Ocean Model Study Research Articles

An Observational and Numerical Modeling of Thermocline Development in the Persian Gulf

An observational and numerical (Princeton Ocean Model) study of the summer thermocline development in the Persian Gulf (PG) shows that as the northwesterly cold winter wind weakens and become warmer, the fresher inflow from Oman Sea penetrates much further into the PG. This is also associated with stronger solar radiation leading to the near surface thermocline development over the whole of the PG. For more realistic monthly averaged wind, the thermocline develops as is indicated by observations. This is particularly marked in the deeper central part in which it grows in depth about 0.2(m) per day. The formation of thermocline decreases the dissolved oxygen in water column due to induced stratification. Over the PG the temperature difference between surface and subsurface increases exponentially from March until May when it levels off, with similar smaller variations for salinity differences than observed.

Accelerating a barotropic ocean model using a GPU

The two-dimensional barotropic vorticity equation is one of the basic equations of ocean dynamics. It is important to have efficient numerical solution techniques to solve this equation. In this paper, we present an implementation of a numerical solution using a Graphics Processing Unit (GPU). The speedup of the calculation on the GPU with respect to that on a CPU depends on the grid size, but reaches a factor of about 50 for resolutions from 2049 × 2049 up to 4097 × 4097. It may therefore be efficient to use green GPUs in future high-resolution ocean modelling studies.

Effect of Atlantic Meridional Overturning Circulation Changes on Tropical Atlantic Sea Surface Temperature Variability: A 2½-Layer Reduced-Gravity Ocean Model Study

Abstract Previous coupled climate model simulations reveal that a dipole-like SST pattern with cooler (warmer) temperature over the north (south) tropical Atlantic emerges in response to a slowdown of the Atlantic meridional overturning circulation (AMOC). Using a 2½-layer reduced-gravity ocean model, a systematic investigation into oceanic processes controlling the tropical Atlantic sea surface temperature (SST) response to AMOC changes by varying the strength of northward mass transport at the open boundaries was conducted. It is found that the North Brazil Current (NBC) reverses its direction in response to a shutdown of the AMOC. Such a circulation change causes a decrease in upper equatorial ocean stratification and warming in the Gulf of Guinea and off the coast of Africa. These findings point to the importance of oceanic dynamics in the equatorial SST response to AMOC changes. Sensitivity experiments further show that the SST response relates nonlinearly to AMOC changes. The strength of the SST response increases dramatically when the AMOC strength falls below a threshold value. This nonlinear threshold behavior depends on the position of a subsurface temperature gradient forming along the boundary between the northern subtropical gyre and the tropical gyre that interacts with the western boundary current. The analysis suggests that, in order for the oceanic dynamics to have a dominant influence on tropical Atlantic SST in response to AMOC changes, two conditions must be satisfied: 1) the AMOC must weaken substantially so that the NBC flows equatorward, permitting water mass exchange between the northern subtropical and tropical gyres, and 2) the subsurface temperature front must be located in an optimal location where subsurface temperature anomalies induced by AMOC change are able to enter the equatorial zone.

Parameterization of rain induced surface roughness and its validation study using a third generation wave model

The effect of raindrops striking water surface and their role in modifying the prevailing sea-surface roughness is investigated. The work presents a new theoretical formulation developed to study rain-induced stress on sea-surface based on dimensional analysis. Rain parameters include drop size, rain intensity and rain duration. The influences of these rain parameters on young and mature waves were studied separately under varying wind speeds, rain intensity and rain duration. Contrary to popular belief that rain only attenuates surface waves, this study also points out rain duration under certain condition can contribute to wave growth at high wind speeds. Strong winds in conjunction with high rain intensity enhance the horizontal stress component on the sea-surface, leading to wave growth. Previous studies based on laboratory experiments and dimensional analysis do not account for rain duration when attempting to parameterize sea-surface roughness. This study signifies the importance of rain duration as an important parameter modifying sea-surface roughness. Qualitative as well quantitative support for the developed formulation is established through critical validation with reports of several researchers and satellite measurements for an extreme cyclonic event in the Indian Ocean. Based on skill assessment, it is suggested that the present formulation is superior to prior studies. Numerical experiments and validation performed by incorporating in state-of-art WAM wave model show the importance of treating rain-induced surface roughness as an essential pre-requisite for ocean wave modeling studies.

