How are large western hemisphere warm pools formed? (original) (raw)
Climate Response to Anomalously Large and Small Atlantic Warm Pools during the Summer
Journal of Climate, 2008
This paper uses the NCAR Community Atmospheric Model to show the influence of Atlantic warm pool (AWP) variability on the summer climate and Atlantic hurricane activity. The model runs show that the climate response to the AWP's heating extends beyond the AWP region to other regions such as the eastern North Pacific. Both the sea level pressure and precipitation display a significant response of low (high) pressure and increased (decreased) rainfall to an anomalously large (small) AWP, in areas with two centers located in the western tropical North Atlantic and in the eastern North Pacific. The rainfall response suggests that an anomalously large (small) AWP suppresses (enhances) the midsummer drought, a phenomenon with a diminution in rainfall during July and August in the region around Central America. In response to the pressure changes, the easterly Caribbean low-level jet is weakened (strengthened), as is its westward moisture transport. An anomalously large (small) AWP weakens (strengthens) the southerly Great Plains low-level jet, which results in reduced (enhanced) northward moisture transport from the Gulf of Mexico to the United States east of the Rocky Mountains and thus decreases (increases) the summer rainfall over the central United States, in agreement with observations. An anomalously large (small) AWP also reduces (enhances) the tropospheric vertical wind shear in the main hurricane development region and increases (decreases) the moist static instability of the troposphere, both of which favor (disfavor) the intensification of tropical storms into major hurricanes. Since the climate response to the North Atlantic SST anomalies is primarily forced at low latitudes, this study implies that reduced (enhanced) rainfall over North America and increased (decreased) hurricane activity due to the warm (cool) phase of the Atlantic multidecadal oscillation may be partly due to the AWP-induced changes of the northward moisture transport and the vertical wind shear and moist static instability associated with more frequent large (small) summer warm pools.
Why Are There Tropical Warm Pools?
Journal of Climate, 2005
Tropical warm pools appear as the primary mode in the distribution of tropical sea surface temperature (SST). Most previous studies have focused on the role of atmospheric processes in homogenizing temperatures in the warm pool and establishing the observed statistical SST distribution. In this paper, a hierarchy of models is used to illustrate both oceanic and atmospheric mechanisms that contribute to the establishment of tropical warm pools. It is found that individual atmospheric processes have competing effects on the SST distribution: atmospheric heat transport tends to homogenize SST, while the spatial structure of atmospheric humidity and surface wind speeds tends to remove homogeneity. The latter effects dominate, and under atmosphere-only processes there is no warm pool. Ocean dynamics counter this effect by homogenizing SST, and it is argued that ocean dynamics is fundamental to the existence of the warm pool. Under easterly wind stress, the thermocline is deep in the west...
Physical processes that drive the seasonal evolution of the Southwestern Tropical Atlantic Warm Pool
Dynamics of Atmospheres and Oceans, 2015
Please cite this article as: Cintra, M.M., Lentini, C.A.D., Servain, J., Araujo, M., Marone, E.,Physical processes that drive the seasonal evolution of the Southwestern Tropical Atlantic Warm Pool, Dynamics of Atmospheres and Oceans (2015), http://dx.ABSTRACT 21 The thermodynamics of the seasonal evolution of the Southwestern Tropical Atlantic 22 Warm Pool (hereafter SWTAWP), which is delimited by the 28°C isotherm, is investigated using 23 the Regional Ocean Modeling System (ROMS). Results indicate that the net heat flux is 24 responsible for the appearance and extinction of the SWTAWP. From March to May, the 25 SWTAWP attains its maximum development and sometimes merges with equatorial warm 26 waters towards the African continent, whose development follows the same period. Along the 27 equator, the combination of oceanic terms (i.e., advection and diffusion) is important to promote 28 the separation -when it occurs -of equatorial warm waters from southwestern tropical waters, A c c e p t e d M a n u s c r i p t 2 which develops off the Brazilian coast. An analysis of the relative contribution of the 30 temperature tendency terms of the mixed layer (ML) heat budget over the appearance, 31 development and extinction of the SWTAWP is also done. The most important term for warming 32 and cooling inside of the ML is the net heat flux at the sea surface. The ML is heated by the 33 atmosphere between October and April, whereas the upper ocean cools down between May and 34 September. The highest heat content values occur during the lower-temperature period (August 35 to October), which is linked to the deepening of the ML during this time period. The horizontal 36 advection along the equator is important, particularly at the eastern domain, which is influenced 37 by the cold tongue. In this area, the vertical diffusive term is also significant; however, it 38 presents values near zero outside the equator. These results contribute to a better understanding 39 of the behavior of the heat budget within the tropical Atlantic, as previous studies over this 40 region focused along the equator only.
