NPZ response to eddy-induced upwelling in a Brazil Current ring: A theoretical approach

This work explores the effect of a cyclonic mesoscale feature of the Brazil Current (BC) moving northward off SE Brazil (20° S–23° S) in the Nutrients-Phytoplankton-Zooplankton (NPZ) dynamics. We employ the contour dynamics technique in a quasi-geostrophic, inviscid, 1½-layer model set as a meridional jet flowing southward along a rigid western boundary. This hydrodynamical model is coupled to a conventional NPZ model. We evaluate two distinct scenarios: (1) Plankton in a mixed layer (ML) with time-variable depth and no entrainment and; (2) Plankton in an ML held at a fixed depth, which allows entrainment. We perform simulations with different ML depths representing summer (42 m) and winter (86 m) for the region of interest. The presence of the western boundary allows the cyclonic ring to propagate northward due to image effect. The eddy motion produces upwelling (downwelling) in the leading (trailing) edge resulting in different responses in the two scenarios examined. In the variable ML depth simulations, Z is favored relative to P because there is no injection of N. On the other hand, in the fixed ML depth simulations, P is favored relative to Z due to dilution. All simulations show P enrichment within the eddy, but winter simulations show a greater response to the physical forcing. We conclude that the cyclonic eddy shed from BC provides favorable conditions for planktonic growth. Winter-like conditions seem to be more favorable than summer-like ones and, therefore, present more intense blooms associated with the ring.     

El Ni?o事件发生和消亡中热带太平洋纬向风应力的动力作用 I. 资料诊断和理论分析

通过资料分析,研究了发生在热带西太平洋海表面西风或东风应力异常与El Ni?o事件的关系。分析结果表明,对应着El Ni?o事件从发生到消亡的过程,热带西太平洋纬向风应力存在着从西风应力异常到东风应力异常的变化,并且在这个过程中,西风应力异常向东传,东风应力异常紧接其后也向东传。本文还根据观测资料的分析结果建立了理想风应力,并利用简单热带海洋模式,对热带西太平洋纬向风应力异常及其东传在ENSO循环中的作用进行了动力学分析,指出了它们在El Ni?o事件发生和消亡中起着重要的作用。西风应力异常通过激发出海洋中东传的暖Kelvin波及其在大洋东边界反射产生的暖Rossby波、以及西风应力异常本身东传到赤道东太平洋,引起正的海洋混合层扰动厚度异常,导致了El Ni?o事件的发生;而异常东风应力则通过激发出东传的冷Kelvin波及其在大洋东边界反射产生的冷Rossby波、以及东风应力异常本身东传到赤道东太平洋,引起负的海洋混合层扰动厚度异常,导致了El Ni?o事件的消亡。对于热带西太平洋上风应力异常的形式是东部为异常西风应力而其西部为异常东风应力,并且它们同时向东传时,则大洋东部混合层厚度对异常风应力的响应随异常东风和西风应力的强度不同而不同,它们强度的相对大小对El Ni?o的持续时间具有重要的作用。     

Simulation of the response of the North Pacific Ocean to the anomalous atmospheric circulation associated with El Niño

During El Niño events when positive sea surface temperature (SST) anomalies form in the equatorial Pacific, SST anomalies also tend to develop in the North Pacific. This study attempts to model and explain the large-scale features of the observed SST anomaly field in the North Pacific during the fall and winter of the El Niño year. The experiment design consists of a mixed layer ocean model of the North Pacific which is forced by atmospheric surface fields from two sets of Community Climate Model (CCM) integrations: the El Niño set with prescribed positive SST anomalies in the tropical Pacific; and the control set which is obtained from an extended CCM integration with prescribed climatological SSTs. The response of the midlatitude ocean to atmospheric surface fields associated with El Niño is obtained by compositing each set of model integrations (El Niño and Control) and then taking the difference between the composites. The ocean model is able to reproduce the general features of the observed midlatitude SST anomaly pattern: warm water in the northeast Pacific and an elliptically shaped cold pool in the central Pacific. In these regions, a large fraction of the temperature anomalies are significant at the 95% level as indicated by a two tailed t-test. The ocean temperature anomalies simulated by the model are primarily caused by changes in the sensible and latent heat flux and to a lesser extent the longwave radiation flux. Entrainment of cold water from below the mixed layer also influences ocean temperatures. However, the entrainment anomaly pattern has a complex spatial structure which does not always coincide with the simulated mixed layer temperature anomalies.This paper was presented at the International Conference on Modelling of Global Climate Change and Variability, held in Hamburg 11–15 September 1989 under the auspices of the Meteorological Institute of the University of Hamburg and the Max Planck Institute for Meteorology. Guest Editor for these papers is Dr. L. Dümenil     

