Time-Lapse Seismic Imaging of Oceanic Fronts and Transient Lenses within South Atlantic Ocean

Oceanic fronts play a pivotal role in controlling water mass transfer, although little is known about deep frontal structure on appropriate temporal and spatial scales. Here, we present a sequence of calibrated time-lapse images from a three-dimensional seismic survey that straddles the Brazil-Malvinas Confluence— a significant feature of the merid- ional overturning circulation. Eight vertical transects reveal the evolution of a major front. It is manifest as a discrete planar surface that dips at less than 2 ◦ and is traceable to 1.5–2 km depth. Its shape and surface expression are consistent with sloping isopycnal surfaces of the calculated potential density field and with coeval sea surface tempera- ture measurements, respectively. Within the top ∼1 km, where cold fresh water subducts beneath warm salty water, a series of tilted lenses are banked up against the sharply de- fined front. The largest of these structures is centered at 700 m depth and is cored by cold fresh water. Time-lapse imagery demonstrates that this tilted lens grows and de- cays over nine days. It has a maximum diameter of < 34 ± 0.13 km and a maximum height of < 750±10 m. Beneath 1 km, where horizontal density gradients are negligi- ble, numerous deforming lenses and filaments on length scales of 10–100 km are being swept toward the advecting front

PICES Press, Vol. 25, No. 1, Winter 2017

PICES science in 2016: A note from the Science Board Chair (pp. 1-8); 2016 PICES awards (pp. 9-12); PICES calendar of events (pp. 13-13); Impressions of PICES from old friends (pp. 14-17); S-CCME Workshop W5, “Modeling effects of climate change on fish and fisheries (pp. 18-22); In memoriam: Professor Emeritus Paul J. Harrison (pp. 23-23); Workshop W9, “The role of the northern Bering Sea in modulating arctic environments” (pp. 24); A symposium to mark the 60th anniversary of Station Papa/Line P (pp. 28-29); To the interface and beyond: Results and legacy of SCOR Working Group 140 (pp. 30-31); Webcam monitoring and modeling of Japanese tsunami marine debris (pp. 32-35); Mapping patterns of marine debris in the main Hawaiian Islands using aerial imagery and spatial analysis (pp. 36-39); New leadership in PICES (pp. 40-44); PICES interns (pp. 45-45); The Bering Sea: Current status and recent trends (pp. 46-49); The state of the western North Pacific during the 2016 warm season (pp. 50-51

Numerical modelling of the Angola Low and the Botswana High during a neutral and two El Ni˜no summers

The Angola Low and Botswana High pressure systems are thought to play a crucial role in the variability of summer rainfall over southern Africa. However, very little is known about their variability during the summer half of the year and how their influence on rainfall patterns during ENSO and non-ENSO summers may vary. In simple terms, a weaker Angola Low is expected to lead to decreased rainfall as is a stronger Botswana High. This study looks at the monthly evolution of the Angola Low and the Botswana High during the neutral summer of 2012/13 and the two strong El Ni˜no summers of 1997/98 and 2015/16 using the WRF model. CFSR and CFSv2 reanalyses, satellite derived winds, GPCC rainfall and TRMM satellite-derived rainfall estimates are used to validate the model. The model was integrated from September through to April for each run with observed sea surface temperature and reanalyses as boundary conditions. During the neutral summer of 2012/13, the Angola Low became clearly evident in the model during the pentad of 6-10 October whereas during the 1997/98 and 2015/16 El Ni˜no summers, it became evident during the pentads of 6-10 November and 16-20 November respectively. In addition to these differences in onset, there were also differences in the date after which the Low was no longer present in the model fields. These dates were 26-28 February 2013 and 26-31 March 1998 while in the 2015/16 case, the Low remained present throughout the whole of March. In each year, the Botswana High was present throughout the entire summer half of the year. The WRF simulation indicated that during the strong El Ni˜no event of 1997/98, the Angola Low did not weaken whereas the Botswana High was weaker than normal. However, during the strong El Ni˜no event of 2015/16, the Angola Low was weaker and the Botswana High was relatively strong. The strengthening of the Angola Low and the weakening of the Botswana High during the strong 1997/98 El Ni˜no led to substantial rainfall over southern Africa. The near to above average rainfall over subtropical southern Africa during 1997/98 was unexpected given the strength of the El Ni˜no and the SST anomalies in the Indian and Pacific Oceans. The weaker Angola Low and stronger Botswana High during the strong 2015/16 El Ni˜no led to severe drought over the region. The study highlights the importance of modulations in the Angola Low and the Botswana High for rainfall anomalies during ENSO and non-ENSO summers as very different rainfall patterns may occur over southern Africa during similar strength ENSO events. The significance of these regional circulation systems is reinforced by the fact that during the 2012/13 neutral summer, the Angola Low was stronger than average and the Botswana High was relatively weak leading to good rainfall. The relationship between the Angola Low, the Botswana High and southern Africa rainfall is found to be relatively strong through the 1979-2017 period. Thus, monitoring and better understanding these regional circulation systems is important and complements ongoing efforts to monitor and predict ENSO

