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Richard Seager
and
Naomi Naik

Abstract

Both naturally occurring La Niña events and model-projected anthropogenic-driven global warming are associated with widespread drying in the subtropics to midlatitudes. Models suggest anthropogenic drying should already be underway but climate variability on interannual to multidecadal time scales can easily obscure any emerging trend, making it hard to assess the validity of the simulated forced change. Here, the authors address this problem by using model simulations and the twentieth-century reanalysis to distinguish between natural variability of, and radiatively forced change in, hydroclimate on the basis of the mechanisms of variations in the three-dimensional moisture budget that drive variations in precipitation minus evaporation (PE). Natural variability of PE is dominated by the El Niño–Southern Oscillation (ENSO) cycle and is “dynamics dominated” in that the associated global PE anomalies are primarily driven by changes in circulation. This is quite well reproduced in the multimodel mean of 15 models used in the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4)/Coupled Model Intercomparison Project 3 (CMIP3). In contrast, radiatively forced PE change is “thermodynamics mediated” in that the rise in specific humidity leads to intensified patterns of moisture transport and PE. But, as for ENSO, the poleward shift of the storm tracks and mean meridional circulation cells also contribute to changes in PE. However, La Niña and radiatively forced changes in the zonal mean flow are distinct in the tropics. These distinctions are applied to the post-1979 record of PE in the twentieth-century reanalysis. ENSO-related variations strongly influence the observed PE trend since 1979, but removal of this influence leaves an emerging pattern of PE change consistent with the predictions of the IPCC AR4/CMIP3 models over this period together with, to some extent, consistent contributions from dynamical and thermodynamical mechanisms and consistent changes in the zonal mean circulation. The forced trends are currently weak compared to those caused by internal variability.

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Gidon Eshel
and
Naomi H. Naik

Abstract

The authors present climatologies of a numerical model of the Red Sea, focusing on the dynamics of winter intermediate water formation. Northward flowing boundary currents are identified as the major dynamical elements. At the northern boundary, the eastern current follows the geometry, eventually turning back to the south. At ∼26°N and the western wall the two boundary currents collide. At the collision site, the denser eastern current subducts under the western boundary current. The subduction forces the western boundary current eastward into the interior. Convection communicates the surface fluxes to the downwelled plume and intermediate water forms. The estimated rate, 0.11 Sv (Sv ≡ 106 m3 s−1), agrees with previous estimates. The authors identify basin-scale sea-surface tilt to the north due to variable thermohaline forcings as the key dynamical variable. The resultant geostrophic eastward cross-channel flow interacts with the boundaries and creates upwelling and surface topography spatial patterns that drive the coastal jets. Upwelling-induced vortex stretching dominates the vorticity balance and governs the separation of the western boundary current from the western wall. The process ceases in the summer.

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Richard Seager
,
Naomi Naik
, and
Gabriel A. Vecchi

Abstract

The mechanisms of changes in the large-scale hydrological cycle projected by 15 models participating in the Coupled Model Intercomparison Project phase 3 and used for the Intergovernmental Panel on Climate Change’s Fourth Assessment Report are analyzed by computing differences between 2046 and 2065 and 1961 and 2000. The contributions to changes in precipitation minus evaporation, PE, caused thermodynamically by changes in specific humidity, dynamically by changes in circulation, and by changes in moisture transports by transient eddies are evaluated. The thermodynamic and dynamic contributions are further separated into advective and divergent components. The nonthermodynamic contributions are then related to changes in the mean and transient circulation. The projected change in PE involves an intensification of the existing pattern of PE with wet areas [the intertropical convergence zone (ITCZ) and mid- to high latitudes] getting wetter and arid and semiarid regions of the subtropics getting drier. In addition, the subtropical dry zones expand poleward. The accentuation of the twentieth-century pattern of PE is in part explained by increases in specific humidity via both advection and divergence terms. Weakening of the tropical divergent circulation partially opposes the thermodynamic contribution by creating a tendency to decreased PE in the ITCZ and to increased PE in the descending branches of the Walker and Hadley cells. The changing mean circulation also causes decreased PE on the poleward flanks of the subtropics because the descending branch of the Hadley Cell expands and the midlatitude meridional circulation cell shifts poleward. Subtropical drying and poleward moistening are also contributed to by an increase in poleward moisture transport by transient eddies. The thermodynamic contribution to changing PE, arising from increased specific humidity, is almost entirely accounted for by atmospheric warming under fixed relative humidity.

