Sensitivity of the tropical Pacific Ocean to precipitation-induced freshwater flux

We have performed experiments using an ocean model to study the sensitivity of tropical Pacific Ocean to variations in precipitation induced freshwater fluxes. Variations in these fluxes arise from natural causes on all time scales. In addition, estimates of these fluxes are uncertain because of differences among measurement techniques. The model used is a quasi-isopycnal model, covering the Pacific from 40?°S to 40?°N. The surface forcing is constructed from observed wind stress, evaporation, precipitation, and sea surface temperature (SST) fields. The heat flux is produced with an iterative technique so as to maintain the model close to the observed climatology, but with only a weak damping to that climatology. Climatological estimates of evaporation are combined with various estimates of precipitation to determine the net surface freshwater flux. Results indicate that increased freshwater input decreases salinity as expected, but increases temperatures in the upper ocean. Using the freshwater flux estimated from the Microwave Sounding Unit leads to a warming of up to 0.6?°C in the western Pacific over?a case with zero net freshwater flux. SST is sensitive to the discrepancies among different precipitation observations, with root-mean-square differences in SST on the order of 0.2–0.3?°C. The change in SST is more pronounced in the eastern Pacific, with difference of over 1?°C found among the various precipitation products. Interannual variation in precipitation during El Niño events leads to increased warming. During the winter of 1982–83, freshwater flux accounts for about 0.4?°C (approximately 10–15% of the maximum warming) of the surface warming in the central-eastern Pacific. Thus, the error of SST caused by the discrepancies in precipitation products is more than half of the SST anomaly produced by the interannual variability of observed precipitation. Further experiments, in which freshwater flux anomalies are imposed in the western, central, and eastern Pacific, show that the influence of net freshwater flux is also spatially dependent. The imposition of freshwater flux in the far western Pacific leads to a trapping of salinity anomalies to the surface layers near the equator. An identical flux imposed in the central Pacific produces deeper and off-equatorial salinity anomalies. The contrast between these two simulations is consistent with other simulations of the western Pacific barrier layer formation.     

Projected changes in tropical cyclone climatology and landfall in the Southwest Indian Ocean region under enhanced anthropogenic forcing

The conformal-cubic atmospheric model, a variable-resolution global model, is applied at high spatial resolution to perform simulations of present-day and future climate over southern Africa and over the Southwest Indian Ocean. The model is forced with the bias-corrected sea-surface temperatures and sea-ice of six coupled global climate models that contributed to Assessment Report 4 of the Intergovernmental Panel on Climate Change. All six simulations are for the period 1961–2100, under the A2 emission scenario. Projections for the latter part of the 21st century indicate a decrease in the occurrence of tropical cyclones over the Southwest Indian Ocean adjacent to southern Africa, as well as a northward shift in the preferred landfall position of these systems over the southern African subcontinent. A concurrent increase in January to March rainfall is projected for northern Mozambique and southern Tanzania, with decreases projected further south over semi-arid areas such as the Limpopo River Basin where these systems make an important contribution as main cause of widespread heavy rainfall. It is shown that the projected changes in tropical cyclone attributes and regional rainfall occur in relation to changes in larger scale atmospheric temperature, pressure and wind profiles of the southern African region and adjacent oceans.     

Modelling the impact of climate change on Pacific skipjack tuna population and fisheries

