Papers by Stephen Van Gorder
Journal of Physical Oceanography, 2003
Recent general circulation simulations suggest that, prior to the closure of the Panama Isthmus (... more Recent general circulation simulations suggest that, prior to the closure of the Panama Isthmus (the narrow strip of land connecting North and South America), low-salinity Pacific Ocean water invaded the Atlantic Ocean via the gap between North and South America. According to this scenario, the invasion decreased the Atlantic Ocean salinity to the point at which North Atlantic Deep Water (NADW) formation was impossible and, consequently, there was probably no ''conveyer belt.'' In line with this scenario, it has been suggested that the closure of the isthmus led to an increased salinity in the Atlantic that, in turn, led to the present-day NADW formation and the familiar meridional overturning cell (MOC). Using simple dynamical principles, analytical modeling, process-oriented numerical experiments, and modern-day wind stress, it is shown that, in the absence of NADW formation, one would expect a westward flow from the Atlantic to the Pacific Ocean through an open Panama Isthmus. This contradicts the suggestion made by the earlier numerical models that imply an eastward flow through the ''Panama Gateway.'' An analogous present-day situation (for a system without deep-water formation) is that of the Indonesian Throughflow, which brings Pacific water to the Indian Ocean rather than the other way around; that is, it is also a westward flow rather than an eastward flow. ''Island rule'' calculations clearly show that the direction of the flow in both situations is determined by the wind field to the east of the gaps. The authors show that exceptionally strong vertical mixing in the Atlantic (as compared with the Pacific) or another means of warm-water removal from the upper layer in the Atlantic (e.g., NADW or strong cooling) could reverse the direction of the flow through the open isthmus. This is most likely what happened in the earlier numerical simulation, which must have invoked (explicitly or implicitly) large quantities of upper-water removal even without NADW formation. On this basis it is suggested that if low-salinity Pacific water did, in fact, invade the Atlantic Ocean prior to the closure of the Panama Isthmus, then this invasion took place via the Bering Strait rather than through the open Panama Isthmus. It is also suggested that, if there were 20 Sv (Sv ϵ 10 6 m 3 s Ϫ1) of NADW formation today and the Panama Isthmus were to be suddenly open today, then Pacific water would indeed invade the Atlantic via the Panama Gateway. In turn, this would either collapse the existing NADW formation rate or reduce it to about 10 Sv, which can be maintained even with an open isthmus. In both cases the final outcome is a westward flow in the open isthmus.
El Nino Prediction Using a Simple Equatorial Heat Content/El Nino Trigger Model
ABSTRACT
A Method for Estimating Wind-Driven Frictional, Time-Dependent, Stratified Shelf and Slope Water Flow
Journal of Physical Oceanography
ABSTRACT
The Propagation of Gravity Currents along Continental Shelves
Journal of Physical Oceanography
Did an open Panama Isthmus correspond to an invasion of Pacific water into the Atlantic?
Existing general circulation modeling studies suggest that, prior to the closure of the Panama is... more Existing general circulation modeling studies suggest that, prior to the closure of the Panama isthmus, low salinity Pacific ocean water invaded the Atlantic ocean via the associated gap between North and South America. According to this scenario, the invasion decreased the Atlantic Ocean salinity to the point where deep water formation was impossible and, consequently, no "conveyer belt" movement was in action. Using simple dynamical principles, analytical modeling and process-oriented numerical experiments, it is shown that one would normally expect a flow from the Atlantic to the Pacific Ocean (rather than from the Pacific to the Atlantic) through an open Panama isthmus. An analogous present-day situation is that of the Indonesian Throughflow which brings Pacific water to the Indian Ocean rather than the other way around. The direction of the flow in both situations is determined by the wind field to the east of the gaps. On this basis it is suggested that if low salinity Pacific water did in fact invade the Atlantic Ocean prior to the closure of the Panama isthmus, then this invasion took place via the Bering Strait rather than through the open Panama Isthmus.
Simple models for the heat flux from the Atlantic meridional overturning cell to the atmosphere
Journal of Marine Research
ABSTRACT
The Robustness of Atlantic Meridional Overturning Cell (AMOC)
ABSTRACT
Does the Oceanic Meridional Overturning Cell Really Have More Than one Stable State ??
