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

2007 ◽  
Vol 20 (10) ◽  
pp. 2133-2146 ◽  
Author(s):  
S-K. Lee ◽  
D. B. Enfield ◽  
C. Wang
Keyword(s):  
Turbulent Mixing ◽  
General Circulation ◽  
Radiation Flux ◽  
Heat Budget ◽  
Warm Pool ◽  
Western Hemisphere ◽  
Vertical Shear ◽  
Boreal Summer ◽  
Clear Sky ◽  
Cooling Mechanism

Abstract 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.

scholarly journals A Synoptic-Scale Cold-Reservoir Hypothesis on the Origin of the Mature-Stage Super Cloud Cluster: A Case Study with a Global Nonhydrostatic Model

2016 ◽  
Vol 56 ◽  
pp. 14.1-14.24 ◽  
Author(s):  
Kazuyoshi Oouchi ◽  
Masaki Satoh
Keyword(s):  
Indian Ocean ◽  
Warm Pool ◽  
Vertical Shear ◽  
Fundamental Aspect ◽  
Boreal Summer ◽  
Squall Line ◽  
Synoptic Scale ◽  
Zonal Circulation ◽  
Nonhydrostatic Model ◽  
Cloud Cluster

Abstract This chapter proposes a working assumption as a way of conceptual simplification of the origin of Madden–Julian oscillation (MJO)-associated convection, or super cloud cluster (SCC). To develop the simplification, the importance of the synoptic-scale cold reservoir underlying the convection and its interaction with the accompanying zonal–vertical circulation is highlighted. The position of the convection with respect to that of climatological warm pool is postulated to determine the effectiveness of this framework. The authors introduce a prototype hypothesis to illustrate the usefulness of the above assumption based on a numerical simulation experiment with a global nonhydrostatic model for the boreal summer season. Premises for the hypothesis include 1) that the cloud cluster (CC) is a basic building block of tropical convection accompanying the precipitation-generated cold reservoir in its subcloud layer and 2) that a warm-pool-induced quasi-persistent zonal circulation is key for the upscale organization of CCs. The theory of squall-line structure by Rotunno, Klemp, and Weisman (hereafter RKW) is employed for the interpretation. No account is taken regarding the influences of equatorial waves as a first-order approximation. Given the premises, an SCC of O(1000) km scale is interpretable as a gigantic analog of a multicellular squall line embedded in the quasi-stationary westerly shear branch of the zonal circulation east of the warm water pool. A CC corresponds to the “cell,” and its successive formation to the east and westward movement represents an upshear-tilting core of intense updraft. The upshear-tilted SCC is favorably maintained with the precipitating area being separated from the gust front boundary between the cold reservoir and a low-level easterly, which is supported in the realm of the RKW theory where two horizontal vortices associated with the cold reservoir and vertical shear are opposite in sign but cold reservoir’s vorticity can be inferred to be larger, leading to upshear-tilted and multicellular behavior. As a counterexample, CCs to the west of the warm pool (Indian Ocean and Arabian Sea) are embedded in the easterly shear and organized into a less coherent cloud cluster complex (CCC) given the situation of RKW where two horizontal vortices associated with the cold reservoir and vertical shear are still opposite in sign, but the smaller vertical shear west of the warm pool causes even more suboptimal vorticity imbalance in the western flank of cold reservoir, leading to larger tilt with height and intermittent, less viable storm situations. A cold pool or cold reservoir, having been prevalent in mesoscale convection research, is argued to be important for the MJO as pointed out by the emerging evidence in the international field campaign for the MJO called Cooperative Indian Ocean Experiment on Intraseasonal Variability (CINDY)/DYNAMO. The simplified and idealistic hypothesis proposed here does not cover all aspects of MJO and its validation awaits further modeling and observational studies, but it can offer a framework for characterizing a fundamental aspect of the origin of MJO-associated convection.

