摘要:

AbstractThe role of the western tropical Pacific Ocean heat content in the South China Sea summer monsoon (SCSSM) onset is investigated in the present paper, by using atmospheric data from NCEP and ocean subsurface temperature data from Japan Meteorology Agency. It is showed from the result that the heat content (HC) of the upper 400 m layer in the western tropical Pacific (WTP), especially in the region of (130°E–150°E, 0°N–14°N) in the last four decades, is a good predictive indicator for the SCSSM onset. Positive (negative) HC anomalies can induce a strong (weak) convection over the WTP, leading to stronger (weaker) Walker circulation and weaker (stronger) western North Pacific subtropical high (WNPSH) in the boreal spring. Consequently, the anomalous westerly (easterly) in the tropical Indian Ocean is favorable (unfavorable) for the airflow into the SCS and for an early (late) WNPSH retreat from the SCS and hence for an early (late) SCSSM onset. It is elucidated that the long‐term trend of SCSSM onset changes its sign around 1993/94 from decline to rise, which is responding and attributed to the WTP HC trend. During the period of 1971–1993, the WTP HC shows a significant decrease trend. In particular, a significant decline trend is observed in the HC difference between the WTP and western tropical Indian Ocean, which causes an easterly trend in the SCS and strengthened WNPSH trend, leading to a late onset trend of SCSSM. The situation is reverse af

ISSN:0148-0227    

DOI:10.1002/2013JC009683

年代:2014

数据来源: WILEY

摘要:

AbstractThis study demonstrates the impact of gridded in situ and Aquarius sea surface salinity (SSS) on coupled forecasts for August 2011 until February 2014. Assimilation of all available subsurface temperature (ASSIM_Tz) is chosen as the baseline and an optimal interpolation of all in situ salinity (ASSIM_Tz_SSSIS) and Aquarius SSS (ASSIM_Tz_SSSAQ) are added in separate assimilation experiments. These three are then used to initialize coupled experiments. Including SSS generally improves NINO3 sea surface temperature anomaly validation. For ASSIM_Tz_SSSIS, correlation is improved after 7 months, but the root mean square error is degraded with respect to ASSIM_Tzafter 5 months. On the other hand, assimilating Aquarius gives significant improvement versus ASSIM_Tzfor all forecast lead times after 5 months. Analysis of the initialization differences with the baseline indicates that SSS assimilation results in an upwelling Rossby wave near the dateline. In the coupled model, this upwelling signal reflects at the western boundary eventually cooling the NINO3 region. For this period, coupled models tend to erroneously predict NINO3 warming, so SSS assimilation corrects this defect. Aquarius is more efficient at cooling the NINO3 region since it is relatively more salty in the eastern Pacific than in situ SSS which leads to increased mixing and upwelling which in turn sets up enhanced west‐to‐east SST gradient and intensified Bjerknes coupling. A final experiment that uses subsampled Aquarius at in situ locations infers that high‐density spatial sampling of Aquarius is the reason for the superior performance of Aquarius versus in sit

ISSN:0148-0227    

DOI:10.1002/2013JC009697

年代:2014

数据来源: WILEY

摘要:

AbstractSea level reconstructions extend spatially dense data sets, such as those from satellite altimetry, by decomposing the data set into basis functions and fitting those functions to in situ tide gauge measurements with a longer temporal record. We compare and evaluate two methods for reconstructing sea level through an idealized study. The compared sea level reconstruction methods differ in the technique for calculating basis functions, i.e., empirical orthogonal functions (EOFs) versus cyclostationary EOFs (CSEOFs). Reconstructions are created using Archiving, Validation, and Interpretation of Satellite Oceanographic (AVISO) satellite altimetry data and synthetic tide gauges. Synthetic tide gauge records are simulated using historical distributions and real high‐frequency signal to test reconstruction skill. The CSEOF reconstructions show high skill in reproducing variations in global mean sea level (GMSL) and ocean climate indices, and are affected less by both limited tide gauge distribution and added high‐frequency tide gauge signal than EOF reconstructions. Typically, CSEOF reconstructions slightly underestimate sea level amplitudes while EOF reconstructions overestimate sea level amplitudes, in some cases, significantly. Both of these results are accentuated with decreasing quality of the synthetic tide gauge data set. Additionally, we investigate how the reconstructions differ when reconstructing with more of the variance retained in the basis functions. Increasing the variance explained by the basis functions from 70% to 90% reduces the efficacy of an EOF reconstruction to reproduce common ocean indices when noise is included in the tide gauge data sets. These results show that in the idealized comparative cases examined the CSEOF method of sea level reconstruction creates more robust reconstructions, especially when less than ideal tide gauge data are u

ISSN:0148-0227    

DOI:10.1002/2014JC009893

年代:2014

数据来源: WILEY

摘要:

AbstractThe 120 m sea‐level drop during the Last Glacial Maximum (LGM; 18–22 kyr BP) had a profound impact on the global tides and lead to an increased tidal dissipation rate, especially in the North Atlantic. Here, we present new simulations of the evolution of the global tides from the LGM to present for the dominating diurnal and semidiurnal constituents. The simulations are undertaken in time slices spanning 500–1000 years. Due to uncertainties in the location of the grounding line of the Antarctic ice sheets during the last glacial, simulations are carried out for two different grounding line scenarios. Our results replicate previously reported enhancements in dissipation and amplitudes of the semidiurnal tide during LGM and subsequent deglaciation, and they provide a detailed picture of the large global changes in M2tidal dynamics occurring over the deglaciation period. We show that Antarctic ice dynamics and the associated grounding line location have a large influence on global semidiurnal tides, whereas the diurnal tides mainly experience regional changes and are not impacted by grounding line shifts in Antar

ISSN:0148-0227    

DOI:10.1002/2013JC009605

年代:2014

数据来源: WILEY

摘要:

AbstractA diurnal warming model is used to create a new data set of global, diurnally varying sea surface temperatures (dSSTs) and surface turbulent heat fluxes over a 5 year period. The magnitude of diurnal warming is primarily a function of low wind speed and net heat flux. Differences between each of the surface turbulent fluxes with and without a diurnally varying SST are examined on hourly, daily, and seasonal time scales. Over a 2 month period, maximum averaged diurnal warming is as large as 0.3°C, and latent heat flux is underestimated by as much as 8 W/m2in the Indian Ocean. They also exceed roughly 0.7°C and 10 W/m2, respectively, up to 25% of the total daytime in the Atlantic. A best‐case approach validation shows the model overestimates peak warming and underestimates the duration of the cycle, though the average error is quite small. The model is tested under a variety of wind speed, solar radiation, and precipitation conditions to examine the impact of potential biases or error in the input data. To test the impact of a positive bias in the wind speeds, diurnal warming magnitudes are recomputed with an adjusted wind under near‐neutral conditions. Compared to the original data, diurnal warming can increase by as much as 1.5°C on an hourly scale but generally is<0.06°C. Although precipitation effects on dSSTs are small compared to winds and radiation, the model configuration wrongly causes diurnal warming to increase by precipitation, contrary to the underlying model

ISSN:0148-0227    

DOI:10.1002/2013JC009489

年代:2014

数据来源: WILEY

摘要:

AbstractThe subtropical North Atlantic exhibits the saltiest surface waters of the open ocean. Eventually that water subducted from the surface and exported toward the Equator, as a subsurface salinity maximum (S‐max) forming the lower limb of the subtropical cell. Climatologically, the winter subtropical surface water, coinciding with the deepest mixed layer of ∼100 m, is saltier and colder than the S‐max. Towed CTD measurements in March/April 2013 (a component of the field program SPURS) within the North Atlantic subtropical surface salinity maximum reveal several relatively fresh, warm anomalies, which deviate strongly from climatological conditions. These features introduce a large amount of freshwater into the subtropical region, exceeding the amount introduced by local rain events. Observed scales and evolution of the features strongly suggest a connection to mesoscale dynamics. This is supported by high‐resolution regional model output, which produces an abundance of features that are similar in scale and structure to those observed. It is hypothesized that turbulent transport in the surface ocean is a crucial process for setting mixed layer characteristics, which spread into S‐max stratum. High variability in the EKE implies a high potential for interannual variability in the resulting S‐max water properties by ocean dynamics in addition to the variability caused by air sea fluxes. This has likely consequences to the meridional transport of heat and freshwater of the subtropical cell in the North Atlantic and to the larger‐scale ocean and c

ISSN:0148-0227    

DOI:10.1002/2013JC009715

年代:2014

数据来源: WILEY

摘要:

AbstractWe examine the snow radar data from the Weddell and Bellingshausen Seas acquired by eight IceBridge (OIB) flightlines in October of 2010 and 2011. In snow depth retrieval, the sidelobes from the stronger scattering snow‐ice (s‐i) interfaces could be misidentified as returns from the weaker air‐snow (a‐s) interfaces. In this paper, we first introduce a retrieval procedure that accounts for the structure of the radar system impulse response followed by a survey of the snow depths in the Weddell and Bellingshausen Seas. Limitations and potential biases in our approach are discussed. Differences between snow depth estimates from a repeat survey of one Weddell Sea track separated by 12 days, without accounting for variability due to ice motion, is −0.7 ± 13.6 cm. Average snow depth is thicker in coastal northwestern Weddell and thins toward Cape Norvegia, a decrease of>30 cm. In the Bellingshausen, the thickest snow is found nearshore in both Octobers and is thickest next to the Abbot Ice Shelf. Snow depth is linearly related to freeboard when freeboards are low but diverge as the freeboard increases especially in the thicker/rougher ice of the western Weddell. We find correlations of 0.71–0.84 between snow depth and surface roughness suggesting preferential accumulation over deformed ice. Retrievals also seem to be related to radar backscatter through surface roughness. Snow depths reported here, generally higher than those from in situ records, suggest dissimilarities in sample populations. Implications of these differences on Antarctic sea ice thickne