Numerical climate and ocean models consistently show that the meridional overturning cell (MOC) h... more Numerical climate and ocean models consistently show that the meridional overturning cell (MOC) has two stable states for the same fresh water flux (into the ocean). One of these two states usually corresponds to a high northward transport of surface water (and, hence, to a warm northern hemisphere climate) whereas the other corresponds to low northward transport (and, hence, to a cool northern hemisphere climate). This two-states scenario leads to the broadly quoted concern that our present day warm climate can perhaps spontaneously flip to a much cooler state. We present new analytical and numerical runs showing that these two states are an art-effect of the high eddy diffusivities most often used in the commonly employed numerical models. For the Atlantic, these diffusivities artificially introduce unrealistic upwelling into the thermocline within the limits of the Atlantic itself. These, in turn, introduce the familiar hysteresis corresponding to the two-states. In the real ocean, the convection in the north Atlantic is too strong and the vertical diffusivities in the thermocline are too small to allow for such upwelling within the limits of the Atlantic itself. As a result, the water which ultimately sinks in the north Atlantic is drawn into the Atlantic from regions far away--the Southern and Indian oceans. Both the analytical and the numerical results show that the two states converge into one in the limit of small diffusivity. This one-state scenario doesn't at all mean that the MOC cannot collapse due to a large fresh water flux--it certainly can do so if the fresh water flux is large enough. Rather, it means that there are no two-states for the same fresh water flux, implying that the transition from warm to a cold state cannot happen spontaneously.
Journal of Physical Oceanography, 2007
Discharge and recharge of the warm water volume (WWV) above the 20°C isotherm in an equatorial Pa... more Discharge and recharge of the warm water volume (WWV) above the 20°C isotherm in an equatorial Pacific Ocean box extending across the Pacific from 156°E to the eastern ocean boundary between latitudes 5°S and 5°N are key variables in ENSO dynamics. A formula linking WWV anomalies, zonally integrated wind stress curl anomalies along the northern and southern edges of the box, and flow into the western end of the box is derived and tested using monthly data since 1993. Consistent with previous work, a WWV balance can only be achieved if the 20°C isotherm surface is not a material surface; that is, warm water can pass through it. For example, during El Niño, part of the WWV anomaly entering the box is cooled so that it is less than 20°C and therefore passes out of the bottom of the box, the 20°C isotherm surface. The observations suggest that the anomalous volume passing through the 20°C isotherm is approximately the same as TЈ W , the anomalous WWV entering the western end of the box. Therefore the observed WWV anomaly can be regarded as being driven by the anomalous wind stress curl along the northern and southern edges of the box. The curl anomaly changes the WWV both by divergent meridional flow at the edges of the box and vortex stretching; that is, the Sverdrup balance does not hold in the upper ocean. A typical amplitude for the rate of change of WWV for the 5°S-5°N box is 9.6 Sv (Sv ϵ 10 6 m 3 s Ϫ1 ). The wind stress curl anomaly and the transport anomaly into the western end of the box are highly correlated with the El Niño index Niño-3.4 [the average sea surface temperature anomaly (SSTA) over the region 5°S-5°N, 170°-120°W] and Niño-3.4 leads minus the WWV anomaly by one-quarter of a cycle. Based on the preceding results, a simple discharge/recharge coupled ENSO model is derived. Only water warmer than about 27.5°-28°C can give rise to deep atmospheric convection, so, unlike past discharge/recharge oscillator models, the west-central rather than eastern equatorial SSTAs are emphasized. The model consists of two variables: TЈ, the SSTA averaged over the region of strong ENSO air-sea interaction in the west-central Pacific equatorial strip 5°S-5°N, 156°E-140°W and DЈ, the 20°C isotherm depth anomaly averaged over the same region. As in the observations, TЈ lags DЈ by one-quarter of a cycle; that is, ץTЈ/ץt ϭ DЈ for some positive constant . Physically, when DЈ Ͼ 0, the thermocline is deeper and warmer water is entrained through the base of the mixed layer, the anomalous heat flux causing ץTЈ/ץt Ͼ 0. Also, when DЈ Ͼ 0, the eastward current anomaly is greater than zero and warm water is advected into the region, again causing ץTЈ/ץt Ͼ 0. Opposite effects occur for DЈ Ͻ 0. A second relationship between TЈ and DЈ results because the water is warm enough that TЈ causes deep atmospheric convection anomalies that drive the wind stress curl anomalies that change the heat storage ץDЈ/ץt. The atmosphere responds essentially instantly to the TЈ forcing and the curl causes a discharge of WWV during El Niño (TЈ Ͼ 0) and recharge during La Niña (TЈ Ͻ 0), so ץDЈ/ץt ϭ ϪTЈ for some positive constant . The two relationships between TЈ and DЈ result in a harmonic oscillator with period 2/ ͌ Ϸ 51 months.
