摘要:

In the equatorial Pacific, zonal displacements of the eastern edge of the warm pool represent an intrinsic manifestation of El Nino Southern Oscillation (ENSO) events, with numerous dynamical and biogeochemical consequences. Following a previous work dedicated to the 1986–1989 Geosat period, we focus on the 1992–1998 zonal displacements of the warm pool using mainly TOPEX/Poseidon data. We also used a simple linear model forced by monthly ERS winds to help in the interpretation of the results. We found that the 1992–1998 zonal displacements of the warm pool resulted mainly from horizontal advection by zonal current anomalies, through a combination of interannual equatorial Kelvin and first meridional mode Rossby waves. The interannual equatorial Kelvin waves were essentially wind forced in the western and central equatorial Pacific, with some minor contribution from reflected Rossby waves on the western Pacific boundary. In particular, westerly wind anomalies and the resulting downwelling Kelvin waves (entailing eastward surface current anomalies and thermocline deepening) contributed strongly to the onset of the 1993, 1994–1995 and 1997–1998 El Niño events. In contrast, easterly wind anomalies and the resulting upwelling Kelvin waves (with westward surface current anomalies and thermocline shoaling) played a role in stopping the 1993 El Niño and in shifting the 1994–1995 and 1997–1998 El Niño into La Niña events. Consistently with the 1987–1988 El Niño‐La Niña scenario, two main downwelling Rossby wave packets, originating from eastern boundary reflections and wind forcing, crossed the entire basin in 1993 and 1994–1995. These waves favored the decay of the corresponding El Niño events, in the sense that their associated current anomalies contributed to shifting the displacements of the eastern edge of the warm pool from eastward to westward. Unlike what happened for the termination of the 1993 and 1994–1995 El Niño events, downwelling Rossby wave packets, mostly reflected from impinging Kelvin waves, did not propagate all the way to the western Pacific during the 1997–1998 El Niño. They stopped propagating in the central basin where they met unfavorable eastward migrating westerly wind anomalies which generated upwelling Rossby waves. Hence reflected downwelling and wind‐forced upwelling Rossby waves opposed each other for shifting the eastern edge of the warm pool. The rapid demise of the 1997–1998 El Niño and its shift into La Niña in mid‐1998 are interpreted as resulting mainly from the effect of upwelling Kelvin waves forced by easterly wind anomalies occurring in the west from the end of 1997. The associated thermocline shoaling was further enhanced by the wind‐forced upwelling Rossby waves in the central basin in mid‐1998, strongly influencing the fast sea surface temperature (SST) cooling at times when the thermocline was very close to the surface at the end

ISSN:0148-0227    

DOI:10.1029/2000JC900113

年代:2000

数据来源: WILEY

摘要:

Sea surface temperature, sea level, and pseudo wind stress anomaly data from late 1996 to early 1998 are assimilated into an intermediate coupled model of the Tropical Pacific. Model data consistency is examined. Impact of the assimilation on forecast is evaluated. The ocean component of the coupled model consists of a shallow water model with two baroclinic modes, an Ekman shear layer, and a mixed layer temperature equation. The atmospheric model is a statistical one (based on dominant covariance of historical surface temperature and pseudo wind stress anomaly data). The adjoint method is used to fit the coupled model to 6 months of data by optimally adjusting the initial state and model parameters. A forecast is performed using the end state of an assimilation experiment as initial conditions and using parameters estimated during the assimilation period. Thus the model state during the assimilation and that during the forecast belong to the same model trajectory in different periods. Such an initialization procedure is useful in avoiding initial shock during forecast due to inconsistency of an initial state with the coupled model physics. As a result of optimal adjustments of initial state and parameters, the model is able to reproduce observed interannual variability of sea surface temperature and sea level reasonably well. The averaged residual model data misfits over various 6 month periods are 0.5°C and 5 cm, respectively. The model has a limited skill in reproducing much of the off‐equatorial wind anomalies. The residual model data misfit in pseudo wind stress anomaly is larger than 10 m2s−2. Forecasts initialized from the assimilation product are overall more realistic than those simply initialized from wind‐forced ocean states. Consistent improvement due to optimal initialization is found for sea surface temperature and sea level anomalies in the central‐eastern Pacific and zonal pseudo wind stress anomaly in the central Pacific, both in terms of root‐mean‐squared deviation from and correlation with the data. The adjustments of parameters in addition to initial state in a coupled context is found to be important to improving the model data consistency during the assimilation and the forecast. In particular, the estimated drag and damping coefficients properly regulate the relative strength of forcing and damping of the ocean state so as to fit the three types of observations during the assimilation (initialization) period, which facilitates the development of a large‐amplitude warming event during the forecast. The study demonstrates the utility of oceanic and atmospheric data to estimate initial state and model parameters in a coupled context, which is useful to the evaluation, improvement, and initialization of El Niño‐Southern Oscillatio