Parameterization of sea surface drag under varying sea state and its dependence on wave age

Momentum and energy exchange at air–sea interface through wind stress is very important for air–sea interaction studies, ocean modeling, and climate studies. The accurate representation of wind stress, in terms of drag coefficient, is a key factor in estimating the momentum transfer at the interface. The drag coefficient, in general, estimated using bulk formulae does not take into account the influence of wave age. This study examines the dependence of wave age on computed surface drag coefficient obtained by combining the Toba 3/2-power law with Froude number scaling, resulting in a new drag formulation (hereafter referred as RP formulation). We demonstrate that our proposed formulation is in good conjunction with established theories for both young and mature waves. Our investigation shows the theoretical formulation advocated earlier by Guan and Xie (hereafter referred as GX) overestimated the surface drag for mature waves as wind speed tends to increase. In addition, the formulation by GX was not verified by observational data. In the present work, for validation purpose, we use time series measurement of meteorological and oceanographic data from a deep water location in the Indian Ocean which was tested with both RP and GX formulations. We find that the proposed RP formulation, which embeds the 3/2 power of wave-age, shows a better match for both young and mature waves with the results of Janssen compared to the hypothesis of conventional wave age used by GX.

Effect of varied atmospheric stability on sea surface drag in shallow seas and its impact on wind-wave growth

This work reports an efficient bulk formulation of sea surface drag that incorporates effect of dynamic stability under varied atmospheric forcing. The proposed formulation exhibits a polynomial dependence of wind speed on air-sea temperature dif- ference based on statistical analysis. Quality checked meteorological and oceanographic data from four shallow water buoys located off Korean seas having measurements at an interval of every 1 h were used for this study. The analyses of in situ records for this region suggest stability ranging from highly stable to very unstable conditions. Importance of this proposed formulation is better reflected during unstable condition where other popular bulk formulations fail. In addition, importance and impact of such a study on wind-wave growth using the state-of-art wave model was also investigated. Finally, we advocate a new drag formulation, which accounts for varied atmospheric stability and suggest that this should be considered as an essential pre-requisite for ocean modeling studies.

Interannual variability of the global carbon cycle (1992–2005) inferred by inversion of atmospheric CO2 and δ13CO2 measurements

We present estimates of the surface sources and sinks of CO2 for 1992–2005 deduced from atmospheric inversions. We use atmospheric CO2 records from 67 sites and 10 δ13CO2 records. We use two atmospheric models to increase the robustness of the results. The results suggest that interannual variability is dominated by the tropical land. Statistically significant variability in the tropical Pacific supports recent ocean modeling studies in that region. The northern land also shows significant variability. In particular, there is a large positive anomaly in 2003 in north Asia, which we associate with anomalous biomass burning. Results using δ13CO2 and CO2 are statistically consistent with those using only CO2, suggesting that it is valid to use both types of data together. An objective analysis of residuals suggests that our treatment of uncertainties in CO2 is conservative, while those for δ13CO2 are optimistic, highlighting problems in our simple isotope model. Finally, δ13CO2 measurements offer a good constraint to nearby land regions, suggesting an ongoing value in these measurements for studies of interannual variability.

Impact of Satellite-Derived Forcings on Numerical Ocean Model Simulations and Study of Sea Surface Salinity Variations in the Indian Ocean

AbstractThis study focuses on two major aspects: the impact of satellite forcings (winds and precipitation) on the simulations of a multilayer Indian Ocean (IO) model (IOM) and the analysis of the processes responsible for salinity variations in the Indian Ocean during dipole years (1994 and 1997). It is observed that the European Remote Sensing Satellite-2 (ERS-2) scatterometer wind-driven solutions describe the interannual variabilities of sea surface temperature (SST) more realistically than the National Centers for Environmental Prediction (NCEP) wind-driven solutions. The equatorial westward current jet [hereafter referred to as reverse Wyrtki jet (RWJ)] originating near the Sumatra coast in response to anomalous easterlies during fall of 1994 and 1997 is quite strong in the scatterometer-forced solutions. This RWJ is found to be weak in the NCEP solution. Two more experiments differing by their precipitation forcings [climatological and interannually varying Global Precipitation Climatology Project (GPCP) rainfall] are carried out. Model-simulated variables like SST, sea surface salinity (SSS), and mixed layer depth (MLD) have been compared with in situ observations to verify the performance of the model. The model suggests a dipolelike structure in surface salinity during late 1994 and 1997, with low salinity in the central equatorial Indian Ocean (EIO) and high salinity near the Sumatra coast. The low-salinity tongue is caused by the transport of fresh surface waters via RWJ, which is further strengthened by a southward branch (which is absent in normal years) coming from the Bay of Bengal. A major inference of the study is that the low-salinity tongue is caused mainly by advection, not by a direct effect of precipitation. On the contrary, the high salinity near the Sumatra coast is due to the strong upwelling caused by anomalous easterlies. Another inference made out of this study is that there is apparently a definite signature of the evolution of the dipole event in the MLD approximately 2 months prior to the peak occurring in SSS in the south EIO.

