An assessment of the CMIP5 models in simulating the Argo geostrophic meridional transport in the North Pacifi c Ocean

Eleven climate system models that participate in the Coupled Model Intercomparison Project phase 5(CMIP5)were evaluated based on an assessment of their simulated meridional transports in comparison with the Sverdrup transports.The analyses show that the simulated North Pacifi c Ocean circulation is essentially in Sverdrup balance in most of the 11 models while the Argo geostrophic meridional transports indicate signifi cant non-Sverdrup gyre circulation in the tropical North Pacifi c Ocean.The climate models overestimated the observed tropical and subtropical volume transports signifi cantly.The non-Sverdrup gyre circulation leads to non-Sverdrup heat and salt transports,the absence of which in the CMIP5 simulations suggests defi ciencies of the CMIP5 model dynamics in simulating the realistic meridional volume,heat,and salt transports of the ocean.

The impact of concurrent variation of atmospheric meridional heat transport in western Baffen Bay and eastern Greenland on summer Arctic sea ice

Based on the climatological reanalysis data of the European Center for Medium-Range Weather Forecasts and the Arctic sea ice data of the National Snow and Ice Data Center, the relationship between the Arctic sea ice area(SIA)and the interannual variation of atmospheric meridional heat transport(AMHT) was analyzed. The results show that the atmospheric meridional heat transported by transient eddy(TAMHT) dominates the June AMHT in midhigh latitudes of the Northern Hemisphere, while the western Baffin Bay(B) and the eastern Greenland(G) are two gates for TAMHT entering the Arctic. TAMHT in the western Baffin Bay(B-TAMHT) and eastern Greenland(G-TAMHT) has a concurrent variation of reverse phase, which is closely related to the summer Arctic SIA.Possible mechanism is that the three Arctic atmospheric circulation patterns(AD, AO and NAO) in June can cause the concurrent variation of TAMHT in the B and G regions. This concurrent variation helps to maintain AD anomaly in summer through wave action and changes the polar air temperature, thus affecting the summer Arctic SIA. Calling the heat entering the Arctic as warm transport and the heat leaving Arctic as cold transport, then the results are classified into three situations based on B-TAMHT and G-TAMHT: warm B corresponding to cold G(WC), cold B corresponding to warm G(CW), cold B corresponding to cold G(CC), while warm B corresponding to warm G is virtually non-existent. During the WC situation, the SIA in the Pacific Arctic sediments and Kara Sea decreases;during the CW situation, the SIA in the Laptev Sea and Kara Sea decreases;during the CC situation, the SIA in the Kara Sea, Laptev Sea and southern Beaufort Sea increases.

Meridional Variation of the 1955-2003 Sea Level Anomalies in the Tropical Pacific Ocean Associated with El Nio Events

The Sea Level Anomaly-Torque （SLAT, relative to a reference location in the Pacific Ocean）, which means the total torque of the gravity forces of sea waters with depths equal to the Sea Level Anomaly （S/A） in the tropical Pacific Ocean, is defined in this study. The time series of the SLAT from merged altimeter data （1993-2003） had a great meridional variation during the 1997-1998 E1 Nifio event. By using historical upper layer temperature data （1955-2003） for the tropical Pacific Ocean, the temperature-based SLAT is also calculated and the meridional variation can be found in the historical E1 Nifio events （1955-2003）, which suggests that the meridional shifts of the sea level anomaly are also intrinsic oscillating modes of the E1 Nifio cycles like the zonal shifts.

Low-frequency variability of the shallow meridional overturning circulation in the South China Sea

The low-frequency variability of the shallow meridional overturning circulation（MOC） in the South China Sea（SCS） is investigated using a Simple Ocean Data Assimilation（SODA） product for the period of 1900-2010. A dynamical decomposition method is used in which the MOC is decomposed into the Ekman, external mode, and vertical shear components. Results show that all the three dynamical components contribute to the formation of the seasonal and annual mean shallow MOC in the SCS. The shallow MOC in the SCS consists of two cells： a clockwise cell in the south and an anticlockwise cell in the north; the former is controlled by the Ekman flow and the latter is dominated by the external barotropic flow, with the contribution of the vertical shear being to reduce the magnitude of both cells. In addition, the strength of the MOC in the south is found to have a falling trend over the past century, due mainly to a weakening of the Luzon Strait transport（LST） that reduces the transport of the external component. Further analysis suggests that the weakening of the LST is closely related to a weakening of the westerly wind anomalies over the equatorial Pacific, which leads to a southward shift of the North Equatorial Current（NEC） bifurcation and thus a stronger transport of the Kuroshio east of Luzon.