Seasonal Cycle of the Mixed Layer Heat Budget in the Northeastern Tropical Atlantic Ocean

2013 ◽  
Vol 26 (20) ◽  
pp. 8169-8188 ◽  
Author(s):  
Gregory R. Foltz ◽  
Claudia Schmid ◽  
Rick Lumpkin
Keyword(s):  
Mixed Layer ◽  
Annual Cycle ◽  
Turbulent Mixing ◽  
Seasonal Cycle ◽  
Heat Budget ◽  
Heat Content ◽  
Boreal Summer ◽  
Tropical Atlantic ◽  
Mixed Layer Heat Budget ◽  
Vertical Turbulent Mixing

Abstract The seasonal cycle of the mixed layer heat budget in the northeastern tropical Atlantic (0°–25°N, 18°–28°W) is quantified using in situ and satellite measurements together with atmospheric reanalysis products. This region is characterized by pronounced latitudinal movements of the intertropical convergence zone (ITCZ) and strong meridional variations of the terms in the heat budget. Three distinct regimes within the northeastern tropical Atlantic are identified. The trade wind region (15°–25°N) experiences a strong annual cycle of mixed layer heat content that is driven by approximately out-of-phase annual cycles of surface shortwave radiation (SWR), which peaks in boreal summer, and evaporative cooling, which reaches a minimum in boreal summer. The surface heat-flux-induced changes in the mixed layer heat content are damped by a strong annual cycle of cooling from vertical turbulent mixing, estimated from the residual in the heat balance. In the ITCZ core region (3°–8°N) a weak seasonal cycle of mixed layer heat content is driven by a semiannual cycle of SWR and damped by evaporative cooling and vertical turbulent mixing. On the equator the seasonal cycle of mixed layer heat content is balanced by an annual cycle of SWR that reaches a maximum in October and a semiannual cycle of turbulent mixing that cools the mixed layer most strongly during May–July and November. These results emphasize the importance of the surface heat flux and vertical turbulent mixing for the seasonal cycle of mixed layer heat content in the northeastern tropical Atlantic.

Mechanism of the Double peaked El Nino

2020 ◽  
Author(s):  
Na-Yeon Shin ◽  
Jong-Seong Kug ◽  
Felicity S. McCormack ◽  
Neil J. Holbrook
Keyword(s):  
General Circulation ◽  
Heat Budget ◽  
Coupled Model ◽  
General Circulation Models ◽  
Warm Pool ◽  
Eastern Pacific ◽  
Dynamical Analysis ◽  
Cold Tongue ◽  
Central Pacific ◽  
Sst Anomalies

<p>   In the past decades, our understanding of the ENSO phenomenon increased steadily. Especially, one of the most interesting topics was the El Niño type because of the different global impacts. The classic classification is the two types of the El Niño and there are various terms to refer this. The conventional El Niño is called the Cold tongue El Niño or the Eastern pacific El Niño. And the other type of the El Niño is called the Warm pool El Niño, the Central pacific El Niño, the El Niño Modoki or the dateline El Niño. However, in Coupled Model Intercomparison Project version 5 (CMIP5) Coupled General Circulation Models (CGCMs) results, those have been shown the Double peaked El Niño events which are the new type of the El Niño due to the climatological cold tongue bias. Double peaked El Niño events are defined as a positive sea surface temperature anomalies are separated into two centers (in Western and Eastern Pacific) and grow individually and simultaneously, and the peak of SST anomalies exceeds the threshold.</p><p>   Double peaked El Niño events are found in not only the models, but also the observations. But there are no dynamical analysis of observations. In this study, the mechanism giving rise to Double peaked El Niño in observation is examined by analyzing the mixed layer heat budget equation and comparing with the Warm Pool El Niño and Cold tongue El Niño.</p><p>   The warm SST anomalies of the western peak and the eastern peak are caused by different dynamic mechanism. Western peaks of Double peaked El Niño are similar to the Warm Pool El Niño. Those can be developed by Zonal advection feedback terms and negative anomalous wind speed, whereas eastern peaks of Double peaked El Niño are different from Warm pool El Niño. Thermocline feedback term considerably contribute to the occurrence of eastern peak. Differences of intensity of the precipitation(4-8N, 195-225E) derive other significant differences of the zonal wind stress(5S-5N, 170-200E), sea level(5S-5N, 230-250E) and zonal current(5S-5N, 230-250E). Thus, the process above can induce the eastern peak of the Double peaked El Niño.</p>

scholarly journals Interhemispheric Influence of the Atlantic Warm Pool on the Southeastern Pacific

2010 ◽  
Vol 23 (2) ◽  
pp. 404-418 ◽  
Author(s):  
Chunzai Wang ◽  
Sang-Ki Lee ◽  
Carlos R. Mechoso
Keyword(s):  
Amazon Basin ◽  
Large Body ◽  
Lower Troposphere ◽  
Late Summer ◽  
Atmospheric Model ◽  
Warm Pool ◽  
Tropical Pacific ◽  
Western Hemisphere ◽  
Boreal Summer ◽  
Atlantic Warm Pool

