Effects of adjusting vertical resolution on the eastern equatorial Pacific cold tongue

The vertical resolution of LICOM1.0 (LASG/IAP Climate System Ocean Model, version 1.0) is adjusted by increasing the level amount within the upper 150 m while keeping the total of levels. It is found that the eastern equatorial Pacific cold tongue is sensitive to the adjustment. Compared with the simulation of the original level scheme, the adjusting yields a more realistic structure of cold tongue extending from the coast of Peru to the equator, as well as a temperature minimum at Costa Rica coast, north of the cold tongue. In the original scheme experiment, the sharp heating by net surface heat flux at the beginning of spin-up leads to a great warm- ing in the eastern equatorial Pacific Ocean. The weak vertical advection due to a too thick mixed layer in the coarse vertical structure also accounts for the warm bias. The fact that most significant improvements of the upper 50 m temperature appear at the region of the thinnest mixed layer indicates the necessity of fine vertical resolution for the eastern equatorial Pacific Ocean. However, the westward extension of equatorial cold tongue, a defect in the original scheme, gets even more serious in the adjusting scheme due to the intensi- fied vertical velocity and hence vertical advection in the central-eastern equatorial Pacific Ocean.

Seasonal Forecast of South China Sea Summer Monsoon Onset Disturbed by Cold Tongue La Niña in the Past Decade

It has been suggested that a warm(cold)ENSO event in winter is mostly followed by a late(early)onset of the South China Sea(SCS)summer monsoon(SCSSM)in spring.Our results show this positive relationship,which is mainly determined by their phase correlation,has been broken under recent rapid global warming since 2011,due to the disturbance of cold tongue(CT)La Niña events.Different from its canonical counterpart,a CT La Niña event is characterized by surface meridional wind divergences in the central-eastern equatorial Pacific,which can delay the SCSSM onset by enhanced convections in the warming Indian Ocean and the western subtropical Pacific.Owing to the increased Indian−western Pacific warming and the prevalent CT La Niña events,empirical seasonal forecasting of SCSSM onset based on ENSO may be challenged in the future.

Impacts of an Improved Low-Level Cloud Scheme on the Eastern Pacific ITCZ-Cold Tongue Complex

A statistically-based low-level cloud parameterization scheme is introduced, modified, and applied in the Flexible coupled General Circulation Model (FGCM-O). It is found that the low-level cloud scheme makes improved simulations of low-level cloud fractions and net surface shortwave radiation fluxes in the subtropical eastern oceans off western coasts in the model. Accompanying the improvement in the net surface shortwave radiation fluxes, the simulated distribution of SSTs is more reasonably asymmetrical about the equator in the tropical eastern Pacific, which suppresses, to some extent, the development of the double ITCZ in the model. Warm SST biases in the ITCZ north of the equator are more realistically reduced, too. But the equatorial cold tongue is strengthened and extends further westward, which reduces the precipitation rate in the western equatorial Pacific but increases it in the ITCZ north of the equator in the far eastern Pacific. It is demonstrated that the low-level cloud-radiation feedback would enhance the cooperative feedback between the equatorial cold tongue and the ITCZ. Based on surface layer heat budget analyses, it is demonstrated that the reduction of SSTs is attributed to both the thermodynamic cooling process modified by the increase of cloud fractions and the oceanic dynamical cooling processes associated with the strengthened surface wind in the eastern equatorial Pacific, but it is mainly attributed to oceanic dynamical cooling processes associated with the strengthening of surface wind in the central and western equatorial Pacific.

Onshore warm tongue and offshore cold tongue in the western Yellow Sea in winter:the evidence

A winter onshore warm tongue extending from the Yellow Sea Warm Current to the southern Jiangsu coast, and an of fshore cold tongue extending from the southern Jiangsu coast to the southwest of Jeju Island(South Korea), are newly identified based on the sea-surface temperature from satellite remote sensing, and further confirmed by the distribution of suspended sediments. In addition, there are two obvious thermal fronts associated with the onshore warm tongue and off shore cold tongue. The narrow gap between the two thermal fronts is supposed to be the pathway for the off shore transport of cold coastal water and suspended sediments. The concurrence of onshore warm and of fshore cold tongues suggests the concurrence of onshore and off shore currents in the western Yellow Sea in winter, which seems to be inconsistent with the previously accepted view that, in winter, the Yellow Sea Coastal Current flows from the Old Huanghe Delta to the southwest of Jeju Island. This distinctive phenomenon helps establish an updated view of the circulation in the western Yellow Sea in winter.

