scholarly journals On the Estimation of Lagrangian Diffusivity: Influence of Nonstationary Mean Flow

2014 ◽  
Vol 44 (10) ◽  
pp. 2796-2811 ◽  
Author(s):  
Yu-Kun Qian ◽  
Shiqiu Peng ◽  
Chang-Xia Liang ◽  
Rick Lumpkin
Keyword(s):  
Seasonal Cycle ◽  
Time Lag ◽  
Eastern Coast ◽  
Seasonal Cycles ◽  
Mean Flow ◽  
Shear Dispersion ◽  
The Indian Ocean ◽  
The Mean ◽  
Monsoon Winds ◽  
Flow Decomposition

Abstract Eddy–mean flow decomposition is crucial to the estimation of Lagrangian diffusivity based on drifter data. Previous studies have shown that inhomogeneous mean flow induces shear dispersion that increases the estimated diffusivity with time. In the present study, the influences of nonstationary mean flows on the estimation of Lagrangian diffusivity, especially the asymptotic behavior, are investigated using a first-order stochastic model, with both idealized and satellite-based oceanic mean flows. Results from both experiments show that, in addition to inhomogeneity, nonstationarity of mean flows that contain slowly varying signals, such as a seasonal cycle, also cause large biases in the estimates of diffusivity within a time lag of 2 months if a traditional binning method is used. Therefore, when assessing Lagrangian diffusivity over regions where a seasonal cycle is significant [e.g., the Indian Ocean (IO) dominated by monsoon winds], inhomogeneity and nonstationarity of the mean flow should be simultaneously taken into account in eddy–mean flow decomposition. A temporally and spatially continuous fit through the Gauss–Markov (GM) estimator turns out to be very efficient in isolating the effects of inhomogeneity and nonstationarity of the mean flow, resulting in estimates that are closest to the true diffusivity, especially in regions where strong seasonal cycles exist such as the eastern coast of Somalia and the equatorial IO.

scholarly journals Annual, Seasonal, and Interannual Variability of Air–Sea Heat Fluxes in the Indian Ocean

2007 ◽  
Vol 20 (13) ◽  
pp. 3190-3209 ◽  
Author(s):  
Lisan Yu ◽  
Xiangze Jin ◽  
Robert A. Weller
Keyword(s):  
Heat Flux ◽  
Indian Ocean ◽  
Interannual Variability ◽  
Seasonal Cycle ◽  
Arabian Sea ◽  
Heat Fluxes ◽  
The Other ◽  
Net Heat Flux ◽  
The Indian Ocean ◽  
The Mean

Abstract This study investigated the accuracy and physical representation of air–sea surface heat flux estimates for the Indian Ocean on annual, seasonal, and interannual time scales. Six heat flux products were analyzed, including the newly developed latent and sensible heat fluxes from the Objectively Analyzed Air–Sea Heat Fluxes (OAFlux) project and net shortwave and longwave radiation results from the International Satellite Cloud Climatology Project (ISCCP), the heat flux analysis from the Southampton Oceanography Centre (SOC), the National Centers for Environmental Prediction reanalysis 1 (NCEP1) and reanalysis-2 (NCEP2) datasets, and the European Centre for Medium-Range Weather Forecasts operational (ECMWF-OP) and 40-yr Re-Analysis (ERA-40) products. This paper presents the analysis of the six products in depicting the mean, the seasonal cycle, and the interannual variability of the net heat flux into the ocean. Two time series of in situ flux measurements, one taken from a 1-yr Arabian Sea Experiment field program and the other from a 1-month Joint Air–Sea Monsoon Interaction Experiment (JASMINE) field program in the Bay of Bengal were used to evaluate the statistical properties of the flux products over the measurement periods. The consistency between the six products on seasonal and interannual time scales was investigated using a standard deviation analysis and a physically based correlation analysis. The study has three findings. First of all, large differences exist in the mean value of the six heat flux products. Part of the differences may be attributable to the bias in the numerical weather prediction (NWP) models that underestimates the net heat flux into the Indian Ocean. Along the JASMINE ship tracks, the four NWP modeled mean fluxes all have a sign opposite to the observations, with NCEP1 being underestimated by 53 W m−2 (the least biased) and ECMWF-OP by 108 W m−2 (the most biased). At the Arabian Sea buoy site, the NWP mean fluxes also have an underestimation bias, with the smallest bias of 26 W m−2 (ERA-40) and the largest bias of 69 W m−2 (NCEP1). On the other hand, the OAFlux+ISCCP has the best comparison at both measurement sites. Second, the bias effect changes with the time scale. Despite the fact that the mean is biased significantly, there is no major bias in the seasonal cycle of all the products except for ECMWF-OP. The latter does not have a fixed mean due to the frequent updates of the model platform. Finally, among the four products (OAFlux+ISCCP, ERA-40, NCEP1, and NCEP2) that can be used for studying interannual variability, OAFlux+ISCCP and ERA-40 Qnet have good consistency as judged from both statistical and physical measures. NCEP1 shows broad agreement with the two products, with varying details. By comparison, NCEP2 is the least representative of the Qnet variabilities over the basin scale.

