The Western Hemisphere Warm Pool (WHWP) – Watts Up With That?

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By Andy May

As seen in the first post of this series the AMO (Atlantic Multidecadal Oscillation) and the WHWP (Western Hemisphere Warm Pool) area are the two climate oscillations that explain most of the variability (64%) in the HadCRUT5 global mean surface temperature reconstruction (GMST) since 1950. Adding the Southern Annular Mode (SAM) explains 77% of HadCRUT5 variability.

The Western Hemisphere warm pool or the WHWP is an anomaly based on the area of the ocean warmer than 28.5°C (that is within the 28.5°C isotherm) and approximately within the rectangular region from 7°N – 27°N and 110°W to 50°W. This area extends from the eastern North Pacific (west of Mexico, Central America, and Columbia) to the Gulf of America, the Caribbean, and well into the Atlantic during the WHWP peak in August and September (Wang & Enfield, 2001) and (Wang & Enfield, 2003). It is significant because deep convection starts at about 28°C (Sud, Walker, & Lau, 1999).

The WHWP nearly disappears in the Northern Hemisphere winter and begins in the eastern Pacific off the coast of Mexico and Central America each spring (think the current Hurricane Erick). It spreads northeastward across Mexico via an atmospheric bridge into the Caribbean and the Gulf of America in June and July. It typically reaches its maximum size in September (see figure 1). Unlike the western Indo-Pacific warm pool which straddles the equator, the WHWP is entirely north of the equator (Wang & Enfield, 2003). Figure 1 shows some key maps of the 1950-2000 average 28.5°C SST isotherm from Wang and Enfield’s 2003 paper.

Figure 1. Key monthly average contour maps of 1950-2000 average SST in the WHWP region. The critical 28.5°C isotherm is shaded. Note the peak area on the Pacific side is in May and the peak Atlantic side area is in September. To see maps for more months see figure 1 in (Wang & Enfield, 2003).

All indices of Atlantic tropical cyclone activity include a multidecadal variation that is consistent with multidecadal variations of the AMO (Goldenberg, Landsea, Mestas-Nuñez, & Gray, 2001) and the Atlantic portion of the WHWP, sometimes called the AWP or the Atlantic Warm Pool (Wang, Lee, & Enfield, 2008). When the Atlantic portion of the WHWP is large it reduces vertical wind shear and increases the instability of the troposphere, both of which increase hurricane activity (Wang, Lee, & Enfield, 2008). The WHWP has strong ties to the AMO and a statistical connection to ENSO (Wang, Lee, & Enfield, 2008) and (Enfield & Mayer, 1997).

Due to the equatorial Atlantic easterly winds and ocean currents, water warmed by the Sun in the Northern Hemispheric summer collects in the Gulf of America and Caribbean forming the core of the AWP. While the Gulf Stream carries away a lot of this heat, it cannot keep up in the summer and the water warms until deep convection starts. The deep convection forms high level clouds that keep longwave radiation from escaping and act as a positive feedback. The increase in SST and evaporation act to lower sea level air pressure further increasing cloudiness and forming organized storms (Wang & Enfield, 2003). Atlantic and Caribbean hurricanes form within the WHWP and act as giant air conditioners that suck heat from the sea surface and take it almost as high as the stratosphere in some strong storms, they also transport heat as far as the North Atlantic and Canada. These processes accelerate the transport of the excess energy to outer space.

Hurricanes often rapidly intensify both south and north of Cuba in August and September. The WHWP very quickly dissipates after October. The heat fluxes in the WHWP are illustrated in figure 2, which is from Wang & Enfield (2003).

Figure 2. 2a: SST, net heat flux, and ocean heat storage. 2b: Heat fluxes, solar is positive heat flux, latent, longwave, and sensible fluxes are negative. The net flux in (a) is solar-latent-longwave-sensible. The zero line in (a) is where the ocean heat storage tendency is balanced, below zero is heat loss from ocean and above zero is heating of the upper ocean.