Spring Cold Bias of SST and Minimal Wind Mixing in the Equatorial Pacific Cold Tongue

The authors investigate the relationship between bias in simulated sea surface temperature （SST） in the equatorial eastern Pacific cold tongue during the boreal spring as simulated by an oceanic general circulation model （OGCM） and minimal wind mixing （MWM） at the surface. The cold bias of simulated SST is the greatest during the boreal spring, at approximately 3℃. A sensi- tivity experiment reducing MWM by one order of magnitude greatly alleviates cold biases, especially in March-April. The decrease in bias is primarily due to weakened vertical mixing, which preserves heat in the uppermost layer and results in warmer simulated SST. The reduction in vertical mixing also leads to a weak westward current in the upper layer, which further contributes to SST warming. These findings imply that there are large uncertainties about simple model parameters such as MWM at the oceanic surface.

The Cold Tongue in the South China Sea during Boreal Winter and Its Interaction with the Atmosphere

A distinct cold tongue has recently been noticed in the South China Sea during the winter monsoon, with the cold tongue temperature minimum occurring in the January or February. This cold tongue shows significant links with the Maritime Continent＇s rainfall during the winter period. The cold tongue and its interaction with the Maritime Continent＇s weather were studied using Reynolds SST data, wind fields from the NCEP-NCAR reanalysis dataset and the quikSCAT dataset. In addition, rainfall from the GOES Precipitation Index （GPI） for the periods 2000 to 2008 was also used. The propagation of the cold tongue towards the south is explained using wind dynamics and the western boundary current. During the period of strong cold tongue, the surface wind is strong and the western boundary current advects the cold tongue to the south. During the period of strong winds the zonal gradient of SST is high [0.5 °C （25 km）-1 ]. The cold tongue plays an important role in regulating the climate over the Maritime Continent. It creates a zonal/meridional SST gradient and this gradient ultimately leads in the formation of convection. Hence, two maximum precipitation zones are found in the Maritime Continent, with a zone of relatively lower precipitation between, which coincides with the cold tongue＇s regions. It was found that the precipitation zones have strong links with the intensity of the cold tongue. During stronger cold tongue periods the precipitation on either side of the cold tongue is considerably greater than during weaker cold tongue periods. The features of convection on the eastern and western sides of the cold tongue behave differently. On the eastern side convection is preceded by one day with SST gradient, while on the western side it is four days.

The tropical Pacific cold tongue mode and its associated main ocean dynamical process in CMIP5 models

The cold tongue mode(CTM),which represents the out-of-phase relationship in sea surface temperature anomaly(SSTA)variability between the Pacific cold tongue region and elsewhere in the tropical Pacific,shows a long-term cooling trend in the eastern equatorial Pacific.In this study,we investigate how well the CTM is reproduced in historical simulations generated by the 20 models considered in Phase 5 of the Coupled Model Intercomparison Project(CMIP5).Qualitatively,all 20 models roughly capture the cooling SSTA associated with the CTM.However,a quantitative assessment(i.e.,Taylor diagrams and the ratio of the trend between the simulations and observations)shows that only five of these 20 models(i.e.,CESM1-CAM5,CMCC-CM,FGOALS-g2,IPSL-CM5B-LR,and NorESM1-M)can reproduce with useful accuracy the spatial pattern and long-term trend of the CTM.We find that these five models generally simulate the main ocean dynamical process associated with the CTM.That is,these models adequately capture the long-term cooling trend in the vertical advection of the anomalous temperature by the mean upwelling.We conclude that the performance of these CMIP5 models,with respect to simulations of the long-term cooling trend associated with the vertical advection,and the related long-term decreasing trend of the vertical gradient of the oceanic temperature anomaly,can play an important role in successful reproduction of the CTM.

Weak ENSO Asymmetry Due to Weak Nonlinear Air–Sea Interaction in CMIP5 Climate Models

State-of-the-art climate models have long-standing intrinsic biases that limit their simulation and projection capabilities. Significantly weak ENSO asymmetry and weakly nonlinear air-sea interaction over the tropical Pacific was found in CMIP5 （Coupled Model Intercomparison Project, Phase 5） climate models compared with observation. The results suggest that a weak nonlinear air-sea interaction may play a role in the weak ENSO asymmetry. Moreover, a weak nonlinearity in air-sea interaction in the models may be associated with the biases in the mean climate--the cold biases in the equatorial central Pacific. The excessive cold tongue bias pushes the deep convection far west to the western Pacific warm pool region and suppresses its development in the central equatorial Pacific. The deep convection has difficulties in further moving to the eastern equatorial Pacific, especially during extreme El Nifio events, which confines the westerly wind anomaly to the western Pacific. This weakens the eastern Pacific El Nifio events, especially the extreme E1 Nifio events, and thus leads to the weakened ENSO asymmetry in climate models. An accurate mean state structure （especially a realistic cold tongue and deep convection） is critical to reproducing ENSO events in climate models. Our evaluation also revealed that ENSO statistics in CMIP5 climate models are slightly improved compared with those of CMIP3. The weak ENSO asymmetry in CMIP5 is closer to the observation. It is more evident in CMIP5 that strong ENSO activities are usually accompanied by strong ENSO asymmetry, and the diversity of ENSO amplitude is reduced.