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Richard Seager
,
Naomi Naik
, and
Laura Vogel

Abstract

The idea that global warming leads to more droughts and floods has become commonplace without clear indication of what is meant by this statement. Here, the authors examine one aspect of this problem and assess whether interannual variability of precipitation P minus evaporation E becomes stronger in the twenty-first century compared to the twentieth century, as deduced from an ensemble of models participating in Coupled Model Intercomparison Project 3. It is shown that indeed interannual variability of PE does increase almost everywhere across the planet, with a few notable exceptions such as southwestern North America and some subtropical regions. The variability increases most at the equator and the high latitudes and least in the subtropics. Although most interannual PE variability arises from internal atmosphere variability, the primary potentially predictable component is related to the El Niño–Southern Oscillation (ENSO). ENSO-driven interannual PE variability clearly increases in amplitude in the tropical Pacific, but elsewhere the changes are more complex. This is not surprising in that ENSO-driven PE anomalies are primarily caused by circulation anomalies combining with the climatological humidity field. As climate warms and the specific humidity increases, this term leads to an intensification of ENSO-driven PE variability. However, ENSO-driven circulation anomalies also change, in some regions amplifying but in others opposing and even overwhelming the impact of rising specific humidity. Consequently, there is sound scientific basis for anticipating a general increase in interannual PE variability, but the predictable component will depend in a more complex way on both thermodynamic responses to global warming and on how tropically forced circulation anomalies alter.

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Naomi Naik
,
Mark A. Cane
,
Semyon Basin
, and
Moshe Israeli

Abstract

A scheme is presented for solving the equation for barotropic ocean circulation, taking into account the special character of the problem: nearly inviscid motion following f/H contours in the ocean interior, with viscous effects closing the flow near western boundaries. Using a special compact finite-difference discretization, the scheme generates boundary layers without spurious oscillations and without demanding very high resolution. Sharp changes in topography and closed f/H contours (e.g., in the vicinity of high sea mounts) are also handled by the scheme in a way that localizes errors due to underresolved topographic features. Strategies are formulated for simplifying the connectedness of the domain by “sinking” the islands.

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Alexander Krupitsky
,
Vladimir M. Kamenkovich
,
Naomi Naik
, and
Mark A. Cane

Abstract

A linear equivalent barotropic (EB) model is applied to study the effects of the bottom topography H and baroclinicity on the total transport and the position of the Antarctic Circumpolar Current (ACC). The model is based on the observation of Killworth that the time mean velocity field of the FRAM Model is self-similar in the vertical.

A realistic large-scale topography H̄ is constructed by filtering 5-minute resolution data with an appropriate smoothing kernel. It is shown that the asymptotic behavior of the solution of the barotropic model (a particular case of the EB model) in the limit of very small bottom friction depends on subtle details of topography and basin geometry. Given the uncertainties of the smoothing procedure the authors conclude that the barotropic model is not robust with respect to possible variations of model topography.

The authors found that the EB model with a vertical profile function similar to that of Killworth reproduces the major features of the time- and depth-averaged FRAM solution, including the position and the transport of the ACC, reasonably well. The solution is robust with respect to uncertainties in H̄. The EB model is much improved by a parameterization of the bottom friction via near-bottom velocity, which tends to shut off the flow in the shallow regions.

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Richard Seager
,
Yochanan Kushnir
,
Celine Herweijer
,
Naomi Naik
, and
Jennifer Velez

Abstract

The causes of persistent droughts and wet periods, or pluvials, over western North America are examined in model simulations of the period from 1856 to 2000. The simulations used either (i) global sea surface temperature data as a lower boundary condition or (ii) observed data in just the tropical Pacific and computed the surface ocean temperature elsewhere with a simple ocean model. With both arrangements, the model was able to simulate many aspects of the low-frequency (periods greater than 6 yr) variations of precipitation over the Great Plains and in the American Southwest including much of the nineteenth-century variability, the droughts of the 1930s (the “Dust Bowl”) and 1950s, and the very wet period in the 1990s. Results indicate that the persistent droughts and pluvials were ultimately forced by persistent variations of tropical Pacific surface ocean temperatures. It is argued that ocean temperature variations outside of the tropical Pacific, but forced from the tropical Pacific, act to strengthen the droughts and pluvials. The persistent precipitation variations are part of a pattern of global variations that have a strong hemispherically and zonally symmetric component, which is akin to interannual variability, and that can be explained in terms of interactions between tropical ocean temperature variations, the subtropical jets, transient eddies, and the eddy-driven mean meridional circulation. Rossby wave propagation poleward and eastward from the tropical Pacific heating anomalies disrupts the zonal symmetry, intensifying droughts and pluvials over North America. Both mechanisms of tropical driving of extratropical precipitation variations work in summer as well as winter and can explain the year-round nature of the precipitation variations. In addition, land–atmosphere interactions over North America appear important by (i) translating winter precipitation variations into summer evaporation and, hence, precipitation anomalies and (ii) shifting the northward flow of moisture around the North Atlantic subtropical anticyclone eastward from the Plains and Southwest to the eastern seaboard and western Atlantic Ocean.

