Large-scale Sea Surface Research Articles

Self-Adaptive TSM-RT for the Fast Analysis of EM Scattering From 3-D Large-Scale Sea Surface

A self-adaptive two-scale model-ray tracing (TSM-RT) method is proposed to further improve the computational efficiency of TSM-RT method in this letter. For the previous TSM-RT method, the mesh size of large triangles does not change with the incident angle. In the contrast, the mesh size of the large triangles is related to the incident angle as well as the roughness of sea surface in the self-adaptive TSM-RT method, where the number of large triangles is further decreased. Correspondingly, the number of initial rays and intersections between the ray and large triangles is also decreased. Therefore, the computational efficiency is improved. In addition, the backscattering radar cross section of the three-dimensional sea surface is calculated. It demonstrated that the simulation results of self-adaptive TSM-RT method show a great agreement with those of the previous TSM-RT method, and a good speedup ratio is achieved.

How much global burned area can be forecast on seasonal time scales using sea surface temperatures?

Large-scale sea surface temperature (SST) patterns influence the interannual variability of burned area in many regions by means of climate controls on fuel continuity, amount, and moisture content. Some of the variability in burned area is predictable on seasonal timescales because fuel characteristics respond to the cumulative effects of climate prior to the onset of the fire season. Here we systematically evaluated the degree to which annual burned area from the Global Fire Emissions Database version 4 with small fires (GFED4s) can be predicted using SSTs from 14 different ocean regions. We found that about 48% of global burned area can be forecast with a correlation coefficient that is significant at a p < 0.01 level using a single ocean climate index (OCI) 3 or more months prior to the month of peak burning. Continental regions where burned area had a higher degree of predictability included equatorial Asia, where 92% of the burned area exceeded the correlation threshold, and Central America, where 86% of the burned area exceeded this threshold. Pacific Ocean indices describing the El Niño-Southern Oscillation were more important than indices from other ocean basins, accounting for about 1/3 of the total predictable global burned area. A model that combined two indices from different oceans considerably improved model performance, suggesting that fires in many regions respond to forcing from more than one ocean basin. Using OCI—burned area relationships and a clustering algorithm, we identified 12 hotspot regions in which fires had a consistent response to SST patterns. Annual burned area in these regions can be predicted with moderate confidence levels, suggesting operational forecasts may be possible with the aim of improving ecosystem management.

Association of Taiwan’s Rainfall Patterns with Large-Scale Oceanic and Atmospheric Phenomena

A 50-year (1960–2009) monthly rainfall gridded dataset produced by the Taiwan Climate Change Projection and Information Platform Project was presented in this study. The gridded data (5 × 5 km) displayed influence of topography on spatial variability of rainfall, and the results of the empirical orthogonal functions (EOFs) analysis revealed the patterns associated with the large-scale sea surface temperature variability over Pacific. The first mode (65%) revealed the annual peaks of large rainfall in the southwestern mountainous area, which is associated with southwest monsoons and typhoons during summertime. The second temporal EOF mode (16%) revealed the rainfall variance associated with the monsoon and its interaction with the slopes of the mountain range. This pattern is the major contributor to spatial variance of rainfall in Taiwan, as indicated by the first mode (40%) of spatial variance EOF analysis. The second temporal EOF mode correlated with the El Niño Southern Oscillation (ENSO). In particular, during the autumn of the La Niña years following the strong El Niño years, the time-varying amplitude was substantially greater than that of normal years. The third temporal EOF mode (7%) revealed a north-south out-of-phase rainfall pattern, the slowly evolving variations of which were in phase with the Pacific Decadal Oscillation. Because of Taiwan’s geographic location and the effect of local terrestrial structures, climate variability related to ENSO differed markedly from other regions in East Asia.