The seasonal cycle of tropical Pacific sea surface temperature in a coupled GCM

 The mechanisms responsible for the seasonal cycle in the tropical central and eastern Pacific sea surface temperature (SST) are investigated using a coupled general circulation model. We find that the annual westward propagation of SST anomalies along the equator is explained by a two-stage process. The first stage sets the phase of the variation at the eastern boundary. The strengthening of the local Hadley Circulation in boreal summer leads to a strengthening of the northward winds that blow across the equator. These stronger winds drive enhanced evaporation and entrainment cooling of the oceanic mixed layer. The resulting change in SST is greatest in the east because the mixed layer is at its shallowest there. As the east Pacific SST cools the zonal SST gradient in the central Pacific becomes more negative. This development signals the onset of the second stage in the seasonal variation of equatorial SST. In response to the anomalous SST gradient the local westward wind stress increases. This increase drives cooling of the oceanic mixed layer in which no single mechanism dominates: enhanced evaporation, wind-driven entrainment, and westward advection all contribute. We discuss the role that equatorial upwelling plays in modulating mixed layer depth and hence the entrainment cooling, and we highlight the importance of seasonal variations in mixed layer depth. In sum these processes act to propagate the SST anomaly westward. Received: 22 February 1999 / Accepted: 20 March 2000     

Influence of the oceanic biology on the tropical Pacific climate in a coupled general circulation model

The influence of chlorophyll spatial patterns and variability on the tropical Pacific climate is investigated by using a fully coupled general circulation model (HadOPA) coupled to a state-of-the-art biogeochemical model (PISCES). The simulated chlorophyll concentrations can feedback onto the ocean by modifying the vertical distribution of radiant heating. This fully interactive biological-ocean-atmosphere experiment is compared to a reference experiment that uses a constant chlorophyll concentration (0.06 mg m⁻³). It is shown that introducing an interactive biology acts to warm the surface eastern equatorial Pacific by about 0.5°C. Two competing processes are involved in generating this warming: (a) a direct 1-D biological warming process in the top layers (0–30 m) resulting from strong chlorophyll concentrations in the upwelling region and enhanced by positive dynamical feedbacks (weaker trade winds, surface currents and upwelling) and (b) a 2-D meridional cooling process which brings cold off-equatorial anomalies from the subsurface into the equatorial mixed layer through the meridional cells. Sensitivity experiments show that the climatological horizontal structure of the chlorophyll field in the upper layers is crucial to maintain the eastern Pacific warming. Concerning the variability, introducing an interactive biology slightly reduces the strength of the seasonal cycle, with stronger SST warming and chlorophyll concentrations during the upwelling season. In addition, ENSO amplitude is slightly increased. Similar experiments performed with another coupled general circulation model (IPSL-CM4) exhibit the same behaviour as in HadOPA, hence showing the robustness of the results.     

Summer Persistence Barrier of Sea Surface Temperature Anomalies in the Central Western North Pacific

The persistence barrier of sea surface temperature anomalies(SSTAs) in the North Pacific was investigated and compared with the ENSO spring persistence barrier.The results show that SSTAs in the central western North Pacific(CWNP) have a persistence barrier in summer:the persistence of SSTAs in the CWNP shows a significant decline in summer regardless of the starting month.Mechanisms of the summer persistence barrier in the CWNP are different from those of the spring persistence barrier of SSTAs in the central and eastern equatorial Pacific.The phase locking of SSTAs to the annual cycle does not explain the CWNP summer persistence barrier.Remote ENSO forcing has little linear influence on the CWNP summer persistence barrier,compared with local upper-ocean process and atmospheric forcing in the North Pacific.Starting in wintertime,SSTAs extend down to the deep winter mixed layer then become sequestered beneath the shallow summer mixed layer,which is decoupled from the surface layer.Thus,wintertime SSTAs do not persist through the following summer.Starting in summertime,persistence of summer SSTAs until autumn can be explained by the atmospheric forcing through a positive SSTAs-cloud/radiation feedback mechanism because the shallow summertime mixed layer is decoupled from the temperature anomalies at depth,then the following autumn-winter-spring,SSTAs persist.Thus,summer SSTAs in the CWNP have a long persistence,showing a significant decline in the following summer.In this way,SSTAs in the CWNP show a persistence barrier in summer regardless of the starting month.     