Abstract 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. It is shown by analyses of observational data and NCAR community atmospheric model simulations that an anomalously large (small) AWP during the boreal summer/fall results in a strengthening (weakening) of the Hadley-type circulation with enhanced descent (ascent) over the southeastern tropical Pacific. It is further demonstrated—by using a simple two-level model linearized about a specified background mean state—that the interhemispheric connection between the AWP and the southeastern tropical Pacific depends on the configuration of the background mean zonal winds in the Southern Hemisphere.

The Impact of Horizontal Resolution on the Tropical Heat Budget in an Atlantic Ocean Model

2005 ◽  
Vol 18 (6) ◽  
pp. 841-851 ◽  
Author(s):  
Markus Jochum ◽  
Raghu Murtugudde ◽  
Raffaele Ferrari ◽  
Paola Malanotte-Rizzoli
Keyword(s):  
Heat Flux ◽  
General Circulation ◽  
Heat Budget ◽  
Circulation Model ◽  
Warm Pool ◽  
Shear Instability ◽  
Entrainment Rate ◽  
Cold Tongue ◽  
Coarse Resolution ◽  
The Impact

Abstract An ocean general circulation model (OGCM) of the tropical Atlantic is coupled to an advective atmospheric boundary layer model. This configuration is used to investigate the hypothesis that resolving tropical instability waves (TIWs) in OGCMs will remove the equatorial cold bias that is a feature common to coarse-resolution OGCMs. It is shown that current eddy parameterizations cannot capture the TIW heat flux because diffusion in coarse-resolution OGCMs removes heat from the warm pool to heat the equatorial cold tongue, whereas TIWs draw their heat mostly from the atmosphere. Thus, they can bring more heat to the equatorial cold tongue without cooling the warm pool, and the SST in the warm pool is higher and more realistic. Contrary to expectations, the SST in the equatorial cold tongue is not significantly improved. The equatorial warming due to TIWs is slightly greater than the warming due to diffusion, but this increased equatorial heat flux in the high-resolution experiment is compensated by increased equatorial entrainment there. This is attributed to the Equatorial Undercurrent being stronger, thereby increasing the entrainment rate through shear instability. Thus, higher resolution does not significantly increase the total oceanic heat flux convergence in the equatorial mixed layer.

scholarly journals Intraseasonal Modulation of the Surface Cooling in the Gulf of Guinea

2013 ◽  
Vol 43 (2) ◽  
pp. 382-401 ◽  
Author(s):  
Julien Jouanno ◽  
Frédéric Marin ◽  
Yves du Penhoat ◽  
Jean-Marc Molines
Keyword(s):  
Mixed Layer ◽  
Gravity Waves ◽  
Heat Budget ◽  
Vertical Shear ◽  
Upper Ocean ◽  
Equatorial Waves ◽  
Gulf Of Guinea ◽  
The Core ◽  
Mixed Layer Heat Budget ◽  
Inertia Gravity Waves

Abstract A regional numerical model of the tropical Atlantic Ocean and observations are analyzed to investigate the intraseasonal fluctuations of the sea surface temperature at the equator in the Gulf of Guinea. Results indicate that the seasonal cooling in this region is significantly shaped by short-duration cooling events caused by wind-forced equatorial waves: mixed Rossby–gravity waves within the 12–20-day period band, inertia–gravity waves with periods below 11 days, and equatorially trapped Kelvin waves with periods between 25 and 40 days. In these different ranges of frequencies, it is shown that the wave-induced horizontal oscillations of the northern front of the mean cold tongue dominate the variations of mixed layer temperature near the equator. But the model mixed layer heat budget also shows that the equatorial waves make a significant contribution to the mixed layer heat budget through modulation of the turbulent cooling, especially above the core of the Equatorial Undercurrent (EUC). The turbulent cooling variability is found to be mainly controlled by the intraseasonal modulation of the vertical shear in the upper ocean. This mechanism is maximum during periods of seasonal cooling, especially in boreal summer, when the surface South Equatorial Current is strongest and between 2°S and the equator, where the presence of the EUC provides a background vertical shear in the upper ocean. It applies for the three types of intraseasonal waves. Inertia–gravity waves also modulate the turbulent heat flux at the equator through vertical displacement of the core of the EUC in response to equatorial divergence and convergence.