Parallelized TSM-RT Method for the Fast RCS Prediction of the 3-D Large-Scale Sea Surface by CUDA

In this paper, a parallelized two-scale model-ray tracing (TSM-RT) method is developed for the fast radar cross-section prediction of the 3-D large-scale sea surface by the compute unified device architecture (CUDA). The TSM-RT method is an accelerated RT method, where the sea surface is constructed into a great deal of large triangles, which consist of many small triangles; this is called the two-scale model. For the TSM-RT method, the large triangles are utilized to determine the asymptotical ray paths, while the small triangles are utilized to calculate the scattered far fields. In addition, the CUDA of NVIDIA takes advantage of the graphics processing units (GPUs) for parallel computing, and greatly improves the speed of computation. The scattering field of each ray in the TSM-RT method is calculated independently. Therefore, a parallelization concept, which is based on the CUDA technology, is presented to improve the computational efficiency. The asynchronous transfer and shared memory are used to further improve the speedup factors. Our method is validated by comparing the numerical results with those obtained using a CPU. At the same time, significant speedup factors are achieved compared with the CPU-based TSM-RT.

An Accelerated Ray Tracing Method Based on the TSM for the RCS Prediction of 3-D Large-Scale Dielectric Sea Surface

An accelerated ray tracing method based on the two-scale model for the radar-cross-section prediction of 3-D large-scale dielectric sea surface is proposed in this letter. For the accelerated ray tracing method, the sea surface is constructed into a great deal of large triangles that consist of many small triangles; this is called the two-scale model. Then, each ray is traced until it does not have any intersection with the large triangles. At the scattered large triangles, the Physical Optics method is performed to obtain the scattered far fields, which is the summation of scattered field from each small triangle. The comparison to the traditional accelerated ray tracing method is given to demonstrate the accuracy and capability of the new accelerated ray tracing method. Compared to the traditional accelerated ray tracing method, the proposed accelerated ray tracing method can dramatically decrease the computational time.

Spatiotemporal Variability and Predictability of Relative Humidity over West African Monsoon Region

Abstract Spatial and temporal variability of relative humidity over the West African monsoon (WAM) region is investigated. In particular, the variability during the onset and retreat periods of the monsoon is considered. A K-means cluster analysis was performed to identify spatially coherent regions of relative humidity variability during the two periods. The cluster average of the relative humidity provides a robust representative index of the strength and timing of the transition periods between the dry and wet periods. Correlating the cluster indices with large-scale circulation and sea surface temperatures indicates that the land–ocean temperature gradient and the corresponding circulation, tropical Atlantic sea surface temperatures (SSTs), and to a somewhat lesser extent tropical Pacific SSTs all play a role in modulating the timing of the monsoon season relative humidity onset and retreat. These connections to large-scale climate features were also found to be persistent over interseasonal time scales, and thus best linear predictive models were developed to enable skillful forecasts of relative humidity during the two periods at 15–75-day lead times. The public health risks due to meningitis epidemics are of grave concern to the population in this region, and these risks are strongly tied to regional humidity levels. Because of this linkage, the understanding and predictability of relative humidity variability is of use in meningitis epidemic risk mitigation, which motivated this research.

BISTATIC RCS PREDICTION OF COMPOSITE SCATTERING FROM ELECTRICALLY VERY LARGE SHIP-SEA GEOMETRY WITH A HYBRID FACET-BASED KA AND SHADOW-CORRECTED GRECO SCHEME

This paper presents a hybrid scheme for fast calculation on the bistatic composite scattering from electrically very large ship-sea geometry at high frequencies. Based on the Kirchhofi approximation (KA), we try to break the large-scale sea surface into myriads of plane facets, then derive the Kirchhofi integration analytically on each individual discretized facet. The analytical expression obtained, so- called the \facet-based Kirchhofi approximation (FBKA), is suitable for a quick scattering calculation on the electrically very large sea surface, since it is beyond the intensively reflned meshes as the usual Monte Carlo implementation does. Meanwhile, combined with graphical electromagnetic computing method (GRECO) to extract the illuminated and shadow facets in accordance with the incident direction, the conventional physical optics method (PO) is improved by employing current marching technique (CMT) to calculate the currents in the shadow region. The shadow-corrected GRECO is presented in this hybrid model to solve the bistatic scattering from complex and very electrically large perfectly electric conducting (PEC) objects. The accuracy of the shadow-corrected GRECO is conflrmed well by exact numerical methods, especially at large scattering angles. The electromagnetic interactions between the ship and sea surface are estimated by the famous \four-path model, which has been proved to be valid for ship scattering at relatively calm sea state. Several numerical examples have been presented to demonstrate the e-ciency and accuracy of the proposed hybrid method.

