGCM–AGCM–RCM of DD, which can be compared to the traditional 2-step approach CGCM–RCM consisting of driving an RCM simulation directly by CGCM-generated BC. In this paper we compare the results obtained with the two approaches, for present and future climates under RCP8.5, using the fifth-generation Canadian Regional Climate Model (CRCM5) with a grid mesh of 0.22° over the North American CORDEX domain, driven by two CMIP5 models: the Canadian Earth System Model of the Canadian Centre for Climate modelling and analysis (CanESM2) and the Earth System Model of the Max-Planck-Institut für Meteorologie (MPI-ESM-MR). The results show that, in current climate, the seasonal-mean 2-m temperature fields simulated with the 3-step DD have generally smaller biases with respect to the observations than those simulated with the 2-step DD; in fact the performance of the 3-step DD simulations often approaches that of the reanalyses-driven simulation. For the seasonal-mean precipitation field, however, the differences between the two DD methods are not conclusive. Differences between the projected climate changes with the two DD methods vary substantially depending upon the variable being considered. Differences are particularly important for temperature: over the bulk of the North American continent, the 3-step DD projects more warming in winter and less in summer. This result highlights the nonlinearities of the climate system, and constitutes an additional measure of uncertainty with DD.

Modeling of Long-Term Evolution of Hydrophysical Fields of the Black Sea

The long-term evolution of the Black Sea dynamics (1980–2020) is reconstructed by numerical simulation. The model of the Black Sea circulation has 4.8 km horizontal spatial resolution and 40 levels in z-coordinates. The mixing processes in the upper layer are parameterized by Mellor-Yamada turbulent model. For the sea surface boundary conditions, atmospheric forcing functions were used, provided for the Black Sea region by the Euro mediterranean Center on Climate Change (CMCC) from the COSMO-CLM regional climate model. These data have a spatial resolution of 14 km and a daily temporal resolution. To evaluate the quality of the hydrodynamic fields derived from the simulation, they were compared with in-situ hydrological measurements and similar results from physical reanalysis of the Black Sea.

Oceanic Forcing of Antarctic Climate Change: A Study Using a Stretched-Grid Atmospheric General Circulation Model

Abstract A variable-resolution atmospheric general circulation model (AGCM) is used for climate change projections over the Antarctic. The present-day simulation uses prescribed observed sea surface conditions, while a set of five simulations for the end of the twenty-first century (2070–99) under the Special Report on Emissions Scenarios (SRES) A1B scenario uses sea surface condition anomalies from selected coupled ocean–atmosphere climate models from phase 3 of the Coupled Model Intercomparison Project (CMIP3). Analysis of the results shows that the prescribed sea surface condition anomalies have a very strong influence on the simulated climate change on the Antarctic continent, largely dominating the direct effect of the prescribed greenhouse gas concentration changes in the AGCM simulations. Complementary simulations with idealized forcings confirm these results. An analysis of circulation changes using self-organizing maps shows that the simulated climate change on regional scales is not principally caused by shifts of the frequencies of the dominant circulation patterns, except for precipitation changes in some coastal regions. The study illustrates that in some respects the use of bias-corrected sea surface boundary conditions in climate projections with a variable-resolution atmospheric general circulation model has some distinct advantages over the use of limited-area atmospheric circulation models directly forced by generally biased coupled climate model output.

Responses of thermal structure and vertical dynamic structure of South China Sea to Typhoon Chanchu

The response of the South China Sea (SCS) to Typhoon Chanchu (2006) was examined using the MM5 and POM model. In the POM model, sea surface boundary conditions were forced by the simulation wind field from MM5, the velocity forcing was introduced in the eastern boundary and the computational schemes of heat fluxes at the surface were introduced. Comparison with the observation data shows that the simulated results are reliable. In the response process of the SCS to Typhoon Chanchu, the influence of the heat fluxes on thermal structure of the SCS was regionally different. Strong wind forcing would lead to upwelling phenomenon in the lateral boundary of deep water basin. Furthermore, the Ekman pumping theory was used to discuss subsurface upwelling and downwelling phenomenon in typhoon forced stage.