Simulating tropical instability waves in the equatorial eastern Pacific with a coupled general circulation model

Satellite observations of SSTs have revealed the existence of unstable waves in the equatorial eastern Pacific and Atlantic oceans. These waves have a 20-40-day periodicity with westward phase speeds of 0.4-0.6 m s-1 and wavelengths of 1000-2000 km during boreal summer and fall. They are generally called tropical instability waves (TIWs). This study investigates TIWs simulated by a high-resolution coupled atmosphere-ocean general circulation model (AOGCM). The horizontal resolution of the model is 120 km in...     

Sensitivity of the Northern Hemisphere climate of the Last Glacial Maximum to sea surface temperatures

The role of prescribing sea surface temperature in paleoclimate atmospheric simulations has been investigated by comparing Last Glacial Maximum AGCMs experiments using different SSTs data sets as well as coupled atmosphere/oceanic mixed layer models. Changes in the SSTs and sea-ice margin generate different patterns of zonal asymmetries in the atmospheric circulation that are responsible for reorganisation of heat and moisture transport, leading to important variations of Northern Hemisphere regional climates, particularly in winter. Additional sensitivity experiments have been carried out to isolate the individual role of North Pacific and North Atlantic SSTs anomalies. We found that changes in North Pacific SSTs have a much stronger impact over all the northern continental surfaces, including Europe and Siberia, than changes in the North Atlantic SSTs. As these SSTs anomalies are of the order of the typical errors generated by coupled ocean-atmosphere models, this suggests that these more complete models will likely still have problems in simulating the regional climate change at the LGM. Received: 11 October 1999 / Accepted: 9 June 2000     

青藏高原和落基山脉对ENSO影响的比较研究

本文利用耦合气候模式研究了“有/无”青藏高原和落基山脉对厄尔尼诺—南方涛动（ENSO）的影响,并从温度变率方程的角度详细分析了ENSO变化的成因,结果表明:移除青藏高原或落基山脉均会造成ENSO变率增强;ENSO变率在无青藏高原试验中增强的幅度比在无落基山脉试验中更大。ENSO变率在地形敏感性试验中的变化与热带太平洋平均气候态的改变密切相关。移除青藏高原后热带太平洋信风减弱,大气对流中心东移,混合层变浅,温跃层变平,呈现出El Ni?o型海温分布,这些平均态的变化使海表风应力敏感性,Ekman抽吸敏感性以及温跃层敏感性幅度增强,最终导致ENSO振幅增大60%。然而,在移除落基山脉的情景下,热带太平洋信风变化更加复杂,大气对流中心稍有东移,混合层加深,温跃层变平,呈现出类La Ni?a型海温分布。这些变化增强了风应力敏感性和温跃层敏感性,最终导致ENSO振幅仅增大15%左右。本文研究表明,在地质时间尺度上青藏高原和落基山脉的抬升均抑制了ENSO变率。     

Impact of Earth’s orbit and freshwater fluxes on Holocene climate mean seasonal cycle and ENSO characteristics

We use a state-of-the-art 3-dimensional coupled model to investigate the relative impact of long term variations in the Holocene insolation forcing and of a freshwater release in the North Atlantic. We show that insolation has a greater effect on seasonality and La Ni?a events and is the major driver of sea surface temperature changes. In contrast, the variations in precipitation reflect changes in El Ni?o events. The impact of ice-sheet melting may have offset the impact of insolation on El Ni?o Southern Oscillation variability at the beginning of the Holocene. These simulations provide a coherent framework to refine the interpretation of proxy data and show that changes in seasonality may bias the projection of relationships established between proxy indicators and climate variations in the east Pacific from present day records.     