Subseasonal SST Variability in the Tropical Eastern North Pacific during Boreal Summer

2008 ◽  
Vol 21 (17) ◽  
pp. 4149-4167 ◽  
Author(s):  
Eric D. Maloney ◽  
Dudley B. Chelton ◽  
Steven K. Esbensen
Keyword(s):  
Costa Rica ◽  
Warm Pool ◽  
Boreal Summer ◽  
Sst Variability ◽  
East Pacific ◽  
Pacific Warm Pool ◽  
Northern Edge ◽  
Precipitation Anomalies ◽  
Sst Anomalies ◽  
Costa Rica Dome

Abstract Boreal summer intraseasonal (30–90-day time scale) sea surface temperature (SST) variability in the east Pacific warm pool is examined using Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) sea surface temperatures during 1998–2005. Intraseasonal SST variance maximizes at two locations in the warm pool: in the vicinity of 9°N, 92°W near the Costa Rica Dome and near the northern edge of the warm pool in the vicinity of 19°N, 108°W. Both locations exhibit a significant spectral peak at 50–60-day periods, time scales characteristic of the Madden–Julian oscillation (MJO). Complex empirical orthogonal function (CEOF) and spectra coherence analyses are used to show that boreal summer intraseasonal SST anomalies are coherent with precipitation anomalies across the east Pacific warm pool. Spatial variations of phase are modest across the warm pool, although evidence exists for the northward progression of intraseasonal SST and precipitation anomalies. Intraseasonal SSTs at the north edge of the warm pool lag those in the vicinity of the Costa Rica Dome by about 1 week. The MJO explains 30%–40% of the variance of intraseasonal SST anomalies in the east Pacific warm pool during boreal summer. Peak-to-peak SST variations of 0.8°–1.0°C occur during MJO events. SST is approximately in quadrature with MJO precipitation, with suppressed (enhanced) MJO precipitation anomalies leading positive (negative) SST anomalies by 7–10 days. Consistent with the CEOF and coherence analyses, MJO-related SST and precipitation anomalies near the Costa Rica Dome lead those at the northern edge of the warm pool by about 1 week.

scholarly journals Impacts of Local Soil Moisture Anomalies on the Atmospheric Circulation and on Remote Surface Meteorological Fields during Boreal Summer: A Comprehensive Analysis over North America

2016 ◽  
Vol 29 (20) ◽  
pp. 7345-7364 ◽  
Author(s):  
Randal D. Koster ◽  
Yehui Chang ◽  
Hailan Wang ◽  
Siegfried D. Schubert
Keyword(s):  
Soil Moisture ◽  
North America ◽  
Atmospheric Circulation ◽  
General Circulation ◽  
Boreal Summer ◽  
Near Surface ◽  
Local Soil ◽  
Lower Mississippi River ◽  
Continental Interior ◽  
The U.S

Abstract A series of stationary wave model (SWM) experiments are performed in which the boreal summer atmosphere is forced, over a number of locations in the continental United States, with an idealized diabatic heating anomaly that mimics the atmospheric heating associated with a dry land surface. For localized heating within a large portion of the continental interior, regardless of the specific location of this heating, the spatial pattern of the forced atmospheric circulation anomaly (in terms of 250-hPa eddy streamfunction) is largely the same: a high anomaly forms over west-central North America and a low anomaly forms to the east. In supplemental atmospheric general circulation model (AGCM) experiments, similar results are found; imposing soil moisture dryness in the AGCM in different locations within the U.S. interior tends to produce the aforementioned pattern, along with an associated near-surface warming and precipitation deficit in the center of the continent. The SWM-based and AGCM-based patterns generally agree with composites generated using reanalysis and precipitation gauge data. The AGCM experiments also suggest that dry anomalies imposed in the lower Mississippi River valley have remote surface impacts of particularly large spatial extent, and a region along the eastern half of the U.S.–Canadian border is particularly sensitive to dry anomalies in a number of remote areas. Overall, the SWM and AGCM experiments support the idea of a positive feedback loop operating over the continent: dry surface conditions in many interior locations lead to changes in atmospheric circulation that act to enhance further the overall dryness of the continental interior.

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