ISSN:0148-0227    

DOI:10.1002/2014JC009943

年代:2014

数据来源: WILEY

摘要:

AbstractThe choice of parameter values is crucial in the course of sea ice model development, since parameters largely affect the modeled mean sea ice state. Manual tuning of parameters will soon become impractical, as sea ice models will likely include more parameters to calibrate, leading to an exponential increase of the number of possible combinations to test. Objective and automatic methods for parameter calibration are thus progressively called on to replace the traditional heuristic, “trial‐and‐error” recipes. Here a method for calibration of parameters based on the ensemble Kalman filter is implemented, tested and validated in the ocean‐sea ice model NEMO‐LIM3. Three dynamic parameters are calibrated: the ice strength parameterP*, the ocean‐sea ice drag parameterCw, and the atmosphere‐sea ice drag parameterCa. In twin, perfect‐model experiments, the default parameter values are retrieved within 1 year of simulation. Using 2007–2012 real sea ice drift data, the calibration of the ice strength parameterP*and the oceanic drag parameterCwimproves clearly the Arctic sea ice drift properties. It is found that the estimation of the atmospheric dragCais not necessary ifP*andCware already estimated. The large reduction in the sea ice speed bias with calibrated parameters comes with a slight overestimation of the winter sea ice areal export through Fram Strait and a slight improvement in the sea ice thickness distribution. Overall, the estimation of parameters with the ensemble Kalman filter represents an encouraging alternative to manual tuning for o

ISSN:0148-0227    

DOI:10.1002/2013JC009705

年代:2014

数据来源: WILEY

摘要:

AbstractThis study deals with standing wave or seiche events using cruise observations, satellite altimeter data, and theoretical analysis. Cruise missions in summer 2007 and 2009 detected internal oscillation signals in the South China Sea (SCS) deep basin. The signals have average wavelengths of 320 and 390 km and the maximum amplitudes of 50–100 m at layers 500–700 m and 1500–1700 m. Satellite altimeter sea level anomaly (SLA) images and the second intrinsic mode function (IMF2) images derived from the empirical mode decomposition (EMD) analysis show that the observed internal oscillations are a portion of 2‐D seiche modes, which lasted for at least 2 weeks. We recognize that the observed internal oscillation signals represent seiche modesH5,3andH5,1derived from a rectangular model ocean basin with a uniform depth, a west‐east length of 1000 km and a north‐south width of 800 km. Statistical analysis of standing wave modesH4,0,H5,1, andH5,3with the average wavelength of 500, 390, and 320 km indicates that from 1993 to 2012 (1045 weeks), total 94 events with total temporal coverage of 218 weeks are affirmed. The total occurrence frequency is 20.9%. Histograms of annual distributions of seiche events and timespans show an interannual variability of about 9 years, with peak years 1993, 1994, 1998, 2001, and 2011. While monthly distributions show an intraseasonal variability double‐peaked in May and October, transit periods of East Asia monso

ISSN:0148-0227    

DOI:10.1002/2014JC009957

年代:2014

数据来源: WILEY

摘要:

AbstractMonthly barrier layer thickness (BLT) estimates are derived from satellite measurements using a multilinear regression model (MRM) within the Indian Ocean. Sea surface salinity (SSS) from the recently launched Soil Moisture and Ocean Salinity (SMOS) and Aquarius SAC‐D salinity missions are utilized to estimate the BLT. The MRM relates BLT to sea surface salinity (SSS), sea surface temperature (SST), and sea surface height anomalies (SSHA). Three regions where the BLT variability is most rigorous are selected to evaluate the performance of the MRM for 2012; the Southeast Arabian Sea (SEAS), Bay of Bengal (BoB), and Eastern Equatorial Indian Ocean (EEIO). The MRM derived BLT estimates are compared to gridded Argo and Hybrid Coordinate Ocean Model (HYCOM) BLTs. It is shown that different mechanisms are important for sustaining the BLT variability in each of the selected regions. Sensitivity tests show that SSS is the primary driver of the BLT within the MRM. Results suggest that salinity measurements obtained from Aquarius and SMOS can be useful for tracking and predicting the BLT in the Indian Ocean. Largest MRM errors occur along coastlines and near islands where land contamination skews the satellite SSS retrievals. The BLT evolution during 2012, as well as the advantages and disadvantages of the current model are discussed. BLT estimations using HYCOM simulations display large errors that are related to model layer structure and the selected BLT methodolog

ISSN:0148-0227    

DOI:10.1002/2013JC009759

年代:2014

数据来源: WILEY