Recent climate variation in the Bering and Chukchi Seas and its linkages to large-scale circulation in the Pacific

The thermal state of the Bering Sea exhibits interdecadal variations, with distinct changes occurred in 1997–1998. After the unusual thermal condition of the Bering Sea in 1997–1998, we found that the recent climate variability (1999–2010) in the Bering Sea is closely related to Pacific basin-scale atmospheric and oceanic circulation patterns. Specifically, warming in the Bering and Chukchi Seas in this period involves sea ice reduction and stronger oceanic heat flux to the atmosphere in winter. The atmospheric response to the recent warming in the Bering and Chukchi Seas resembles the North Pacific Oscillation (NPO) pattern. Further analysis reveals that the recent climate variability in the Bering and Chukchi Seas has strong covariability with large-scale climate modes in the Pacific, that is, the North Pacific Gyre Oscillation and the central Pacific El Niño. In this study, physical connections among the recent climate variations in the Bering and Chukchi Seas, the NPO pattern and the Pacific large-scale climate patterns are investigated via cyclostationary empirical orthogonal function analysis. An additional model experiment using the National Center for Atmospheric Research Community Atmospheric Model, version 3, is conducted to support the robustness of the results.     

Role of the Atlantic multidecadal variability in modulating East Asian climate

We assess the effects of the North Atlantic Ocean Sea Surface Temperature (NASST) on North East Asian (NEA) surface temperature. We use a set of sensitivity experiments, performed with MetUM-GOML2, an atmospheric general circulation model coupled to a multi-level ocean mixed layer model, to mimic warming and cooling over the North Atlantic Ocean. Results show that a warming of the NASST is associated with a significant warming over NEA. Two mechanisms are pointed out to explain the NASST—North East Asia surface temperature relationship. First, the warming of the NASST is associated with a modulation of the northern hemisphere circulation, due to the propagation of a Rossby wave (i.e. the circumglobal teleconnection). The change in the atmosphere circulation is associated with advections of heat from the Pacific Ocean to NEA and with an increase in net surface shortwave radiation over NEA, both acting to increase NEA surface temperature. Second, the warming of the NASST is associated with a cooling (warming) over the eastern (western) Pacific Ocean, which modulates the circulation over the western Pacific Ocean and NEA. Additional simulations, in which Pacific Ocean sea surface temperatures are kept constant, show that the modulation of the circumglobal teleconnection is key to explaining impacts of the NASST on NEA surface temperature.

     

Low-frequency modulation of the Atlantic warm pool by the Atlantic multidecadal oscillation

This paper investigates the low-frequency modulation of the Atlantic warm pool (AWP) by the Atlantic multidecadal oscillation (AMO). Consistent with previous study, it shows that the time series of AWP area varies in phase with the AMO on multidecadal timescales. However, the variability of AWP area is out of phase with the AMO: A small (large) variance of AWP area is associated with the AMO warm (cold) phase. In addition, the modulation of AWP area variability by the AMO has a large seasonality, with a small (large) modulation in summer (fall). The modulation of the annual AWP area variability is primarily determined by the low frequency changes in the Pacific ENSO and the local heat flux feedback, and countered by the low frequency changes in the North Atlantic Oscillation and the ocean mixed layer depth. The local heat flux feedback and mixed layer depth change also play important roles in the AMO-modulated seasonality of the AWP area variability.     

The climate regime shift over the Pacific during 1996/1997

A climate regime shift (CRS) in the Pacific sea surface temperature (SST) pattern was identified in 1996/1997. This decadal SST change is characterized by a warming over the equatorial western Pacific (EWP) and mid-latitude North and South Pacific and a cooling in the equatorial central Pacific (ECP). The large-scale atmospheric circulation change associated with this CRS exhibits a pair of low-level anticyclonic (cyclonic) gyres off the EWP (ECP) and a zonal-vertical overturning circulation anomaly along the equator. Both the empirical orthogonal function and singular vector decomposition analyses indicate that the CRS signal in 1996/1997 is robust. A mixed layer heat budget analysis suggests that the abrupt change of SST in the EWP and ECP is attributed to different physical processes. The abrupt warming over the EWP was initiated by a short wave radiation (SWR) anomaly in association with a preceding warming in the ECP. The cooling in the ECP happened about 6 months later than that of the EWP and was primarily attributed to anomalous oceanic zonal and vertical temperature advections.     

