Tropical Pacific–Driven Decadel Energy Transport Variability

Abstract The atmospheric energy transport variability associated with decadal sea surface temperature variability in the tropical Pacific is studied using an atmospheric primitive equation model coupled to a slab mixed layer. The decadal variability is prescribed as an anomalous surface heat flux that represents the reduced ocean heat transport in the tropical Pacific when it is anomalously warm. The atmospheric energy transport increases and compensates for the reduced ocean heat transport. Increased transport by the mean meridional overturning (i.e., the strengthening of the Hadley cells) causes increased poleward energy transport. The subtropical jets increase in strength and shift equatorward, and in the midlatitudes the transients are affected. NCEP–NCAR reanalysis data show that the warming of the tropical Pacific in the 1980s compared to the early 1970s seems to have caused very similar changes in atmospheric energy transport indicating that these atmospheric transport variations were driven from the tropical Pacific. To study the implication of these changes for the coupled climate system an ocean model is driven with winds obtained from the atmosphere model. The poleward ocean heat transport increased when simulated wind anomalies associated with decadal tropical Pacific variability were used, showing a negative feedback between decadal variations in the mean meridional circulation in the atmosphere and in the Pacific Ocean. The Hadley cells and subtropical cells act to stabilize each other on the decadal time scale

Climate scenarios of sea level rise for the northeast Atlantic Ocean: a study including the effects of ocean dynamics and gravity changes induced by ice melt

Here we present a set of regional climate scenarios of sea level rise for the northeast Atlantic Ocean. In this study, the latest observations and results obtained with state-of-the-art climate models are combined. In addition, regional effects due to ocean dynamics and changes in the Earth’s gravity field induced by melting of land-based ice masses have been taken into account. The climate scenarios are constructed for the target years 2050 and 2100, for both a moderate and a large rise in global mean atmospheric temperature (2 °C and 4 °C in 2100 respectively). The climate scenarios contain contributions from changes in ocean density (global thermal expansion and local steric changes related to changing ocean dynamics) and changes in ocean mass (melting of mountain glaciers and ice caps, changes in the Greenland and Antarctic ice sheets, and (minor) terrestrial water-storage contributions). All major components depend on the global temperature rise achieved in the target periods considered. The resulting set of climate scenarios represents our best estimate of twenty-first century sea level rise in the northeast Atlantic Ocean, given the current understanding of the various contributions. For 2100, they yield a local rise of 30 to 55 cm and 40 to 80 cm for the moderate and large rise in global mean atmospheric temperature, respectively. <br/

Arctic decadal variability in a warming world

Natural decadal variability of surface air temperature might obscure Arctic temperature trends induced by anthropogenic forcing. It is therefore imperative to know how Arctic decadal variability (ADV) will change as the climate warms. In this study, we evaluate ADV characteristics in three equilibrium climates with present-day, double, and quadrupled atmospheric CO2 forcing. The dominant region of variability, which is located over the Barents and Greenland Sea at present, shifts to the central Arctic and Siberian regions as the climate warms. The maximum variability in sea ice cover and surface air temperature occurs in the CO2 doubling climate when sea ice becomes more vulnerable to melt over vast stretches of the Arctic. Furthermore, the links between dominant atmospheric circulation modes and Arctic surface climate characteristics vary strongly with climate change. For instance, a positive Arctic Oscillation index is associated with a colder Arctic in warmer climates, instead of a warmer Arctic at present. Such changing relationships are partly related to the retreat of sea ice because altered wind patterns influence the sea ice distribution and hence the associated local surface fluxes. The atmospheric pressure distributions governing ADV and the associated large-scale dynamics also change with climate warming. The changing character of the ADV shows that it is vital to consider (changes in) ADV when addressing Arctic warming in climate model projections

Impact of the Midlatitude Storm Track on the Upper Pacific Ocean

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

The effect of vertical ocean mixing on the tropical Atlantic in a coupled global climate model