Sea surface salinity interannual variability in the western tropical Atlantic: An ocean general circulation model study

Interannual variability of sea surface salinity in the northwest tropical Atlantic is investigated in several ocean general circulation model eddy‐permitting simulations using different air‐sea freshwater forcing. The only simulation able to reproduce some features of the observed variability from 1989 to 1993, in particular in the northwest South American region up to 20°N, east of the Lesser Antilles, is one with no relaxation to a salinity climatology. The model variability results from advection of freshwater originating from the large South American continental shelf river runoff and has a seasonal cycle associated with the displacement of the intertropical convergence zone: Anomalies develop from March to May and then propagate to a larger area within the next months with the horizontal currents (where both geostrophic and Ekman currents contribute). Later on, from July to October, the anomalies are damped through entrainment across the mixed layer base. The vertical diffusion through the mixed layer base does not weaken when anomalously fresh surface water is present (i.e., no barrier layer effect). According to the European Centre for Medium‐Range Weather Forecasts ERA15 reanalysis, air‐sea freshwater fluxes have little impact on the large (over 0.5 practical salinity scale) sea surface salinity (SSS) variability in this region. In the model, eddy variability plays little role in the interannual SSS variability, maybe because of inadequate resolution resulting in too weak and too few North Brazil Current eddies.

The solubility of sulfur hexafluoride in water and seawater

The concentration of sulfur hexafluoride (SF 6) in the atmosphere has been rapidly increasing during the past several decades. This long-lived compound enters the surface ocean by air–sea gas exchange and is potentially a very useful transient tracer for studying ocean circulation and mixing. SF 6 has also been directly injected into the ocean at a minimal number of locations as a part of deliberate tracer release experiments to study gas exchange and sub-surface mixing rates. In this study, laboratory measurements of the solubility of SF 6 in water and seawater were made over the temperature range of ∼−0.5°C to 40°C. Volumes of water and seawater held at constant temperature in glass chambers were equilibrated with a gas mixture containing SF 6 and CFC-12 (CF 2Cl 2) at parts-per-trillion levels in nitrogen. Small volume water samples were analyzed by electron capture gas chromatography. Using the method of least squares, equations previously used in describing gas solubility as a function of temperature and salinity were fit to the SF 6 and CFC-12 measurements. The CFC-12 results were in good agreement with previous work, while substantial differences were found between these SF 6 results and those reported in earlier studies. The mean error for the analytical measurements is estimated to be ∼0.5%. Based on errors in the fits and the analytical errors, we estimate the overall accuracy of the SF 6 solubility function to be of the order of 2%. The results from this work should be useful in determining equilibrium concentrations for SF 6 in ocean observation and modeling studies.

On relationships between cannibalism, climate variability, physical transport, and recruitment success of Bering Sea walleye pollock (Theragra chalcogramma)

Walleye pollock (Theragra chalcogramma) is the single most abundant fish species in the Bering Sea and comprises the bulk of the commercial catch. Juvenile pollock are an important forage fish for older pollock, other fish, marine mammals, and birds. We examine the interaction between cannibalism, climate variability, and related patterns in physical transport. Our analysis of adult and juvenile pollock abundance and distribution time series, ocean current modelling studies, and information on climate variability indicates that cannibalism is a major determinant of interannual recruitment variability. In turn, the intensity of cannibalism appears to be dependent on the degree of spatial separation of adults and juveniles. Strong year classes occur when juvenile pollock are transported inshore and away from adults in spring - conditions typical of warm years. In cold years, transport is reduced and juveniles remain on the outer shelf in proximity to adults. Co-occurring distribution patterns of adults and juveniles resulting from these conditions lead to potentially increased cannibalism and subsequent weak year classes.