IPCC-type climate models have produced simulations of the oceanic environment that can be used to drive models of upper trophic levels to explore the impact of climate change on marine resources. We use the Spatial Ecosystem And Population Dynamics Model (SEAPODYM) to investigate the potential impact of Climate change under IPCC A2 scenario on Pacific skipjack tuna (Katsuwonus pelamis). IPCC-type models are still coarse in resolution and can produce significant anomalies, e.g., in water temperature. These limitations have direct and strong effects when modeling the dynamics of marine species. Therefore, parameter estimation experiments based on assimilation of historical fishing data are necessary to calibrate the model to these conditions before exploring the future scenarios. A new simulation based on corrected temperature fields of the A2 simulation from one climate model (IPSL-CM4) is presented. The corrected fields led to a new parameterization close to the one achieved with more realistic environment from an ocean reanalysis and satellite-derived primary production. Projected changes in skipjack population under simple fishing effort scenarios are presented. The skipjack catch and biomass is predicted to slightly increase in the Western Central Pacific Ocean until 2050 then the biomass stabilizes and starts to decrease after 2060 while the catch reaches a plateau. Both feeding and spawning habitat become progressively more favourable in the eastern Pacific Ocean and also extend to higher latitudes, while the western equatorial warm pool is predicted to become less favorable for skipjack spawning.     

热带太平洋地区海气系统的耦合振荡

本文讨论了东太平洋赤道海温和太平洋月平均云量距平的关系,结果表明:(1)东太平洋赤道海温距平和中太平洋赤道云量距平有很好的正相关,而与东、西太平洋赤道云量距平有很好负相关。所以太平洋赤道上空应该存在二个距平的东西向环流。(2)云和海温存在周期为34—38个月的耦合振荡,我们提出了云-辐射-海温机制来作解释。(3)在东太平洋海温暖水月的前12个月到后6个月期间,东太平洋赤道云量是负距平的(即偏少),这表明此期间可能是大气在影响海洋,亦即Walker环流影响海温变化,而不是海温影响了Walker 环流。海温和Walker环流是相互作用的,这种相互作用组成了大气和海洋之间的一种耦合振荡。(4)东太平洋海温、中太平洋云和北半球中、西太平洋信风亦有很好的相关。     

Projected future changes in tropical cyclone-related wave climate in the North Atlantic

Tropical cyclones are a major hazard for numerous countries surrounding the tropical-to-subtropical North Atlantic sub-basin including the Caribbean Sea and Gulf of Mexico. Their intense winds, which can exceed 300 km h⁻¹, can cause serious damage, particularly along coastlines where the combined action of waves, currents and low atmospheric pressure leads to storm surge and coastal flooding. This work presents future projections of North Atlantic tropical cyclone-related wave climate. A new configuration of the ARPEGE-Climat global atmospheric model on a stretched grid reaching ~ 14 km resolution to the north-east of the eastern Caribbean is able to reproduce the distribution of tropical cyclone winds, including Category 5 hurricanes. Historical (1984–2013, 5 members) and future (2051–2080, 5 members) simulations with the IPCC RCP8.5 scenario are used to drive the MFWAM (Météo-France Wave Action Model) spectral wave model over the Atlantic basin during the hurricane season. An intermediate 50-km resolution grid is used to propagate mid-latitude swells into a higher 10-km resolution grid over the tropical cyclone main development region. Wave model performance is evaluated over the historical period with the ERA5 reanalysis and satellite altimetry data. Future projections exhibit a modest but widespread reduction in seasonal mean wave heights in response to weakening subtropical anticyclone, yet marked increases in tropical cyclone-related wind sea and extreme wave heights within a large region extending from the African coasts to the North American continent.

     

Modeling the tropical Pacific Ocean using a regional coupled climate model

A high-resolution tropical Pacific general circulation model (GCM) coupled to a global atmospheric GCM is described in this paper. The atmosphere component is the 5°×4°global general circulation model of the Institute of Atmospheric Physics (IAP) with 9 levels in the vertical direction. The ocean component with a horizontal resolution of 0.5°, is based on a low-resolution model (2°×1°in longitude-latitude).Simulations of the ocean component are first compared with its previous version. Results show that the enhanced ocean horizontal resolution allows an improved ocean state to be simulated; this involves (1) an apparent decrease in errors in the tropical Pacific cold tongue region, which exists in many ocean models,(2) more realistic large-scale flows, and (3) an improved ability to simulate the interannual variability and a reduced root mean square error (RMSE) in a long time integration. In coupling these component models, a monthly "linear-regression" method is employed to correct the model's exchanged flux between the sea and the atmosphere. A 100-year integration conducted with the coupled GCM (CGCM) shows the effectiveness of such a method in reducing climate drift. Results from years 70 to 100 are described.The model produces a reasonably realistic annual cycle of equatorial SST. The large SSTA is confined to the eastern equatorial Pacific with little propagation. Irregular warm and cold events alternate with a broad spectrum of periods between 24 and 50 months, which is very realistic. But the simulated variability is weaker than the observed and is also asymmetric in the sense of the amplitude of the warm and cold events.     

