William Llovel

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.

Evaluation of the Global Mean Sea Level Budget Between 1993 and 2014

Evaluating global mean sea level (GMSL) in terms of its components—mass and steric—is useful for both quantifying the accuracy of the measurements and understanding the processes that contribute to GMSL rise. In this paper, we review the GMSL budget over two periods—1993 to 2014 and 2005 to 2014—using multiple data sets of both total GMSL and the components (mass and steric). In addition to comparing linear trends, we also compare the level of agreement of the time series. For the longer period (1993–2014), we find closure in terms of the long-term trend but not for year-to-year variations, consistent with other studies. This is due to the lack of sufficient estimates of the amount of natural water mass cycling between the oceans and hydrosphere. For the more recent period (2005–2014), we find closure in both the long-term trend and for month-to-month variations. This is also consistent with previous studies.

Observed southern upper-ocean warming over 2005–2014 and associated mechanisms

The climate system is gaining heat owing to increasing concentration of greenhouse gases due to human activities. As the worlds oceans are the dominant reservoir of heat in the climate system, an accurate estimation of the ocean heat content change is essential to quantify the Earths energy budget and global mean sea level rise. Based on the mean estimate of the three Argo gridded products considered, we provide a decadal ocean heat content estimate (over 2005–2014), down to 2000 m, of 0.76 ± 0.14 W m−2 and its spatial pattern since 2005 with unprecedented data coverage. We find that the southern hemisphere explains 90% of the net ocean heat uptake located around 40°S mainly for the Indian and Pacific oceans that corresponds to the center of their subtropical gyres. We find that this rapid upper ocean warming is linked to a poleward shift of mean wind stress curl enhancing Ekman pumping for the 45°S–60°S band. Therefore, the increase of Ekman pumping steepens the isopycnal surface and can enhance heat penetration into the deeper layers of the ocean. We also highlight a relative consistency between the year-to-year net top-of-the-atmosphere flux inferred by satellite measurements and the ocean heating rates (correlation coefficient of 0.53). We conclude that there is no strong evidence of missing energy in the climate system because of remaining large uncertainties in the observing system.

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.