Historical growth of Bristol Bay and Yukon River, Alaska chum salmon (Oncorhynchus keta) in relation to climate and inter- and intraspecific competition

We examined Bristol Bay and Yukon River adult chum salmon scales to determine whether climate variability, such as changes in sea surface temperature and climate indices, and high pink and Asian chum salmon abundance reduced chum salmon growth. Annual marine growth increments for 1965–2006 were estimated from scale growth measurements and were modeled as a function of potential explanatory variables using a generalized least squares regression approach. First-year growth of salmon originating from Bristol Bay and the Yukon River showed increased growth in association with higher regional ocean temperatures and was negatively affected by wind mixing and ice cover. Third-year growth was lower when Asian chum salmon were more abundant. Contrary to our hypothesis, warmer large-scale sea surface temperatures in the Gulf of Alaska were also associated with reduced third-year growth. Negative effects of high abundances of Russian pink salmon on third-year growth provided some evidence for interspecific interactions, but the effects were smaller than the effects of Asian chum salmon abundance and Gulf of Alaska sea surface temperature. Although the relative effects of Asian chum salmon and sea surface temperature on the growth of Yukon and Bristol Bay chum salmon were difficult to untangle, we found consistent evidence that high abundances of Asian chum salmon contributed to a reduction in the growth of western Alaska chum salmon.

Currents off the west coast of Northland, New Zealand

The concept of a southward-flowing West Auckland Current off the west coast of Northland has become an accepted feature of the ocean circulation around New Zealand. Knowledge of this flow is based on sparse data collected in the 1950s. Here we quantify the flows in the area for the first time using a current meter array deployed for a year (2003–2004) across the shelf and slope offshore of Hokianga Harbour and three hydrographic surveys along a transect coincident with the current meter array. Flows are found to be weak, and dominated by the variability. There is a southeastward drift in the mean flow offshore of the 1000-m isobath, with a northwestward mean found inshore of this. The offshore flows appear to result from large-scale sea surface height gradients, while the inshore flows respond to wind forcing in a complex manner.

A partitioning analysis of tropical rainfall based on cloud budget

Precipitation in the deep tropical convective regime is investigated through the partitioning analysis of grid-scale data from a two-dimensional cloud-resolving model simulation based on cloud budget. The model is forced by large-scale vertical velocity, zonal wind, horizontal advection, and sea surface temperature observed and derived from TOGA COARE. In cloud budget, rainfall is associated with cloud microphysical processes denoted by the net condensation and dynamical processes denoted by hydrometeor change/convergence. Thus, three rainfall types are separated in this scheme. Time-mean analysis shows that the rainfall type associated with hydrometeor loss/convergence only accounts for 27% of total rainfall, which corresponds to downward motions and downward water vapor and hydrometeor mass flux in the mid and lower troposphere. The two other rainfall types associated with the net condensation contribute equally to total rainfall (36%), which are generally related to upward motions and upward water vapor and hydrometeor mass flux throughout the troposphere.