Coupled atmosphere�Cmixed layer ocean response to ocean heat flux convergence along the Kuroshio Current Extension

The winter response of the coupled atmosphere?Cocean mixed layer system to anomalous geostrophic ocean heat flux convergence in the Kuroshio Extension is investigated by means of experiments with an atmospheric general circulation model coupled to an entraining ocean mixed layer model in the extra-tropics. The direct response consists of positive SST anomalies along the Kuroshio Extension and a baroclinic (low-level trough and upper-level ridge) circulation anomaly over the North Pacific. The low-level component of this atmospheric circulation response is weaker in the case without coupling to an extratropical ocean mixed layer, especially in late winter. The inclusion of an interactive mixed layer in the tropics modifies the direct coupled atmospheric response due to a northward displacement of the Pacific Inter-Tropical Convergence Zone which drives an equivalent barotropic anomalous ridge over the North Pacific. Although the tropically driven component of the North Pacific atmospheric circulation response is comparable to the direct response in terms of sea level pressure amplitude, it is less important in terms of wind stress curl amplitude due to the mitigating effect of the relatively broad spatial scale of the tropically forced atmospheric teleconnection.     

太平洋区域季内振荡的一种负反馈过程

利用1981—2002年美国国家气象中心（National Meteorological Center,NMC）逐日海表温度（sea surface temperature,SST）、10 m高处风场（V）及逐月混合层厚度（mixed layer depth,mld）资料,研究了太平洋区域海表温度季内振荡的气候及异常特征,重点探讨了北太平洋区域海表温度季内振荡的维持机制。研究发现,太平洋区域海表温度存在3个季内振荡强度气候高值区,即热带东太平洋（终年存在）、西北太平洋（北半球春、夏、秋存在）、西南太平洋（南半球夏季前后存在）,它们出现在气候混合层厚度最小的区域和季节。海表温度季内振荡强度年际异常与混合层厚度年际异常存在显著负相关,在物理上,这种关系比它与海表温度异常的关系更直接。北太平洋区域5—9月地面风场与海表温度季节内振荡的基本耦合模态揭示出以漂流和感热输送为动力的一个负反馈过程,它存在于薄混合层海区,这是该海区强海表温度季内振荡的维持机制。     

Evidence bearing on the mechanism of rapid deglaciation

Six northeast Atlantic cores contain planktonic foraminiferal records implying a very abrupt glacial/interglacial surface-ocean warming roughly coincident with the last deglaciation (isotopic termination II) at 127 000 yr B.P. These faunal composition curves have, however, been substantially altered by sediment mixing processes on the sea floor; they are translated downward in the core record and made to look steeper than they actually were. The reason for this abnormally large mixing impact is an interval of sediment with very low to negligible concentrations of all microfossils (surface ocean and bottom living). These low concentrations reflect a several-thousand-year interval of low productivity and little or no life in the overlying surface waters. We interpret this thorough suppression of productivity as a consequence of meltwater and icebergs flooding into the subpolar Atlantic gyre from the surrounding Northern Hemisphere ice sheets during deglaciation. The meltwater influx inhibited warm-season productivity by maintaining a well-stratified low-salinity surface layer; in winter, the low salinity layer froze, stopping nutrientrich deep waters from surfacing in normal cold-season convection. The earth's orbital configuration during this deglaciation created an unusually strong summer insolation maximum and winter insolation minimum in the Northern Hemisphere. Rapid melting and disintegration of the Northern Hemisphere ice sheets induced by strong summer insolation apparently created the meltwater influx; combined with very low winter insolation, the presence of this low-salinity meltwater layer led to unusually extensive sea-ice formation. The existence of a large region of winter sea ice across the subpolar North Atlantic during deglaciation implies a reduced supply of moisture in winter to the wasting Northern Hemisphere ice sheets. This includes the loss of winter moisture both locally from ice-covered northern waters and regionally from low-latitude winter storms no longer penetrating northward. The winter sea-ice cover thus acts as an amplifier providing positive feedback to the insolation-driven deglaciation process.     