ISSN:0148-0227    

DOI:10.1029/2000JC900118

年代:2000

数据来源: WILEY

摘要:

Shipboard biological and physical measurements made during 1996, 1997, and 1998 in the equatorial Pacific are used to quantify the effect of the 1997–1998 El Niño on the phytoplankton community. This El Niño was by some measures the strongest ever observed and resulted in extremely low phytoplankton biomass and productivity throughout this normally moderately productive region. At the height of the event in late 1997 to early 1998, in the central Pacific, nitrate was absent throughout the entire euphotic zone (∼100 m), resulting in chlorophyll concentrations (0.05 μgL−1) that were among the lowest ever observed in the region and rates of primary production (∼0.41 g C m−2d−1) that were approximately half the climatological mean. These conditions persisted until May 1998 when the trade winds resumed and upwelling, with its associated supply of nutrients, was restored along the equatorial Pacific. The phytoplankton community quickly recovered, and by June 1998, nitrate, chlorophyll, and primary productivity levels were comparable to, or in excess of, their respective climat

ISSN:0148-0227    

DOI:10.1029/1999JC000056

年代:2000

数据来源: WILEY

摘要:

Drastic changes have occurred in the vertical structure of the deep waters of the eastern Mediterranean in the early 1990s, as dense water of Aegean origin has displaced lighter waters of Adriatic origin at the bottom of the deep basins. This work suggests that the initiation of this process took place in the North Aegean in the winter of 1986/1987 and was intensified by another formation event in 1992/1993. The available observations from the North Aegean support such a scenario. Furthermore, we propose that the outflow of Black Sea waters into the Aegean through the Dardanelles could act as an insulator of the deeper layer from the atmosphere, thus absorbing a large part of the heat and salt exchange; despite this fact, the existence of the densest bottom water of the Mediterranean in the North Aegean, and the continuation of density increase for a large period of time, suggests that it is a region of formation, thus that the insulation layer may at times be penetrated. We suggest that reduced Black Sea outflow into the North Aegean could facilitate dense water formation during the passage of cold atmospheric fronts in the winter.

ISSN:0148-0227    

DOI:10.1029/2000JC900131

年代:2000

数据来源: WILEY

摘要:

The large scale Indian Ocean circulation is estimated from a global hydrographic inverse geostrophic box model with a focus on the meridional overturning circulation (MOC). The global model is based on selected recent World Ocean Circulation Experiment (WOCE) sections which in the Indian Basin consist of zonal sections at 32°S, 20°S and 8°S, and a section between Bali and Australia from the Java‐Australia Dynamic Experiment (JADE). The circulation is required to conserve mass, salinity, heat, silica and “PO” (170PO4+O2). Near‐conservation is imposed within layers bounded by neutral surfaces, while permitting advective and diffusive exchanges between the layers. Conceptually, the derived circulation is an estimate of the average circulation for the period 1987–1995. A deep inflow into the Indian Basin of 11±4 Sv is found, which is in the lower range of previous estimates, but consistent with conservation requirements and the global data set. The Indonesian Throughflow (ITF) is estimated at 15±5 Sv. The flow in the Mozambique Channel is of the same magnitude, implying a weak net flow between Madagascar and Australia. A net evaporation of −0.6±0.4 Sv is found between 32°S and 8°S, consistent with independent estimates. No net heat gain is found over the Indian Basin (0.1 ± 0.2PW north of 32°S) as a consequence of the large warm water influx from the ITF. Through the use of anomaly equations, the average dianeutral upwelling and diffusion between the sections are required and resolved, with values in the range 1–3×10−5cm s−1for the upwelling and 2