Journal of Physical Oceanography, 1999
A different way of looking at the meridional warm water ( Ͻ 26.8) flux in the South and North Atl... more A different way of looking at the meridional warm water ( Ͻ 26.8) flux in the South and North Atlantic is proposed. The approach involves the blending of observational aspects into analytical modeling, which allows one to circumvent finding a detailed solution to the complete wind-thermohaline problem. The method employs an integration of the momentum equations along a ''horseshoe'' path in a rectangular basin that is open on the southern side.
A Different Outflow Length Scale?
Journal of Physical Oceanography, 2004
ABSTRACT
The Connection between the Boreal Spring Southern Oscillation Persistence Barrier and Biennial Variability
Journal of Climate, 1999
ABSTRACT
On Fitting a Straight Line to Data when the “Noise” in Both Variables Is Unknown*
Journal of Atmospheric and Oceanic Technology, 2013
ABSTRACT
Forecasting Long-Lead Rainfall Probability with Application to Australia’s Northeastern Coast
Journal of Applied Meteorology and Climatology, 2010
... Yvette Everingham ... 1). This region experiences wet summers and dry winters with considerab... more ... Yvette Everingham ... 1). This region experiences wet summers and dry winters with considerable rainfall variability from one year to the next, much of this ... The objective is to complete the harvest before the onset of the summer rains, when conditions are too boggy for machinery ...
International Journal of Climatology, 2007
Rainfall variability is a crucial element that impinges on the success of sugarcane growing regio... more Rainfall variability is a crucial element that impinges on the success of sugarcane growing regions around the world. As the scientific community and industry personnel gain more experience at working participatively, the ability of long-range rainfall forecasts to reduce the risk and uncertainty associated with decisions impacted by rainfall variability has become increasingly recognized. Some important decisions, however, require knowing the chances of rain at early lead times that span the austral autumn period. These types of decisions remain largely unassisted by climate forecasting technologies owing to the boreal spring (austral autumn) persistence barrier. Taking the Australian sugar industry as a case study example, this article explores the capability of a long lead statistical El Niño Southern Oscillation phenomenon (ENSO) prediction model to reduce the risk associated with decisions that must be made before autumn and are effected by rainfall anomalies post-autumn. Results shown across all regions considered in this study indicated a higher risk of obtaining an above-median rainfall index when the statistical model predicted La Niña type conditions to emerge post-spring. For selected regions, this risk was reduced when the model predicted El Niño type conditions for the same period. In addition, the model would have provided an earlier indication of the likelihood of disruption due to wet harvest conditions in a year that devastated the Australian sugar industry. This benchmark study has highlighted the potential of an ENSO prediction model to aid industry decisions that have previously been made in isolation of probabilistic knowledge about future rainfall conditions.
Geophysical Research Letters, 2003
Thoughts on a variable meridional overturning cell and a variable heat-flux to the atmosphere
Geophysical & Astrophysical Fluid Dynamics, 2011
ABSTRACT The atmospheric response to a potential slowdown of the Atlantic meridional overturning ... more ABSTRACT The atmospheric response to a potential slowdown of the Atlantic meridional overturning cell (AMOC) is examined using the nonlinear analytical approach (for Heinrich events) introduced by Sandal and Nof (Sandal, C. and Nof, D., A new analytical model for Heinrich events and climate instability. J. Phys. Oceanogr. 2008, 38, 451–466; SN, hereafter). Most numerical global climate models predict that the atmosphere should cool in response to the increased freshwater-fluxes (“hosing”) that slow the AMOC down and significantly reduce the heat-flux to the atmosphere. Our application of SN to the modern day climate suggests that the answer to the question of how the atmosphere responds to a slowing AMOC is not that simple. Within the (admittedly limited) dynamics which SN invoke, we find that, as the global numerical climate models predict, a slowdown of the AMOC will indeed cause the mean atmosphere of the entire Northern Hemisphere to cool. However, in contrast to the numerical predictions, our analytical approach suggests that a region in the immediate vicinity of the Atlantic convection (up to a distance of ∼O(1000 km)) may warm up, not cool down (roughly 3°C for 50% mass-transport reduction). For some extreme conditions of a constant atmospheric transport independent of the AMOC (which is not a part of the dynamics involved by SN), the atmosphere can indeed locally cool, but the cooling is minimal (less than 0.3°C for a 50% ocean mass transport reduction), and the associated reduction in heat flux from the ocean to the atmosphere is almost totally negligible. We also place the SN results on a somewhat firmer ground by examining in detail about its closure condition and the most critical assumption adapted by SN. The first has to do with the ratio of the atmospheric and oceanic mass transports (assumed unity in SN) and the second involves up-to-date maps of the ocean–atmosphere heat-fluxes. We show that the system of governing equations admits physically relevant solutions only for particular relationships between the atmospheric and ocean mass transports participating in the ocean–atmosphere heat exchange. Still, as the analytics misses critical atmospheric components such as moisture and variability in the heat exchange interaction area, our results can only serve as an indicator of the problem complexity.