On the predictability of decadal changes in the North Pacific

 The predictability of decadal changes in the North Pacific is investigated with an ocean general circulation model forced by simplified and realistic atmospheric conditions. First, the model is forced by a spatially fixed wind stress anomaly pattern characteristic for decadal North Pacific climate variations. The time evolution of the wind stress anomaly is chosen to be sinusoidal, with a period of 20 years. In this experiment different physical processes are found to be important for the decadal variations: baroclinic Rossby waves dominate the response. They move westward and lead to an adjustment of the subtropical and subpolar gyre circulations in such a way that anomalous temperatures in the central North Pacific develop as a delayed response to the preceding wind stress anomalies. This delayed response provides not only a negative feedback but also bears the potential for long-term predictions of upper ocean temperature changes in the central North Pacific. It is shown by additional experiments that once these Rossby waves have been excited, decadal changes of the upper ocean temperatures in the central North Pacific evolve without any further anomalous atmospheric forcing. In the second part, the model is forced by surface heat flux and wind stress observations for the period 1949–1993. It is shown that the same physical processes which were found to be important in the simplified experiments also govern the evolution of the upper ocean in this more realistic simulation. The 1976/77 cooling can be mainly attributed to anomalously strong horizontal advection due to the delayed response to persistent wind stress curl anomalies in the early 1970s rather than local anomalous atmospheric forcing. This decadal change could have been predicted some years in advance. The subsequent warming in the late 1980s, however, cannot be mainly explained by advection. In this case, local anomalous atmospheric forcing needs to be considered. Received: 6 July 1998 / Accepted: 16 October 1999     

Sensitivity of the tropical Pacific to a change of orbital forcing in two versions of a coupled GCM

 The changes of the variability of the tropical Pacific ocean forced by a shift of six months in the date of the perihelion are studied using a coupled tropical Pacific ocean/global atmosphere GCM. The sensitivity experiments are conducted with two versions of the atmospheric model, varied by two parametrization changes. The first one concerns the interpolation scheme between the atmosphere and ocean models grids near the coasts, the second one the advection of water vapor in the presence of downstream negative temperature gradients, as encountered in the vicinity of mountains. In the tropical Pacific region, the parametrization differences only have a significant direct effect near the coasts; but coupled feedbacks lead to a 1 °C warming of the equatorial cold tongue in the modified (version 2) model, and a widening of the western Pacific large-scale convergence area. The sensitivity of the seasonal cycle of equatorial SST is very different between the two experiments. In both cases, the response to the solar flux forcing is strongly modified by coupled interactions between the SST, wind stress response and ocean dynamics. In the first version, the main feedback is due to anomalous upwelling and leads to westward propagation of SST anomalies; whereas the version 2 model is dominated by an eastward-propagating thermocline mode. The main reason diagnosed for these different behaviors is the atmospheric response to SST anomalies. In the warmer climate simulated by the second version, the wind stress response in the western Pacific is enhanced, and the off-equatorial curl is reduced, both effects favoring eastward propagation through thermocline depth anomalies. The modifications of the simulated seasonal cycle in version 2 lead to a change in ENSO behavior. In the control climate, the interannual variability in the eastern Pacific is dominated by warm events, whereas cold events tend to be the more extreme ones with a shifted perihelion. Received: 14 December 1999 / Accepted: 24 May 2000     

Projected changes in the tropical Pacific Ocean of importance to tuna fisheries

Future physical and chemical changes to the ocean are likely to significantly affect the distribution and productivity of many marine species. Tuna are of particular importance in the tropical Pacific, as they contribute significantly to the livelihoods, food and economic security of island states. Changes in water properties and circulation will impact on tuna larval dispersal, preferred habitat distributions and the trophic systems that support tuna populations throughout the region. Using recent observations and ocean projections from the CMIP3 and preliminary results from CMIP5 climate models, we document the projected changes to ocean temperature, salinity, stratification and circulation most relevant to distributions of tuna. Under a business-as-usual emission scenario, projections indicate a surface intensified warming in the upper 400 m and a large expansion of the western Pacific Warm Pool, with most surface waters of the central and western equatorial Pacific reaching temperatures warmer than 29 °C by 2100. These changes are likely to alter the preferred habitat of tuna, based on present-day thermal tolerances, and in turn the distribution of spawning and foraging grounds. Large-scale shoaling of the mixed layer and increases in stratification are expected to impact nutrient provision to the biologically active layer, with flow-on trophic effects on the micronekton. Several oceanic currents are projected to change, including a strengthened upper equatorial undercurrent, which could modify the supply of bioavailable iron to the eastern Pacific.