Re-emerging ocean temperature anomalies in late-2010 associated with a repeat negative NAO

Northern Europe was influenced by consecutive episodes of extreme winter weather at the start and end of the 2010 calendar year. A tripole pattern in North Atlantic sea surface temperature anomalies (SSTAs), associated with an exceptionally negative phase of the North Atlantic Oscillation (NAO), characterized both winter periods. This pattern was largely absent at the surface during the 2010 summer season; however equivalent sub-surface temperature anomalies were preserved within the seasonal thermocline throughout the year. Here, we present evidence for the re-emergence of late-winter 2009/10 SSTAs during the following early winter season of 2010/11. The observed re-emergence contributes toward the winter-to-winter persistence of the anomalous tripole pattern. Considering the active influence of the oceans upon leading modes of atmospheric circulation over seasonal timescales, associated with the memory of large-scale sea surface temperature anomaly patterns, the re-emergence of remnant temperature anomalies may have also contributed toward the persistence of a negative winter NAO, and the recurrence of extreme wintry conditions over the initial 2010/11 winter season.

Large-scale anomaly patterns associated to temperature variability inside Scarisoara Ice Cave

The relationship between temperature variability inside Scarisoara Ice Cave (NW Romania) and large-scale atmospheric circulation and sea surface temperature (SST) anomalies is investigated. A composite analysis reveals that high (low) temperatures inside the cave are associated with high (low) air temperature over a large area that covers central and Eastern Europe. Coherent large-scale patterns are identified in the field of SST anomalies. The corresponding atmospheric circulation patterns favour relatively warm (cold) air advection towards the cave region during high (low) temperatures inside the cave. A correlation analysis reveals that the atmospheric circulation patterns associated with cave temperature variability is mainly the result of the East Atlantic-Western Russia (r = − 0.38) teleconnection pattern. Our results have strong implications for the interpretation of climate variability in the region as recorded by the ice block within the Scarisoara Ice Cave, which is more than 3000 years old. Copyright © 2011 Royal Meteorological Society

Interannual Biases Induced by Freshwater Flux and Coupled Feedback in the Tropical Pacific

Abstract Freshwater flux (FWF) forcing–induced feedback has not been represented adequately in many coupled ocean–atmosphere models of the tropical Pacific. Previously, various approximations have been made in representing the FWF forcing in climate modeling. In this article, using a hybrid coupled model (HCM), sensitivity experiments are performed to examine the extent to which this forcing and related feedback effects can contribute to tropical biases in interannual simulations of the tropical Pacific. The total FWF into the ocean, represented by precipitation (P) minus evaporation (E), (P − E), is separated into its climatological part and interannual anomaly part: FWFTotal = (P − E)clim + FWFinter. The former can be prescribed (seasonally varying); the latter can be captured using an empirical model linking with large-scale sea surface temperature (SST) variability. Four cases are considered with different FWFinter specifications: interannual (P − E) forcing [FWFinter = (P − E)inter], interannual P forcing (FWFinter = Pinter), interannual E forcing (FWFinter = −Einter), and climatological (P − E) forcing (FWFinter = 0.0), respectively. The HCM-based experiments indicate that different FWFinter approximations can modulate interannual variability in a substantial way. The HCM with the interannual (P − E) forcing, in which a positive SST − (P − E)inter feedback is included explicitly, has a reasonably realistic simulation of interannual variability. When FWFinter is approximated in some ways, the simulated interannual variability can be modulated significantly: it is weakened with the climatological (P − E) forcing and is even more damped with the interannual E forcing, but is exaggerated with the interannual P forcing. Quantitatively, taking the interannual (P − E) forcing run as a reference, the Niño-3 SST variance can be reduced by about 12% and 26% in the climatological (P − E) forcing run and interannual E forcing run, respectively, but overestimated by 11% in the Pinter forcing run. It is demonstrated that FWF can be a clear bias source for coupled model simulations in the tropical Pacific.