scholarly journals Strong Intraseasonal Variability of Meridional Currents near 5°N in the Eastern Indian Ocean: Characteristics and Causes

2017 ◽  
Vol 47 (5) ◽  
pp. 979-998 ◽  
Author(s):  
Gengxin Chen ◽  
Weiqing Han ◽  
Yuanlong Li ◽  
Michael J. McPhaden ◽  
Ju Chen ◽  
...  
Keyword(s):  
Indian Ocean ◽  
Rossby Waves ◽  
Intraseasonal Variability ◽  
Upper Ocean ◽  
Mean Flow ◽  
Eastern Boundary ◽  
Typical Period ◽  
The Indian Ocean ◽  
The Mean

AbstractThis paper reports on strong, intraseasonal, upper-ocean meridional currents observed in the Indian Ocean between the Bay of Bengal (BOB) and the equator and elucidates the underlying physical processes responsible for them. In situ measurements from a subsurface mooring at 5°N, 90.5°E reveal strong intraseasonal variability of the meridional current with an amplitude of ~0.4 m s−1 and a typical period of 30–50 days in the upper 150 m, which by far exceeds the magnitudes of the mean flow and seasonal cycle. Such prominent intraseasonal variability is, however, not seen in zonal current at the same location. Further analysis suggests that the observed intraseasonal flows are closely associated with westward-propagating eddylike sea surface height anomalies (SSHAs) along 5°N. The eddylike SSHAs are largely manifestations of symmetric Rossby waves, which result primarily from intraseasonal wind stress forcing in the equatorial waveguide and reflection of the equatorial Kelvin waves at the eastern boundary. Since the wave signals are generally symmetric about the equator, similar variability is also seen at 5°S but with weaker intensity because of the inclined coastline at the eastern boundary. The Rossby waves propagate westward, causing pronounced intraseasonal SSHA and meridional current in the upper ocean across the entire southern BOB between 84° and 94°E. They greatly weaken in the western Indian Basin, but zonal currents near the equator remain relatively strong.

scholarly journals Intermediate-Depth Circulation of the Indian and South Pacific Oceans Measured by Autonomous Floats

2005 ◽  
Vol 35 (5) ◽  
pp. 683-707 ◽  
Author(s):  
Russ E. Davis
Keyword(s):  
Indian Ocean ◽  
Seasonal Variability ◽  
World Ocean ◽  
South Pacific ◽  
Western Boundary ◽  
Mean Flow ◽  
The North ◽  
The Indian Ocean ◽  
The Mean ◽  
Mean Circulation

