Hindumathi Palanisamy

Estimating ENSO Influence on the Global Mean Sea Level, 1993–2010

Interannual global mean sea level (GMSL) variations and El Nino-Southern Oscillation (ENSO) are highly correlated, with positive/negative GMSL anomalies during El Nino/La Nina events. In a previous study, we showed that interannual GMSL and total land water storage variations are inversely correlated, with lower-than-average total water storage on land and higher-than-average GMSL during El Nino. This result is in agreement with the observed rainfall deficit/excess over land/oceans during El Nino (and vice versa during La Nina). It suggests that the positive GMSL anomaly observed during El Nino is likely due to an ocean mass rather than thermal expansion increase. Here, we analyze the respective contribution of the Atlantic, Indian, and Pacific oceans to the interannual (ENSO-related) GMSL anomalies observed during the altimetry era (i.e., since 1993) with an emphasis on the 1997/1998 El Nino event. For each oceanic region, we compute the steric contribution, and remove it from the altimetry-based mean sea level to estimate the ocean mass component. We find that mass changes of the tropical Pacific Ocean, mainly in the region within 0–25°N, are mostly responsible for the observed 1997/1998 ENSO-related GMSL anomaly. The ocean mass excess of this region almost perfectly compensates the total land water deficit during the 1997/1998 El Nino. An estimate of the ocean-atmosphere water balance of this region shows that the time derivative of the ocean mass component is well correlated with net P-E (precipitation minus evaporation) over most of the study period, except during the 1997/1998 ENSO event, where there is a temporary ocean mass increase, not compensated by the net P-E. We thus propose that the 1997/1998 ocean mass increase of this north tropical Pacific area be linked to an imbalance between the inflow/outflow entering/leaving the north tropical Pacific. A preliminary qualitative analysis indicates that a significant reduction of the Makassar Strait transport, (about 80% of the total Indonesian throughflow), as previously reported in the literature during the strong 1997/1998 El Nino event, could explain the north tropical Pacific Ocean mass excess reported in this study, hence the observed positive GMSL anomaly.

Regional sea level change and variability in the Caribbean sea since 1950

Regional sea level change and variability in the Caribbean sea since 1950 We investigate the regional variability in sea level in the Caribbean Sea region over the past 60 years (1950-2009) using an Empirical Orthogonal Function (EOF)-based 2-dimensional past sea level reconstruction (a mean of 3 reconstructions based on few long tide gauge records and different sea level grids from satellite altimetry and ocean circulation models) and satellite altimetry data for the last two decades. We find that over the past 60 years, the mean rate of sea level rise in the region was similar to the global mean rise (~1.8 mm/yr). The interannual mean sea level of the placeCaribbean region appears highly correlated with El Nino-Southern Oscillation (ENSO) indices. Interpolation of the sea level reconstruction grid at different sites, in particular at the Caribbean Islands where tide gauge records are either very short or inexistent, shows that locally, the sea level trend is on the order of 2 mm/yr, i.e. only slightly larger than the mean trend over the region. Besides, correlation with ENSO is in general good, especially since the mid-1980s. We also find a significant correlation between the interannual variability in sea level and hurricane activity, especially over the past decade during which hurricane intensity and sea level interannual variability have both increased.

Spatial Patterns of Sea Level Variability Associated with Natural Internal Climate Modes

Sea level rise (SLR) can exert significant stress on highly populated coastal societies and low-lying island countries around the world. Because of this, there is huge societal demand for improved decadal predictions and future projections of SLR, particularly on a local scale along coastlines. Regionally, sea level variations can deviate considerably from the global mean due to various geophysical processes. These include changes of ocean circulations, which partially can be attributed to natural, internal modes of variability in the complex Earth’s climate system. Anthropogenic influence may also contribute to regional sea level variations. Separating the effects of natural climate modes and anthropogenic forcing, however, remains a challenge and requires identification of the imprint of specific climate modes in observed sea level change patterns. In this paper, we review our current state of knowledge about spatial patterns of sea level variability associated with natural climate modes on interannual-to-multidecadal timescales, with particular focus on decadal-to-multidecadal variability. Relevant climate modes and our current state of understanding their associated sea level patterns and driving mechanisms are elaborated separately for the Pacific, the Indian, the Atlantic, and the Arctic and Southern Oceans. We also discuss the issues, challenges and future outlooks for understanding the regional sea level patterns associated with climate modes. Effects of these internal modes have to be taken into account in order to achieve more reliable near-term predictions and future projections of regional SLR.