Asymmetric distribution of convection in tropical cyclones over the western North Pacific Ocean

Forecasts of the intensity and quantitative precipitation of tropical cyclones(TCs) are generally inaccurate, because the strength and structure of a TC show a complicated spatiotemporal pattern and are affected by various factors. Among these, asymmetric convection plays an important role. This study investigates the asymmetric distribution of convection in TCs over the western North Pacific during the period 2005–2012, based on data obtained from the Feng Yun 2(FY2)geostationary satellite. The asymmetric distributions of the incidence, intensity and morphology of convections are analyzed.Results show that the PDFs of the convection occurrence curve to the azimuth are sinusoidal. The rear-left quadrant relative to TC motion shows the highest occurrence rate of convection, while the front-right quadrant has the lowest. In terms of intensity, weak convections are favored in the front-left of a TC at large distances, whereas strong convections are more likely to appear to the rear-right of a TC within a 300 km range. More than 70% of all MCSs examined here are elongated systems, and meso-β enlongated convective systems(MβECSs) are the most dominant type observed in the outer region of a TC. Smaller MCSs tend to be more concentrated near the center of a TC. While semi-circular MCSs [MβCCSs, MCCs(mesoscale convective complexes)] show a high incidence rate to the rear of a TC, elongated MCSs [MβECSs, PECSs(persistent elongated convective systems)] are more likely to appear in the rear-right quadrant of a TC within a range of 400 km.     

Effects of tropical instability wave (TIW)-induced surface wind feedback in the tropical Pacific Ocean

Tropical instability waves (TIWs) arise from oceanic instability in the eastern tropical Pacific and Atlantic Oceans, having a clear atmospheric signature that results in coupled atmosphere–ocean interactions at TIW scales. In this study, the extent to which TIW-induced surface wind feedback influences the ocean is examined using an ocean general circulation model (OGCM). The TIW-induced wind stress (τ_TIW) part is diagnostically determined using an empirical τ_TIW model from sea surface temperature (SST) fields simulated in the OGCM. The interactively represented TIW wind tends to reduce TIW activity in the ocean and influence the mean state, with largest impacts during TIW active periods in fall and winter. In December, the interactive τ_TIW forcing induces a surface cooling (an order of ?0.1 to ?0.3 °C), an increased heat flux into the ocean, a shallower mixed layer and a weakening of the South Equatorial Current in the eastern equatorial Pacific. Additionally, the TIW wind effect yields a pronounced latitudinal asymmetry of sea level field across the equator, and a change to upper thermal structure, characterized by a surface cooling and a warming below in the thermocline, leading to a decreased temperature gradient between the mixed layer and the thermocline. Processes responsible for the τ_TIW–induced cooling effects are analyzed. Vertical mixing and meridional advection are the two terms in the SST budget that are dominantly affected by the TIW wind feedback: the cooling effect from the vertical mixing on SST is enhanced, with the maximum induced cooling in winter; the warming effect from the meridional advection is reduced in July–October, but enhanced in November–December. Additional experiments are performed to separate the relative roles the affected surface momentum and heat fluxes play in the cooling effect on SST. This ocean-only modeling work indicates that the effect of TIW-induced wind feedback is small but not negligible, and may need to be adequately taken into account in large-scale climate modeling.     