Abyssal gyres
Geophysical & Astrophysical Fluid Dynamics, 1991
The flow of Antarctic Bottom Water in the western part of the Atlantic Ocean is modeled in terms ... more The flow of Antarctic Bottom Water in the western part of the Atlantic Ocean is modeled in terms of an annulus consisting of a coupled frontal current (i.e., a cold filament) moving along the internal surface of a cone. Namely, the interface bounding the (reduced-gravity) cyclonic current from above intersects the bottom along two curves, one on its right hand
Deep Sea Research Part I: Oceanographic Research Papers, 2000
The established`island rulea and the recently introduced`separation formulaa are combined to yiel... more The established`island rulea and the recently introduced`separation formulaa are combined to yield an analytical expression for the total upwelling into the thermocline in the Paci"c. The combination of the two is achieved with the use of a hybrid model containing a strati"ed upper layer, a thick (slowly moving) homogenous intermediate layer and an inert lower layer. Both the upper and the intermediate layers are subject to diabatic cooling and heating (which need not be speci"ed) and there is an exchange of mass between the two active layers. An attempt is made to examine the above analytical (hybrid) model numerically. Ideally, this should be done with a complete two-and-a-half layer model (with upwelling and downwelling), but such a model is much too complex for process-oriented studies (due to the required parameterization of vertical mixing). Consequently, we focus our attention on verifying that the separation formula and the island rule are consistent with each other in a much simpler, layer-and-a-half model (without upwelling). We "rst veri"ed that the new`separation formulaa provides a reasonable estimate of the wind-induced transport in an island-free basin. We then compare the wind-induced transport predicted by the separation formula and the island rule in an idealized basin containing an island. We show that in these idealized situations the two methods give results that are consistent with each other and the numerics. We then turned to an application of the (hybrid) two-and-a-half layer model to the Paci"c where, in contrast to the idealized layer-anda-half models (where the two methods address the same water mass), the two methods address two diwerent water masses. While the separation formula addresses only thermocline water ( F (26.20), the island rule addresses all the water down to 27.5 F (i.e., both the upper and intermediate layer). This is why the application of the two methods to the Paci"c gives two di!erent results * an application of the formula gives zero warm water transport whereas an 0967-0637/00/$ -see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 -0 6 3 7 ( 0 0 ) 0 0 0 2 9 -7
Deep Sea Research Part I: Oceanographic Research Papers, 1999
An upper bound for the nonlinear eastward propagation rate of the Paci"c warm pool is derived ana... more An upper bound for the nonlinear eastward propagation rate of the Paci"c warm pool is derived analytically using an inertial two-and-a-half-layer model on a plane. The model is based on the familiar idea that, in most years, the eastward migration tendency is arrested by the drag imposed on the ocean by the westward trade winds. During El Nino years, however, when the wind partially (or completely) relaxes, the pool is freed to move toward the east. The upper bound that we focus on corresponds to a rapid migration associated with a complete relaxation of the westward winds. Nonlinear analytical solutions to the above state are constructed by integrating the (inviscid) horizontal momentum equations over a control volume in a coordinate system moving steadily toward the east. A balance between the eastward #owforce (i.e., the momentum #ux resulting from the eastward density gradient) and the opposing westward form-drag (exerted by the westward #owing intermediate #uid diving under the pool) is examined. It involves integrated pressure forces, integrated inertia and the integrated Coriolis forces. In the limit of a control volume with an in"nitesimal north}south extent, no recirculation (i.e., no lateral exchange of mass between the fraction of the pool occupying the immediate vicinity of the equator and regions immediately to the north and south), and no crossequatorial #ows, the governing equations reduce to the equations that govern the nonrotating (i.e., ,0) intrusion of warm water into a resting two-layer system. This essentially means that the Coriolis force does not have any zonal component along the equator. For such conditions, the nonlinear eastward speed is found to be [2g( / )H ] [1!(H /H )], where is the density di!erence between the pool and the intermediate water underneath (i.e., the so-called intermediate layer), H the undisturbed thickness of the intermediate layer ahead of the pool, and H is the intermediate layer thickness under the pool. Typical values for the Paci"c give a bounding propagation rate of 50}60 cm s\, which is in good agreement with the observed migration rate during both the 1982}83 El Nino and the 1997 El Nino, the only ones 0967-0637/99/$ -see front matter
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Papers by Stephen Van Gorder