Abstract As part of the World Ocean Circulation Experiment, 306 autonomous floats were deployed in the tropical and South Pacific Ocean and 228 were deployed in the Indian Ocean to observe the basinwide circulation near 900-m depth. Mean velocities, seasonal variability, and lateral eddy diffusivity from the resultant 2583 float-years of data are presented. Area averages, local function fits, and a novel application of objective mapping are used to estimate the mean circulation. Patterns of mean circulation resemble those at the surface in both basins. Well-developed subtropical gyres, twice as strong in the Indian Ocean as in the Pacific, feed western boundary currents. Tropical gyres are separated by eastward flow along the equator in both hemispheres of both basins, although the Indian subcontinent splits the north Indian tropical gyre. The Antarctic Circumpolar Current (ACC) and west wind drifts are prominent in both basins, generally tending slightly southward but deviating to the north behind the Del Cano, Kerguelen, and Campbell Plateaus and, of course, South America. Remarkably, the eastern boundaries of the southern subtropical gyres in all three basins apparently occur in the ocean interior, away from land. The Indian Ocean’s subtropical gyre, and perhaps part of the South Atlantic’s, reaches east to a retroflection just upstream of the Campbell Plateau south of New Zealand. Seasonal variability at 900 m is focused around the equator with weaker variability found near certain bathymetric features. There is a remarkable agreement between the observed seasonable variability and that predicted by the Jet Propulsion Laboratory (JPL)–Estimating the Circulation and Climate of the Ocean (ECCO) data-assimilating numerical model. Aside from seasonal effects, eddy variability is greatest along the equator, in tropical and subtropical western basins, and along the ACC. Integrals of velocity across regional passages (Tasman Sea, Mozambique Channel) provide useful reference for hydrographic analyses of transport. Across whole ocean basins, however, the uncertainty associated with the appropriate continuity relation for horizontal flow (e.g., geostrophy vs nondivergence) is comparable to the mean flow.

Characteristics of the Near-Surface Currents in the Indian Ocean as Deduced from Satellite-Tracked Surface Drifters. Part II: Lagrangian Statistics

2015 ◽  
Vol 45 (2) ◽  
pp. 459-477 ◽  
Author(s):  
Shiqiu Peng ◽  
Yu-Kun Qian ◽  
Rick Lumpkin ◽  
Ping Li ◽  
Dongxiao Wang ◽  
...  
Keyword(s):  
Indian Ocean ◽  
Time Scale ◽  
Eastern Coast ◽  
Length Scale ◽  
Time Lags ◽  
Horizontal Shear ◽  
Mean Flow ◽  
Near Surface ◽  
Lagrangian Statistics ◽  
The Indian Ocean

AbstractLagrangian statistics of the surface circulation in the Indian Ocean (IO) are investigated using drifter observations during 1985–2013. The methodology isolates the influence of low-frequency variations and horizontal shear of mean flow. The estimated Lagrangian statistics are spatially inhomogeneous and anisotropic over the IO basin, with values of ~6–85 × 107 cm2 s−1 for diffusivity, ~2–7 days for integral time scale, and ~33–223 km for length scale. Large diffusivities (>20 × 107 cm2 s−1) occur in the central-eastern equatorial IO and the eastern African coast. Small diffusivities (~6–8 × 107 cm2 s−1) appear in the subtropical gyre of the southern IO and the southeastern Arabian Sea. The equatorial IO has the largest zonal diffusivity (~85 × 107 cm2 s−1), corresponding to the largest time scale (~7 days) and length scale (~223 km), while the eastern coast of Somalia has the largest meridional diffusivity (~31 × 107 cm2 s−1). The minor component of the Lagrangian length scale is approximately equal to the first baroclinic Rossby radius (R1) at midlatitudes (R1 ~ 30–50 km), while the major component equals R1 in the equatorial region (R1 > 80 km). The periods of the energetic eddy-containing bands in the IO in Lagrangian spectra range from several days to a couple of months, where anticyclones dominate. A significant result is that the drifter-derived diffusivities asymptote to constant values in relatively short time lags (~10 days) for some subregions of the IO if they are correctly calculated. This is an important contribution to the ongoing debate regarding drifter-based diffusivity estimates with relatively short Lagrangian velocity time series versus tracer-based estimates.

scholarly journals VELOCITY AND STRESS MEASUREMENTS IN A TIDAL INLET

1978 ◽  
Vol 1 (16) ◽  
pp. 77
Author(s):  
David A. Huntley ◽  
Dag Nummedal
Keyword(s):  
Single Point ◽  
Time Lag ◽  
Low Frequency ◽  
Fast Response ◽  
Significant Feature ◽  
Wave Direction ◽  
Mean Flow ◽  
Wave Velocities ◽  
Wave Effects ◽  
The Mean