Factors influencing the climatological mixed layer depth in the South China Sea: numerical simulations

The mixed layer depth (MLD) in the upper ocean is an important physical parameter for describing the upper ocean mixed layer. We analyzed several major factors influencing the climatological mixed layer depth (CMLD), and established a numerical simulation in the South China Sea (SCS) using the Regional Ocean Model System (ROMS) with a high-resolution (1/12°×1/12°) grid nesting method and 50 vertical layers. Several ideal numerical experiments were tested by modifying the existing sea surface boundary conditions. Especially, we analyzed the sensitivity of the results simulated for the CMLD with factors of sea surface wind stress (SSWS), sea surface net heat flux (SSNHF), and the difference between evaporation and precipitation (DEP). The result shows that of the three factors that change the depth of the CMLD, SSWS is in the first place, when ignoring the impact of SSWS, CMLD will change by 26% on average, and its effect is always to deepen the CMLD; the next comes SSNHF (13%) for deepening the CMLD in October to January and shallowing the CMLD in February to September; and the DEP comes in the third (only 2%). Moreover, we analyzed the temporal and spatial characteristics of CMLD and compared the simulation result with the ARGO observational data. The results indicate that ROMS is applicable for studying CMLD in the SCS area.

Influence of Oceanic Boundary Conditions in Simulations of Antarctic Climate and Surface Mass Balance Change during the Coming Century

Abstract This article reports on high-resolution (60 km) atmospheric general circulation model simulations of the Antarctic climate for the periods 1981–2000 and 2081–2100. The analysis focuses on the surface mass balance change, one of the components of the total ice sheet mass balance, and its impact on global eustatic sea level. Contrary to previous simulations, in which the authors directly used sea surface boundary conditions produced by a coupled ocean–atmosphere model for the last decades of both centuries, an anomaly method was applied here in which the present-day simulations use observed sea surface conditions, while the simulations for the end of the twenty-first century use the change in sea surface conditions taken from the coupled simulations superimposed on the present-day observations. It is shown that the use of observed oceanic boundary conditions clearly improves the simulation of the present-day Antarctic climate, compared to model runs using boundary conditions from a coupled climate model. Moreover, although the spatial patterns of the simulated climate change are similar, the two methods yield significantly different estimates of the amplitude of the future climate and surface mass balance change over the Antarctic continent. These differences are of similar magnitude as the intermodel dispersion in the current Intergovernmental Panel on Climate Change (IPCC) exercise: selecting a method for generating boundary conditions for a high-resolution model may be just as important as selecting the climate model itself. Using the anomaly method, the simulated mean surface mass balance change over the grounded ice sheet from 1981–2000 to 2081–2100 is 43-mm water equivalent per year, corresponding to a eustatic sea level decrease of 1.5 mm yr−1. A further result of this work is that future continental-mean surface mass balance changes are dominated by the coastal regions, and that high-resolution models, which better resolve coastal processes, tend to predict stronger precipitation changes than models with lower spatial resolution.

On sensitivity of ocean circulation to sea surface salinity

Sensitivity of global ocean circulation to details of sea surface boundary conditions has been revisited in a set of numerical experiments. The focus is on the role of sea surface salinity (SSS) asymmetries between different ocean basins. The results indicate that the basin-wide inter-basin salinity contrasts are the key elements of success for many idealized ocean models that have realistically mirrored ocean circulation despite strongly simplified sea surface conditions. We suggest that exact knowledge of the spatial distribution of sea surface salinity may not be crucial to the simulation of the thermohaline ocean circulation (THC) in many process-oriented climate studies. Therefore, simulations based on limited sea surface data, for example in paleoclimate studies, can yield acceptable circulation patterns, as long as the inter-basin surface salinity contrasts are retained. Possible feedbacks of both concurrent northern and southern freshwater impacts and the building up of inter-basin sea surface salinity contrasts are discussed.