Case studies in interannual to decadal climate predictability

The predictability of ocean and climate variables is investigated, using a perfect model-based case study approach that recognises that predictability is dependent on the initial climate state. In line with previous studies, large scale ocean variables show predictability for several years or more; by contrast, the predictability of climate variables is generally limited to 2 years at most. That predictability shows high sensitivity to the initial state is demonstrated by predictable climate signals arising in different regions, variables and seasons for different initial conditions. The predictability of climate variables in the second year is of particular interest, because this is beyond the timescale that is usually considered to be the limit of seasonal predictability. For different initial conditions, second year predictability is found in: temperatures in southeastern North America (winter) and western Europe (winter and summer), and precipitation in India (summer monsoon) and in the tropical South Atlantic. Second year predictability arises either from persistence of large-scale sea surface temperature (SST) and related ocean heat content anomalies, particularly in regions such as the North Atlantic and Southern Ocean, or from mechanisms that involve El Nino Southern Oscillation (ENSO) dynamics.

Predictability of SST-Modulated Westerly Wind Bursts

AbstractWesterly wind bursts (WWBs), a significant player in ENSO dynamics, are modeled using an observationally motivated statistical approach that relates the characteristics of WWBs to the large-scale sea surface temperature. Although the WWB wind stress at a given location may be a nonlinear function of SST, the characteristics of WWBs are well described as a linear function of SST. Over 50% of the interannual variance in the WWB likelihood, zonal location, duration, and fetch is explained by changes in SST. The model captures what is seen in a 17-yr record of satellite-derived winds: the eastward migration and increased occurrence of wind bursts as the western Pacific warm pool extends. The WWB model shows significant skill in predicting the interannual variability of the characteristics of WWBs, while the prediction skill of the WWB seasonal cycle is limited by the record length of available data. The novel formulation of the WWB model can be implemented in a stochastic or deterministic mode, where the deterministic mode predicts the ensemble-mean WWB characteristics. Therefore, the WWB model is especially appropriate for ensemble prediction experiments with existing ENSO models that are not capable of simulating realistic WWBs on their own. Should only the slowly varying component of WWBs be important for ENSO prediction, this WWB model allows a shortcut to directly compute the slowly varying ensemble-mean wind field without performing many realizations.

Spatial Coherence of Monsoon Onset over Western and Central Sahel (1950–2000)

Abstract The spatial coherence of boreal monsoon onset over the western and central Sahel (Senegal, Mali, Burkina Faso) is studied through the analysis of daily rainfall data for 103 stations from 1950 to 2000. Onset date is defined using a local agronomic definition, that is, the first wet day (&amp;gt;1 mm) of 1 or 2 consecutive days receiving at least 20 mm without a 7-day dry spell receiving less than 5 mm in the following 20 days. Changing either the length or the amplitude of the initial wet spell, or both, or the length of the following dry spell modifies the long-term mean of local-scale onset date but has only a weak impact either on its interannual variability or its spatial coherence. Onset date exhibits a seasonal progression from southern Burkina Faso (mid-May) to northwestern Senegal and Saharian edges (early August). Interannual variability of the local-scale onset date does not seem to be strongly spatially coherent. The amount of common or covariant signal across the stations is far weaker than the interstation noise at the interannual time scale. In particular, a systematic spatially consistent advance or delay of the onset is hardly observed across the whole western and central Sahel. In consequence, the seasonal predictability of local-scale onset over the western and central Sahel associated, for example, with large-scale sea surface temperatures, is, at best, weak.

The Southern Annular Mode and opposite-phased Basin Mode of the Southern Ocean circulation

Over the Southern Ocean the dominant modes of the atmospheric field are known as the Southern Annular Mode (SAM) or Antarctic Oscillation, and the Pacific South American (PSA) pattern. Statistical analysis of sea surface wind (SSW) from satellite observation revealed two leading modes of SAM-like and PSA patterns. In the high latitudes, the SAM-like pattern of the SSW had a large amplitude over the Bellingshausen Basin and Australian-Antarctic Basin, with opposite phase between the two basins. On the intraseasonal time scale, large-scale sea surface height (SSH) also had notable variability, showing a basin-scale anti-phase mode over the two basins. To explain the response of oceanic variations to these atmospheric modes, we analyzed the relationship between the dominant modes of wind stress and large-scale SSH on the intraseasonal time scale. The SAM-like pattern of wind stress was correlated with the SSH variation over the two basins. The SSH basin mode was most simply explained by a simple barotropic response to the SAM-like mode of wind stress, with the curl of opposite phase between the two basins. We conclude that the zonal asymmetry of the wind field of the SAM plays an important role in driving the antiphase SSH basin modes.