Interannual Sea Level Variations in the South China Sea Over 1950–2009

Spatial patterns of interannual sea level variations in the South China Sea (SCS) are investigated by analyzing an EOF-based 2-dimensional past sea level reconstruction from 1950 to 2009 and satellite altimetry data from 1993 to 2009. Long-term tide gauge records from 14 selected stations in this region are also used to assess the quality of reconstructed sea levels and determine the rate of sea level along the coastal area. We found that the rising rate of sea levels derived from merged satellite altimetry data during 1993–2009 and past sea level reconstruction over 1950–2009 is about 3.9 ± 0.6 mm/yr and 1.7 ± 0.1 mm/yr, respectively. For the longer period, this rate is not significantly different from the global mean rate (of 1.8 ± 0.3 mm/yr). The interannual mean sea level of the SCS region appears highly correlated with Niño 4 indices (a proxy of El Niño-Southern Oscillation/ENSO), suggesting that the interannual sea level variations over the SCS region is driven by ENSO events. Interpolation of the reconstructed sea level data for 1950–2009 at sites where tide gauge records are of poor quality (either short or gapped) show that sea level along the Chinese coastal area is rising faster than the global mean rate of 1.8 mm/yr. At some sites, the rate is up to 2.5 mm/yr.

Internal Variability Versus Anthropogenic Forcing on Sea Level and Its Components

In this paper we review and update detection and attribution studies in sea level and its major contributors during the past decades. Tide gauge records reveal that the observed twentieth-century global and regional sea level rise is out of the bounds of its natural variability, evidencing thus a human fingerprint in the reported trends. The signal varies regionally, and it partly depends on the magnitude of the background variability. The human fingerprint is also manifested in the contributors of sea level for which observations are available, namely ocean thermal expansion and glaciers’ mass loss, which dominated the global sea level rise over the twentieth century. Attribution studies provide evidence that the trends in both components are clearly dominated by anthropogenic forcing over the second half of the twentieth century. In the earlier decades, there is a lack of observations hampering an improved attribution of causes to the observed sea level rise. At certain locations along the coast, the human influence is exacerbated by local coastal activities that induce land subsidence and increase the risk of sea level-related hazards.

Effect of La Niña on The Global Mean Sea Level And North Pacifc Ocean Mass Over 2005-2011

Abstract Interannual fluctuations of the global mean sea level are highly correlated with El Niño-Southern Oscillation (ENSO) events, with positive/negative anomalies during El Niño/La Niña. In a previous study we showed that during the 1997 - 1998 El Niño, a positive anomaly observed in the global mean sea level was mostly caused by an increase of the ocean mass component rather than by steric (thermal) effects. This result was related to an increase of precipitation over the tropical ocean and a deficit in land water storage. In the present study, we investigate the effect of the recent 2008 and 2011 La Niña events on the satellite altimetry-based global mean sea level. We find that the large global mean sea level drop associated with the 2011 La Niña results from the combined decrease of the steric and ocean mass components, with a slightly dominant contribution from the latter. We show that the ocean mass contribution to the global mean sea level drop is spatially confined over the north eastern tropical Pacific (just as was found previously for the 1997 - 1998 El Niño, but with opposite sign). Corresponding ocean mass spatial pattern is closely correlated to observed sea level and steric spatial patterns over the duration of the La Niña event. This is also observed for previous El Niño and La Niña events. Such a drop in ocean mass during ENSO in the eastern part of the tropical Pacific has not been reported before. It is possibly related to a temporary decrease in the net precipitation over the north eastern Pacific (opposite situation was found during the 1997 - 1998 El Niño).

L'influence d'El Niño et de La Niña sur le niveau de la mer

The detrended global mean sea level displays positive/negative anomalies of a few millimetres amplitude during El Nino/La Nina events that are inversely correlated to total terrestrial water storage variations.This result is in agreement with the observed rainfall def icit/excess over land/oceans during El Nino (and vice versa during La Nina). It suggests that the positive anomaly observed during El Nino in the global mean sea level is likely due to the ocean mass rather than thermal expansion. A detailed analysis over each oceanic region shows that the global mean sea level anomaly observed during the strong 1997-1998 El Nino resulted from an excess of mass of the north tropical Pacific Ocean with almost perfect compensation with the total terrestrial water deficit during this El Nino.