How are large western hemisphere warm pools formed? (original) (raw)

A Further Study of the Tropical Western Hemisphere Warm Pool

Journal of Climate, 2003

Variability of the tropical Western Hemisphere warm pool (WHWP) of water warmer than 28.5ЊC, which extends seasonally over parts of the eastern North Pacific, the Gulf of Mexico, the Caribbean, and the western tropical North Atlantic (TNA), was previously studied by Wang and Enfield using the da Silva data from 1945-93. Using additional datasets of the NCEP-NCAR reanalysis field and the NCEP SST from 1950-99, and the Levitus climatological subsurface temperature, the present paper confirms and extends the previous study of Wang and Enfield. The WHWP alternates with northern South America as the seasonal heating source for the Walker and Hadley circulations in the Western Hemisphere. During the boreal winter a strong Hadley cell emanates northward from the Amazon heat source with subsidence over the subtropical North Atlantic north of 20ЊN, sustaining a strong North Atlantic anticyclone and associated northeast (NE) trade winds over its southern limb in the TNA. This circulation, including the NE trades, is weakened during Pacific El Niñ o winters and results in a spring warming of the TNA, which in turn induces the development of an unusually large summer warm pool and a wetter Caribbean rainy season. As the WHWP develops in the late boreal spring, the center of tropospheric heating and convection shifts to the WHWP region, whence the summer Hadley circulation emanates from the WHWP and forks into the subsidence regions of the subtropical South Atlantic and South Pacific. During the summers following El Niñ o, when the warm pool is larger than normal, the increased Hadley flow into the subtropical South Pacific reinforces the South Pacific anticyclone and trade winds, probably playing a role in the transition back to the cool phase of ENSO.

What Drives the Seasonal Onset and Decay of the Western Hemisphere Warm Pool?

Journal of Climate, 2007

The annual heat budget of the Western Hemisphere warm pool (WHWP) is explored using the output of an ocean general circulation model (OGCM) simulation. According to the analysis, the WHWP cannot be considered as a monolithic whole with a single set of dominating processes that explain its behavior. The three regions considered, namely the eastern north Pacific (ENP), the Gulf of Mexico (GoM), and the Caribbean Sea (CBN), are each unique in terms of the atmospheric and oceanic processes that dominate the corresponding heat budgets. In the ENP region, clear-sky shortwave radiation flux is responsible for the growth of the warm pool in boreal spring, while increased cloud cover in boreal summer and associated reduction in solar radiation play a crucial role for the ENP warm pool’s demise. Ocean upwelling in the Costa Rica Dome connected to surrounding areas by horizontal advection offers a persistent yearlong cooling mechanism. Over the Atlantic, the clear-sky radiation flux that increases monotonically from December to May and decreases later is largely responsible for the onset and decay of the Atlantic-side warm pool in boreal summer and fall. The CBN region is affected by upwelling and horizontal advective cooling within and away from the coastal upwelling zone off northern South America during the onset and peak phases, thus slowing down the warm pool’s development, but no evidence was found that advective heat flux divergence is important in the GoM region. Turbulent mixing is also an important cooling mechanism in the annual cycle of the WHWP, and the vertical shear at the warm pool base helps to sustain the turbulent mixing. Common to all three WHWP regions is the reduction of wind speed at the peak phase, suggestive of a convection–evaporation feedback known to be important in the Indo-Pacific warm pool dynamics.

The Heat Balance of the Western Hemisphere Warm Pool

Journal of Climate, 2005

The thermodynamic development of the Western Hemisphere warm pool and its four geographic subregions are analyzed. The subregional warm pools of the eastern North Pacific and equatorial Atlantic are best developed in the boreal spring, while in the Gulf of Mexico and Caribbean, the highest temperatures prevail during the early and late summer, respectively. For the defining isotherms chosen (Ն27.5°, Ն28.0°, Ն28.5°C) the warm pool depths are similar to the mixed-layer depth (20-40 m) but are considerably less than the Indo-Pacific warm pool depth (50-60 m). The heat balance of the WHWP subregions is examined through two successive types of analysis: first by considering a changing volume ("bubble") bounded by constant temperature wherein advective fluxes disappear and diffusive fluxes can be estimated as a residual, and second by considering a slab layer of constant dimensions with the bubble diffusion estimates as an additional input and the advective heat flux divergence as a residual output. From this sequential procedure it is possible to disqualify as being physically inconsistent four of seven surface heat flux climatologies: the NCEP-NCAR reanalysis (NCEP1) and the ECMWF 15-yr global reanalysis (ERA-15) because they yield a nonphysical diffusion of heat into the warm pools from their cooler surroundings, and the unconstrained da Silva and Southampton datasets because their estimated diffusion rates are inconsistent with the smaller rates of the better understood Indo-Pacific warm pool when the bubble analysis is applied to both regions. The remaining surface flux datasets of da Silva and Southampton (constrained) and Oberhuber have a much narrower range of slab surface warming (ϩ25 Ϯ 5 W m Ϫ2 ) associated with bubble residual estimates of total diffusion of -5 to -20 W m Ϫ2 (Ϯ5 W m Ϫ2 ) and total advective heat flux divergence of -2 to -14 W m Ϫ2 (Ϯ5 W m Ϫ2 ). The latter are independently confirmed by direct estimates using wind stress data and drifters for the Gulf of Mexico and eastern North Pacific subregions.

Impact of the Atlantic Warm Pool on the Summer Climate of the Western Hemisphere

Journal of Climate, 2007

The Atlantic warm pool (AWP) is a large body of warm water that comprises the Gulf of Mexico, the Caribbean Sea, and the western tropical North Atlantic. Located to its northeastern side is the North Atlantic subtropical high (NASH), which produces the tropical easterly trade winds. The easterly trade winds carry moisture from the tropical North Atlantic into the Caribbean Sea, where the flow intensifies, forming the Caribbean low-level jet (CLLJ). The CLLJ then splits into two branches: one turning northward and connecting with the Great Plains low-level jet (GPLLJ), and the other continuing westward across Central America into the eastern North Pacific. The easterly CLLJ and its westward moisture transport are maximized in the summer and winter, whereas they are minimized in the fall and spring. This semiannual feature results from the semiannual variation of sea level pressure in the Caribbean region owing to the westward extension and eastward retreat of the NASH.

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.

Interhemispheric Influence of the Atlantic Warm Pool on the Southeastern Pacific

Journal of Climate, 2010

The Atlantic warm pool (AWP) is a large body of warm water comprising the Gulf of Mexico, Caribbean Sea, and western tropical North Atlantic. The AWP can vary on seasonal, interannual, and multidecadal time scales. The maximum AWP size is in the boreal late summer and early fall, with the largest extent in the year being about 3 times the smallest one. The AWP alternates with the Amazon basin in South America as the seasonal heating source for circulations of the Hadley and Walker type in the Western Hemisphere. During the boreal summer/fall, a strong Hadley-type circulation is established, with ascending motion over the AWP and subsidence over the southeastern tropical Pacific. This is accompanied by equatorward flow in the lower troposphere over the southeastern tropical Pacific, as dynamically required by the Sverdrup vorticity balance.

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.