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Elena Yulaeva and John M. Wallace

Abstract

Global temperature anomalies associated with ENSO are investigated, making use of a 13-year record of gridded temperature and precipitation data from the microwave sounding unit (MSU). The warm phase of the ENSO cycle during this period was characterized by an overall warming of the tropical troposphere, superimposed upon a distinctive equatorially symmetric dumbbell-shaped pattern straddling the equator near 140°W, accompanied by negative anomalies along the equator over the western Pacific. By means of singular value decomposition (SVD) analysis it is shown that this pattern fluctuated in phase with the displacements of convective activity over the equatorial Pacific, as reflected in the anomalies in outgoing longwave radiation (OLR) and MSU precipitation fields. Fluctuations in mean tropical tropospheric temperature lagged the OLR anomalies and the related temperature pattern by about 3 months. The same dumbbell-shaped pattern was evident, with reversed polarity, in the lower stratosphere, together with the zonally symmetric signature of the quasi-biennial oscillation.

The dumbbell-shaped temperature pattern is related to the off-equatorial upper-tropospheric gyres that have been identified in previous studies. It can be interpreted as the dynamical response to shifts in the distribution of diabatic heating in the equatorial belt. It resembles the linear response to an equatorial heat source, but its major centers of action are shifted slightly eastward. It is detectable in SVD analysis for each season, but appears to be best organized around March, the season in which the equatorial cold tongue is weakest and precipitation anomalies associated with the ENSO cycle impact the equatorial dry zone most strongly.

The fluctuations in mean tropical tropospheric temperature that occur in association with the ENSO cycle are highly coherent with the fluctuations in surface air temperature over the tropical landmasses and sea surface temperatures over the tropical Indian and North Atlantic Oceans. It is argued that these fluctuations represent a thermodynamic response to the perturbations in the surface heat balance induced by the ENSO cycle in the eastern equatorial Pacific. They can be simulated by means of a simple thermodynamic model with a linear damping, an empirically determined heat capacity, and forcing proportional to the observed sea surface temperature anomalies in the cold tongue region of the equatorial eastern Pacific.

The warming of the tropical troposphere is accompanied by a strengthening of the zonally averaged jet stream in both hemispheres induced by an intensified Hadley circulation.

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Elena Yulaeva, Niklas Schneider, David W. Pierce, and Tim P. Barnett

Abstract

Potential predictability of low-frequency climate changes in the North Pacific depends on two main factors. The first is the sensitivity of the atmosphere to ocean-induced anomalies at the sea surface in midlatitudes. The second is the degree of teleconnectivity of the tropical low-frequency variability to midlatitudes. In contrast to the traditional approach of prescribing sea surface temperature (SST) anomalies, the response of a coupled atmospheric general circulation (CCM3)–mixed layer ocean model to oceanic perturbations of the mixed layer heat budget is examined. Since positive oceanic heat flux perturbations partially increase SST anomalies (locally), and partially are vented directly into the atmosphere, expressing boundary forcing on the atmosphere by prescribing upper-ocean heat flux anomalies allows for better understanding of the physical mechanism of low-frequency variability in midlatitudes. In the framework of this approach SST is considered to be a part of the adjustment of the coupled system rather than an external forcing. Wintertime model responses to mixed layer heat budget perturbations of up to 40 W m−2 in the Kuroshio extension region and in the tropical central Pacific show statistically significant anomalies of 500-mb geopotential height (Z500) in the midlatitudes. The response to the tropical forcing resembles the well-known Pacific–North American pattern, one of the leading modes of internal variability of the control run. The amplitude of the Z500 geopotential height reaches 40 m in the region of the Aleutian low. The response of Z500 to forcing in the Kuroshio Current extension region resembles the mixture of western Pacific and Pacific–North American patterns, the first two modes of the internal variability of the atmosphere. In midlatitudes this response is equivalent barotropic, with the maximum of 80 m at (60°N, 160°W). Examination of the vorticity and thermodynamic budgets reveals the crucial role of submonthly transient eddies in maintaining the anomalous circulation in the free atmosphere.

At the surface the response manifests itself in changes of surface temperature and the wind stress. The amplitude of response to the tropical forcing in the SST field at the Kuroshio Current extension region is up to 0.3 K (in absolute value) that is 2 times weaker than SST anomalies induced by midlatitude forcing of the same amplitude. In addition, the spatial structures of the responses in the SST field over the North Pacific are different. While tropical forcing induces SST anomalies in the central North Pacific, the midlatitude forcing causes SST anomalies off the east coast of Japan, in the Kuroshio–Oyashio extension region. Overall, remote tropical forcing appears to be effective in driving anomalies over the central North Pacific. This signal can be transported westward by the oceanic processes. Thus tropical forcing anomalies can serve as a precursor of the changes over the western North Pacific.