Projected changes in the western North Pacific subtropical high under six global warming targets

西北太平洋副热带高压(简称西太副高)是影响东亚夏季天气气候的关键环流系统。本文利用CMIP5的历史气候模拟试验和RCP8.5路径下的未来气候变化预估试验数据,采用扰动位势高度,流函数等多种变量描述西太副高,分析了西太副高在6个全球变暖阈值(1.5℃, 2.0℃, 2.5℃,3.0℃, 3.5℃和4.0℃)下相对于当代气候的变化情况。在对流层中层(500 h Pa),西太副高在1.5℃阈值下几乎没有变化,而在2.0℃阈值下迅速减弱并东退约2.5°。当升温大于2.5℃时,西太副高呈线性减弱趋势,在4.0℃阈值下将东退约6.0°。在对流层低层(850 h Pa),西太副高在1.5℃阈值下增强西伸,但在升温到2.0℃的过程中变化不大。当变暖达到4.0℃阈值时,西太副高将西伸约2.0°。     

热带西太平洋边界层的多层结构分析

本文主要利用第一次“中美热带西太平洋海气相互作用联合调查”所获得的海洋水文、气象及大气探测资料,分析了热带西太平洋暖水区边界层大气层结特征,指出热带西太平洋边界层大气具有多层结构和多层送温现象,边界层内各层厚度及气温、位温、混合比随高度的变化趋势如下:超绝热层位于地面以上50m左右,气温、位温、混合比迅速减小,系绝热不稳定层;第一层为混合层,平均厚度为351m,位温、混合比变化不明显;混合层上又是逆温层大多为30—100m厚,位温迅速增大,混合比迅速减小,系绝对稳定层;700—1000m为浅对流云活跃层,     

On the climate response of the low-latitude Pacific Ocean to changes in the global freshwater cycle

Under global warming, the predicted intensification of the global freshwater cycle will modify the net freshwater flux at the ocean surface. Since the freshwater flux maintains ocean salinity structures, changes to the density-driven ocean circulation are likely. A modified ocean circulation could further alter the climate, potentially allowing rapid changes, as seen in the past. The relevant feedback mechanisms and timescales are poorly understood in detail, however, especially at low latitudes where the effects of salinity are relatively subtle. In an attempt to resolve some of these outstanding issues, we present an investigation of the climate response of the low-latitude Pacific region to changes in freshwater forcing. Initiated from the present-day thermohaline structure, a control run of a coupled ocean–atmosphere general circulation model is compared with a perturbation run in which the net freshwater flux is prescribed to be zero over the ocean. Such an extreme experiment helps to elucidate the general adjustment mechanisms and their timescales. The atmospheric greenhouse gas concentrations are held constant, and we restrict our attention to the adjustment of the upper 1,000 m of the Pacific Ocean between 40°N and 40°S, over 100 years. In the perturbation run, changes to the surface buoyancy, near-surface vertical mixing and mixed-layer depth are established within 1 year. Subsequently, relative to the control run, the surface of the low-latitude Pacific Ocean in the perturbation run warms by an average of 0.6°C, and the interior cools by up to 1.1°C, after a few decades. This vertical re-arrangement of the ocean heat content is shown to be achieved by a gradual shutdown of the heat flux due to isopycnal (i.e. along surfaces of constant density) mixing, the vertical component of which is downwards at low latitudes. This heat transfer depends crucially upon the existence of density-compensating temperature and salinity gradients on isopycnal surfaces. The timescale of the thermal changes in the perturbation run is therefore set by the timescale for the decay of isopycnal salinity gradients in response to the eliminated freshwater forcing, which we demonstrate to be around 10–20 years. Such isopycnal heat flux changes may play a role in the response of the low-latitude climate to a future accelerated freshwater cycle. Specifically, the mechanism appears to represent a weak negative sea surface temperature feedback, which we speculate might partially shield from view the anthropogenically-forced global warming signal at low latitudes. Furthermore, since the surface freshwater flux is shown to play a role in determining the ocean’s thermal structure, it follows that evaporation and/or precipitation biases in general circulation models are likely to cause sea surface temperature biases.     

