¹Lamont-Doherty Earth Observatory of Columbia University Palisades, New York, USA

Kushnir@Ideo.columbia.edu

　

²University of Illinois at Urbana-Champaign Urbana, Illinois, USA

　

It is well known that sea surface temperature (SST) anomalies in the extratropical ocean are forced by the atmosphere through changes in surface heat and momentum exchange. The question that continues to puzzle us is, to what extent do these SST anomalies affect the overlying atmosphere and whether they can imprint their time scale on the latter. Recent experiments with atmospheric models forced with prescribed SST anomalies show that a weak positive feedback can occur under some circumstances and that it is mediated by the response of the baroclinic-eddy storm tracks to these SST anomalies. Here we present evidence from observations that such feedback exists in the North Atlantic Basin, along the sharp surface temperature front associated with the Gulf Stream. The leading pattern of SST variability in the Basin is associated with changes in the strength of this temperature gradient. These changes evolve through the winter season and with them dose the intensity of the Atlantic storm track. The changes in eddy activity in the storm tracks feed back positively on the equivalent barotropic wind perturbation that force the SST anomaly in the first place. Such feedback can explain the enhanced wintertime persistent of the prominent atmospheric low-frequency pattern - the North Atlantic Oscillation (NAO) - and may also be the cause of its significant interannual persistence.

　

　

¹Hokkaido University Sapporo, JAPAN

tanimoto@ees.hokudai.ac.jp

　

²IPRC/Univerisity of Hawaii Honolulu, Hawaii, USA

xie@soest.hawaii.edu

　

Tropical Atlantic shipboard observations show that the meridional gradient of sea surface temperature anomaly (SSTA) varies on the decadal time scales. Associated local maxima of this SSTA gradient located at 10-15 degrees latitude in either side of the equator have comparable amplitude with an opposite polarity to one another. Given positive (negative) SSTAs in the northern (southern) tropics, negative (positive) sea surface pressure anomalies (SLPAs) are formed, respectively. Then, this SLPA gradient induces the northward surface wind across the equator. The Colioris force turns the surface wind to the eastward (westward) north (south) of the equator and weaken (enhance) the climatological trade wind. Changes in trade wind tend to keep the polarity of the local SSTA via latent heat flux variability. This wind-evaporation-SST feedback is partly presented from observed meteorological variables.

　

However, a closer look to the atmospheric signal south of the equator bares weaker signal of SLPA and surface wind anomalies compared to those north of the equator despite comparable amplitudes of two local SSTA maxima. As an additional process in forming SSTAS in the southern tropics, we find low-level cloud shielding of incoming solar radiation. Coherent cloudiness anomalies are observed to be associated with the SST decadal variations. Since surface convergence is not associated with these cloud changes, low-level clouds are stratus that is trapped at the top of the planetary boundary layer. Warm (cold) SSTAs decreases (increases) stratification via changes in vertical temperature gradient. Weakened (enhanced) capping of the boundary layer leads to stratus decreases (increases), which will in turn cause a further SST warming (cooling). These processes complete a positive feedback loop between local SST and stratus.

　

　

Department of Earth and Planetary Science, Graduate School of Science The University of Tokyo, Tokyo, JAPAN

Masumoto@eps.s.u-tokyo.ac.jp

　

The Indonesian throughflow has been considered to be an important component of the current systems that affect the climate change of the earth. It connects the equatorial Pacific Ocean to the tropical Indian Ocean through the Indonesian archipelago. In this paper, the intraseasonal-to-interaunual variability of the Indonesian throughflow simulated in ocean general circulation models (OGCM) is described in detail, and mechanisms responsible for the variability are demonstrated.

　

The intraseasonal variability of the throughflow in a high-resolution OGCM is associated with mesoscale eddies in the Sulawesi Sea. The cyclonic eddies, with a diameter of about 400km, are generated at the entrance of the Sulawesi Sea between the Mindanao and the Halmahera Islands with 40 days interval. The intraseasonal eddies significantly affect the volume transport through northern passages of the Indonesian archipelago. However, they are highly damped within the Indonesian seas in the present model.