A 2D Coupled Atmosphere–Ocean Model Study of Air–Sea Interactions during a Cold Air Outbreak over the Gulf Stream

Abstract The two-dimensional, Advanced Regional Prediction System (ARPS) has been coupled with the Princeton Ocean Model to study air–sea interaction processes during an extreme cold air outbreak over the Gulf Stream off the southeastern United States. Emphases have been placed on the development of the mesoscale front and local winds in the lower atmosphere due to differential fluxes over the land, the cold shelf water, and the warm Gulf Stream, and on how the mesoscale front and the local winds feed back to the ocean and modify the upper-ocean temperature and current fields. Model results show that a shallow mesoscale atmospheric front is generated over the Gulf Stream and progresses eastward with the prevailing airflow. Behind the front, the wind intensifies by as much as 75% and a northerly low-level wind maximum with speeds near 5 m s−1 appears. The low-level northerly winds remain relatively strong even after the front has progressed past the Gulf Stream. The total surface heat flux in the coupled e...

A 2D Coupled Atmosphere–Ocean Model Study of Air–Sea Interactions during a Cold Air Outbreak over the Gulf Stream

A 2D Coupled Atmosphere–Ocean Model Study of Air–Sea Interactions during a Cold Air Outbreak over the Gulf Stream

Decadal Variabilities of the Upper Layers of the Subtropical North Atlantic: An Ocean Model Study

Numerical simulations of the Atlantic Ocean during the period 1950 to 1989, using a sigma coordinate, free surface numerical model, show long-term variabilities in the upper ocean subtropical gyre similar to those obtained from observations. The simulations show how westward propagating planetary waves, originated in the eastern North Atlantic, affect interdecadal variabilities of ocean properties such as the Bermuda sea level, the Gulf Stream position and strength, and subsurface temperature anomalies in the western North Atlantic. Special attention is given to the dramatic sea level drop at Bermuda in the early 1970s, which is accompanied by cooling of subsurface layers in the western North Atlantic and a northward shift and weakening of the Gulf Stream. Following these events, between 1970 and 1980, the cold temperature anomalies in the upper layers of the western North Atlantic slowly propagated eastward and downward; the strongest propagating signal in the model is found at 200-m depth, suggesting that advection of anomalies downstream by the Gulf Stream current and changes in winter mixing are involved. Significant correlations were found between the sea level anomalies at Bermuda and sea level anomalies in the eastern North Atlantic up to eight years earlier. Sensitivity experiments with different atmospheric forcing fields are used to study the ocean response to observed sea surface temperature and wind stress anomalies. It is shown that on decadal timescales, the ocean model responds in a linear fashion to the combined effect of SST and wind stress anomalies, a fact that might be exploited in future climate prediction studies.

Evaluation of regional numerical weather prediction model surface fields over the Middle Atlantic Bight

Coastal ocean models often rely on the surface fields from numerical weather prediction (NWP) models for realistic surface boundary conditions, but the errors in these fields are poorly understood. We evaluate the surface meteorological and flux fields provided by three of the regional NWP models in operation during 1996 and 1997 at the U.S. National Centers for Environmental Prediction (NCEP): the Eta‐48, Eta‐29, and Rapid Update Cycle (RUC‐1) models. These model fields are compared to in situ measurements made from an air‐sea interaction buoy deployed from July 1996 to June 1997 at a midshelf location in the Middle Atlantic Bight during the Coastal Mixing and Optics experiment. In addition, data from six National Data Buoy Center buoys are used to evaluate spatial errors in the model fields. The Eta‐29 and RUC‐1 models overestimate the net ocean‐to‐atmosphere heat flux by an average 83 and 74 W m−2, respectively, with notable errors in each of the individual heat flux components. The poorly resolved sea surface temperature fields used in the 1996–1997 regional NWP models lead to significant errors in the latent and sensible heat fluxes over the continental shelf and slope. Moreover, wind speeds are slightly overestimated in the Eta‐48 and Eta‐29 models while the RUC‐1 model underestimates them by more than 1 m s−1. All of the models have mean wind direction errors of 7° to 13° east of north. In light of these evaluations, considerations for improving the accuracy of the surface flux fields for use in future ocean modeling studies are discussed.

