Entrainment Of Subsurface Water Research Articles

Simulation of salinity variability and the related freshwater flux forcing in the tropical Pacific: An evaluation using the Beijing normal university earth system model (BNU-ESM)

The climatology and interannual variability of sea surface salinity (SSS) and freshwater flux (FWF) in the equatorial Pacific are analyzed and evaluated using simulations from the Beijing Normal University Earth System Model (BNU-ESM). The simulated annual climatology and interannual variations of SSS, FWF, mixed layer depth (MLD), and buoyancy flux agree with those observed in the equatorial Pacific. The relationships among the interannual anomaly fields simulated by BNU-ESM are analyzed to illustrate the climate feedbacks induced by FWF in the tropical Pacific. The largest interannual variations of SSS and FWF are located in the western-central equatorial Pacific. A positive FWF feedback effect on sea surface temperature (SST) in the equatorial Pacific is identified. As a response to El Nino–Southern Oscillation (ENSO), the interannual variation of FWF induces ocean processes which, in turn, enhance ENSO. During El Nino, a positive FWF anomaly in the western-central Pacific (an indication of increased precipitation rates) acts to enhance a negative salinity anomaly and a negative surface ocean density anomaly, leading to stable stratification in the upper ocean. Hence, the vertical mixing and entrainment of subsurface water into the mixed layer are reduced, and the associated El Nino is enhanced. Related to this positive feedback, the simulated FWF bias is clearly reflected in SSS and SST simulations, with a positive FWF perturbation into the ocean corresponding to a low SSS and a small surface ocean density in the western-central equatorial Pacific warm pool.

Effects of interannual salinity variability and freshwater flux forcing on the development of the 2007/08 La Niña event diagnosed from Argo and satellite data

Oceanic salinity and its related freshwater flux (FWF) forcing in the tropical Pacific have been of increased interest recently due to their roles in the El Niño-Southern Oscillation (ENSO), the global climate and water cycle. A comprehensive data analysis is performed to illustrate the significant effects of interannual salinity variability and FWF forcing during the 2007/08 La Niña event using three-dimensional temperature and salinity fields from Argo profiles, and some related fields derived from the Argo and satellite-based data, including the mixed layer depth (MLD), heat flux, freshwater flux, and buoyancy flux (QB). It is demonstrated that during the developing phase of 2007/08 La Niña, a negative FWF anomaly and its associated positive sea surface salinity (SSS) anomaly in the western-central basin act to increase oceanic density and de-stabilize the upper ocean. At the same time, the negative FWF anomaly tends to reduce a positive QB anomaly and deepen the mixed layer (ML). These related oceanic processes act to strengthen the vertical mixing and entrainment of subsurface water at the base of ML, which further enhance cold sea surface temperature (SST) anomalies associated with the La Niña event, a demonstration of a positive feedback induced by FWF forcing.

Distinguished Effects of Interannual Salinity Variability on the Development of the Central-Pacific El Niñ Events

AbstractEl Nino events in the central equatorial Pacific (CP) are gaining increased attention, due to their increasing intensity within the global warming context. Various physical processes have been identified in the climate system that can be responsible for the modulation of El Nino, especially the effects of interannual salinity variability. In this work, a comprehensive data analysis is performed to illustrate the effects of interannual salinity variability using surface and subsurface salinity fields from the Met Office ENSEMBLES (EN3) quality controlled ocean dataset. It is demonstrated that during the developing phase of an El Nino event, a negative sea surface salinity (SSS) anomaly in the western-central basin acts to freshen the mixed layer (ML), decrease oceanic density in the upper ocean, and stabilize the upper layers. These related oceanic processes tend to reduce the vertical mixing and entrainment of subsurface water at the base of the ML, which further enhances the warm sea surface temp...

Trace element variations in fossil corals from Tahiti collected by IODP Expedition 310: Reconstruction of marine environments during the last deglaciation (15 to 9 ka)

Climate responses and changes in marine environments during the last deglaciation have been controversial and few paleoceanographic data are available from the tropical South Pacific, though this region is crucial in the investigations of ocean–atmosphere interactions. Integrated Ocean Drilling Program Expedition 310 was conducted to establish the time course of the postglacial sea-level rise at Tahiti in the South Pacific. A principal objective of this expedition was to examine the variation of marine environments during the last deglaciation. As fossil Porites coral is ideal for assessing past marine environments, we selected only Porites specimens from the many coral specimens retrieved, examined them by XRD, and dated them by the 14C method. In all, we obtained 17 pristine Porites specimens composed of only aragonite with ages from 15 to 9 ka. Then, we measured Mg/Ca, Ba/Ca, and U/Ca ratios and Cd contents as proxies for upwelling and sea surface temperature. Higher Ba/Ca ratios and Cd content together with lower reconstructed SSTs using U/Ca ratios in the coral specimens between 12.6 and 9.8 cal ka compared to around 15 cal ka suggest that upwelling and/or entrainment of subsurface water into mixed layer was enhanced around Tahiti during this period. This finding is consistent with previous reports and supports the idea that the South Pacific was characterized by La Niña-like conditions at least from 12.6 to 9.8 cal ka.

