Precession cycles of the El Niño/Southern oscillation-like system controlled by Pacific upper-ocean stratification

Modern observations have presented linkages between subsurface waters of the western Pacific warm pool and both El Niño/Southern Oscillation-related and extratropic-controlled upper-ocean stratification on interannual timescales. Moreover, studies have showed that such controls may operate on orbital cycles, although the details remain unclear. Here we present paired temperature and salinity reconstructions for the surface and thermocline waters in the central western Pacific warm pool over the past 360,000 years, as well as transit modeling results from an Earth system model. Our results show that variations in subsurface temperature and salinity in the western Pacific warm pool have consistently correlated with the shallow meridional overturning cell over the past four glacial-interglacial cycles, and they vary on eccentricity and precession cycles. The shallow meridional overturning cell regulates subsurface waters of the western Pacific warm pool by changing subtropical surface water density and thus equatorial upper-ocean stratification, acting as an El Niño/Southern Oscillation-like process in the precession band. Therefore, the western Pacific warm pool is critical in connecting the austral shallow meridional overturning cell to the Earth’s climate system on orbital timescales.

The Wind Effect on Biogeochemistry in Eddy Cores in the Northern South China Sea

Cyclonic and anticyclonic eddies are usually characterized by upwelling and downwelling, respectively, which are induced by eddy pumping near their core. Using a repeated expendable bathythermograph transect (XBT) and Argo floats, and by cruise experiments, we determined that not all eddies in the northern South China Sea (NSCS) were accompanied by eddy pumping. The weakening of background thermocline was attributed to the strengthening of eddy pumping, affected by (1) wind-induced meridional Sverdrup transports and (2) Kuroshio intrusion into the NSCS. Higher particulate organic carbon (POC) fluxes (> 100 mg-C m−2 day−1) were found near the eddy cores with significant eddy pumping (defined by a depth change of 22°C isotherm near the thermocline for over 10 m), although the satellite-estimated POC fluxes were inconsistent with the in-situ POC fluxes. nitrogen limitation transition and high POC flux were even found near the core of a smaller mesoscale (diameter < 100 km) cyclonic eddy in May 2014, during the weakening of the background thermocline in the NSCS. This finding provides evidence that small mesoscale eddies can efficiently provide nutrients to the subsurface, and that they can remove carbon from the euphotic zone. This is important for global warming, which generally strengthens upper ocean stratification.

A Novel Freshwater to Marine Evolutionary Transition Revealed within Methylophilaceae Bacteria from the Arctic Ocean

Global warming is profoundly influencing the Arctic Ocean. Rapid ice melt and increased freshwater input is increasing ocean stratification, driving shifts in nutrient availability and the primary production that supports marine food webs.

Observational evidence of stratification control of upwelling and pelagic fishery in the eastern Arabian Sea

Upwelling is a physical phenomenon that occurs globally along the eastern boundary of the ocean and supports pelagic fishery which is an important source of protein for the coastal population. Though upwelling and associated small pelagic fishery along the eastern Arabian Sea (EAS) is known to exist at least for the past six decades, our understanding of the factors controlling them are still elusive. Based on observation and data analysis we hypothesize that upwelling in the EAS during 2017 was modulated by freshwater-induced stratification. To validate this hypothesis, we examined 17 years of data from 2001 and show that inter-annual variability of freshwater influx indeed controls the upwelling in the EAS through stratification, a mechanism hitherto unexplored. The upper ocean stratification in turn is regulated by the fresh water influx through a combination of precipitation and river runoff. We further show that the oil sardine which is one of the dominant fish of the small pelagic fishery of the EAS varied inversely with stratification. Our study for the first time underscored the role of freshwater influx in regulating the coastal upwelling and upper ocean stratification controlling the regional pelagic fishery of the EAS.

Uncertainty of Tropical Cyclone Wind Radii on Sea Surface Temperature Cooling

<p>The sea surface temperature (SST) beneath a tropical cyclone (TC) is of great importance to its dynamics; therefore, understanding and accurately estimating the magnitude of SST cooling is of vital importance.  Existing studies have explored important influences on SST such as TC translation speed, maximum surface winds, ocean thermal condition and ocean stratification.  But the influence of the TC wind radii (or collectively called the TC size) on SST has been largely overlooked.  In this study we assess the influence of wind radii uncertainty on SST cooling by a total of 15,983 numerical simulations for the western North Pacific during the 2014-2018 seasons.  Results show a 6-20% SST cooling error induced using wind radii from the Joint Typhoon Warning Center official forecast and a 35-40% SST cooling error using wind radii from the operational runs of the Hurricane Weather Research and Forecasting (HWRF) model.  Our results indicate that SST cooling is most sensitive to the radius of 64 kt winds.  The correlation between SST cooling induced by the TC and its size is 0.49, which is highest among all the parameters tested.  This suggests that it is extremely important to get TC size correct in order to predict the SST cooling response, which then impacts TC evolution in numerical weather prediction models.</p>

Southeast tropical Atlantic changing from subtropical to tropical conditions

<div> <p><span>A warming and freshening trend of the mixed layer in the upper southeast tropical Atlantic Ocean (SETA) is observed by the Argo observation array during the time period of 2006 to 2019. Thus, the ocean surface density is reducing. This has an impact on the upper-ocean stratification which intensified by more than 30 % in the SETA region since 2006. The initial typical subtropical stratification with a salinity maximum at the surface is shifted to more tropical conditions characterized by warmer and fresher surface waters and a subsurface salinity maximum. </span></p> </div><div> <p><span>A more detailed analysis of isopycnals shows a continuous upward displacement of isopycnal surfaces suggesting that wind stress curl-driven upwelling has to play an essential role. Therefore, ASCAT wind stress changes are examined, revealing that increased open ocean wind curl-driven upwelling but also partly counteracting reduced coastal upwelling due to weakened alongshore southerly winds are present. Changing alongshore winds might be a reason why tropical surface waters spread further southward reaching more into the SETA region. Besides, atmospheric fluxes could further impact upper ocean characteristics. </span></p> </div><div> <p><span>Changes in the upper-ocean stratification matter as they affect not only physical ocean dynamics such as ocean ventilation processes but also biogeochemical and ecological activities such as nutrient fluxes and fisheries. Nevertheless, the consequences of increased stratification for upwelling regions are not yet fully understood. The SETA upwelling system is a key region for enhanced nutrient supply to the euphotic zone and hence, a core nutrient source for high coastal primary productivity. </span></p> </div><p>We aim to assess the recent change of upper-ocean stratification towards tropical conditions at the sea surface in the SETA region and explore its driving mechanisms as well as possible consequences for the primary productivity and fisheries off Angola and Namibia, in order to improve our understanding of what is happening as a result of intensified upper-ocean stratification in upwelling regions.</p>