Declining Coral Calcification on the Great Barrier Reef

Reef-building corals are under increasing physiological stress from a changing climate and ocean absorption of increasing atmospheric carbon dioxide. We investigated 328 colonies of massive Porites corals from 69 reefs of the Great Barrier Reef (GBR) in Australia. Their skeletal records show that throughout the GBR, calcification has declined by 14.2% since 1990, predominantly because extension (linear growth) has declined by 13.3%. The data suggest that such a severe and sudden decline in calcification is unprecedented in at least the past 400 years. Calcification increases linearly with increasing large-scale sea surface temperature but responds nonlinearly to annual temperature anomalies. The causes of the decline remain unknown; however, this study suggests that increasing temperature stress and a declining saturation state of seawater aragonite may be diminishing the ability of GBR corals to deposit calcium carbonate.

Decadal Variability in the Large-Scale Sea Surface Height Field of the South Pacific Ocean: Observations and Causes

Abstract Large-scale sea surface height (SSH) changes in the extraequatorial South Pacific Ocean are investigated using satellite altimetry data of the past 12 yr. The decadal SSH signals in the 1990s were dominated by an increasing trend in the 30°–50°S band and a decreasing trend in the central South Pacific Ocean poleward of 50°S. In recent years since 2002 there has been a reversal in both of these trends. Spatially varying low-frequency SSH signals are also found in the tropical region of 10°–25°S where the decadal SSH trend is negative in the eastern basin, but positive in the western basin. To clarify the causes for these observed spatially varying SSH signals, a 1½-layer reduced-gravity model that includes the wind-driven baroclinic Rossby wave dynamics and the responses forced by SSH changes along the South American coast was adopted. The model hindcasts the spatially varying decadal trends in the midlatitude and the eastern tropical regions well. Accumulation of the wind-forced SSH anomalies along Rossby wave characteristics is found to be important for both previously reported long-term trends and their reversals in recent years. The boundary forcing associated with the time-varying SSH signals along the South American coast is crucial for understanding the observed SSH signals of all time scales in the eastern tropical South Pacific basin, but it has little impact upon the midlatitude interior SSH signals.

The Prestige crisis: Operational oceanography applied to oil recovery, by the Basque fishing fleet

On 19th November 2002, the oil tanker Prestige (containing 77,000 tonnes of heavy fuel no. 2 (M100)) sank in 3500 m of water, off the coast of northwestern Spain. Intermittent discharge of oil from the stricken tanker, combined with large-scale sea surface dispersion, created a tracking and recovery problem. Initially, conventional oil recovery approaches were adopted, close to the wreck. With time and distance from the source, the oil dispersed dramatically and became less viscous. Consequently, a unique monitoring, prediction and data dissemination system was established, based upon the principles of ‘operational oceanography’; this utilised in situ tracked buoys and numerical (spill trajectory) modelling outputs, in combination with remote sensing (satellite sensors and visual observation). Overall, wind effects on the surface waters were found to be the most important mechanism controlling the smaller oil slick movements. The recovery operation involved up to 180 fishing boats, 9–30 m in length. Such labour-intensive recovery of the oil (21,000 tonnes, representing an unprecedented ratio of 6.6 tonnes at sea, per tonne recovered on land) continued over a 10 month period. The overall recovery at sea, by the fishing vessels, represented 63% of the total oil recovered at sea; this compares to only 37% recovered by specialised ‘counter- pollution’ vessels.