In figure 2a SST, net heat flux, and ocean heat storage are plotted by average 1950-2000 monthly values. The horizontal blue line is at zero ocean heat storage to divide ocean cooling from ocean warming, the boundaries are in February and August. SST changes follow heat flux changes by three to four months. The individual heat fluxes are plotted in figure 2b, the net flux in (a) is the shortwave (solar) flux minus the net longwave, net latent (evaporation), and net sensible fluxes which are all negative (Wang & Enfield, 2003).

The longwave radiation is computed using the graybody flux from the ocean surface and factoring in the back radiation from clouds. The latent flux takes into account evaporation, which is a function of SST and average windspeed. Sensible heat flux is mostly a function of wind speed. The average depth of the mixed layer, and thus the SSTs shown in figure 1, is about 25 meters.

The WHWP is closely correlated to both the Niño-3 anomaly and the tropical North Atlantic anomaly, R2 = 0.68 and 0.63 respectively (Wang & Enfield, 2003). Unsurprisingly, the eastern North Pacific portion of the WHWP is very closely correlated with Niño-3 with a zero time-lag. Niño-3 and the overall WHWP have a three-month lag. Figure 3 displays the full year WHWP and its 5-year running mean.

Figure 3. The full year WHWP average and its 5-year running mean. Data from NOAA.

As we saw in post one, the WHWP is closely related to the global mean surface temperature (GMST) something also pointed out in Wang and Enfield, 2003. The annual development and destruction of the WHWP correlates closely with seasonal precipitation, temperature, and storminess over North and Central America. The WHWP nearly disappears every winter, so the key months for the WHWP are from May through October. Figure 4 plots the average just for these critical months, I added the HadCRUT5 GMST for comparison. The close relationship between HadCRUT5 and WHWP is easily seen.

Figure 4. The “summer average,” that is the average of the months May through October, the active WHWP period. Data from NOAA. The heavy gray curve is the HadCRUT5 average for the whole year.

Although the WHWP is not discussed as much as the AMO, PDO, ENSO, and other oscillations it is a good predictor of the HadCRUT5 global mean surface temperature. In combination with the Antarctic Oscillation or Southern Annular Mode and the AMO it does a very good job. This suggests that The North Atlantic and the Southern Hemisphere circulation patterns correlate very well with global climate trends, CO2 may fit in there somewhere, but it must share the spotlight with these natural oscillations.

Enfield, D. B., & Mayer, D. A. (1997). Tropical Atlantic sea surface temperature variability and its relation to El Niño-Southern Oscillation. Journal of Geophysical Research: Oceans, 102(C1). doi:10.1029/96JC03296

Goldenberg, S. B., Landsea, C. W., Mestas-Nuñez, A. M., & Gray, W. M. (2001). The Recent Increase in Atlantic Hurricane Activity: Causes and Implications. Science, 293(5529), 474-479. doi:10.1126/science.1060040

Sud, Y. C., Walker, G. K., & Lau, K. M. (1999). Mechanisms Regulating Sea-Surface Temperatures and Deep Convection in the Tropics. Geophysical Research Letters, 26(8), 1019-1022. doi:10.1029/1999GL900197

Wang, C., & Enfield, D. B. (2001). The Tropical Western Hemisphere Warm Pool. Geophysical Research Letters, 28(8). doi:10.1029/2000GL011763

Wang, C., & Enfield, D. B. (2003). A Further Study of the Tropical Western Hemisphere Warm Pool. Journal of Climate, 16(10), 1476-1493. doi:10.1175/1520-0442(2003)016<1476:AFSOTT>2.0.CO;2

Wang, C., Lee, S.-K., & Enfield, D. B. (2008). Atlantic Warm Pool acting as a link between Atlantic Multidecadal Oscillation and Atlantic tropical cyclone activity. Geochemistry, Geophysics, Geosystems, 9(5). doi:10.1029/2007GC001809

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