Freshwater Flux (FWF)-Induced Oceanic Feedback in a Hybrid Coupled Model of the Tropical Pacific

Abstract The impacts of freshwater flux (FWF) forcing on interannual variability in the tropical Pacific climate system are investigated using a hybrid coupled model (HCM), constructed from an oceanic general circulation model (OGCM) and a simplified atmospheric model, whose forcing fields to the ocean consist of three components. Interannual anomalies of wind stress and precipitation minus evaporation, (P − E), are calculated respectively by their statistical feedback models that are constructed from a singular value decomposition (SVD) analysis of their historical data. Heat flux is calculated using an advective atmospheric mixed layer (AML) model. The constructed HCM can well reproduce interannual variability associated with ENSO in the tropical Pacific. HCM experiments are performed with varying strengths of anomalous FWF forcing. It is demonstrated that FWF can have a significant modulating impact on interannual variability. The buoyancy flux (QB) field, an important parameter determining the mixing and entrainment in the equatorial Pacific, is analyzed to illustrate the compensating role played by its two contributing parts: one is related to heat flux (QT) and the other to freshwater flux (QS). A positive feedback is identified between FWF and SST as follows: SST anomalies, generated by El Niño, nonlocally induce large anomalous FWF variability over the western and central regions, which directly influences sea surface salinity (SSS) and QB, leading to changes in the mixed layer depth (MLD), the upper-ocean stability, and the mixing and the entrainment of subsurface waters. These oceanic processes act to enhance the SST anomalies, which in turn feedback to the atmosphere in a coupled ocean–atmosphere system. As a result, taking into account anomalous FWF forcing in the HCM leads to an enhanced interannual variability and ENSO cycles. It is further shown that FWF forcing is playing a different role from heat flux forcing, with the former acting to drive a change in SST while the latter represents a passive response to the SST change. This HCM-based modeling study presents clear evidence for the role of FWF forcing in modulating interannual variability in the tropical Pacific. The significance and implications of these results are further discussed for physical understanding and model improvements of interannual variability in the tropical Pacific ocean–atmosphere system.

Biological and chemical characteristics of high-chlorophyll, low-temperature water observed near the Sulu Archipelago

Abstract According to ocean-color remote sensing data, a low phytoplankton biomass exists in the central parts of the Sulu and Celebes seas, whereas higher concentrations of chlorophyll a are observed in the waters near the Sulu Archipelago, which separates these water bodies. These high-chlorophyll a waters have lower temperatures than those of surrounding waters, indicating vertical mixing of nutrient-rich subsurface water that is attributable to the topographical features of the Sulu Ridge. Differences in photosynthetic parameters, primary productivity and the phytoplankton community structure between the high-chlorophyll area near the Sulu Archipelago and the oligotrophic basin area in the Philippine, Sulu and Celebes seas were investigated in November 2002 during the R.V. Hakuho-Maru KH-02-4 cruise. Entrainment of subsurface water and vertical mixing by intense tidal flow from the Sulu Archipelago to the Celebes Sea created a low-temperature, high-nutrient condition at the southeastern edge of the Sulu Ridge. Chlorophyll a concentrations greater than 2 μg l−1 were observed at stations 5–15 km offshore at the edge of the Sulu Ridge. This high-chlorophyll tongue in the Celebes Sea showed high primary productivity: it was 1.6–18 times higher than those measured in oligotrophic central basin regions of the Philippine, Sulu, and Celebes seas. Microscopic observation of these high-chlorophyll waters showed the dominance of chain-forming large centric diatoms. Extension of high-chlorophyll waters was not observed at the Sulu Sea side, where the surface water salinity was lower than in the Celebes Sea and consequently prevented intrusion of the Celebes Sea water into the surface layer. Occasional extensions of productive, chlorophyll-rich surface water from the archipelagic zone to the surrounding basin area seems to be an important factor causing spatiotemporal variability in the phytoplankton biomass in the southern Sulu Sea and the western Celebes Sea.

Offshore wind forcing in the Gulf of Tehuantepec, Mexico: The asymmetric circulation

Since the early surveys carried out by the Eastern Tropical Pacific (EASTROPIC) and Scripps Tuna Oceanographic Research (STOR) projects in the tropical Pacific off Mexico, the northerly winds which blow over the Gulf of Tehuantepec were described as an important factor controlling the dynamics of this coastal ocean. In January–February 1989 an international team carried out the experiment Tehuano, designed to study the response of the gulf to these wind pulses. The complete evolution of the coastal ocean after an event of moderate intensity was observed. The forcing is characterized by a mostly symmetric, fanshaped, offshore wind jet, which in turn produces a remarkably asymmetric upper ocean response. While analytical results based on Ekman theory forced by a symmetric offshore wind predict the formation of a symmetric dipolar circulation, the observed flow consists mainly of a large (200 km in diameter) anticyclonic warm‐core eddy in the western gulf, with only a weak cyclonic counterpart in the eastern gulf. Intense surface cooling under the wind jet is caused by entrainment of subsurface water into the upper layer. The thermocline in the west deepens with the development of the eddy, which is formed initially by the advection of warm surface waters from west of the gulf. East of the axis of the wind, the mixed layer deepens due to wind‐induced entrainment, while, at the same time, shoaling and compression of the deeper isotherms by curl‐induced upwelling (Ekman pumping) strengthen the thermocline.