Fast-response electromagnetic flowmeters were used in a marginal flood channel of an ebb tidal delta to assess the importance of wave contributions to the flood dominance of these channels. Measurements were made at a single point in the channel in both ebb and flood currents. The oscillatory motion of waves was a very significant feature of the velocity records, and its magnitude was comparable with the mean flow at all stages of the tide. This observation shows that flowmeters capable of responding accurately to wave velocities are needed to obtain accurate values of mean flow. Some earlier measurements made with slow response flowmeters are probably unreliable. Wave contributions to the mean flow were assessed by looking at the correlation between the low frequency (>17.5s) oscillations of the along-channel current and the low frequency envelope of the wave velocities. Surprisingly little correlation was found for any time lag, suggesting that wave effects were not important in the mean tidal currents in the channel studied. However, close to low tide on the ebb, conditions existed which appear to have been favourable for the "wave pump" mechanism suggested by Bruun andViggisson (1973). Significant correlation between the wave envelope and low frequency fluctuations was observed at this time. It is therefore suggested that wave effects can be important to the mean flow in marginal channels with rapidly converging and shoaling mouths which are oriented towards the dominant incident wave direction.

scholarly journals Comparison of Precipitation from Observed Data and General Circulation Models over the Iberian Peninsula

2006 ◽  
Vol 19 (17) ◽  
pp. 4254-4275 ◽  
Author(s):  
Susana Nieto ◽  
Concepción Rodríguez-Puebla
Keyword(s):  
Iberian Peninsula ◽  
Interannual Variability ◽  
Seasonal Cycle ◽  
General Circulation ◽  
Large Scale ◽  
Mean Flow ◽  
Storm Activity ◽  
Model Simulations ◽  
Teleconnection Indices ◽  
The Mean

Abstract In this paper, the ability of model outputs from the Intergovernmental Panel on Climate Change (IPCC) to describe the natural internal variability of precipitation observations is evaluated. The analysis is focused on the Iberian Peninsula for December–February (DJF). The study was performed with observed data from National Meteorological Institutes, reanalysis data from the National Centers for Environmental Prediction–National Center for Atmospheric Research, teleconnection indices, and model simulations. First, the seasonal cycle, mean winter pattern, and tendency for nine model simulations were evaluated. Then, four models were selected to obtain interannual variability and to diagnose the links between precipitation and large-scale circulation. This intercomparison is based on the modes obtained by the empirical orthogonal function (EOF) and spectral analyses to investigate the temporal properties of the most significant spatial patterns. The models well reproduce the observed seasonal cycle and the mean winter pattern; however, they poorly capture the interannual variability found in observed data. To ascertain the reasons for these results, features affecting the precipitation process are considered by analyzing the relationships with the dominant modes of large-scale atmospheric fields, such as sea level pressure, storm activity, jet stream, moisture flux, and teleconnection indices. The precipitation response to the mean flow suggests signs of potential seasonal predictability in DJF.

Storm surges in the North Sea, 11 to 30 December 1954

1958 ◽  
Vol 251 (991) ◽  
pp. 139-160 ◽  
Keyword(s):  
North Sea ◽  
Time Lag ◽  
Storm Surges ◽  
Geostrophic Wind ◽  
Wind Effect ◽  
Mean Flow ◽  
The North ◽  
The North Sea ◽  
Transverse Gradient ◽  
The Mean

The disturbances produced by the stormy conditions of December 1954 have been analyzed. The basic data used were tidal observations at ten British and fifteen continental stations in the North Sea and English Channel, together with records of the mean flow of water through the Straits of Dover as represented by electromagnetically induced currents in a telephone cable running from St Margaret’s Bay to Sangatte. The main purpose of the investigation has been to determine the relative magnitudes of the various factors contributing to the phenomena. The effect of westerly winds has been shown to depend upon whether the wind system is confined to the North Sea, or to the north-western approaches to the sea, or is a broad airstream covering both areas. Evidence has been put forward for the existence of a 'return’ surge, or southward return of water previously expelled from the North Sea, on 15 December. Co-disturbance charts have been constructed for the large surges of 20 to 25 December, and the water movements thus deduced exhibit marked geostrophic effects in all cases. An example of an external surge has been noted. Representing the sea by a rectangle, a correlation of 0-96 was found between the longitudinal water gradient and the geostrophic wind; this analysis led to a value of the wind stress coefficient, y 2 , of 2-7 x 10- 3 . The transverse gradient has been shown to be composed of a direct wind effect and a larger geostrophic effect. Estimates have been made of the mean level of the sea which, during surge peaks, was some 2-5 ft. above normal, and these have been represented satisfactorily by winds in and to the north of the sea. The cable measurements throughout the whole period have been analyzed at intervals of 25 h, and, after Bowden (1956), provided estimates of y2 (2-1 x 10- 3 ) and the bottom friction coefficient k (3-5 x 10- 3 ). A similar analysis of the data at 3 h intervals gave values of 2-3 x 10~3 and 4-5 x 10“3, respectively. The subject of the oscillatory development of storm surges has been reviewed with particular reference to the North Sea, and the conclusion reached that for this area the motion appears to be so heavily damped that positive surges may be represented, for practical purposes, by equilibrium conditions modified by a time lag. Negative surges, however, exhibit a strong tendency towards oscillations, as evidenced by the existence of return surges