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Moshe Israeli
,
Naomi H. Naik
, and
Mark A. Cane

Abstract

A finite-difference scheme for solving the linear shallow water equations in a bounded domain is described. Its time step is not restricted by a Courant–Friedrichs–Levy (CFL) condition. The scheme, known as Israeli–Naik–Cane (INC), is the offspring of semi-Lagrangian (SL) schemes and the Cane–Patton (CP) algorithm. In common with the latter it treats the shallow water equations implicitly in y and with attention to wave propagation in x. Unlike CP, it uses an SL-like approach to the zonal variations, which allows the scheme to apply to the full primitive equations. The great advantage, even in problems where quasigeostrophic dynamics are appropriate in the interior, is that the INC scheme accommodates complete boundary conditions.

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Wilco Hazeleger
,
Richard Seager
,
Martin Visbeck
,
Naomi Naik
, and
Keith Rodgers

Abstract

Transient eddies in the atmosphere induce a poleward transport of heat and moisture. A moist static energy budget of the surface layer is determined from the NCEP reanalysis data to evaluate the impact of the storm track. It is found that the transient eddies induce a cooling and drying of the surface layer with a monthly mean maximum of 60 W m−2. The cooling in the midlatitudes extends zonally over the entire basin. The impact of this cooling and drying on surface heat fluxes, sea surface temperature (SST), water mass transformation, and vertical structure of the Pacific is investigated using an ocean model coupled to an atmospheric mixed layer model. The cooling by atmospheric storms is represented by adding an eddy-induced transfer velocity to the mean velocity in an atmospheric mixed layer model. This is based on a parameterization of tracer transport by eddies in the ocean. When the atmospheric mixed layer model is coupled to an ocean model, realistic SSTs are simulated. The SST is up to 3 K lower due to the cooling by storms. The additional cooling leads to enhanced transformation rates of water masses in the midlatitudes. The enhanced shallow overturning cells affect even tropical regions. Together with realistic SST and deep winter mixed layer depths, this leads to formation of homogeneous water masses in the upper North Pacific, in accordance to observations.

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Wilco Hazeleger
,
Richard Seager
,
Mark A. Cane
, and
Naomi H. Naik

Abstract

Pacific Ocean oceanic heat transport is studied in an ocean model coupled to an atmospheric mixed-layer model. The shallow meridional overturning circulation cells in the Tropics and subtropics transport heat away from the equator. The heat transport by the horizontal gyre circulation in the Tropics is smaller and directed toward the equator. The response of the Pacific oceanic heat transport to El Niño–like winds, extratropical winds, and variations in the Indonesian Throughflow is studied. Large, opposing changes are found in the heat transport by the meridional overturning and the horizontal gyres in response to El Niño–like winds. Consequently, the change in total heat transport is relatively small. The overturning transport decreases and the gyres spin down when the winds decrease in the Tropics. This compensation breaks down when the Indonesian Throughflow is allowed to vary in the model. A reduced Indonesian Throughflow, as observed during El Niño–like conditions, causes a large reduction of poleward heat transport in the South Pacific and affects the ocean heat transport in the southern tropical Pacific. Extratropical atmospheric anomalies can affect tropical ocean heat transport as the tropical thermocline is ventilated from the extratropics. The authors find that changes in the heat loss in the midlatitudes affect tropical ocean heat transport by driving an enhanced buoyancy-driven overturning that reaches into the Tropics. The results are related to observed changes in the overturning circulation in the Pacific in the 1990s, sea surface temperarture changes, and changes in atmospheric circulation. The results imply that the ratio of heat transport in the ocean to that in the atmosphere can change.

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