Journal of Climate, 2004
Seasonal and interannual variations of the net surface heating (FmT) and sea surface temperature tendency (T,/dt) in the tropical eastern Indian and western Pacific Oceans are studied. The surface heat fluxes are derived from the Special Sensor mcrowavehager and Japanese Geostationary Meteorological Satellite radiance measurements for the period October 1997-September 2000. It is found that the magnitude of solar heating is lager than that of evaporative cooling, but the spatial variation of the latter is significantly large than the former. As a result, the spatial variations of seasonal and interannual variability of F, , , follow closely that of evaporative cooling. Seasonal variations of FNET and T,/dt are significantly correlated, except for the equatorial western Pacific. The high correlation is primarily attributable to high correlation between seasonal cycles of solar heating and T,/dt. The change of FNET between 1997-98 El Nino and 1998-99 La Nina is significantly larger in the tropical eastern Indian Ocean than tropical western Pacific. For the former region, the reduced evaporative cooling arising from weakened winds during the El Nino is generally associated with enhanced solar heating due to decreased cloudiness, and thus increases the interannual variability of FNET. For the latter region, the reduced evaporative cooling due to weakened winds is generally associated with but exceeds the reduced solar heating arising from increased cloudiness, and vise versa. Thus the interannual variability of FNET is reduced due to this offsetting effect. Interannual variations of FNET and T,/dt have very low correlation. This is most likely related to interannual variability of ocean dynamics, which includes the variations of solar radiation penetrating through oceanic mixed layer, upwelling of cold thermocline water, Indonesian throughflow for transporting heat from the Pacific to Indian Ocean, and interhemispheric transport in the Indian Ocean.
Journal of Geophysical Research, 2012
The seasonal and interannual variabilities of warm pool properties in the Pacific and Indian Ocean sectors are examined and contrasted. The properties examined are the size, mean and maximum sea surface temperatures (SSTs), and central position. The seasonal variability is more vigorous in the Indian Ocean sector, but the interannual variability is comparable in the Pacific and Indian Ocean sectors. The variability is associated with significant longitudinal and latitudinal displacements on seasonal time scales but only with longitudinal displacements on interannual time scales. As for the controlling factors, while the seasonal variability of the warm pool is controlled by the annual march of the Sun in the Pacific sector and by the Indian summer monsoon in the Indian Ocean sector, the interannual variability in both sectors is related mostly to El Niño-Southern Oscillation (ENSO). ENSO is closely correlated with the size variations and longitudinal displacements of the warm pool. Interestingly, the warm pool intensity in both sectors is not highly correlated with ENSO until 5 to 6 months after ENSO peaks. The possible causes of this delayed ENSO influence are discussed. Only size and intensity (i.e., mean SST) variations in the Indian Ocean warm pool are significantly correlated with quasi-biennial variability in the Indian monsoon, which indicates that the Indian Ocean warm pool may be a potential predictor for Indian monsoon variations.
El Nin-Southern Oscillation Displacements of the Western Equatorial Pacific Warm Pool
Science, 1990
The western equatorial Pacific warm pool (sea-surface temperatures >29^circC) was observed to migrate eastward across the date line during the 1986-1987 El Nino-Southern Oscillation event. Direct velocity measurements made in the upper ocean from 1986 to 1988 indicate that this migration was associated with a prolonged reversal in the South Equatorial Current forced by a large-scale relaxation of the trade winds. The data suggest that wind-forced zonal advection plays an important role in the thermodynamics of the western Pacific warm pool on interannual time scales.
Inhomogeneous influence of the Atlantic warm pool on United States precipitation
On interannual time scales, the warming of the Atlantic warm pool (AWP) is associated with a tripole sea surface temperature (SST) pattern in the North Atlantic and leads to more rainfall in the central and eastern US. On decadal-to-multidecadal time scales, the AWP warming corresponds to a basin-wide warming pattern and results in less precipitation in the central and eastern US. The inhomogeneous relationship between the AWP warming and US rainfall on different time scales is largely due to the sign of mid-latitude SST anomaly. The negative mid-latitude SST anomaly associated with the tripole pattern may enhance the low sea level pressure over the northeastern North American continent and also enhance the barotropic response there of the AWP-induced barotropic Rossby wave. This strengthened low pressure system, which is not exhibited when the warming is basin-wide, results in a different moisture transport variation and thus the rainfall pattern over the United States.
Journal of Geophysical Research, 2005
A series of ocean general circulation (OGCM) model experiments is carried out using a hybrid coordinate ocean model (HYCOM) to determine the annual cycle of Western Hemisphere Warm Pool (WHWP) heat budget and to assess the appropriateness of commonly used surface flux data sets in driving HYCOM simulations of the WHWP. Among the eight surface heat flux data sets addressed in this study, we find that the simulated SST is closest to the observations when the Southampton constrained (SHC) heat flux data are used, consistent with the conclusion of the data-based study of Enfield and Lee (2005). However, the modeled thermocline water is warmer and its stratification is weaker than observed regardless of the surface heat flux data used, possibly because of the low vertical resolution of the model used in this study. A preliminary heat budget analysis suggests that the surface net heat flux serves as the dominant forcing mechanism in the WHWP regions except in the equatorial Atlantic, where advective processes associated with the equatorial cold tongue are more important. A process of winter overturning that warms the upper layer by convection marks the Gulf of Mexico, while horizontal advection is of little importance there. The eastern north Pacific and Caribbean are affected significantly by vertical and horizontal advection during the onset and peak phases, slowing down the warming considerably.