scholarly journals The Indian Summer Monsoon in MetUM-GOML2.0: Effects of air-sea coupling and resolution

2018 ◽  
Author(s):  
Simon C. Peatman ◽  
Nicholas P. Klingaman
Keyword(s):  
Indian Ocean ◽  
Summer Monsoon ◽  
Indian Summer Monsoon ◽  
Seasonal Cycle ◽  
Global Ocean ◽  
Boreal Summer ◽  
Correction Technique ◽  
The Indian Ocean ◽  
The Mean ◽  
Mean State

Abstract. The fidelity of the simulated Indian Summer Monsoon is analysed in the UK Met Office Unified Model Global Ocean Mixed Layer configuration (MetUM-GOML2.0), in terms of its boreal summer mean state and propagation of the Boreal Summer IntraSeasonal Oscillation (BSISO). The model produces substantial biases in mean June--September precipitation, especially over India, in common with other MetUM configurations. Using a correction technique to constrain the mean seasonal cycle of ocean temperature and salinity, the effects of regional air-sea coupling and atmospheric horizontal resolution are investigated. Introducing coupling in the Indian Ocean degrades the atmospheric basic state, compared with prescribing the observed seasonal cycle of sea surface temperature (SST). This degradation of the mean state is attributable to small errors (±0.5 C) in mean SST. However, coupling slightly improves the simulation of northward BSISO propagation over the Indian Ocean, Bay of Bengal and India. Increasing resolution from 200 km to 90 km grid spacing (approximate value at the equator) improves the atmospheric mean state, but increasing resolution again to 40~km offers no substantial improvement. The improvement to intraseasonal propagation at finer resolution is similar to that due to coupling.

scholarly journals The Indian summer monsoon in MetUM-GOML2.0: effects of air–sea coupling and resolution

2018 ◽  
Vol 11 (11) ◽  
pp. 4693-4709 ◽  
Author(s):  
Simon C. Peatman ◽  
Nicholas P. Klingaman
Keyword(s):  
Indian Ocean ◽  
Summer Monsoon ◽  
Indian Summer Monsoon ◽  
Seasonal Cycle ◽  
Global Ocean ◽  
Boreal Summer ◽  
Correction Technique ◽  
The Indian Ocean ◽  
The Mean ◽  
Mean State

Abstract. The fidelity of the simulated Indian summer monsoon is analysed in the UK Met Office Unified Model Global Ocean Mixed Layer configuration (MetUM-GOML2.0) in terms of its boreal summer mean state and propagation of the boreal summer intraseasonal oscillation (BSISO). The model produces substantial biases in mean June–September precipitation, especially over India, in common with other MetUM configurations. Using a correction technique to constrain the mean seasonal cycle of ocean temperature and salinity, the effects of regional air–sea coupling and atmospheric horizontal resolution are investigated. Introducing coupling in the Indian Ocean degrades the atmospheric basic state compared with prescribing the observed seasonal cycle of sea surface temperature (SST). This degradation of the mean state is attributable to small errors (±0.5 ∘C) in mean SST. Coupling slightly improves some aspects of the simulation of northward BSISO propagation over the Indian Ocean, Bay of Bengal, and India, but degrades others. Increasing resolution from 200 to 90 km grid spacing (approximate value at the Equator) improves the atmospheric mean state, but increasing resolution again to 40 km offers no substantial improvement. The improvement to intraseasonal propagation at finer resolution is similar to that due to coupling.

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