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Ocean Surface Topography from Space


Jean Tournadre - (IFREMER - LOS)

  Fanny Girard-Ardhuin
Frédérique Remy


Mean probility of presence of iceberg in the southern ocean from 1992 to 2012 from the ALTIBERG database combining TOPEX/Poseidon, Envisat, Jason-1 , Jason-2 and Cryosat altimeter data.
Interest in icebergs has been growing in recent years (see for example the recent review by Smith 2011) as they may account for a significant part of the freshwater flux in the southern ocean [Silva et al., 2006; Martin and Adcroft , 2010; Gladstone et al., 2001; Jongma et al., 2009], and because they have been shown to transport nutriments (in particular labile iron) that could have a significant impact on ocean primary productivity [Schodlok et al., 2006; Raiswell et al., 2008; Lancelot et al., 2009; Schwarz and Schodlok, 2009]. It was also demonstrated that small iceberg can have a significant impact on sea state modelling (Ardhuin et al 2011). Contrary to large tabular icebergs that are routinely tracked and monitored using scatterometer data Long et al. [2002], the distribution of smaller icebergs, i.e. less than 2-3 km in length, in the southern ocean is known mainly from ship-based observations with a limited temporal and spatial coverage [Jacka and Giles, 2007; Wadhams, 1988; Romanov et al., 2008]. Indeed, they are difficult to detect operationally using satellite borne sensors. Visible and infrared sensors are often blinded by clouds whilst low resolution microwave sensors, such as radiometers, cannot detect such small features. Microwave scatterometer have been used regionally and with limitations to establish statistics on icebergs in coastal areas [Young et al., 1998]. Synthetic Aperture Radars, although their detecting capabilities have been demonstrated [Gladstone and Bigg, 2002; Silva and Bigg, 2005] have yet to be used on an operational basis to produce iceberg distribution or climatology, mainly due to the large amount of data to be processed.

In a recent studies, Tournadre et al. [2008] and Tournadre et al [2012] demonstrated that small icebergs, at least in open water, have a significant signature in the noise part of high resolution (HR) altimeter waveforms and that the analysis of Jason altimeter HR waveforms over the southern ocean enables to determine the small iceberg distribution on a monthly basis. The method they proposed also allow to estimate, under assumptions on the freeboard height and on the ice backscatter, the iceberg surface and thus the ice volume. Using the complete Jason-1 archive (from 2002 to 2010) they created a small iceberg data base for the southern ocean which gives an unprecedented description of the small iceberg (100m-2800 m) distribution at unprecedented time and space resolutions.

The iceberg size, which follows a log-normal distribution with an overall mean length of 630 m, has a strong seasonal cycle reflecting the melting of icebergs during the austral summer estimated at 1.5 m/day. The total volume of ice in the southern ocean has an annual mean value of about 400 Gt, i.e. about 35% of the mean yearly volume of large tabular icebergs estimated from the National Ice Center database of 1979-2003 iceberg tracks and a model of iceberg thermodynamics. They can thus play a significant role in the injection of melt water in the ocean. The distribution of ice volume which has strong seasonal cycle presents a very high spatial and temporal inter-annual variability which is much contrasted in the three ocean basins (South Atlantic, Indian and Pacific oceans). The analysis of the relationship between small and large (>5 km) icebergs shows that a majority of small icebergs are directly associated with the large ones but that there are vast regions, such as the eastern branch of the Wedell gyre, where the transport of ice is made only through the smaller ones.

Following these studies, the ALTIBERG project was submitted, selected and founded by CNES (TOSCA). Its main goal is to create the most complete small iceberg database using the high resolution waveforms of all past and present altimeters. The project started in January 2012 and the archive containing the HR waveforms of ERS-1 and 2, Topex/Poseidon, Jason1 and 2 and Envisat has already been created at LOS. The adaptation of the existing Jason-1 processing chain to the other altimeter configuration is underway as well as the processing of the archive. It is planned that within the year small icebergs database covering the whole altimeter archive will be available for scientific studies as well as for use by the modeling (ocean and sea state) community. Contacts have already be taken with interested French teams to define the format of the small icebergs products (monthly probability, volume of ice, distribution of size,...).