Intraseasonal oscillation intensity over the western North Pacific:Projected changes under global warming

本文利用8个CMIP5模式的日资料,预估了RCP4.5和RCP8.5情景下全球增温达1.5℃和2.0℃时西北太平洋夏季30～60天和10～20天季节内振荡(ISO)强度的变化情况.大多数模式都认为,无论增温水平或情景如何,预估结果均显示从中南半岛南部到菲律宾以东的带状区域内ISO强度增加,并且关键气象要素背景的变化会对...     

Predictability of Northwest Pacific climate during summer and the role of the tropical Indian Ocean

A seasonal forecast system based on a global, fully coupled ocean?Catmosphere general circulation model is used to (1) evaluate the interannual predictability of the Northwest Pacific climate during June?CAugust following El Ni?o [JJA(1)], and (2) examine the contribution from the tropical Indian Ocean (TIO) variability. The model retrospective forecast for 1983?C2006 captures major modes of atmospheric variability over the Northwest Pacific during JJA(1), including a rise in sea level pressure (SLP), an anomalous anticyclone at the surface, and a reduction in subtropical rainfall, and increased rainfall to the northeast over East Asia. The anomaly correlation coefficient (ACC) for the leading principal components (PCs) of SLP and rainfall stays above 0.5 for lead time up to 3?C4?months. The predictability for zonal wind is slightly better. An additional experiment is performed by prescribing the SST climatology over the TIO. In this run, designated as NoTIO, the Northwest Pacific anticyclone during JJA(1) weakens considerably and reduces its westward extension. Without an interactive TIO, the ACC for PC prediction drops significantly. To diagnose the TIO effect on the circulation, the differences between the two runs (Control minus NoTIO) are analyzed. The diagnosis shows that El Nino causes the TIO SST to rise and to remain high until JJA(1). In response to the higher than usual SST, precipitation increases over the TIO and excites a warm atmospheric Kelvin wave, which propagates into the western Pacific along the equator. The decrease in equatorial SLP drives northeasterly wind anomalies, induces surface wind divergence, and suppresses convection over the subtropical Northwest Pacific. An anomalous anticyclone forms in the Northwest Pacific, and the intensified moisture transport on its northwest flank causes rainfall to increase over East Asia. In the NoTIO experiment, the Northwest Pacific anticyclone weakens but does not disappear. Other mechanisms for maintaining this anomalous circulation are discussed.     

Variability of tropical cyclones over the southwest Pacific Ocean using a high-resolution climate model

Summary Knowledge of the variability in tropical cyclone (TC) frequency and distribution is essential in determining the possible impact of natural or human-induced climate change. This variability can be investigated using the available TC data bases and by carrying out long-term climate model simulations for both past and future climates. A coupled ocean-atmosphere climate model (referred to here as the OU-CGCM) is described and applied with a higher resolution (50 km) nested domain in the southwest Pacific region. Six-member ensembles of simulations with the OU-CGCM have been run for 80 years, from 1970 to 2050. During the period 1970–2000, the OU-CGCM runs were compared with the observed TC data base. For the period 2000–2050, two ensembles of simulations were performed, one with constant greenhouse gas concentrations and the second with increasing greenhouse gases. The OU-CGCM simulated well the observed TC frequency and distribution in the southwest Pacific during the period 1970–2000. It also produced clear interannual and interdecadal TC variability in both the fixed and enhanced greenhouse gas simulations during the period 2000–2050. The variability in TC frequencies was associated with the typical atmospheric and SST anomaly patterns that occur in periods of quiet and active TC frequencies. The main findings from the enhanced greenhouse gas scenario for the period 2000–2050 are: no change in the mean decadal number of TCs relative to the control run, but a marked increase of about 15% in the mean decadal number of TCs in the most severe WMO categories 4 and 5; the likelihood of TCs during the next 50-year period that are more intense than ever previously experienced in the Australian region; a poleward extension of TC tracks; and a poleward shift of over 2 degrees of latitude in the TC genesis region.     

