The First Decade of the New Century: A Cooling Trend for Most of Alaska
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During the first decade of the 21st century most of Alaska experienced a cooling shift, modifying the long-term warming trend, which has been about twice the global change up to this time. All of Alaska cooled with the exception of Northern Regions. This trend was caused by a change in sign of the Pacific Decadal Oscillation (PDO), which became dominantly negative, weakening the Aleutian Low. This weakening results in less relatively warm air being advected from the Northern Pacific. This transport is especially important in winter when the solar radiation is weak. It is during this period that the strongest cooling was observed. In addition, the cooling was especially pronounced in Western Alaska, closest to the area of the center of the Aleutian Low. The changes seen in the reanalyzed data were confirmed from surface observations, both in the decrease of the North-South atmospheric pressure gradient, as well as the decrease in the mean wind speeds for stations located in the Bering Sea area.Keywords:
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Decadal variability in the North Pacific is typically described by the Pacific Decadal Oscillation (PDO). While many aspects of Pacific climate anomalies are associated with the PDO, variations of oceanic salinity, nutrients and the ecosystem off the coast of North America are independent of the PDO. A 50+ year ocean hindcast in the North-East Pacific suggests that the North Pacific Gyre Oscillation (NPGO; Di Lorenzo et al. 2008), the second empirical orthogonal function of sea surface height, reproduces these time series in the California Current and Alaskan Gyre. The mode describes a spin-up (or down) of the subtropical and Alaskan gyres, and an acceleration of the North Pacific Current. Associated sea surface temperature patterns are similar to the Victoria Mode, the second EOF of North Pacific surface temperature anomalies. With a lag of a few years, the NPGO is linked to strength of Kuroshio Extension via Rossby wave dynamics. The NPGO is the oceanic expression of the North Pacific Oscillation, a dipole sea level pressure pattern that results from intrinsic atmospheric mid-latitude variability and interactions with the El Nino/Southern Oscillation.
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Empirical orthogonal functions
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During the first decade of the 21st century most of Alaska experienced a cooling shift, modifying the long-term warming trend, which has been about twice the global change up to this time. All of Alaska cooled with the exception of Northern Regions. This trend was caused by a change in sign of the Pacific Decadal Oscillation (PDO), which became dominantly negative, weakening the Aleutian Low. This weakening results in less relatively warm air being advected from the Northern Pacific. This transport is especially important in winter when the solar radiation is weak. It is during this period that the strongest cooling was observed. In addition, the cooling was especially pronounced in Western Alaska, closest to the area of the center of the Aleutian Low. The changes seen in the reanalyzed data were confirmed from surface observations, both in the decrease of the North-South atmospheric pressure gradient, as well as the decrease in the mean wind speeds for stations located in the Bering Sea area.
North Pacific High
Siberian High
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The relationship between the tropical and North Pacific sea surface temperature (SST) variations is reexamined following the results of Deser and Blackmon (1995, DB95) based on a much longer period of data (1949–2010). As in DB95, the two leading SST modes, the El Niño‐Southern Oscillation (ENSO) mode and the North Pacific mode, represent the SST variations in the Pacific domain before 1992. Considering the period after 1992, however, one needs to consider a new mode of SST variation along with the two modes mentioned to understand the relationship between the tropical and North Pacific SST variations. A new SST mode, known as the Warm Pool mode, exhibits a strong variance in the warm pool region and undergoes a phase shift after the mid‐1990s, reflecting a warming in the warm pool region and a cooling in the central and eastern equatorial Pacific. It is found that the Warm Pool mode accompanies the North Pacific Oscillation‐like atmospheric variability over the North Pacific. Through this teleconnection, the Warm Pool mode mostly shows a relationship between the warm pool SST and the associated North Pacific SST component and which has some similarities with the North Pacific Gyre Oscillation.
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El Niño–Southern Oscillation (ENSO) events are accompanied by an anomalous zonal sea surface temperature (SST) gradient over the west Pacific Ocean, defined here as the west Pacific SST gradient (WPG). The WPG is defined as the standardized difference between area-averaged SST over the central Pacific Ocean (Niño-4 region) and west Pacific Ocean (0°–10°N, 130°–150°E). While the direction of the WPG follows ENSO cycles, the magnitude of the gradient varies considerably between individual El Niño and La Niña events. In this study, El Niño and La Niña events are grouped according to the magnitude of the WPG, and tropical SST, circulations, and precipitation are examined for the period 1948–2011. Until the 1980s the WPG showed little trend as the west and central Pacific warmed at similar rates; however, the west Pacific has recently warmed faster than the central Pacific, which has resulted in an increased WPG during La Niña events. The temporal evolution and distribution of tropical Pacific SST as well as the near-surface tropical Pacific zonal wind, divergence, and vertical velocity are considerably different during ENSO events partitioned according to the strength of the WPG. Modifications to the tropical circulation, resulting in changes to Indo– west Pacific precipitation, are linked to strong and consistent circulation and precipitation modifications throughout the Northern Hemisphere during winter.
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Atmospheric Circulation
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The anomalies of sea surface temperature(SST) in equatorial central and eastern Pacific are important indexes for the research of ENSO(El Ni?o-South Oscillation),. Based on the oceanic reanalysis, the relationship between the fluctuations of the water(T≥28℃) of the western Pacific pool(WPWP) and SST anomalies in equatorial central and eastern Pacific was investigated with focus on the interannual variability of the three-dimensional WPWP water, and from the view of the evolution of three-dimensional distribution of the WPWP water preceding the emergence of SST anomalies in key Ni?o regions and its important role in controlling the pattern of mass and heat in upper ocean, the intrinsic consistency among the anomalous changes in SST in equatorial central and eastern Pacific was probed into, which was expected to provide the new clue and basis for the mechanism and forecast of ENSO. The analysis suggests that the dominant pattern of spatio-temporal variation on ENSO time- scale as for three-dimensional WPWP water is the zonal oscillation with a zonal-averaged equilibrium line around 170 oE. At the same time, its remarkable characteristic is the formation of an anomalous double cantilever warm pool in the central and eastern tropical Pacific at the early stage(leading by around 6 months) of variations of SST anomalies in key Ni?o regions. The interannual zonal oscillation of the WPWP water is one of prerequisites for the anomalous changes of SST in key Ni?o regions. In virtue of the development of the anomalous warm pool with different intensity(generally refer to the volume of water and the magnitude of water depth and extent) in central and eastern Pacific which favors the zonal reconfiguration of the mass and heat in the upper ocean, the WPWP zonal oscillation is very likely to play decisive role as a original driver in the ocean to trigger the different EI Nino events, namely, that there is a sense in which the different flavors of ENSO events are to a great extent homeomorphous.
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Anomaly (physics)
Surface air temperature
Atmospheric Circulation
North Pacific High
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