The ALTIBERG-2 project, in direct continuation of the ALTIBERG project, aims at developing the scientific use of the 20-year iceberg database and to prepare operational iceberg products for future missions. Four main axes of study corresponding to open questions on the role of icebergs will be investigated..

Firstly, the possible retro-action between icebergs and ice volume and sea ice extension. A recent study by Lancelot et al. (2009) based on numerical modeling of the southern ocean including sea ice and iceberg has shown that higher icebergs concentration can lead to higher sea ice extent. However, no direct validation has yet been possible because of the lack of reliable iceberg data. The joint analysis of the 20-year small iceberg database and the archive of sea ice data from microwave radiometry and scatterometer opens a new way of a direct analysis of the retro-action between iceberg and sea ice.

Secondly, Romanov et al (2008) investigated the relationship between icebergs and ENSO and showed that several regions in the Atlantic and Indian Ocean sector of Antarctica have been identified where yearly anomalies of the iceberg occurrence exhibits a noticeable correlation with ENSO events. The strongest ENSO effect was observed in the region east of Drake Passage where the iceberg concentration increased by about 50% during years of the negative Southern Oscillation Index phase. Their study is based on a limited ship-based iceberg observation (from 1970 to 2005) that only allows the analysis of yearly variations of iceberg distribution. The more detailed description (in time and space) of the iceberg concentration and ice volume (which might be a more pertinent parameter to explain possible ENSO relationship) provided by the iceberg database will allow a better analysis of the links between iceberg and ENSAO but also other atmospheric mode such the Southern Annular Mode (SAM) that has been shown to play a significant role in the southern ocean mixed layer depth and ventilation.

Thirdly, icebergs cool their immediate surroundings as they melt, increasing water density. These effects can alter the vertical density profile, affecting the character of the pycnocline and the stability of the upper water column (Jenkins, 1999). Icebergs can thus influence deep water formation. Furthermore, a freshening and cooling of surface waters might be expected to facilitate the formation of sea-ice, influencing the climate through several feedbacks. The cooling and dilution of ocean water by large tabular icebergs has been studied using in situ and satellite measurements (Helly et al, 2011). showing that the surface meltwater effects were detectable as far away as 19 km from the iceberg and persisted for at least 10 days. However, no study has yet been conducted on the cooling induced by patches of small icebergs. For a equivalent volume of ice, groups of small icebergs might have a stronger cooling and dilution effect than a large iceberg because of the increase of surface of contact between water and ice. It is thus important to better quantify their impact on the upper ocean. The same question arises when considering the iceberg impact on biology. Several studies published on this subject have quoted icebergs as "hot spots' for the enrichment of the southern ocean. Helly et al (2011) showed that the chemical and biological effects were detected at the same space and time scales as the physical properties with decreasing chlorophyll concentration near the iceberg. However, ten days after the passage of the iceberg, chlorophyll-a had increased by 15%. These results are consistent with hypotheses regarding the role of icebergs as mediators of a localized geophysical disturbance as well as promoters of chlorophyll-a production. Combining and co-localizing small icebergs and coincident satellite sea surface temperature data (from infrared and microwave sensors) as well as satellite ocean color Chl-a data (Seawifs, MODIS) will allow a direct estimate of the cooling effect of small icebergs and their potential impact on Chl-a production.

Fourthly, the analysis of the Jason-1 small iceberg database showed (Tournadre et al, 2012) the strong relationship between large and small icebergs. The decay of large icebergs creates patches of small icebergs that drift far away from their origin and follow quite different routes than the larger one. They can for example drift within the eastern branch of the Wedell gyre where large icebergs are almost never detected. The analysis of small iceberg can not be complete without the pending analysis of the larger one. The Bringham Young University maintains a large iceberg data base (Long et al 2002) that will be extensively used to estimate the relative impact of small and large icebergs on the upper ocean and on sea ice. Using the waveform archive we will also investigate the potential of HR altimetry to better study the larger icebergs. Tests on the collocation of large icebergs positions and altimeter data are very promising. Several hundred of co-located data have been found. The example presented in figure 1 shows that the high resolution waveforms over large icebergs holds information on the iceberg freeboard, the shape of the iceberg surface and on the backscatter. Based on the experience on the inversion of altimeter waveforms in term of high resolution backscatter (Tournadre et al, 2011) and on the modeling of altimeter waveforms we will defined specific processing to estimate the shape and volume of large icebergs.

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