　

The large amplitude interannual variation of the throughflow is simulated by an OGCM, driven by the ERS satellite winds from July 1992 to June 1997. The simulated net Indonesian throughflow shows large transport from the Pacific to the Indian Ocean during 1994/95 and small transport during 1992 and 1997. It turns out that the interannual variation has a good association with the upper ocean variability in the Indian Ocean, which is strongly related to the Indian Ocean Dipole events. Contrary, the correspondence with the Pacific ENSO events is rather low during the period of simulation.

　

　

¹International Pacific Research Center, Univerisity of Hawaii Honolulu, Hawaii, USA

tjensen@hawaii.edu

　

²Frontier Research System for Global Change Yokohama City, Kanagawa, JAPAN

　

³Japan Marine Science and Technology Center Yokosuka City, Kanagawa, JAPAN

　

The North Equatorial Current bifurcates when it reaches the continental shelf by the Philippines Islands. It branches into the northward flowing Kuroshio and the southward flowing Mindanao Current. The bifurcation latitude is about 14゜N on average, but a recent reanalysis of observations by Qu and Lukas shows that the bifurcation latitude varies with the seasons and with depth. During the boreal summer the bifurcation latitude is shifted equatorward, while a poleward shifi is found during the northern winter.

　

Simulations from three different numerical ocean models have been analyzed and compared to the bifurcation latitudes from the reanalysis. A wind-driven 4.5 layer model of the global ocean is able to reproduce the main characteristics of the bifurcation, which demonstrates that the controlling factor mainly is wind stress related. Experiments with different wind stress climatologies also show sensitivity to wind forcing. Simulations with two GCMs, one covering the Pacific and Indian Oceans and based on the Princeton Ocean Model and a second, the global JAMSTEC model based on the Modular Ocean Model, show a seasonal cycle in phase with the observations. However, the amplitude is smaller and the increase in bifurcation latitude with depth is less than found in observations.

　

　

IPRC, Univerisity of Hawaii Honolulu, Hawaii, USA

humiom@soest.hawaii.edu

and

Frontier Research System for Global Change Yokohama, JAPAN

　

Western-boundary currents are of special interest for climate issues because they transport so much heat and salt, and because they are primary pathways of intergyre communication. In the North Pacific basin, the Kuroshio and the Oyashio are the western-boundary currents associated with the subtropical and the subpolar gyre, respectively. They join off the coast of Japan, forming the Mixed Water Region where a large amount of intergyre exchange appears to take place. They then flow eastward to form the Kuroshio and Oyashio Extensions that export heat, salt, and tracers into the ocean interior. Decadal SST variations have been found to be most pronounced in the mid-latitude North Pacific, rather than the tropics, particularly in the Kuroshio-Oyashio confluence zone, and they are linked to the PNA.

　

In this paper we will present results of simulation using a high-resolution regional model of the Kuroshio-Oyashio confluence, focusing on the processes in the intermediate layer where water masses of the subtropical and subarctic origin meets and mix. The model represents the major characteristics of the Kuroshio and the Oyashio system well such as the separation of the Kuroshio from the Japanese coast around 35 degrees north and southward intrusion of the Oyashio to 38 degrees north.

　

Pathways of the subpolar water to the subtropics, which closely resembles those of CTD surveys, have been successfully simulated in this model. The main pathway forms in a close vicinity of the Japanese coast. The subpolar water is characterized by fresh and low-Potential Vorticity (PV) water that outflows from the Sea of Okhotsk. Impacts of this outflow is identified by conducting an experiment in which the exchange of water mass between the Pacific Ocean and the Sea of Okhotsk is blocked. The impacts are striking. Without the exchange, the warm and salty water originated from the Kuroshio occupies whole region of the MWR. This is a consequence of an abnormally strong cyclonic recirculation north of the Kuroshio Extension, which advects the Kuroshio water northward up to the Oyashio Front via mean giotoropic pathways. The low-PV Oyashio intrusion reduces this cyclonic circulation greatly. These results indicate that the Oyashio southward intrusion is important in the dynamics of the confluence zone.