In the case of midlatitude forcing, the response in the wind stress field alters Ekman pumping in such a way that the expected change of the oceanic gyre, as measured by the Sverdrup transport, would counteract the prescribed forcing in the Kuroshio extension region, thus causing a negative feedback. This response is consistent with the hypothesis that quasi-oscillatory decadal climate variations in the North Pacific result from midlatitude ocean–atmosphere interaction.

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Elena Yulaeva, James R. Holton, and John M. Wallace

Abstract

An analysis of monthly mean, zonally averaged temperatures from the lower-stratospheric channel of the microwave sounding unit (MSU-4) shows that on the month-to-month time scale, there is nearly complete compensation between temperature changes in the tropics and in the extratropics. For the annual cycle the MSU-4 data show a similar compensation between temperatures in the tropics and those in high latitudes, with only a small residual variation in the global mean. The tropics are coldest in January and warmest in July, compensating for the warmer wintertime temperatures in the Northern Hemisphere compared to those in the Southern Hemisphere. These out-of-phase temperature variations between the tropics and extratropics are interpreted as the signature of an annual cycle in the strength of the wave-driven, Lagrangian mean meridional circulation, which warms the high-latitude winter hemisphere and cools the tropics. The observed phase of the annual cycle in tropical lower-stratospheric temperatures is thus determined by the stronger orographic and thermal forcing of the wintertime planetary waves in the Northern Hemisphere, which drives a stronger Lagrangian mean meridional circulation. In the absence of fluctuations in the global mean diabatic heating, the compensation would be complete and the globally averaged temperature would be constant. The small annual cycle in the globally averaged temperature in the MSU-4 data is nearly in phase with the annual cycle in the tropics and is consistent with the annual cycle in diabatic heating by ozone.

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Andrew S. Delman, Julie L. McClean, Janet Sprintall, Lynne D. Talley, Elena Yulaeva, and Steven R. Jayne

Abstract

Eddy–mean flow interactions along the Kuroshio Extension (KE) jet are investigated using a vorticity budget of a high-resolution ocean model simulation, averaged over a 13-yr period. The simulation explicitly resolves mesoscale eddies in the KE and is forced with air–sea fluxes representing the years 1995–2007. A mean-eddy decomposition in a jet-following coordinate system removes the variability of the jet path from the eddy components of velocity; thus, eddy kinetic energy in the jet reference frame is substantially lower than in geographic coordinates and exhibits a cross-jet asymmetry that is consistent with the baroclinic instability criterion of the long-term mean field. The vorticity budget is computed in both geographic (i.e., Eulerian) and jet reference frames; the jet frame budget reveals several patterns of eddy forcing that are largely attributed to varicose modes of variability. Eddies tend to diffuse the relative vorticity minima/maxima that flank the jet, removing momentum from the fast-moving jet core and reinforcing the quasi-permanent meridional meanders in the mean jet. A pattern associated with the vertical stretching of relative vorticity in eddies indicates a deceleration (acceleration) of the jet coincident with northward (southward) quasi-permanent meanders. Eddy relative vorticity advection outside of the eastward jet core is balanced mostly by vertical stretching of the mean flow, which through baroclinic adjustment helps to drive the flanking recirculation gyres. The jet frame vorticity budget presents a well-defined picture of eddy activity, illustrating along-jet variations in eddy–mean flow interaction that may have implications for the jet’s dynamics and cross-frontal tracer fluxes.

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Elen M. C. Cutrim, David W. Martin, Dean G. Butzow, Isa M. Silva, and Elena Yulaeva

Abstract

This paper presents results of a pilot study of rainfall along the part of the Amazon River that flows through Brazil. Rain was measured at three stations, one for each of three regimes: coastal, interior bottomland, and interior upland. For each station the record from 1 January 1988 through 31 December 1990 was parsed into accumulation periods of 1 h. Storms on the coast tended to be more showery than those in the interior and storms in the interior upland tended to be more showery than those in the interior lowland. The diurnal cycle varied with distance from the Amazon River as well as with distance from the Atlantic coast.

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Simon J. Mason, Lisa Goddard, Nicholas E. Graham, Elena Yulaeva, Liqiang Sun, and Philip A. Arkin

The International Research Institute for Climate Prediction (IRI) was formed in late 1996 with the aim of fostering the improvement, production, and use of global forecasts of seasonal to interannual climate variability for the explicit benefit of society. The development of the 1997/98 El Niño provided an ideal impetus to the IRI Experimental Forecast Division (IRI EFD) to generate seasonal climate forecasts on an operational basis. In the production of these forecasts an extensive suite of forecasting tools has been developed, and these are described in this paper. An argument is made for the need for a multimodel ensemble approach and for extensive validation of each model's ability to simulate interannual climate variability accurately. The need for global sea surface temperature forecasts is demonstrated. Forecasts of precipitation and air temperature are presented in the form of “net assessments,” following the format adopted by the regional consensus forums. During the 1997/98 El Niño, the skill of the net assessments was greater than chance, except over Europe, and in most cases was an improvement over a forecast of persistence of the latest month's climate anomaly.

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