Effect of Chlorophyll-a Spatial Distribution on Upper Ocean Temperature in the Central and Eastern Equatorial Pacific

Effect of the spatial distributions of chlorophyll-a concentration on upper ocean temperature and currents in the equatorial Pacific is investigated through a set of numerical experiments by using an ocean general circulation model. This study indicates that enhanced meridional gradient of chlorophyll-a between the equator and off-equatorial regions can strengthen zonal circulation and lead to a decrease in equatorial sea surface temperature （SST）. However, the circulation changes by themselves are not effective enough to affect SST in the equatorial cold tongue （CT） region. The comparison between the experiments indicates that the CT SST are more sensitive to chlorophyll-a distribution away from the equator. The off-equatorial chlorophyll-a traps more solar radiation in the mixed layer, therefore, the temperature in the thermoeline decreases. The cold water can then be transported to the equator by the meridional circulation within the mixed layer. Furthermore, the relation among CT SST, the surface heat flux, and the equatorial upwelling are discussed. The study implies the simulation biases of temperature on the equator are not only related to the local ocean dynamics but also related to some deficiency in simulating off-equatorial processes.

Physical structure and phytoplankton community off the eastern Hainan coast during summer 2015

Based on satellite remote sensing dataset and survey data during upwelling season of 2015,the spatial structures of phytoplankton biomass and community for the first time in the eastern Hainan upwelling(EHU)and its adjacent area,the eastern Leizhou Peninsula upwelling(ELPU)were illustrated.It is found that a significant cold tongue with high salinity and low temperature along the eastern Hainan coast driven by upwelling-favorable summer monsoon.The ELPU was relative weaker than the EHU because of its wide and gentle continental slope.Due to mixing by tides and waves,DO concentration with high value(>6.0 mg/L)were almost homogenous from surface to 30 m depth at the EHU.Beneath that,low DO water(<6.0 mg/L,anoxia)were pumped upward from bottom by the upwelling.The ELPU has worse DO condition compared with the EHU where bottom DO were lower than 3.5 mg/L owing to abundant DO consumption.The phytoplankton biomass reached maximal value about 1.5 mg/m3 at 30 m depth layer rather than surface layer at the EHU indicating the impact limit of upwelling on phytoplankton growth and DO distribution.Nourished by rich nutrient input,the phytoplankton biomass at the ELPU were much higher than the EHU where the maximal value can reach about 4.0 mg/m3.The phytoplankton biomass were reduced to about 0.2–0.3 mg/m3 at the offshore areas of the EHU and ELPU which were close to the value at open sea.At the inshore of the EHU,the phytoplankton community was dominated by diatom which accounted for about 50%of phytoplankton biomass.And prokaryotes(about 40%),green algae(about 20%)and prochlorococcus(about 20%)became main species at the offshore of the EHU.At the ELPU,diatom accounted for about 80%of phytoplankton biomass followed by green algae,indicating a different ecosystem at this region compared with the EHU.

NUMERICAL STUDY ON THE ROLE OF VORTICAL AND DIVERGENT COMPONENTS OF WIND STRESS IN ENSO CYCLE

The 1960-1991 monthly mean FSU (Florida State University)wind stress data are decomposed into a vortical and a divergent component with each of which to force the model ocean in the context of a two-layer tropical Pacific model.Evidence suggests that for the seasonal variation the ocean forcing does not produce a realistic cold tongue using either of the components and the tongue will not be effectively improved in its intensity and pattern even if the components are doubled or halved:the utilization of climatic mean wind stress(no decomposition is done of the wind stress)that contains its seasonal variation will lead to a realistic SST distribution on which is imposed,separately,the interannual anomalies of each of the components so as to get the SSTA pattern:under the action of the interannual anomaly of the vortical(divergent)component there arises qnite intense SSTA oscillation marked by noticeable ENSO periods(feeble SSTA with higher oscillation frequency for obscure ENSO periods),thereby illustrating that the roles of the two components differ from each other in the genesis of SST variation on a seasonal and an interannual basis such that a realistic cold tongue pattern follows under the joint effects on the model ocean of the two components of wind stress while rational E1 Nino/La Nina phenomena result under the forcing of an anomalous wind stress vortical component.Moreover,the divergent component is innegligible in generating a mean climatic condition of the ocean sector but of less importance compared to the vortical component in ENSO development.