Heat and salt transport throughout the North Pacific Ocean

Absolute geostrophic currents in the North Pacific Ocean are calculated using the P-vector method and gridded Argo profiling data from January 2004 to December 2012. Three-dimensional structures and seasonal variability of meridional heat transport(MHT) and meridional salt transport(MST) are analyzed. The results show that geostrophic and Ekman components are generally opposite in sign, with the southward geostrophic component dominating in the subtropics and the northward Ekman component dominating in the tropics. In combination with the net surface heat flux and the MST through the Bering Strait, the MHT and MST of the western boundary currents(WBCs) are estimated for the fi rst time. The results suggest that the WBCs are of great importance in maintaining the heat and salt balance of the North Pacifi c. The total interior MHT and MST in the tropics show nearly the same seasonal variability as that of the Ekman components, consistent with the variability of zonal wind stress. The geostrophic MHT in the tropics is mainly concentrated in the upper layers, while MST with large amplitude and annual variation can extend much deeper. This suggests that shallow processes dominate MHT in the North Pacific, while MST can be affected by deep ocean circulation. In the extratropical ocean, both MHT and MST are weak. However, there is relatively large and irregular seasonal variability of geostrophic MST, suggesting the importance of the geostrophic circulation in the MST of that area.

A Global Ocean Biogeochemistry General Circulation Model and its Simulations

An ocean biogeochemistry model was developed and incorporated into a global ocean general circulation model （LICOM） to form an ocean biogeochemistry general circulation model （OBGCM）. The model was used to study the natural carbon cycle and the uptake and storage of anthropogenic CO2 in the ocean. A global export production of 12.5 Pg C yr-1 was obtained. The model estimated that in the pre-industrial era the global equatorial region within ~15~ of the equator released 0.97 Pg C yr-1 to the atmosphere, which was balanced by the gain of CO2 in other regions. The post-industrial air sea CO2 flux indicated the oceanic uptake of CO2 emitted by human activities. An increase of 20-50 ~mol kg-1 for surface dissolved inorganic carbon （DIC） concentrations in the 1990s relative to pre-industrial times was obtained in the simulation, which was consistent with data-based estimates. The model generated a total anthropogenic carbon inventory of 105 Pg C as of 1994, which was within the range of estimates by other researchers. Various transports of both natural and anthropogenic DIC as well as labile dissolved organic carbon （LDOC） were estimated from the simulation. It was realized that the Southern Ocean and the high-latitude region of the North Pacific are important export regions where accumulative air-sea CO2 fluxes are larger than the DIC inventory, whereas the subtropical regions are acceptance regions. The interhemispheric transport of total natural carbon （DIC＋LDOC） was found to be northward （0.11 Pg C yr-1）, which was just balanced by the gain of carbon from the atmosphere in the Southern Hemisphere.

Interbasin exchanges and their roles in global ocean circulation:A study based on 1 400 years' spin up of MOM4p1

A global prognostic model based on MOM4p1,which is a primitive equation nonBoussinesq numerical model,has been integrated with 1 400 years from the state of rest based on the realistic topography to study the long-term pattern of combined wind-driven and thermodynamically-driven general circulation.The model is driven by monthly climatological mean forces and includes 192×189 horizontal grids and 31 pressure-based vertical levels.The main objective is to investigate the mass and heat transports at interbasin passages and their compensations and roles in the global ocean circulation under equilibrium state of long-term spin up.The kinetic energy analysis divides the spin up process into three stages：the quasi-stable state of wind driven current,the growing phase of thermodynamical circulation and the equilibrium state of thermohaline circulation.It is essential to spin up over a thousand years in order to reach the thermohaline equilibrium state from a state of rest.The Arctic Throughflow from the Bering Strait to the Greenland Sea and the Indonesian Throughflow（ITF） are captured and examined with their compensations and existing data.Analysis reveals that the slope structures of sea surface height are the dynamical driving mechanism of the Pacific-Arctic-Atlantic throughflow and ITF.The analysis denotes,in spite of O（1.4×106m3/s） of the southward volume transport in the northern Atlantic,that there is still O（1 PW） of heat transported northward since the northward currents in the upper layer carry much higher temperature water than the southward flowing northern Atlantic deep water（NADW）.Meridional volume and heat transports are focused on the contributions to NADW renewals and Atlantic meridional overturning circulation（AMOC）.Quantitative descriptions of the interbasin exchanges are explained by meridional compensations and supported by previous observations and numerical modeling results.Analysis indicates that the volume and heat exchanges on the interbasin passages proposed in this article manifest their hub roles in the Great Ocean Conveyor System.