The use of remote sensing for soil moisture estimation using downscaling and soil water balance modelling in Malmesbury and the Riebeek Valley

>Magister Scientiae - MScSoil moisture forms an integral part of the hydrological cycle and exerts considerable influence on hydrological processes at or near the earth’s surface. Knowledge of soil moisture is important for planning and decision-making in the agricultural sector, land and water conservation and flood warning. Point measurements of soil moisture, although highly accurate, are time consuming, costly and do not provide an accurate indication of the soil moisture variation over time and space as soil moisture has a high degree of spatial and temporal variability. The spatial variability of soil moisture is due to the heterogeneity of soil water holding properties, the influence of plants, and land uses. The downscaling of satellite microwave soil moisture estimates and soil water balance modelling was investigated at six transects in the semi-arid, Western Cape Province of South Africa, as alternatives to in situ soil measurements. It was found that microwave soil moisture estimates compared well to in situ measurements at the six transects (study sites), with coefficient of determination (r2) values greater than 0.7 and root mean square error (RMSE) values less than 1.5%. Downscaling using the universal triangle method, performed well at 4 of the 6 transects, with r2 values great than 0.65 and low to moderate RMSE values (0.5-12%). Soil water balance modelling similarly performed well in comparison with in situ measurements at 4 of the transects with regards to r2 values (>0.6) but had moderate to high RMSE (4.5-19%). Poor downscaling results were attributed to fine scale (within 1 km) surface heterogeneity while poor model performance was attributed to soil hydrological and rainfall heterogeneity within the study areas

Validation and calibration of ASCAT using CMOD5.n

11 pages, 10 figures.-- © 2010 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other worksThe Advanced Scatterometer (ASCAT) onboard the Metop-A satellite became operational shortly after launch in 2006, and an absolute calibration using three transponders was achieved in November 2008. In this paper, we describe how the CMOD5.n ocean backscatter geophysical model function (GMF), which was derived using data from previous scatterometers onboard the European Remote Sensing 1 and 2 satellites (ERS-1 and ERS-2), was used to derive backscatter bias correction factors. The purpose is to remove the bias between ASCAT backscatter data and the CMOD5.n GMF output which allows these data to be used in place of ERS data in existing wind processing algorithms. The ASCATWind Data Processor, developed at the Royal Netherlands Meteorological Institute (KNMI), applies the bias correction factors to ASCAT data and uses CMOD5.n to retrieve wind vectors in order to produce an operational wind product. This resulted in a stable and high-quality ASCAT wind product since February 2007. We validate this product by comparing it to the European Centre for Medium-range Weather Forecasts (ECMWF) winds and buoy measurements. The bias correction factors indicate that ASCAT data and the GMF differ by roughly 0.3 dB below 55° and up to 0.8 dB above 55°. A possible explanation lies in CMOD5.n which has been poorly validated in this incidence angle regime. Validation of ASCAT data using the ocean calibration method confirms this result and also indicates that bias-corrected data are everywhere within 0.3 dB of CMOD5.n. The wind product validation shows an rms error of 1.3 m . s-1 in wind speed and 16° in wind direction when compared to ECMWF winds. This is better than the results achieved using ERS scatterometer data. Against buoy winds, we find an rms error wind component error of approximately 1.8 m . s-1. These results show that the ASCAT wind product is of high quality and satisfies its wind component accuracy requirement of 2 mAWDP has been funded by EUMETSAT in the context of the Numerical Weather Prediction (NWP) SAF and can be obtained free of charge from the NWP SAF Web sitePeer Reviewe