The Dynamics of a Simple Baroclinic Model of the Wind-Driven Circulation

The authors study the dynamics of a two-layer approximation to the steadily forced baroclinic circulation in a closed ocean basin with the aim of understanding its temporal variability and the onset of low-frequency variability. It is found that, for a range of dissipation that includes values used in a number of ocean modeling studies in the past five years, if one waits a sufficient length of time, the asymptotic behavior of the system is characterized by only a very small number of degrees of freedom. By varying the dissipation as a control parameter, the authors identify abrupt transitions in the form of the long-term circulation exhibited by the model. One type of transition, from a time-varying circulation dominated by two frequencies to a chaotic circulation, is accompanied by the appearance of low-frequency variability. This constitutes an internal mechanism for the production of variability at climatological timescales. The model used is a two-layer, quasigeostrophic model forced by a steady wind stress with a uniform cyclonic curl. Dissipation is modeled by a lateral diffusion of momentum with a uniform eddy viscosity. In two sets of experiments with two internal deformation radii and layer depth ratios, the eddy viscosity is varied and the types of circulation that result are reported. The stable steady circulation seen in the viscous limit gives way to time-dependent circulations of increasing temporal complexity. Spatially, the circulations fall into two types, those dominated by large recirculating gyres in the western part of the basin and those with a strong (and strongly meandering) peripheral current reminiscent of that seen in the Black Sea. From an examination of the linear eigenmodes of the steady circulation, the initial transition to time dependence may be characterized as a baroclinic instability of the western recirculation gyre.

Dynamics of the Gulf Stream/Deep Western Boundary Current Crossover. Part I: Entrainment and Recirculation

A regional primitive equation model is applied to the study of the interaction between the Gulf Stream and the deep western boundary current (DWBC) where they cross at Cape Hatteras. It is found that for a wide range of forcing parameters the upper core of the DWBC is split into two mean paths at the crossover, one flowing toward the south along the western boundary and the other flowing toward the cast under the Gulf Stream. The eastward branch is entrained Into the southern recirculation gyre and, after being diverted into the interior for up to 1500 km, eventually returns to the western boundary current and continues to flow southward. This recirculation and mixing is shown to have a significant impact on the separation point and mean path of the Gulf Stream, the basin-scale stratification, and the properties of the DWBC south of the crossover. For most configurations, the lower DWBC remains largely on the western boundary and interacts only weakly with the interior. The entrainment of the upper core is shown to be driven by a baroclinic, time-dependent process of DWBC water eddy formations into the recirculation gyres under the Gulf Stream. Potential vorticity considerations are key to understanding the entrainment mechanism and its sensitivity to variations in the model forcing and configuration. A scaling estimate of the amount of entrained DWBC water as a function of the eddy field is derived. The mean paths of the upper and lower DWBCs and strength of the eddy fluxes compare well with various observational estimates. The importance of such entrainment and mixing processes on large-scale ocean modeling and climate studies is discussed.

Data Assimilation and Model Evaluation Experiment Datasets

The Institute for Naval Oceanography, in cooperation with Naval Research Laboratories and universities, executed the Data Assimilation and Model Evaluation Experiment (DAMEE) for the Gulf Stream region during fiscal years 1991-1993. Enormous effort has gone into the preparation of several high-quality and consistent datasets for model initialization and verification. This paper describes the preparation process, the temporal and spatial scopes, the contents, the structure, etc., of these datasets. The goal of DAMEE and the need of data for the four phases of experiment are briefly stated. The preparation of DAMEE datasets consisted of a series of processes: (1) collection of observational data; (2) analysis and interpretation; (3) interpolation using the Optimum Thermal Interpolation System package; (4) quality control and re-analysis; and (5) data archiving and software documentation. The data products from these processes included a time series of 3D fields of temperature and salinity, 2D fields of surface dynamic height and mixed-layer depth, analysis of the Gulf Stream and rings system, and bathythermograph profiles. To date, these are the most detailed and high-quality data for mesoscale ocean modeling, data assimilation, and forecasting research. Feedback from ocean modeling groups who tested this data was incorporated into its refinement. Suggestions for DAMEE data usages include (1) ocean modeling and data assimilation studies, (2) diagnosis and theoretical studies, and (3) comparisons with locally detailed observations.