Arctic forcing of decadal variability in the tropical Pacific Ocean in a high-resolution global coupled GCM

The hypothesis that northern high-latitude atmospheric variability influences decadal variability in the tropical Pacific Ocean by modulating the wind jet blowing over the Gulf of Tehuantepec (GT) is examined using the high-resolution configuration of the MIROC 3.2 global coupled model. The model is shown to have acceptable skill in replicating the spatial pattern, strength, seasonality, and time scale of observed GT wind events. The decadal variability of the simulated GT winds in a 100-year control integration is driven by the Arctic Oscillation (AO). The regional impacts of the GT winds include strong sea surface cooling, increased salinity, and the generation of westward-propagating anticyclonic eddies, also consistent with observations. However, significant nonlocal effects also emerge in concert with the low-frequency variability of the GT winds, including anomalously low upper ocean heat content (OHC) in the central tropical Pacific Ocean. It is suggested that the mesoscale eddies generated by the wind stress curl signature of the GT winds, which propagate several thousand kilometers toward the central Pacific, contribute to this anomaly by strengthening the meridional overturning associated with the northern subtropical cell. A parallel mechanism for the decadal OHC variability is considered by examining the Ekman and Sverdrup transports inferred from the atmospheric circulation anomalies in the northern midlatitude Pacific directly associated with the AO.     

Roles of Indian and Pacific Ocean air–sea coupling in tropical atmospheric variability

Sea surface temperature (SST) variations include negative feedbacks from the atmosphere, whereas SST anomalies are specified in stand-alone atmospheric general circulation simulations. Is the SST forced response the same as the coupled response? In this study, the importance of air–sea coupling in the Indian and Pacific Oceans for tropical atmospheric variability is investigated through numerical experiments with a coupled atmosphere-ocean general circulation model. The local and remote impacts of the Indian and Pacific Ocean coupling are obtained by comparing a coupled simulation with an experiment in which the SST forcing from the coupled simulation is specified in either the Indian or the Pacific Ocean. It is found that the Indian Ocean coupling is critical for atmospheric variability over the Pacific Ocean. Without the Indian Ocean coupling, the rainfall and SST variations are completely different throughout most of the Pacific Ocean basin. Without the Pacific Ocean coupling, part of the rainfall and SST variations in the Indian Ocean are reproduced in the forced run. In regions of large mean rainfall where the atmospheric negative feedback is strong, such as the North Indian Ocean and the western North Pacific in boreal summer, the atmospheric variability is significantly enhanced when air–sea coupling is replaced by specified SST forcing. This enhancement is due to the lack of the negative feedback in the forced SST simulation. In these regions, erroneous atmospheric anomalies could be induced by specified SST anomalies derived from the coupled model. The ENSO variability is reduced by about 20% when the Indian Ocean air–sea coupling is replaced by specified SST forcing. This change is attributed to the interfering roles of the Indian Ocean SST and Indian monsoon in western and central equatorial Pacific surface wind variations.     

热带太平洋与印度洋相互作用的年代际变化及其数值模拟

利用全球海表温度资料和NCEP／NCAR再分析资料,发现热带印度洋偶极子事件与热带太平洋ENSO事件存在相互作用,但其相互作用关系在1961年前后发生了明显的跃变。通过CCM3（community climate model version3）模式,研究了不同年代热带太平洋和热带印度洋SST（seasur—face temperature）变化对其上空大气环流影响的变化,结果表明：1961年后,热带印度洋发生正偶极子事件时,两大洋的垂直环流异常的耦合很强,热带太平洋上空大气环流对印度洋偶极子事件的响应,给太平洋暖事件的异常发展提供了有利条件;同样,热带太平洋暖事件通过对热带印度洋上空大气环流的影响,给印度洋偶极子的异常发展提供了有利条件。