Analysis of Decadal Climate Variability in the Tropical Pacific by Coupled GCM

This study presents the spatial and temporal structures of the decadal variability of the Pacific from an extended control run of a coupled global climate model (GCM).The GCM used was version-g2.0 of the Flexible Global Ocean Atmosphere Land System (FGOALS-g2.0) developed at LASG/IAP.The GCM FGOALS-g2.0 re-produces similar spatial-temporal structures of sea surface temperature (SST) as observed in the Pacific decadal os-cillation (PDO) with a significant period of approximately 14 years.Correspondingly,the PDO signals were closely related to the decadal change both in the upper-ocean temperature anomalies and in the atmospheric circulation.The present results suggest that warm SST anomalies along the equator relax the trade winds,causing the SSTs to warm even more in the eastern equatorial Pacific,which is a positive feedback.Meanwhile,warm SST anomalies along the equator force characteristic off-equa-torial wind stress curl anomalies,inducing much more poleward transport of heat,which is a negative feedback.The upper-ocean meridional heat transport,which is asso-ciated with the PDO phase transition,links the equatorial to the off-equatorial Pacific Ocean,acting as a major mechanism responsible for the tropical Pacific decadal variations.Therefore,the positive and negative feedbacks working together eventually result in the decadal oscilla-tion in the Pacific.

IMPACTS OF THE ONSET OF THE SOUTHEAST ASIAN SUMMER MONSOON ON THE BEGINNING OF THE RAINY SEASON IN YUNNAN

Summer monsoon in Southeast Asia can cause large-scale precipitation in the region in early summer, which is featured by prevailing low-level southwesterly from the Bay of Bengal to South China Sea （SCS）. It has characteristics of its own as well as those of Asian monsoons in general. As found in studies over recent years on East Asian monsoons, the earliest onset of the Southeast Asian summer monsoon occurs in early summer over the SCS, among all members of the monsoon system. It then advances westward to India and northward to eastern China, Japan and Korean Peninsula. As pointed out by Lau and Yang121, the end of April is the earliest time when the Asian monsoon sets up at the southern tip of Indo-china Peninsula. Being the earliest signal for the whole summer monsoon system in Asia, it may be of some predictive value for the establishment of Asian summer monsoon （ASM）.

Simulated Spatiotemporal Response of Ocean Heat Transport to Freshwater Enhancement in North Atlantic and Associated Mechanisms

The Atlantic Meridional Overturning Circulation （AMOC） transports a large amount of heat to northern high latitudes, playing an important role in the global climate change. Investigation of the freshwater perturbation in North Atlantic （NA） has become one of the hot topics in the recent years. In this study, the mechanism and pathway of meridional ocean heat transport （OHT） under the enhanced freshwater input to the northern high latitudes in the Atlantic are investigated by an ocean-sea ice-atmosphere coupled model. The results show that the anomalous OHT in the freshwater experiment （FW） is dominated by the meridional circulation kinetic and ocean thermal processes. In the FW, OHT drops down during the period of weakened AMOC while the upper tropical ocean turns warmer due to the retained NA warm currents. Conversely, OHT recovers as the AMOC recovers, and the mechanism can be generalized as： 1） increased ocean heat content in the tropical Southern Ocean during the early integration provides the thermal condition for the recovery of OHT in NA; 2） the OttT from the Southern Ocean enters the NA through the equator along the deep Ekman layer; 3） in NA, the recovery of OHT appears mainly along the isopycnic layers of 24.70- 25.77 below the mixing layer. It is then transported into the mixing layer from the ＂outcropping points＂ in northern high latitudes, and finally released to the atmosphere by the ocean-atmosphere heat exchange.