ISSN:0148-0227    

DOI:10.1029/2000JC900122

年代:2000

数据来源: WILEY

摘要:

The circulation of the Indian Ocean in the Hadley Centre Climate Model is characterized by a very strong Indonesian Throughflow of O(24 Sv) compared with observational estimates of O(10 Sv) and very weak deep inflow across 32°S compared with observational estimates. The model throughflow has an upper core and a lower core of approximately equal magnitudes. We examine the driving terms for the throughflow and find that the upper core is driven by the wind stress curl and the lower core is driven by bottom pressure torques. The heat gained by the Indian Ocean from the atmosphere is approximately zero in the model, which is at odds with the observational estimates, which indicate a net heating of the Indian Ocean of between 0.4 and 1.2 PW. We hypothesize that the small heat flux is a direct consequence of the large throughflow and weak overturning. Two sensitivity experiments with different throughflows are described. We find that the magnitude of the deep overturning in the coupled model is strongly related to the magnitude of the lower core of the throughflow. The deep overturning increases from 3 to 9 Sv, as the magnitude of the lower core is reduced from 12 to 0 Sv. We find that the sensitivity of the heat gain of the Indian Ocean to the magnitude of the throughflow is O(0.005 PW) per sverdrup. This weak sensitivity may be due to the fact that the changes in the throughflow in the sensitivity experiments take place in the lower (colder) core

ISSN:0148-0227    

DOI:10.1029/1999JC000083

年代:2000

数据来源: WILEY

摘要:

A global three‐dimensional primitive equation general circulation model is used to estimate the upper ocean heat budget and subduction rates in the eastern North Atlantic during the period of the Subduction Experiment, June 1991 through July 1993. The seasonal cycle is dominated by one‐dimensional processes throughout the subtropical gyre, i.e., a local balance between the net heat flux into the ocean through the surface and a vertical redistribution due to vertical mixing. However, closure of the heat budget on long timescales involves redistribution by Ekman transports (both horizonal and vertical), geostrophic advection, and vertical diffusion. Subgridscale parameterizations of eddy processes weakly restratify the upper ocean. The annual rate at which waters are subducted from the mixed layer into the permanent thermocline is estimated using both kinematic and thermodynamic methods. These subduction rates and patterns are generally consistent with each other and also with previous estimates of the large‐scale subduction rate based on climatologies and models. Consideration of a finite thickness Ekman layer is shown to reduce the thermodynamic estimate of the subduction rate at low latitudes byO(50%), in much better agreement with the kinematic method. The effects of convergent eddy fluxes in the mixed layer and a diabatic thermocline on the subduction rate are also calculated and found to be small but not negligible. The seasonal and interannual evolution of the sea surface temperature and mixed layer depth in the model compare well with in situ measurements at five mooring locations taken as part of the Subduction Experiment. These results demonstrate that a global, low‐resolution general circulation model forced with surface fluxes of heat and freshwater can accurately reproduce the evolution of the upper ocean thermal structure and provide a useful tool for the analysis of air‐sea interaction and climate variability on seasonal to interannual t

ISSN:0148-0227    

DOI:10.1029/2000JC000228

年代:2000

数据来源: WILEY

摘要:

The objective of this study is to compare the mean and seasonal variability of the circulation in the southwest Atlantic with observations. The results used in the comparison are from the last 200 years of a 300 year control integration of the Climate System Model (CSM). The area of study includes the confluence region between the subtropical and subpolar waters represented by the Brazil and Malvinas Currents. The seasonal variation of transport and its relationship to changes in the wind stress forcing and in the sea surface temperature are examined and compared to available oceanographic observations. This study shows that a coarse resolution climate model, such as the CSM, can successfully reproduce major characteristics of the Brazil‐Malvinas confluence seasonally, although the mesoscale features involving recirculation and meander dynamics are not resolved. The CSM transport values in the region of 38°S are consistent with hydrographically derived values. The transport of the CSM Brazil Current is higher during austral summer and smaller during austral winter. Conversely, the Malvinas Current transport is weaker during austral summer and stronger during austral winter. This is also consistent with observations. The CSM seasonal cycle in transport associated with both the Brazil and Malvinas Currents and its meridional displacement is closely linked to the seasonal variations in the local wind stress curl. However, the displacement is much smaller in the model than in observations. The CSM results show that the latitudinal displacement of the 24°C and 17°C at the South American coast beween austral summer and winter is 20° and 12°, respectively. This is very similar to the displacement seen in observ

ISSN:0148-0227    

DOI:10.1029/1999JC000134

年代:2000

数据来源: WILEY

摘要:

The influence of quasi‐realistic high‐frequency wind variability for the probability distribution of surface waves and for the skill of the predictions with the wave model WAM is investigated. So far, the sensitivity of the surface wave field to high‐frequency wind variability has been demonstrated only for idealized wave conditions or particular events using rather simplified high‐frequency wind forcings. In this study the problem is addressed more generally by estimating the high‐frequency wind variability (for periods shorter than 6 hours) and by assessing its relevance for a particular North Atlantic winter. Wind observations with 20 min time resolution are used to build a statistical wind generator that reconstructs (in a statistical sense) that fraction of the wind variability that is missing in the analyzed model wind fields. These quasi‐realistic wind fluctuations, superimposed on analyzed 6 hourly wind fields, produced an increase of the air‐sea momentum flux and resulted in a moderate but systematic increase of the average wave heights and in their short‐term variability. While the results are qualitatively consistent with the findings of earlier studies under simplified conditions, they differ quantitatively. The change of the distribution of the wave data is found to be less pronounced for young sea states in the storm track region than for older sea states in the low latitudes. The response yields everywhere a significant amplification of the spectral variance for periods below 12 hours and an increase of the probability of extreme wave heights. Thus the increased air‐sea momentum flux is seen to be effective for predictions of the probability distribution of wave data and may also influence predictions of the

ISSN:0148-0227    

DOI:10.1029/1999JC000066

年代:2000

数据来源: WILEY

摘要:

Eddy motion in the Caribbean Sea is described on the basis of sea level anomalies deduced from ERS‐1 altimetry data corrected with TOPEX/Poseidon data during the 15 months of the Exact Repeat Mission (October 1992 to December 1993). Both cyclones and anticyclones were observed in the satellite data as anomalies originating in the Venezuelan Basin or entering the Caribbean through the Antillean passages, mainly the St. Lucia Channel, Anegada Passage, and north of Trinidad. The diameter of the eddies ranged from a few tens of kilometers to 700 km. Advection speeds were typically 20–30 cm s−1and the eddies were energetic (kinetic energy>0.6 m2s−2). Their lifetime of 3–4 months was determined, in general, by their interaction with topography. Most eddy activity was eroded and disappeared at the Central American Rise area, although a few eddies crossed into the Cayman Sea through the Chibcha Channel. Some eddies also entered the Cayman Sea from outside the Caribbean through the Windward Passage. The Panama‐Colombia Gyre was evident only during the tropical rainy season. A large cyclonic eddy was formed there during the period of maximum precipitation, when strong meridional salinity and wind speed gradients occurred. Eddy production in the central Caribbean appears to be associated with the interaction of the meandering Caribbean Current and the stron

ISSN:0148-0227    

DOI:10.1029/2000JC000300

年代:2000

数据来源: WILEY