The Sustained North American Warming of 1997 and 1998

North America experienced sustained and strong surface warming during 1997 and 1998. This period coincided with a dramatic swing of the El Niño–Southern Oscillation (ENSO), with El Niño in 1997 rapidly replaced by La Niña in 1998. An additional aspect of the sea surface temperatures (SSTs) was the warmth of the world oceans as a whole for the entire period, with unprecedented amplitudes within the recent instrumental record. Using a suite of dynamical and empirical model simulations, this study examines the causes for the North American warming, focusing on the role of the sea surface boundary conditions. Two sets of atmospheric general circulation model experiments, one forced with the observed global SSTs and the other with the tropical east Pacific portion only, produce similar North American–wide warming during fall and winter of 1997. The GCM results match empirical estimates of the canonical temperature response related to a strong El Niño and confirm that east equatorial Pacific SST forcing was a major factor in the continental warming of 1997. Perpetuation of that warming from spring through fall of 1998 is shown to be unrelated to equatorial east Pacific SSTs and thus cannot be attributed to the ENSO cycle directly. Yet, simulations using the observed global SSTs are shown to reproduce realistically the continuation of North American warming throughout 1998. The continental warmth occurs in tandem with a warming of the troposphere that, initially confined to tropical latitudes during El Niño’s peak in 1997, spreads poleward and covers the entire globe in 1998. This evolutionary aspect of the global circulation anomalies during 1997 and 1998 is found to be a response to global SSTs and not linked directly to ENSO’s evolution. Results presented here demonstrate that a significant fraction of the North American warming in 1997 and 1998 is explainable as the forced response to sea surface boundary conditions. The hand-over in the impact of those SSTs, with a classic ENSO driven signal in 1997 but an outwardly independent signal in 1998 related to the disposition of global SSTs outside the ENSO region is emphasized. The high potential predictability of North American climate during this 2-yr period raises new questions on the role of global SSTs in climate variability and the ability to predict them skillfully.

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.

Origins of Extreme Climate States during the 1982–83 ENSO Winter

Abstract The origin of extreme climate states during the exceptional 1982–83 El Nino event has continued to be a source of controversy and debate. On the one hand, empirical analyses of extratropical climate patterns during past El Nino events suggests the observed anomalies during 1982–83 were consistent with tropical forcing. On the other hand, the large amplitude of those anomalies have not been replicated in atmospheric general circulation model (AGCM) simulations for that period performed as part of the Atmospheric Model Intercomparison Project (AMIP). It has recently become apparent, however, that the sea surface boundary conditions used to drive the multitude of AMIP simulations were deficient, in that at least 30% of available tropical Pacific SST observations were discarded in the analysis cycle due to excessive trimming constraints. It is shown from a reanalysis of the sea surface temperatures that the observed east equatorial Pacific waters were 1.5°C warmer than original estimates. In order to...

The influence of wave groups on the dual‐frequency Doppler spectrum: Experimental and theoretical approach

Experimental output data from a multifrequency, continuous wave, C band system have been investigated and discussed with reference to physical principles involved in modulation of small‐scale centimetric water waves by larger‐scale wave trains. Attention payed at field and laboratory measurements described in the literature show that the propagation velocity of certain wave components below the spectral peak occasionally differs from predictions of conventional linear theory. A one‐dimensional theoretical analysis of the dual‐frequency microwave response, including Doppler features and modulational effects induced by modulated wave trains, has been developed to throw light on this phenomenon. The theoretical expressions derived predict that wave train modulation should manifest itself as two narrow, Doppler‐shifted peaks in the cross‐product power spectrum generated from two distinct microwave returns. The wave field involved in the modulational process, is modeled as not only consisting of independently propagating Fourier components obeying the linear dispersion relation but also being composed of wave groups of Stokeslike waves, forming independent wave packets. Furthermore, each isolated packet is constructed to be a perturbation solution of the wave equations representing the sea surface boundary conditions. Experimentally, pronounced spectral components were observed with variations in magnitude and Doppler behavior which are consistent with proposed theory, surface truth, and results presented earlier.