Sea surface temperature (SST) biases in the tropical Atlantic are a long-standing problem among coupled global climate models (CGCMs). They occur in equilibrated state, as well as in initialised seasonal to decadal simulations. The bias is typically characterised by too high SST in upwelling regions and associated errors of wind and precipitation. We examine the SST bias in the state-of-the-art CGCM EC-Earth by means of an upper ocean heat budget analysis. Horizontal advection processes affect the SST bias development only to a small extent, and surface heat fluxes mostly dampen the warm bias. Subgrid-scale upper ocean vertical mixing is too low in EC-Earth when compared to estimates from reanalysis data, potentially giving rise to the warm bias. We perform sensitivity experiments to examine the effect of enhanced vertical mixing on the SST bias in quasi equilibrium present day climate and its impact on projected climate change. Enhanced mixing in historical simulation mode (MixUp pr) reduces the SST bias in the tropical Atlantic compared to the control experiment (Control pr). Associated atmospheric biases of precipitation and surface winds are also reduced in MixUp pr. We further perform climate projections under the RCP8.5 emission scenario (Control fu and MixUp fu). Under increasing greenhouse gas forcing, the tropical Atlantic warms by up to 4.5∘C locally, and maritime precipitation increases in boreal winter and spring. We show that the vertical mixing parameterisation influences future climate. In MixUp fu, SSTs remain 0.5∘C colder in boreal winter and spring, but increase with the same amplitude in summer and fall. The strength and location of the projected intertropical convergence zone also depends on the ocean vertical mixing efficiency. The rain band moves southward in summer, and its strength increases in winter in MixUp fu as compared to Control fu.</p

An EC-Earth coupled atmosphere–ocean single-column model (AOSCM.v1_EC-Earth3) for studying coupled marine and polar processes

Single-column models (SCMs) have been used as tools to help develop numerical weather prediction and global climate models for several decades. SCMs decouple small-scale processes from large-scale forcing, which allows the testing of physical parameterisations in a controlled environment with reduced computational cost. Typically, either the ocean, sea ice or atmosphere is fully modelled and assumptions have to be made regarding the boundary conditions from other subsystems, adding a potential source of error. Here, we present a fully coupled atmosphere–ocean SCM (AOSCM), which is based on the global climate model EC-Earth3. The initial configuration of the AOSCM consists of the Nucleus for European Modelling of the Ocean (NEMO3.6) (ocean), the Louvain-la-Neuve Sea Ice Model (LIM3) (sea ice), the Open Integrated Forecasting System (OpenIFS) cycle 40r1 (atmosphere), and OASIS3-MCT (coupler). Results from the AOSCM are presented at three locations: the tropical Atlantic, the midlatitude Pacific and the Arctic. At all three locations, in situ observations are available for comparison. We find that the coupled AOSCM can capture the observed atmospheric and oceanic evolution based on comparisons with buoy data, soundings and ship-based observations. The model evolution is sensitive to the initial conditions and forcing data imposed on the column. Comparing coupled and uncoupled configurations of the model can help disentangle model feedbacks. We demonstrate that the AOSCM in the current set-up is a valuable tool to advance our understanding in marine and polar boundary layer processes and the interactions between the individual components of the system (atmosphere, sea ice and ocean)

Oceanic heat transport into the Arctic under high and low CO2 forcing

Enhanced ocean heat transport into the Arctic is linked to stronger future Arctic warming and polar amplification. To quantify the impact of ocean heat transport on Arctic climate, it is imperative to understand how its magnitude and the associated mechanisms change in other climate states. This paper therefore assesses the ocean heat transport into the Arctic at 70∘N for climates forced with a broad range of carbon dioxide concentration levels, ranging from one-fourth to four times modern values. We focused on ocean heat transports through the Arctic entrances (Bering Strait, Canadian Archipelago, and Nordic Seas) and identified relative contributions of volume and temperature to these changes. The results show that ocean heat transport differences across the five climate states are dominated by heat transport changes in the Nordic Seas, although in the warmest climate state heat transport through the Bering Strait plays an almost equally important role. This is primarily caused by changes in horizontal currents owing to anomalous wind responses and to differential advection of thermal anomalies. Changes in sea ice cover play a prominent role by modulating the surface heat fluxes and the impact of wind stresses on ocean currents. The Atlantic meridional overturning circulation and its associated heat transport play a more modest role in the ocean heat transport into the Arctic. The net effect of these changes is that the poleward ocean heat transport at 70∘N strongly increases from the coldest climate to the warmest climate state