Comparison of special sensor microwave imager vector wind stress with model‐derived and subjective products for the tropical Pacific

A new source of vector wind stress data is assessed relative to existing analyses of the surface wind field. The large‐scale variability of vector wind stress generated by Atlas et al. (1991) and based on the special sensor microwave imager (SSM/I) remotely sensed observations of surface wind speed is compared with five operational and subjectively analyzed wind products across the tropical Pacific basin for the first year of SSM/I, July 1987 through June 1988. The conventional wind stress data considered are the operational wind products from European Centre for Medium Range Weather Forecasts (ECMWF), National Meteorological Center (NMC), and Fleet Numerical Oceanography Center (FNOC), and the subjective analyses from Florida State University and the University of Hawaii. The spatial and temporal variability of the zonal component, meridional component, and curl of the wind stress are examined relative to their future use in wind‐driven ocean model studies of tropical Pacific Ocean circulation. The basin‐scale structure of the SSM/I data fall within a range bracketed by ECMWF, the University of Hawaii, NMC, and the Florida State products. The SSM/I data are shown to be most similar to the ECMWF analysis and the subjective analysis of satellite‐derived cloud motion wind vectors (SAWIN) performed at the University of Hawaii. The basin‐wide mean rms difference between the SSM/I data and these two products is 0.18 dyn cm−2 for τx and 0.10–0.12 dyn cm−2 for τy. In order to place these differences within context, the basin‐wide mean standard deviation for the temporal variability is found to be 0.23–0.26 dyn cm−2 for τx and 0.15–0.16 dyn cm−2 for τy. On regional scales, some of the differences among these products are greater than 0.3 dyn cm−2. The ECMWF and SAWIN analyses are the most similar (basin‐wide mean rms difference τx = 0.16 dyn cm−2, τy = 0.11 dyn cm−2) of the 15 possible product versus product comparisons. This may indicate a high weight given to cloud motion wind vectors in the ECMWF analysis for data sparse regions of the tropical Pacific. The wind data from the FNOC analysis forecast system in use in 1987–1988 (and since upgraded) was the most dissimilar wind product. The relatively dense space‐time coverage of the SSM/I satellite data (order of 27 observations per 2° × 2.5° grid square every 1.5 days), together with the large‐scale similarity with conventional wind products, suggests that the SSM/I‐based analysis represents a new source of surface wind information suitable for ocean modeling studies. As a result of the potential demonstrated here, strong consideration should be given to the use of these wind data in forcing ocean circulation modeling studies. Furthermore, the prospects of processing the surface wind speed retrievals from spaceborne passive and active microwave sensors dating back for more than 10 years should be examined.

North Atlantic Deep Water cools the southern hemisphere

A standard explanation for coupling climate variations in the northern and southern hemispheres involves fluctuations in North Atlantic Deep Water (NADW) production. However, I suggest that the “NADW‐Antarctic” connection may work opposite to that conjectured by many investigators; that is, when NADW production rates are high, southern hemisphere temperatures decrease rather than increase. The revised interpretation is consistent with observations and ocean modeling studies which demonstrate that, although upwelling of relatively warm NADW water around Antarctica promotes sea ice meltback, a second and more important negative feedback is also operating. In order to conserve volume, southward export of NADW across the equator is accompanied by import of an equivalent volume of considerably warmer water from shallower oceanic layers in the South Atlantic. The southern hemisphere loses heat as a result of this exchange. The hemispherically averaged net heat loss may be as high as 4 W/m², an amount comparable to a CO2 doubling. It is suggested that this more comprehensive view of the role of NADW may explain both decadal‐scale variations in South Atlantic sea surface temperatures in this century and two significant problems in Pleistocene climatology: why southern hemisphere temperatures decreased before CO2 levels decreased at the end of the last interglacial and why southern hemisphere temperature changes precede changes in northern hemisphere ice volume. It is shown that when NADW production was reinitiated during the last interglacial (120,000 B.P.), high‐latitude southern hemisphere temperatures decreased. The estimated magnitude of altered southern hemisphere heat export is comparable to the ice‐age CO2 signal and may be able to account for the observed cooling even when CO2 levels were high. When cast into a frequency domain framework, this interpretation may also help explain why southern hemisphere temperatures lead global ice volume changes.